We’re thrilled by the high level of interest and the questions that enriched the discussion, allowing for a direct exchange on real cases and daily challenges in the industry. A special thanks to our internal experts: Matteo Margiri, Andrea Panvini, Ermo Fusè, Matteo Colombo, and Marco Scaramuccia, for offering concrete insights drawn from their experience at Bruschi foundries, making the webinar even more relevant and practical.

Stay tuned for future in-depth events and updates on industry developments.

Thank you once again for your participation and interest!

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In both cases, the surface of the die-cast must be in suitable conditions to receive the foreseen treatments. Therefore, it is crucial to avoid defects created on the products during the die-casting process that do not allow the painting treatment or the galvanic coating to adhere correctly to the piece.

Below, we will analyze the primary casting defects that occur during the zinc alloy die-casting process, analyzing the possible causes and the relative remedies to be adopted in the production phase.

Flow Marks

It is the most common defect of the die-casting process and is generated by the cooling of the liquid metal in contact with the surface of the mold during the cavity-filling phase. The surface of the die-cast with flow marks has streaks and veins that reproduce the metal’s flow lines, similar to a geographical map. In the injection phase, the zinc alloy comes into contact with the mold in a few moments. It cools abruptly, creating thin superimposed plates that give rise to the characteristic pattern typical of the flow marks on the piece’s surface once it has cooled.

Causes of flow marks

The leading causes of marbling are mainly three:

•  Excessive mold cavity filling time
• Mold temperature and filling speed are too low
• Improperly designed supply channels

In literature, as in daily foundry practice, it is now clear that it is almost impossible to obtain a die-cast utterly free from this defect: it is, therefore, important to know how to evaluate and define the degree of acceptability. As previously anticipated, an excessive degree of flow marks can cause defects in the painting or galvanic treatment phase due to the irregularities of the surface. The superficial micro-cracks of the flow marks can, in fact, trap gas or air, which are then released in the various phases of the painting or galvanic processes, generating aesthetic defects such as microbubbles, pitting, micro craters, which are not tolerated by both aesthetically and functionally.

Solutions: How to avoid flow marks

The elimination or, at least, the reduction of the marbling problem is obtained through the correct regulation of the main molding parameters: such as the filling speed and the temperature of the mold surface. In this way, it is possible to speed up the filling time, reducing it to a few milliseconds, and avoid the formation of sheets of cold material on the surface of the die-cast.

This type of casting defect can, therefore, already be foreseen in the mold design phase. In fact, a correct analysis using the simulation program allows the design of the injection points and the shapes of the optimal feeding channels to obtain flow rates and filling speeds suitable to avoid thus the problem of flow marks in the zinc die-casting process.

Blistering

Blistering is a porosity defect due to gas trapping within a sub-surface region of the part. This casting defect is characterized by a small surface area that deforms when the internal pressure of the porosity due to the gas under the surface is high enough to distort the thin metal layer that covers it plastically. Generally, the size of the bubbles can vary from 100 µm to several mm.

Causes of blistering

Metal deformation occurs quickly at relatively high temperatures when castings are ejected from the mold or during subsequent heat treatments. This type of defect is mainly attributable to the turbulent filling of the mold and the consequent entrapment of air present in the feed channels and the mold cavities. That is a defect well known to painters since, in the powder coating process, the final phase involves a passage in the oven at temperatures of around 200°C and, at this temperature, any gas or air trapped in the die casts expands and in the case of thin-walled products or air inclusions close to the surface, it generates blistering.

Solutions: how to avoid blisterings

Let’s see below what are the different precautions to avoid the onset of this defect:

Analyses

The first activity to be carried out is an in-depth analysis of the data provided by the filling simulation program. In fact, this program makes identifying the product areas at the highest risk of air entrapment possible. Following this information, it is possible to size the injection sections and the supply channels in order to avoid entrapments. The modern machines that are protagonists (Pre-filling, FDS) of the zinc die-casting process are equipped with systems that reduce the introduction of air during the filling phase, obviously reducing the phenomenon but not completely eliminating it.

Vacuum molding

The most important and efficient solution is the use of the vacuum molding system: that is, by extracting the air from the mold cavities before the metal reaches them. This system guarantees a drastic reduction of waste due to the presence of surface bubbles, both in the molding phase and in the subsequent surface treatment phase.

