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Zinc alloys are an excellent material for the production of resistant, aesthetically appealing and complex shaped components: they can indeed be used to meet the requests of various and different industries, from automotive to building sector, ensuring high performance standards, both from a functional and an aesthetical point of view. Taken into account the different characteristics required by each sector it is therefore fundamental to conduct a thorough analysis of zinc alloys with the aim of identifying the most appropriate one for the purpose we would like to reach, examining the requirements of the product and looking for a zinc alloy with the right properties to meet these demands.

In order to conduct this analysis it is essential to know zinc alloys classification, which can be divided into two main categories: ZAMAK alloys, whose name is composed of the word Alloy followed by a number on the basis of their sequential development, and zinc alloys classified with the prefix ZA, which have a remarkable higher percentage of aluminum compared to the previous ones and are therefore provided with a greater resistance.

At the beginning ZAMAK was a trademark registered by New Jersey Zinc Co., the first company to use this specific alloy, but with the passing of time it has been chosen as the common name to define all zinc alloys for die casting. Only with the development of numerous types of alloys the nomenclature of zinc alloys has become more structured, as we have seen with the distinction between ZAMAK and ZA alloys: however, ZAMAK is still considered the most common and conventional name to indicate a zinc alloy.

This acronym indeed defines a zinc alloy composed of four main metals: zinc, aluminum, magnesium and copper (Kupfer in German). These elements occur in zinc alloys in different percentages: alloys indeed differ on the basis of the concentration of each metal, thus showing different characteristics that affect the features of the diecast. These characteristics are crucial for the choice of the most suitable zinc alloy for one’s own technology and production process. ZAMAK can indeed be worked with different production processes: hot chamber die casting, cold chamber die casting, spin casting, sludge casting and gravity die casting. In this article we are going to focus on the best zinc alloys for hot chamber die casting, the technology that Bruschi employs for its business.

The main zinc alloys used for hot chamber die casting are:

• Alloy 2
• Alloy 3
• Alloy 5
• ZA8

ZA8 is the only alloy, among the previous ones, that does not belong to ZAMAK classification because it contains a high quantity of aluminum that categorizes it as ZA: it is however an especially suitable alloy for hot chamber die casting and for this reason it has been included in this analysis.

As anticipated, these alloys have different compositions: they are indeed composed of the same elements but they are dosed in different percentages, so as to enhance specific characteristics such as resistance or fluidity. The following table shows the composition of the previously listed zinc alloys, defined by the European regulations EN 12844 European Standard for Zinc Alloy Castings and EN 1774 Zinc and zinc alloys – Alloys for foundry purposes – Ingot and liquid:

Zinc alloys chemical composition

The table highlights the different concentrations of the materials in the different zinc alloys, especially aluminum and copper concentrations: these two elements are indeed the ones that mainly influence the characteristics of the alloys.

Zinc alloys for hot chamber die casting

Alloy 2

Alloy 2 is characterized by high resistance and hardness: it is indeed the most resistant alloy among all zinc alloys. Aluminum percentage is equivalent to that of Alloy 3 and Alloy 5, while copper quantity is definitely higher: it can reach 3,3% and it is precisely this high copper percentage that provides an excellent resistance to the alloy. However, over time such a high amount can lead to the alteration of specific characteristics of the alloy: during metal aging it is indeed possible to observe some issues related to dimensions, such as a small dimensional variation. Furthermore, due to aging, the alloy can be subjected to performance decrease, which can reach levels similar to those of aluminum alloys: as a matter of fact, the main disadvantage when employing Alloy 2 is related to performance reduction, particularly a reduction of ductility over time. Despite this performance loss caused by aging, Alloy 2 is an excellent material for die casting thanks to its exceptional castability and creep performance, as well as its ability to maintain high resistance and hardness standards also during long-term aging.

Alloy 3

Alloy 3 is the most popular zinc alloy in North America: it is indeed the mostly used alloy because of its excellent castability and dimensional stability over time. Alloy 3 has a superior dimensional stability compared to the other alloys, however this high stability degree is only significant when very narrow tolerances are required. As far as resistance is concerned, Alloy 3 is characterized by a low percentage of copper that implies a lower resistance compared to the other zinc alloys but, at the same time, it reduces the likelihood of alterations due to material aging, thus avoiding dimensional shrinkage and maintaining a constant performance over time. This zinc alloy is also perfect for the production of components that need surface treatments, such as plating, painting and chroming.

Alloy 5

Alloy 5 is the most commonly used zinc alloy in Europe. This alloy has outstanding castability properties and, compared to Alloy 3, it contains a slightly higher copper percentage that gives the alloy a superior resistance and hardness, as well as a better creep performance. This higher copper quantity is though responsible, as previously seen, for an inferior ductility that can affect the processability of the alloy during secondary operations such as bending, riveting, pressing or crimping. Similarly to Alloy 3, Alloy 5 is an excellent choice for products that require surface finish treatments.

ZA 8

ZA8, as suggested by the classification ZA and not Alloy, is characterized by a chemical composition that has a high aluminum amount, considerably higher compared to the other zinc alloys. This high aluminum concentration provides the alloy with a superior resistance, hardness and creep performance in relation to the other alloys, except for Alloy 2 that has very similar features. ZA 8, like the other alloys, is appropriate for surface finish processes such as chroming and painting.

Main characteristics of zinc alloys

Through the analysis of the chemical composition of zinc alloys it is therefore possible to define which alloy is the most appropriate for one’s own sector, not only on the basis of mechanical and physical properties of zinc alloys, but also with regard to the features of the product that has to be realized.

The choice of the best zinc alloy for hot chamber die casting indeed starts from an examination of the characteristics of the component:

• Which are its most significant features?
• Is it an aesthetical or a functional product?
• Will it undergo surface treatments?
• Does it require a higher resistance or a greater dimensional stability?

Answer similar questions and therefore conduct a careful analysis of the requirements of the product is indeed the first step for the choice of the raw material.

Once the main characteristics of the product have been identified, it is possible to analyze the properties of the different zinc alloys.

As their previous descriptions report, the most significant characteristics of zinc alloys are castability, resistance, hardness, dimensional stability, creep performance and suitability for surface treatments. Now we will see these properties in detail.

Castability

The four zinc alloys mainly used for hot chamber die casting have all an excellent castability: it is exactly this feature that makes them specifically suitable for hot chamber die casting. High castability levels allow the die caster to produce components characterized by very thin walls, which are lighter compared to components produced with other metals. This ability to obtain minimum thickness leads also to a remarkable saving in terms of costs.

Resistance and hardness

Zinc alloys with the highest resistance and hardness are Alloy 2 and ZA8. Alloy 2 indeed contains a high copper percentage that gives it a particular resistance, while ZA8, despite having a copper amount similar to that of Alloy 5, is more resistant and hard thanks to the extremely high aluminum percentage (8,0-8,8%). These two alloys are therefore perfect for the production of components that need a great resistance and hardness but do not require specific dimensional parameters.

Dimensional stability

On the contrary, products that need a high dimensional stability over time should be produced using Alloy 3 or Alloy 5. These two alloys have indeed the right copper percentage to ensure a good resistance, while at the same time assuring dimensional stability over time. They are an excellent choice for the production of components with very complex shapes that require minimum dimensional shrinkage in order not to compromise the functionality of the product as time goes by. Thanks to this balance between resistance and dimensional stability Alloy 3 and Alloy 5 are the most widely used zinc alloys respectively in North America and Europe.

Creep performance

As far as creep performance is concerned, the most efficient alloys are Alloy 2 and ZA8. However, also Alloy 5 has a good creep performance, superior to that of Alloy 3, consequently it is more suitable to be used at high temperatures under a continuous stress.

Surface treatments

All zinc alloys listed in this post are appropriate for surface treatments: high fluidity of ZAMAK allows the die caster to apply various kinds of finishes, from chroming to powder coating. Zinc components can be worked in order to achieve almost every type of surface finish desired, thus obtaining aesthetically appealing products thanks to a shining chrome finish or a painting with intense and vivid colors, as well as smooth and silky surfaces thanks to satin finishing.

How to choose the most appropriate zinc alloy

Ultimately, to choose the most appropriate zinc alloy for one’s own product it is essential to focus on two main concepts: which are the characteristics that the final product should have and which are the properties of the zinc alloy that are fundamental to achieve these characteristics. If the component must be specifically resistant because it will be exposed to a high stress during its life cycle, then it will be more advantageous to choose Alloy 2 or ZA8. On the contrary, if the component needs a high dimensional stability level because it has very narrow tolerances, then it will be more convenient to choose Alloy 3 or Alloy 5.

Each product has specific and unique technical features that must be carefully examined in order to identify the most suitable zinc alloy, thus enhancing as much as possible the final outcome. The die cast supplier is consequently a valuable help when looking for the best zinc alloy to use because, thanks to the experience gained in different sectors and to the deep knowledge of the raw material, he or she can give precious advice for the achievement of the requested performance.

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The case study concerns a component produced for the building industry and the related mold, which was built 10 years ago. Over the years the mold began to cause problems due to its wear condition: it was therefore necessary to study and examine mold design and to introduce significant changes, in order to reduce casting defects that emerged on the component.

In accordance with the lesson learned approach, Bruschi engineers have analyzed production performances of the component and defined new parameters on the basis of the knowledge they assimilated through the years. Furthermore, cutting edge technology, such as simulation software, has allowed Bruschi design department to foresee material reactions and to make accurate changes to mold design, in order to reduce casting defects. In this way, they have figured out new solutions with the aim of achieving a component that could meet the requested quality standards.

Elements that influence superficial quality of the diecast

Superficial quality of a component produced with die casting technology can, indeed, be determined by multiple factors, such as mold thermoregulation and temperature of the material injected in the cavity, but it mainly depends on the filling method of the feeder. The design of an appropriate feeder, which allows a symmetrical filling of the cavities, is therefore a fundamental requirement to avoid superficial defects on the product. On the contrary, an asymmetric or divergent feeder can cause air entrapment in the die cast, as well as areas with evident cold laps caused by irregular filling.

Case study

This case study is about a component that is part of a hinge for a door. After several years of component production with the first mold, the latter was worn out and it caused superficial defects on the product, such as cold laps, lack of material and porosity. Bruschi engineers have conducted an analysis of the parameters and of the geometry of the mold, after that they have identified the leading cause of the defects: the incorrect filling of the cavities. In order to solve this problem they have thus decided to design and build a new mold. With a view to reduce casting defects the design department has therefore modified mold design, more specifically feeder design.

Phase 1 – Simulation

The first phase of mold design project concerned simulation of the filling of the cavities with thermofluidodynamic simulation software Magmasoft, which allows foreseeing to a good approximation the behavior of the material during filling phase. With Magmasoft it is indeed possible to examine different parameters, such as temperature of the metal in every stage of the filling, air pressure in the cavity, material speed and presence of areas with air entrapments. For further information on simulation, here are some previous posts:

• Simulation for HPDC: surface aesthetical quality in automotive
• Simulation for HPDC: shrinkage porosity case study
• Simulation for HPDC: die maintenance and optimization of set up

Simulation has highlighted three main problems:

• Asymmetrical filling
• Divergent feeder
• Lack of homogeneity in the material temperature

To best define the corrective actions to implement engineers had to carefully observe product’s shape: the component has, indeed, a thicker upper section and a thinner lower section. The product’s geometry has thus requested a specific care during the design of the feeder because, in order to achieve a symmetrical and regular filling, it was necessary to add further elements that could facilitate the dynamic behavior of the filling fluid.

Phase 2 – Upper section design

Simulation phase has revealed that the filling of the upper section was asymmetrical and therefore caused air entrapment in the die cast, with consequent air bubbles presence on the component’s surface. The cause of the asymmetrical filling was ascribed to the different speeds at which the material started to fill the cavity. As a matter of fact, the fluid entered from the injection point with a high kinetic energy and continued its way by following the cavity geometry, which drove the material to fill specific areas that were thus quickly and more filled, reaching instead only at a later stage those areas that were not in the direction of the main flow.