Sink

A sink is a surface depression created during the die castings’ cooling phase due to a shrinkage porosity under the surface. This casting defect appears as a concave sink, even of several mm, usually in correspondence with high thicknesses or significant changes in the thickness of the product, and is frequently associated with ribs.

Cause of the sinking phenomenon

It is a phenomenon that occurs when, during the solidification of the casting, a hot spot is found near the surface of the mould: the “skin” layer: it forms as a result of the cooling of the zamak in contact with the mould, it is unable to support the stresses deriving from the contraction of the underlying region in solidification (shrinkage) and therefore plastically deforms.
The effects of this defect translate into imperfections in the finished product.

Solutions to the sink defect

The primary remedy is the correct design of the components, i.e., making sure that there are no significant changes in thickness or areas of the product with excessive thickness. Unfortunately, it is not always possible to create the ideal shape for the functional needs of the product,
It is, therefore, crucial, during the design phase of a new product, to have a close collaboration between the designer and the die caster in order to identify the correct shapes to reduce hollows while respecting the product’s functionality.
To avoid sinks during the production process, it is also possible to act on the pressure and speed of the molten metal during the injection phase. Lastly, even more, important is the management of the heat balance of the mould, creating adequate cooling systems to avoid the formation of hot spots. Also, in this case, the cooling simulation analysis helps us identify the areas of the die casting with the most extended cooling times. It allows us to create the most efficient cooling circuit, suitable for avoiding the sink.

In conclusion

As mentioned in the incipit of this post, we have dealt with the main problems concerning possible casting defects in the zinc die-casting process. As previously explained, all issues can be foreseen and solved or defined as acceptable: the important thing is to know how to recognize and prevent them. There are other issues besides flow marks, blistering, and sinks in zinc die casting. Problems to be discussed in depth: look for these topics in our blog.

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Introduction

In a previous post we discussed the importance of the metrology laboratory in a zinc alloys die casting productive plant. The metrology laboratory, in fact, assures the quality of the products throughout the production chain by guaranteeing the compliance to the requirements of the customer from a dimensional, structural and chemical point of view. The activities of the metrology laboratory described in the post Dimensional measurement: production control and reverse engineering are:

– Constant monitoring of each production cycle
– Fast checks of new projects and sampling
– Checks of projects that have undergone changes over time
– Control of parts incoming from external suppliers
– Reverse engineering, both on the product and the mold

To deepen these themes, we suggest you to read the post.

In this post we will instead examine different devices paying special attention to the need to choose the most appropriate tool on the basis of the characteristics and the requirements of the product.

Importance of the metrology laboratory

The presence of a metrology laboratory in the production plant is not enough to guarantee the quality of the products: in fact, this result can only be achieved through the employment of appropriate tools for the component to be tested. The set of devices must be up-to-date, advanced and comprehensive of different technologies, in order to allow the minimization of risks that is necessary to meet the requirements of the customer.

This is particularly true in a plant that employs zinc die casting technology for products intended for different market sectors: the variety of products corresponds to a diversification of the tools and of the technologies that the metrology laboratory needs in order to carry out its analysis.

To be specialized in different market sectors allows the competences acquired in a sector to be transferred in another and this is also true for the tests of the metrology laboratory. The choice of the tool in fact is not only determined by the requirements of the customer or by the necessary documentation in the different stages of a project, but it is also defined by the necessity to obtain the most indicative and effective data for the assessment of the compliance of a component with the design standards. The expertise acquired in the measurements of the components intended for a sector can then be transferred to the analysis that are necessary in another sector with advantages that affect the entire production chain.

The tools of the metrology laboratory

After depicting the importance of the metrology laboratory let’s now analyze the tools that allow the metrology tests to be complete and effective.

Industrial computed tomography

The technology of industrial computed tomography, referred to zinc alloys die casting, has a double usefulness. In fact, it allows defect analysis on the internal structure of the components aimed at detecting defects in the internal distribution of the material and thus at identifying porosity and air inclusions. Moreover, thanks to tomographic analysis, it is possible to evaluate the success of the assembly of components. In the case of the assembly of different materials the tomographic analysis allows a functional evaluation of the clearances between assembled parts and the verification of the absence of flaws such as air inclusions or deformations.