The first problem to solve was therefore related to the asymmetrical entrance of the material in the gate: in order to symmetrize material entry in the cavity, designers have added a damper at the end of the main feeder. With the new configuration the flow, before dividing, fills the damper while losing kinetic energy and continues its way towards the cavity at a constant speed.

The second problem, still concerning the upper section of the die cast, was related to the divergence of the feeder in specific sections, thus causing the appearance of turbulences. Consequently, engineers have designed a convergent feeder, which has helped to better channel the material and to facilitate the filling of the cavity, thus reducing boundary layer separation.

Phase 3 – Lower section design

The lower section of the product is visibly different from the upper section. Indeed, it is a thinner part that, with the first mold, was filled by material at low temperatures thus causing the presence of cold laps and lack of material. By observing the component’s shape, it was indeed clear that a uniform filling was difficult to obtain, particularly in the lower section: the material filled in the first place the whole upper section, which is characterized by an important dimension, while it reached only at a later stage the lower section. Consequently, the material started to fill the lower section at a too cold temperature, such as not to guarantee an appropriate level of superficial quality.

With the aim of solving this problem, Bruschi team has designed a devoted feeding system through the addition of two auxiliary runners. The runners have been added to the lower section of the component in order to achieve a regular filling, by maintaining a stable material temperature in the entire die cast. The two auxiliary runners, as well as the main feeder, have been designed in order to obtain a symmetrical flow: to achieve this result engineers have added two further dampers to the auxiliary runners.

In addition to that, considering the particular shape of the product, in order to further facilitate the filling in the whole cavity, engineers have designed a partially tangential in-gate capable of channel the material in critical areas. The lower section presents, indeed, some inserts that, without this type of gate, could potentially obstruct the material flow, thus impacting on filling uniformity. This kind of solution therefore determines a better material distribution, by facilitating its entrance in the cavities and the reaching of the most critical areas.

Achievements

After having modified mold design engineers have conducted the simulation of the filling of the cavities with the software Magmasoft and have obtained excellent results: the previously observed critical issues have indeed remarkably decreased. Specifically, through the design of a new feeder, designers have achieved a higher filling uniformity, which has consequently determined the reduction of casting defects on the component’s surface.

This case study therefore confirms the importance of knowing how to draw on previous experiences for the resolution of new problems, by combining know how and technology in order to achieve increasingly outstanding results. Mold design and Magmasoft have allowed designers to test various solutions before proceeding with the actual production of the component, thus obtaining relevant benefits in terms of quality, time and costs.

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This post is part of a series in which we explain the importance of simulation for HPDC (High Pressure Die Casting) through the presentation of real life cases.

You can find a full list of discussed topics in our first post on the subject, by clicking here.

CASE STUDY DIE MAINTENANCE: automotive industry

The product we are going to analyze in this post is a steering lock housing assembly for automotive industry.

The critical part was the upper cover of the steering block assembly, containing an electronic card. The part itself was not flawed at first, but soon the signs of mold erosion started to show up. The erosion of the mold left distinctive marks on the surface of the component: these irregular dark spots were the sign of die wear.

The identified die wear was caused by the high speed of flow during the filling phase and by the specific geometry of the part. The consequences of poor die maintenance are rather severe as they can bring on a malfunctioning of the part, caused by surface spurs resulting from die wear, in addition to increasing the need for maintenance work. In order to solve the problem, a simulation of the filling process was run.

OBJECTIVE AND PHASES OF THE SIMULATION

The objective of the simulation was to identify opportunities for slowing down or eliminating erosion on an existing die: to achieve the wanted result, our engineering department run an analysis of alternative feeder solutions.

The first step was to identify the cause of erosion: erosion is caused by the development and non-stationary implosion of alloy bubbles against the surface of the die. Due to high speeds of the flow through the narrow ducts in proximity of feeder, the pressure falls below the vapor pressure of the alloy, causing it to evaporate instantaneously.

Subsequently, the alloy enters the die with a larger cross section than the feeders, causing an expansion that reduces the speed of the flow. The pressure rises rapidly causing the implosion of vapor bubbles and freeing up energy. This entire process is called cavitation and is the main cause of wear to the internal surface of the die.

RESULTS

The filling velocity field represents the speed of the flow when it initially enters the control volume.

Since cavitation is a non-stationary process that takes place in the first phase of filling, which is also the phase of highest instability, the study of filling velocity field represent the highest opportunity to identify the critical areas.

As the following images show, the new geometry positively influences the expansion of the flow, mitigating the velocity gradients along the green line.

Internal part of the housing

The area with largest cavitation is in the center of the field, where the green line demarcates the separation between two areas with different speeds. Exactly in this area, because the flow slows down and pressure increases, the alloy bubbles implode and damage the die.

With the new feeder duct the green line is moved generating a broadening of flow diffusion. This broadening should shift the cavitation area and reduce the risk of cavitation because of the lower speed gradient in the critical area of the die.

External part of the housing

The green line moved on the fixed side of the die as well, broadening the diffusion and diminishing the transversal speed gradient.

In conclusion, the use of simulation allowed to identify the critical area in which cavitation was occurring and to design a broaden flow to shift cavitation, preventing wear of the die and reducing the need of die maintenance.

We will now move on with the next case study, which focuses on small domestic appliances and set up parameters optimization.

CASE STUDY OPTIMIZATION OF SET UP PARAMETERS: small domestic appliances

The subject of our second case study is a chrome plated handle. The product is a component for a pad type coffee machine: in this case there was not a problem to fix, but it was necessary to identify the correct filling of the die to optimize the production.

In order to achieve a high level of esthetical quality, the die has to be maintained at very high temperatures: high temperature in the die reduces alloy cooling during the filling phase, reducing flow marks. However, high temperatures implies longer cooling time: as a consequence, the cycle time expands because of the prolonged solidifying phase of the part. Therefore, it is necessary to establish an operating temperature range that allows a high quality level while keeping the cycle time as low as possible, to increase productivity.

Once identified, the die casting conditioning parameters can be set to keep the die within the range. These conditioning parameters are entry temperature and the type of coolant.

OBJECTIVES AND PHASES OF THE SIMULATION

The objective of the simulation is to estimate the change in surface quality and cycle time at the varying of die temperatures.

Die temperature depends on two factors: the geometry of the conditioning circuit, defined in engineering phase, and the temperature and nature of the coolant applied in production phase.

In the simulation, a homogeneous die temperature was imposed in proximity of the cavity. Surface quality has been determined by the average value of the alloy temperature at the end of the filling phase, whereas the production cycle has been derived from the necessary time for the component to solidify and cool to a temperature of 340°C.

The evolution of the two quantities has been calculated across a series of simulations in which the temperature was made variable.

RESULTS

The following graph clearly indicates the results of the series of simulations. Technically, this is not an optimization analysis because the two monotonously increasing curves do not allow to find an absolute minimum or maximum: it can be considered as a tradeoff analysis to define the optimal temperature range.

The 180 – 228°C temperature range is where the surface quality curve grows most significantly, while the product cycle time stays fairly constant before increasing almost exponentially.

For these reasons we choose the range within bandwidth of about 10°C, placed at the extreme right of the interval (218°C < T < 228°C).

In this case, simulation of the component has helped determine the ideal die temperature range to obtain the required surface quality, guiding the setting of entry temperature and the choice of the coolant.

In conclusion, throughout this series of post we have seen different applications of simulation for die casting: scrap reduction, optimization of esthetical quality, improvement of mechanical characteristics, improved die maintenance and optimization of set up parameters are only some of the many advantages offered by the use of this technology.

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You can find a full list of discussed topics in our first post on the subject, by clicking here.

CASE STUDY CONSTRUCTION: Mechanical characteristics

The product we are going to study in today’s post is a door hinge. This mechanical component was designed to be subjected to medium-low stress intensity so, in contrast to other products in building sector, for this project esthetical characteristics were not relevant, while mechanical characteristics and resistance to wear were fundamental.

OBJECTIVE AND PHASES OF THE SIMULATION

The simulation objective was to minimize the defects that may influence mechanical characteristics of the door hinge: porosity and holes. The requirements for the component were:

• to be free of internal cavities and bubbles, caused by air entrapment, in the holes;
• to be filled correctly;
• no missing details;
• low shrinkage porosity both internally and in proximity of the feeders.

Thanks to previous experience in the production of similar pieces, our technicians knew that porosity is an extremely critical aspect for door hinges: the presence of holes and cavities, caused by the shrinkage of the part in proximity of the feeder, could reduce wear resistance and even cause the component to break when subjected to stress. In addition, incorrect cooling of the parts causes hot spots on the surface of the die, leading to the formation of cavities on the component surface.

The simulation analysis focuses mainly on the solidifying phase, with the objective of finding a configuration that allows homogeneous cooling of the part, thus avoiding the creation of hotter areas and prevent porosity.

RESULTS

The study of the filling phase focused on the analysis of the distribution of entrapped air. As you can see from the following picture, the quantity of air is minimal and evenly distributed in small amounts across the part.

Porosity is analyzed observing the behavior of the part during the solidifying phase: the most critical point is the feeder duct, where the alloy solidifies and contracts during cooling phase.

Contrary to the configuration used in similar parts that were previously made, where the feeder duct is perpendicular to the axis of the holes, in this simulation the duct is orientated parallel to the axis and its geometrical configuration is optimized to avoid the creation of hot spots on the part’s surface.

As can be observed in the image, in the new configuration the surface next to the feeders solidifies much more rapidly, subsequently diminishing the risk of shrinkage porosity thanks to the alloy flowing back into the feeder ducts or into the part itself.

The junction area solidifies quickly (colored in light blue), together with the rest of the product surface, lowering the surface porosity risk. The most critical area is the one right below the junction – as seen in the red circle – where the cooling is much slower because of the proximity to the feeder.

But as can be seen from the picture tagged “50% liquid”, the criticality should now be solved: the circled area is solid (colored in blue) while the core of the part is still liquid (colored in yellow).

The image above displays mold simulation: the part volume near the casting ingate leads to a really delayed solidification and to shrinking porosity as a direct consequence. In comparison, the junction re-positioning has reduced the issue of porosity thanks to a quicker cooling of molten metal.

The results of the simulation have been confirmed during the utilization of the die: the problem of shrinkage porosity, typical of this type of product, has been fully eliminated.

To sum up, the experience gained dealing with similar cases allowed Bruschi to identify the criticalities already in die designing phase and to study a solution through the use of simulation. The objective was to improve the resistance of the component by reducing shrinkage porosity: this makes the component more resistant to wear and prevents it from breaking under stress. The simulation study of solidification phase proved that placing the feeder duct parallel to holes’ axis increases cooling speed, thus reducing the risk of porosity.

Once again, this successful case study shows the importance of simulation in die casting: the application of new technologies can make a competitive difference on the market even in a millenarian industry such as metal working.

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With casting process there is often no need of further machining after casting: not only its accuracy is up to 99.8%, but casted products also can be used raw, since they have a pleasant finish. Zinc die casting usage is almost 28% in automotive industry, followed by building and hardware sectors.

Zinc has become one of the most important metals in automotive parts industry, especially for products like door lock housing, pawls, retractor gears and pulleys in seat belt system, but also for the camshaft and sensor components. By using this metal and its alloys, it is possible to reach a strength, ductility and malleability that will not be possible with other materials.

Moreover, zinc can be the right choice to obtain high quality aesthetic components, with  strict tolerances impossible to achieve with other materials, and to obtain embossing and grooves for mechanical or gear components.

Automotive zinc die casting components

As said before, automotive industry is the most common application of die casting: by using zinc and its alloys is possible to produce components capable of reaching high aesthetic quality , with strict and narrow tolerances for shapes morphology. Zinc alloy is also used for plating purposes because of its many benefits, such as enhancing the already impressive anti-corrosion zinc properties.