3D Scanner

3D Scanner is particularly suitable in the case of components with complex shapes. The device is less accurate than a Coordinate Measuring Machine (CMM), but it allows the capture, without contact, of all the visible surface of the measured object and it makes the comparison with the theoretical model possible. For this reason, whereas a test with CMM would be too time-consuming for the necessity of a complete reconstruction of the shape of the object, the 3D scanner allows the result to be obtained rapidly and effectively.

CMM

In a metrology laboratory CMM is surely the most common technology. The machine allows the selection of the desired areas and the execution of very accurate checks to be performed. For this reason, it is particularly suitable for the measurement of elements of components with simple shapes for which the tolerances are very narrow. However, the CMM does not guarantee the same results with different materials. In the case of zinc alloys, it is particularly suitable because the metal, differently from other materials such as plastics, does not run the risk of deforming during measurement.

Polyworks

Polyworks is the Innovmetric software that allows the tools of the metrology laboratory not only to be complete, but also complementary. What previously said about CMM and 3D scanner must not suggest that, during testing, a measurement method is to be chosen rather than another one. In fact, thanks to the software, it is possible to make the acquired data interact and to overlap the measurements of the 3D scanner with those of the CMM. For this reason, the software, allowing different data to interconnect, makes it possible to the metrology laboratory to unify its technologies for a complete and detailed final result.

UTM Machine

The Universal Testing Machine is the device employed in order to test tensile and compressive strength of a component. It is a very versatile machine that makes it possible to perform different kinds of tests on a wide range of materials and components. For example, concerning tests on zinc alloy die casts, the UTM is fundamental to work with the automotive sector. The verification of the resistance of products intended for this sector is in fact a fundamental part of the documentation that is necessary for the approval and the good outcome of projects.

In-line machine for the 100% dimensional control of production

This in-house designed and developed machine is intended to the verification of the compliance with the dimensional tolerances of automotive components. The device uses a camera for the detection of dimensions, volume and superficial quality of the components and a profilometer. The profilometer, thanks to laser technology, allows the reconstruction of the profile of the measured component and it is particularly useful for the reconstruction of the profile of complex shapes such as threads.

Other devices

Eventually, other common but fundamental devices in a Zamak die casts productive plant are the spectrometer, the density scale and the electronic microscope. The spectrometer allows the chemical composition of Zamak to be analyzed and it is of fundamental importance when, on the same machine, the mold is changed, and we move from the production of a component to that of another in different alloy. Thanks to the spectrometer in fact it is possible to verify that Zamak bath is concluded and that all the material employed in the process is the new one. The density scale is used to verify the compactness of the component and to exclude the presence of air inclusions or other similar flaws. Finally, the microscope makes it possible to detect superficial defects that would not be identifiable at sight, proving to be a valid aid in detecting cold laps, laminations, cracks and lakes.

For further information on the most common die casting defects we suggest you to read the article Die casting defects: internal and superficial.

Other tests

The various tests carried on in a metrology laboratory are aimed at measuring the dimensions of the components, their weight, the internal distribution of the material and their resistance. Moreover, other kinds of tests can be carried out, such as for example salt spray testaimed at evaluating the resistance of the component to environmental corrosion or the leak test that through the employment of dedicated tools, aims at verifying the absence of air inclusions in the component.

Conclusions

With this brief overview on the tools of the metrology laboratory we wanted to focus the attention on the importance of this department.

If you are interested in this theme and you want to be updated on the news of the sector we suggest you to visit Metrology News web site, an important online review rich in information on metrology tools and their employment in different industrial sectors.

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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.

To get updates on trends and innovations in the Zinc Die Casting industry, you are welcome to subscribe to our blog.

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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.

To learn more about the use of simulation software for zinc die casting, subscribe to our blog.

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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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When products require a high aesthetic impact, painting is often the best choice thanks to its versatility: not only does it offer a wide variety of colors, but it also determines the final texture of the product. Different painting processes make it possible to achieve matt textures, shiny textures and even metallic textures at a lower price than galvanization. Moreover, painted products gain extra surface resistance thanks to additional layers and to passivating treatments preceding coating.