Hereunder you can find a selection of examples of what is possible to produce with zinc plating:

• Interior aesthetic parts
• Sunroofs parts
• Mechanical parts
• Engine and other under-the-hood components
• Power steering systems
• Brake parts and systems
• Air-conditioning components and systems
• Chassis hardware
• Parts in seat belt system
• Climate control components
• Fuel systems

In the next paragraph, we will proceed with a list of zinc alloys advantages.

Zinc alloys advantages

Zinc alloys in die casting allows to develop products and to simplify production, providing a durable end-product. Zinc strength, ductility and stability allows specialists to obtain a better and more resistant product. It is this high ductility that makes zinc ideal for die casting process, in which it is necessary to sustain pressure and expansion without fracturing. Other zinc alloys advantages are used in die casting, such as:

• Electrical and thermal conductivity
• Raw material low cost
• Strong stability, which grants a long-lasting end product with high quality finishing properties
• Thin walls: they contribute to corrosion resistance for end products, increasing their durability in harsh environment
• Economical for small parts
• Low melting point, which means it requires less energy for production
• Dies have a longer tool life
• Complex geometries and quality finishing: by using zinc it is possible to cast intricate, thin walled and technically demanding parts with high precision
• Top quality coating and chrome plating: zinc die casting components are fit for electroplating

Zinc vs other materials

Each alloy has its own characteristics in terms of physical and mechanical properties: these factors are fundamental to determine longevity, strength and functionality of products. Product complexity, its wall thicknesses and the required precision will influence how castable it is: these elements should be considered main points to better organize a die casting production.

There are different methods of die casting, depending on design application and metal choice: hot chamber or cold chamber are just two of them. Hot chamber die casting is ideal for metal with low melting temperatures, for example zinc or magnesium, while cold chamber processes are used for alloys such as aluminum, which have higher melting temperatures.

Zinc alloys compete with other materials in die casting industry: aluminum, cast iron, machined brass, machined steel and magnesium, together with zinc, are the most commonly used metal in die casting products. Choosing alloys is a tough operation: the die caster has to analyze application, material density, tensile strength, yield strength, melting temperature and elasticity to obtain a better outcome with all its functional and aesthetical requirements.

1. Zinc alloys vs Aluminum

One of the main difference between aluminum and zinc alloys is zinc low melting temperature and its need for lower pressure during die casting. For these reasons zinc alloys are the most castable ones. Having a low melting temperature allows zinc die casting tools to last longer, and since dies are expensive, using them for a longer time is a cost-effective approach to high volume of production.

Moreover, zinc can be cast with HPDC, which is less expensive than cold chamber process; by using HPDC it is possible to increase production speed. Aluminum alloys, on the other hand, are normally casted with cold chamber procedure, which is less productive than hot chamber process used for zinc.

In addition, zinc is more suitable for very detailed castings and very little machining or finishing work is needed after ejection of the component from die: low pressure and low melting point reduce thermal shock during casting, giving a smoother surface to the component. Moreover, the superior zinc formability and machinability gives costs advantages in finishing and assembly operations.

There is another downside to aluminum die casting: it tends to produce sparks after impacting against iron or steel. This forbids its use in risky environment where explosive atmospheres can develop. Instead, zinc is a non-sparking material and thus it is free from this risk and it is suitable even for those environments.

In general, zinc has a better precision, smaller draft angles, smaller and longer cored holes, much longer tools life, thinner wall sections possible, superior tensile strength, better machinability, better formability, lower casting costs, superior pressure tightness, more finishing options and it is a non-sparking material when compared to aluminum.

2. Zinc alloys vs cast iron

Zinc, in comparison to cast iron, has lower casting costs, more precision and superior thermal and electrical conductivity. However, it is difficult to compare cast iron with zinc alloys because there are a large range of materials for iron casting, each with different properties, so that is impossible to strictly define them.

3. Zinc alloys vs machined brass

Zinc alloys have lower process costs, lower material costs, less process scrap and equivalent or superior tolerances achievable than machined brass. As before, the term brass regards a wide range of cast materials, with many different mechanical properties and is impossible to define each of them.

4. Zinc alloys vs machined steel

Compared to machined steel, zinc has lower process costs, a superior corrosion resistance and equivalent or superior tolerances achievable.

5. Zinc alloy vs magnesium die castings

One of the main differences between magnesium and zinc alloys is that zinc alloys need lower pressure and temperature for casting. Unlike magnesium, which requires a special treatment for corrosion resistance and finishing, zinc alloys have excellent corrosion resistance and a better surface right after being ejected from the die.

Magnesium has a really low density and, just like zinc, magnesium alloys can be die casted using HPDC: for this reason, they have faster cycle times than aluminum. However, zinc properties are superior to those of magnesium in every aspect.

Zinc, when compared to magnesium, has lower processing costs, lower draft angles, better precision, superior stiffness, superior tensile strength, better corrosion resistance, superior formability, longer tool life, more finishing options and it is a non-sparking material.

CASE STUDIES

The use of zinc in die casting processes allows a series of finishes that would be unachievable with other alloys, leading to products with both high technical coefficient and important aesthetic value.

Bruschi has 70 years of experience with clients of several industries, including automotive, lightning engineering, household electrical appliance, doors and windows locks. Having a wide know-how that extends from one field to another allows the designer to provide his clients with alternative solutions inspired from other industries and always ensuring top quality in material, processes, manufacturing and durability.

In addition, an analysis phase preceding mold design leads to proposals aimed at reducing scraps,  minimizing production costs.

Hereunder a selection of case studies focused on automotive sector.

CASE STUDY n° 1 – New finishing to avoid blistering

An important automotive player asked Bruschi to produce a component in order to find a solution for its scrap rate: it was already being produced by another supplier, but it had a scrap rate of over 25% due to blistering, appearing after treatment of galvanizing coating.

Bruschi worked focusing its efforts, involving its supplier network, to avoid blistering. The solution was to  change the finish: shifting from galvanizing coating to passivation. This meant a great reduction of scrap rate caused by blistering, that decreased from 25% to only a few parts per million.

CASE STUDY n° 2 – New finishing to improve SST resistance

An important automotive player asked Bruschi a solution to improve the salt spray test resistance of a  product. The component needed a higher SST resistance for a new project, in which the component would become part of luxury cars.

The SST resistance is given by a particular kind of finishing, called Fe/Zn: for subcompact cars the guaranteed resistance is 120 hours. However, this treatment was not suitable for luxury cars. The new goal was to push the resistance up to 1000 hours. Thanks to Bruschi expertise and network, our client reached its goal by applying a special galvanic treatment, which improved SST resistance to over 1000 hours.

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In today’s post, after a short introduction, we are going to analyze a real case from automation sector dealing with blistering and air trapping problems that were solved through the use of simulation software.

In the process of zinc alloy die casting the element that most influences the final casting is the feeder duct, which leads the alloy into the die. A correct feeding of the die has crucial importance for quality, strength and finish of the product, and can also prolong mold life span.

The simulation of feeding, filling and solidification phases offers a real advantage: it helps to predict possible defects caused by non-uniform liquid flow, high temperature gradient, local increase in velocity, and other issues. With simulation software it is possible to optimize the entire production with positive effect on duration of die life and on the final product in terms of aesthetics and efficiency. These simulations can be run through the use of specific software, such as Magmasoft.

Magmasoft is an innovative software package specifically developed for simulation for HPDC. It allows simulating the die feed and the casting solidification, calculating alloy flow, alloy temperature and the possible presence of gases or porosity in the finished product.

Thanks to the simulation it is possible to identify the best configuration for the feeders and to achieve the best results in terms of precision and reliability, reduce risks to a minimum and save time and resources.

In this series, we are going to analyze case studies in which simulation analysis has contributed to improve current production runs and avoid engineering mistakes that would have affected the finished product by generating waste of material, time and resources.

In detail, we are going to discuss five distinct cases in which the use of the software influenced important decisions in the choice of technology for die casting components. Here is a list of the topics we will see in each post:

1. Scrap reduction
2. Esthetical quality
3. Mechanical characteristics
4. Maintenance and life cycle of the die
5. Optimization of set up parameters

Now we are going to present the first case study, related to scrap reduction for a product used in the sector of energy management and automation.

SCRAP REDUCTION CASE STUDY: ENERGY MANAGEMENT AND AUTOMATION

This case study is about a support for an emergency switch. Due to its function, the product is in sight but high esthetical quality is not required. However, the product required a painting process that brought out surface defects.

The die of this product has been in use for several years and has always generated 5% scrap due to the formation of bubbles during the painting process. The origin of this phenomenon is the presence of air in the alloy which is forced out of the product by the high temperatures that are reached during the painting process and stays trapped between product and paint layer. This kind of defect is known as blistering.

OBJECTIVE AND PHASES OF THE SIMULATION

In this case the simulation focuses on avoiding bubbles in the post-production phase. The objective of the simulation was to reduce the quantity of scrapped parts caused by entrapped air. We examined the behavior of the flow during the filling phase to ascertain the causes of this phenomenon. Subsequently, alternative ways of feeding were studied.

RESULTS OF ANALYSIS

The analysis shows reflux and air encapsulation areas in proximity of the edges, where bubbles are later spotted. This turbulence is caused by the speed of the flow and by the geometrical characteristics of the part.

In the following image, you can see the result of a numerical simulation of the current die configuration:

Once the cause of the problem was identified, it was possible to search for a solution that could reduce the speed of the flow.

SOLUTION

The solution that is depicted in the image represents an alternative feeding, with the feeder ducts rotated by 45° around the axis of the product. The results of the modification are shows consecutively.

Through the use of a simulation, the reflux area has been fully eliminated and the filling flow results more uniform.

2nd configuration:

Comparison:

After the simulation, the die has been modified with results confirming the validity of the study: the quantity of rejects caused by bubbles was reduced by approximately 95%.

In conclusion, thanks to the use of simulation for HPDC it was possible to clearly identify the problematic areas where air trappings formed. The rotation of feeder ducts allowed the speed of the flaw to be regulated without modifying the shape of the component, preventing reflux in the previously identified areas. After the modification of the feeding system, the scrap rate caused by blisters was significantly reduced.

This is just one of the many examples in which the use of simulation for HPDC can help improving production. To learn more about the use of simulation and the advantages it can bring, subscribe to our blog.

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Surface finishing has a crucial role in zinc alloy components production: often the main reason to choose Zamak for a component is the wide range of finishing and colorings available, either through plating or painting. For this reason it is important to know and learn to prevent defects linked to the final surface quality of the diecast.

Die casting surface treatments defects can be linked to many different causes, ranging from the quality of the casting to issues in pre-finishing or finishing phases. The term pre-finishing includes all the operations necessary to prepare the product for following treatments of electrodeposition or painting, such as deburring, vibrating, cleaning and polishing. Depending on the root cause, the kind of defect may vary, but some are more common than others.

Among the most widespread defects in die casting surface treatments, we find blisters and pitting: they can be caused by a number of factors, therefore it may be necessary a more in-depth analysis to identify the cause and proceed to correction.

In addition to blisters and pitting, there are three types of surface defects easily recognizable: burnings, nodules and flaking.

Blisters

Blisters look like small bulges on the surface of the components and their dimension can vary depending on the cause of their formation. They can be caused by errors in casting, pre-finishing phases or during electrodeposition.

Casting blisters

This kind of blisters can be caused by contaminants in the alloy, such as lead and cadmium, or by intergranular corrosion, but they can also be linked to defects in casting such as cold laps and surface porosity.

In this case, blisters are round-shaped and big, and they feel hard to the touch. For a better analysis it will be necessary to cut the blister with a sharp blade: it will be then possible to observe that, by sectioning the blister, the diecast will be cut too. These traits identify blisters formed due to underlying porosity in die casting phase: in order to solve this issue, it will be necessary to reduce surface porosity.