However, like every industrial process, there are many factors that can compromise the final result. When working with die casting, some errors are more common than others. For example:

• Hue variation
• Fizzing
• Blisters
• Deformation
• Peeling
• Lack of paint
• Pitting
• Thin or scarce layer
• Orange peel

Now we are going to describe these defects, along with their causes and how to prevent them.

Hue variation

Hue variations can affect a single component, which could present different colors in some areas or darker shades, but they can also affect different batches of the same product, so that the first batch would be of a different hue than the followings.

These discoloring can be caused by expired or poorly stored powder, by fumes blackening some parts of the product, by variations in temperature or in baking time between each batch.

During painting, components are heated up to ease the application of powder painting and could be exposed to UVA rays or heated up again, depending on the treatment and the required finishing. Because of this, it is fundamental that time and temperature set for the first batch are exactly the same as the following batches and that they aren’t changed during the production. In addition, temperature control should be performed with carefully calibrated pyrometers.

Fizzing

Fizzing appears like small bubbles, very similar to blisters but smaller in dimensions. They are usually in groups and sometimes bubbles can have a central hole. In most cases they are caused by water or solvents trapped in porosities on the piece surface and emerged after heating.

These trappings can be avoided by carefully cleaning and drying the components, for example by pre-heating them up to 100°C or by using warm air instead of steam to dry them.

Blister

Blisters look like bubbles adhering on the metal surface, usually not bigger than 8 mm. Generally, blisters aren’t properly a defect in painting, but defects in die casting caused by air trappings and porosity laying under the surface that emerged after component’s heating.

In order to avoid this problem, it is possible to improve the distribution of porosity in casting through the use of a simulation and to test each component by pre-heating it before moving on with the painting phase. As an alternative, it is possible to reduce baking time and use the lowest temperature possible to avoid blisters from emerging.

Deformation

This kind of defect is rarer than the previous one, but it is still important to know how to identify it: it occurs when a component can’t bear the heat and creeps after baking, or when painting supports apply an excessive strength on the component, deforming it. It can be caused by some defects in die casting process or by a bad design.

To avoid this problem, it is advisable not to use high temperatures and to check positioning on the hangers and strength applied by supports. In some cases it could be necessary to modify the product design: asking suggestions to a trustworthy supplier would be advisable, to make sure that the design is suitable for the chosen industrial processes.  This may sound like a simple advice, but it can prevent further complications and improve the overall functionality of the component.

Peeling

Peeling are areas in which the paint coating was applied unevenly or is easily removable. This usually happens due to surface contamination caused by oils, fats, oxides, powders but also fingerprints. It is an easily avoidable problem: a careful cleaning and the use of gloves to manipulate each component should be enough to prevent the formation of this defect.

Lack of paint

This defect is typical of electrostatic powder painting: with this particular technique, an electric field is generated between the tip of the painting gun and the component. Normally this helps powder paint to stick to the surface evenly, but when a component has sockets, hollows or ducts on the surface, a Faraday Cage could form and  make the electric field head toward low resistivity zones, such as channels edges, dragging along powder painting particles. As a result, there will be heaps of paint on the borders of the hollow, and a lack of it on the internal surface.

To avoid these situations and make sure that varnish may reach the inside of hollows, it is advisable to reduce painting gun voltage: as a consequence, strength of the electric field near the component surface will decrease too, improving penetration rate of particles through Faraday cage by weakening the force pulling particles towards hollows’ borders.

When designing a component destined to electrostatic powder painting, it is advisable to avoid deep hollows and steep curves, to prevent the Faraday cage effect.

Pitting

Pitting looks like small granules underlying paint coating. They usually appear in groups and are caused by presence of dust on the component. To avoid their formation it is necessary to assure a meticulous cleaning of both working environment and  components, even in storage phases, by cleaning them with electrostatic cloth or with compressed air.

Thin or scarce layer

The expression identifies those zones in which the paint coating is transparent or inadequate, which are usually located near the edges of a component. They can be caused by scarce quantity, linked to a low powder dosage, or by wrong spraying parameters, unsuited for the component shape.

In addition, there could be problems with grounding that cause interferences between electric field, preventing the correct sticking process. For all of these cases, the solution is controlling and correcting process parameters and defects.