Pre-finishing blisters

As stated above, not all blisters are caused by casting defects: some can form due to errors in pre-finishing phases. For example, during the preparation of a piece for electrodeposition, special attention should be dedicated to the detection of oxidation, corrosion and other superficial contaminants. If the surface isn’t perfectly clean, groups of round blisters will form after electrodeposition: by cutting through these blisters, the underlying metal will be left exposed and, observing its section at a microscope, it would be easy to spot  the oxidated layer that caused the defect.

In order to avoid these situations, it is advisable to prevent delays between casting, cleaning and electrodeposition so as to reduce the chance of surface oxidation. When this is not possible, for example when relaying on an external supplier, another way to prevent blistering is to apply a corrosion inhibitive solution to the components and remove it right before plating. If, despite taking all the mentioned precautions, traces of oxidation are found on the surface, it is possible to remove it through mechanical vibrating or cleaning it with acids.

Blisters caused by electrodeposition

Another easily recognizable kind of blister is the one caused by a lack of adherence between nickel and copper layers, or between nickel and chrome. These blisters often pop up in large groups, have an irregular shape and can be found on the whole surface of the component, sometimes far from one another. Unlike casting blisters, they are soft and squishy and, after removal, the underlying copper or nickel coating will be visible.

The formation of these blisters can be prevented by making sure that the copper layer is sufficiently thick, by leaving the shortest possible time lapse between nickeling and copper plating and by regularly checking baths for contaminants.

Pitting

Unlike blisters, pitting is always caused by mistakes during pre-finishing or finishing phases. They look like small lumps whose shape and position can vary depending on the root cause. There are 4 main causes of pitting: solvents or corrosive liquids on the surface, gas trappings and hydrogen bubbles in the galvanic bath.

When pitting is due to solvent residues that prevent the correct adherence of copper layer, lumps will be disposed in circle or chains, whereas when the cause is an unexpected splash of corrosive liquid, pitting will be distributed in an irregular shape.

In the first case, to solve the problem it is possible either to modify the degreasing system, or to add a cooling bath between degreasing phase and plating phase. Temperature plays a crucial role in these phases: sometimes it can be enough to lower the temperature of the casting or to wait longer between liquid and steaming treatments to prevent the formation of pitting.

Instead, when pitting appears in irregular shapes, the solution can be to control the source of splashes and fix it, or to apply a thicker copper layer so as to cover the defect.

Sometimes pitting can be randomly disposed and look like small craters with high edges: this happens because during the nickel-plating phase some hydrogen bubbles in the galvanic bath stick to component’s surface. The presence of these bubbles can be caused by different factors:

• Inadequate mixing of galvanic bath
• Low level of surfactants
• Oily layer on the surface of nickeling bath
• Dust particles sticking to the piece’s surface

To prevent the formation of this defect, it will be necessary to monitor the amount of surfactants and the mixing of the galvanic bath, in addition to protecting the surroundings to prevent infiltration of powders.

One last kind of pitting is caused by the presence of gas and bubbles in the galvanic bath: bubbles stick to the surface of the component, creating round and deep pitting, close to each other and covering the whole surface. To prevent the formation of these defects, it will be necessary to spot the gas source and to check baths’ state.

Sometimes the solution itself is naturally rich in gas, which gets released as soon as the mixture reaches a certain temperature. In times like these it is possible to remove gas by overheating the solution, bringing it to an higher temperature than the one requested for galvanization process and cooling it down again before using it.

Burnings

They look like a brown spot and a burnt crown, and are caused by a high pitch in current density or by a contamination of the bath: this contamination can derive from the use of the solution, because during the plating process some zinc particles may detach from the component and alter the solution.

To avoid these formations is then sufficient to periodically change the bath mix or to dilute the solution by adding more electrolytes.

Nodules

Nodules look like small bumps that make the surface of the casting grainy: they derive from particulate matter depositing on the piece or in the nickel-plating tank. To avoid these formations it is advisable to periodically filter galvanic mixture and to be careful when cleaning the pieces.

Flaking

Flaking is caused by a faulty attachment of the external nickel layer: it looks like compact and long vines, with different shapes and dimensions. They are easily scratched with nails or a blade, and when removed, they leave the underlying coating on display.  

The root causes of these defects are identifiable thanks to their position: if detaching is formed between copper and nickel coatings, it may be caused by contaminated solution, delays in the process or insufficient activation. Flaking can also be formed between polished layer and semilucid nickel layer, and their cause is to be found in the passivation of the semilucid coating, whereas if the formation is located inside the polished nickel layer, it is usually due to flicks or current interruptions during the plating.

Generally, to avoid the creation of flaking it is necessary to keep the solution pure, or to monitor the current flow to avoid interruptions during the process.

To sum up, we have seen how working with chroming and electrodeposition on zinc alloy products can induce many surface defects that can be linked to casting defects or processing errors. Some can however be avoided through constant monitoring of galvanic baths and solutions, and a careful cleaning of the diecast and the working environment.

To learn more about zinc die casting surface treatments and how to fix common defects in die casting, subscribe to our blog or contact us for a tailor-made solution using the form at the end of the page.

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In order to enhance efficiency of the process, various topics can be explored:

• Design of the production process
• Risk analysis
• Choice of production tools
• Planning of activities

Design of the production process for Die Casting Cycle Time Reduction

The most advanced technology that is available for die casters today includes software that can simulate production and therefore optimize process parameters.

These include:

• Sequence of the casting process
• Shot curve
• Temperature of melt
• Thermal regulation

The results of the simulation can be represented in graphs or videos that show the various phases of the process: filling the die, solidification of the material and possible defects that could happen and that we will discuss in the next paragraph under risk analysis.

During the design of the process one shouldn’t forget additional operations to be performed after the die casting phase. Beyond their impact on time, which in such case will lengthen and must be included to establish the delivery date, additional wall thickness must be considered to maintain the tolerances as required by the customer.

Risk analysis

The possibility to simulate the entire die casting process provides a precious opportunity which allows to foresee possible defects and eliminate them by acting on the causes before production starts.

The most common defect is burr, requiring additional machining for elimination, increasing the production time and the cost of the final product.

Another aspect to be analyzed in the preliminary phase is the surface quality of the part: based on the more or less correct filling of the die, flow marks and cold laps could appear.

Shrinking of the material during the cooling phase must also be calculated: we are referring to porosity, being the appearance of cavities inside the die cast product while solidifying. The main cause of this defect is the differing speed of cooling and hence solidification in the various zones of the die. A preventive analysis of material flow inside the die allows to optimize this delicate effect and render speed and temperature the most homogeneous possible.

For the die caster, being able to identify defects and analyze them in each and every aspect (quantity, shape, exact location, etc.) is crucial because by establishing the causes of defects one can choose to modify the productive process or to modify the die.

In these cases, a choice that might seem more complex could induce a time and cost saving by eliminating additional machining.

The choice of production tools

In the die casting sector, when choosing a supplier it is fundamental to evaluate if they use cutting-edge machinery, if they effect periodical checks on the tools and if they offer the possibility to develop automation processes in-house.

An expert die caster is able to execute efficient implementation of automation even when involving third parties.

Numerical control of all equipment allows to intervene in every phase to optimize and reduce production time.

To this point, industry philosophy 4.0 foresees to make the entire industrial process digital. The die caster’s engineering team receives feedback from both the internal quality control department and from the client when product defects are identified. The analysis of the entire process also has the objective to make production times more efficient.

With data at hand, the analysis repeats itself the same way as in the preliminary design phases of the production process.

Planning of activities

The initial planning covers the definition of the technologies to use, the production phases and the process parameters such as which die to utilize, casting temperature of the material etc.

At a later stage one can be more precise and define:

• Runner design
• Gating design
• Detailed process parameters

From product co-design in strict collaboration between client and die caster, to simulation of the production to prevent defects, from die casting cycle times to reduction of additional machining operations and surface treatments, up to final in-house assembly avoiding additional manipulation, the whole productive chain can be analyzed and made to the highest level of efficiency.

Speaking of surface finishing and surface treatment and again pointing to die casting cycle time reduction, the surface characteristics of the end product as requested by the client can be achieved in a more efficient way when the entire productive process is analyzed.

Starting with the client component design idea, some characteristics can be optimized to render the subsequent machining less complex and avoid issues with wall thickness or defects.

If the part needs to be delivered painted, for example, a chromate conversion coating is usually applied to prepare the surface for an improved life span of the paint finish. If a product with smooth finishing is needed, the surface of the product needs to be prepared before chromate conversion with a vibrating treatment.

Placing confidence in expert suppliers, you can count on a quality product at contained costs, because analyses to identify die casting cycle time reductions are included in all phases of the project to always guarantee an excellent result which is conform to the client’s expectations.

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In this article we will discuss zinc die casting finishes applied to the most common products or components in building sector:

• Chroming
• Gilding
• White zinc plating
• Painting

Zinc Die Casting Finishes for Building Sector

The use of zinc die casting components is common across many industrial sectors thanks to the exceptional properties of zinc, both in terms of accuracy of the finished product and in terms of production cost, certainly lower than many other materials.

Every sector however has different requirements concerning functional and aesthetical characteristics of the product.

When the element is not exposed to view, die casters will focus on functional product aspects, whereas if the product is visible, as in the building industry, beyond functional aspects the product must satisfy precise appearance requirements. In any case, the surface treatment covers an important role in determining product characteristics, both for resistance to wear and for aesthetics requirements.

We will now illustrate and analyze the most common Zinc die casting finishes for the building sector.

Chroming

Chroming is one of the most used die casting finishes. It is a chrome coating applied on a product or component for functional and aesthetical reasons.

For components needing both an aesthetical finish and good resistance to corrosion, an initial application of 30/40 micron of copper is required. Without this copper film, the part would be corroded by the subsequent nickel film.

The last phase is then galvanic chroming with a chrome flash that confers shine to the product.

Beyond aesthetics, this treatment confers a thicker surface film and increases the resistance to:

• Abrasion
• Dents
• Scratches
• Vapors
• Heat
• Smoke

It is necessary to bear in mind that chroming a zinc die cast component will add 60 to 70 micron to product dimensions.

Gilding

Gilding is an aesthetical process for metal surfaces. The product is immersed into a galvanic bath and exposed to electric currents which deposit material particles on the part. Exactly as in chroming, gilding requires a preventive protection by copper plating, followed by a nickel barrier and a gold flash providing the color effect and shine as required.

It might be obvious but material cost should be considered and limits the use of gilding techniques to components where the aesthetical value is essential and in high-end use.

As aforementioned, in gilding case it should equally be considered that the final dimensions of the product will increase by 60 to 70 micron.

White Zinc Plating

This treatment is often applied to components such as door hinges, hatches and handles.

The aim of white zinc plating is to improve corrosion resistance by atmospheric agents, other than product aesthetical aspects. This type of treatment will usually resist 30 hours of saline mist exposure.

Painting

Depending on the client’s requirements, alternative die casting finishes such as painting are available. In this case the treatment is not galvanic, such as for chroming, gilding or zinc plating, but electrostatic. Before painting, parts are exposed to a preliminary treatment of vibration or sand blasting to eliminate flash and sharp edges and improve adherence of the paint.

The powdered paint polymerizes on the component at a temperature of approximately 200°C.

Focus on co-design activity

It is good to point out the importance of involving the die caster during the component design phase that needs assembly, if not during the entire product design phase. The experience of a high quality supplier is essential during initial phases of the project to ensure excellent result at reasonable cost.

Often the difficulty for die casters to satisfy customer requirements is that they have to work with a design that has already been approved by a technical (hence executive) team and that they cannot suggest modifications which could vastly improve process efficiency and product quality.

A very useful service is called co-design, where the die caster helps the customer as off the very first phases of engineering the product and suggests, based on his experience, potential modifications to product and material and then engineers the mold and the process around it, resulting in optimized production within the customer’s deadline and budget.

The supplier may suggest various types of Zinc Die Casting finishing based on the process or required outcome. An experienced Die Caster will be able to identify the most appropriate treatment in order to obtain both functional and aesthetical required properties while maintaining low production costs.

If you would like to get more information on zinc die casting processes you can comment here below or contact us directly.

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