Orange peel

As the name suggests, this defect makes the painted component surface similar to orange peel: instead of being polished and smooth, it displays small bulges that makes it feel rough to the touch. It is a defect caused by excessive thickness of the painting layers: in order to prevent it, it is necessary to check the correct operation of nozzles and their positioning, and to modify system settings to spray a thinner layer.

To sum up, we have listed the most common defects in die casting painting processes, analyzing for each of them the root cause and explaining how to prevent them. To avoid this kind of defects, the best solution is to relay on expert and trusted suppliers, capable of suggesting the best kind of pre-finishing for the desired painting process.

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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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As a premise to go deeply the subject of mold maintenance it is important to underline that at the end of the die casting process the molds that were used in the production phase are in need of customized treatment to preserve their functionality and to increase their lifetime. These routine operations allow the maintenance department to keep productive standards consistent through time. For this reason every foundry is backed by a small maintenance department, which is dedicated to dies upkeep: the operations herein run are of the utmost importance for the quality of the final result. These operations include mold cleaning, extractors greasing, check/correction of cooling system and more.

As stated above, all foundries can count on a die maintenance department, ranging from small and simple ones to more complex ones, but not all these departments include a tooling department.

The presence of a tooling department becomes a distinctive factor and a competitive advantage for a foundry.

The tooling department, in addition to repairing mild damages, is fundamental to deeply analyze issues related to slowdowns or stops in the production process and, moreover, to develop and carry out activities in order to fix defects. These solutions are possible thanks to stored machines and appliances for a punctual analysis of defects, that help the operator to preserve the production standard and to notice particular reiterate problems. This allows to continuously improve processes and tools that, thanks to these activities, are constantly analyzed and updated for better performances.

Foundries that can count on an integrated tooling department can definitely improve their production process and their mold maintenance workflow, due to lower production costs in the long run and to a minimization of both risks and related time losses. On the other hand, foundries devoid of a tooling department need to rely on external mold makers for reparations, which are often located far from the production area.

Disadvantages of third-part mold repair

There are many downsides related to this procedure: sending a mold to an external mold maker naturally requires a larger amount of time, since delivery time adds to fixing time. Moreover, often third-parts workers do not have the same know-how of productive processes and error recurrences, as workers of the original production plant have. In worst-case scenarios roughly executed local repairs due to inadequate tools can negatively affect the whole production.

The need to send molds in for repair implies a slowdown even for those companies in which dies are self-produced, despite the mold-making plant being close to the foundry: since the reparations are usually pressing, they interfere with the planned workflow of the mold production department, leading to delays in fixed dies delivery. Moreover, unit saturation could cause the lack of an adequate break cause analysis, therefore leading to an error’s repetition that may have a severe impact on the production cycle.

Consequently, the lack of data prevents the identification of repeated problems, that will be treated as unique, thus requiring extended resolution time. It is not uncommon that a radical intervention is needed, involving the mold design, in order to permanently solve the issue.

Benefits of a tooling and maintenance department integrated in the foundry

Choosing to include a tooling department in the maintenance service, preferring a direct contact with the foundry and the Engineering department, offers a number of advantages, such as:

• Identification of the root cause;
• An accurate analysis of the breaking cause;
• A chance to perform more precise and quicker modifications;
• A more frequent mold maintenance;

Thanks to an adequate equipment and experience is possible to deeply study all defects and issues, performing all analysis and tests needed to identify the causes, allowing anomalies’ resolving. Furthermore, repairs can be achieved more complex and complete than the ones usually obtainable in a small maintenance department.

For example, when dealing with multi-cavity molds, if the tooling department is not adequate, it is a common practice to solve damages effecting just one single cavity by sealing it. This technique may appear practical, since it allows for a quick return of the die in the productive cycle, but it actually has a negative impact on the whole production: a unusable cavity implies one less product for every productive cycle, thus offsetting time tables and number of expected pieces, and increasing energy costs while obtaining the same produced quantity.

On the contrary, it is possible to avoid this kind of situation through the use of dedicated equipment and thanks to an experienced staff capable of executing complex processing, thus averting the need for extra-cycle activities.

It seems now evident how this is not just abstract theory, but the improvements can be measured through real factors and their effect on the productive capacity.

To sum up, the inclusion of the maintenance and the tooling departments in the productive plant brings direct advantages, such as the correction of criticalities and defects typical of the die casted production, improved schedules and a reduction of the extra-cycle activities.

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