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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Preliminary Phase

Before starting with the actual mold design it is necessary to verify the manufacturability of the piece with high pressure die casting technology. In this phase the practicability of the process should be judged from a dimensional and geometrical point of view.

• Dimensional issues
  In order to calculate the volume of the casting and the opening force we need to know the dimension of the component and the required number of cavities for each casting. These two data prove essential for studying the feasibility of the process.
• Geometrical issues
  The geometry of the component should include the drawing of the parting line. This line splits mold in two parts which allow mold opening and casting ejection. Component surfaces tilt depends on their position from parting line: surfaces have to be oriented in mold opening direction. Moreover, the geometrical tolerances of quotes, which can be found on the 2d model or can be indicated by the costumer, can be difficult to achieve due to the shrinkage caused by metal cooling. Quotes with identical tolerances can be more or less critical, depending on their nominal value: the higher the nominal value of a quote, the higher the difficulty to obtain the same value on the casting. This is called “manufacturing precision” and it is present in every technological process.

Because of this, it would be advisable for the manufacturer to make an arrangement with his client to elaborate the project within a co-design service, to find an agreement on the modifications of critical tolerances, on the geometry of the component and on the position of the die’s parting lines.

Once the manufacturability of the component has been verified, it is possible to move on with the actual mold design. The design of pressure die casting mold begins with the knowledge and the definition of the piece that is going to be casted.

In order to deeply analyze the different design phases, it is necessary to reconsider the previously obtained data regarding the weight (or volume) of the component, its projection area towards the die opening, and the number of  cavities needed.

Number of cavities

To calculate the number of cavities it’s very important to consider hypothetical cycle time, number of pieces which have to be produced and cavities orientation on mold. It will be possible to choose between a single cavity mold or a multi-cavity mold.

When considering a multi-cavity mold, it is necessary to keep in mind that not only the complexity of filling and ejection phases will increase, but also that the handling of the production process may be affected by cavities’ disposition and dimensions of the products. An example of this can be found in logistics, since an higher number of impressions means a direct increased flow of material to move.

Projection Area

The projection area identifies the theoretical surface that can be obtained by the projection of a cavity on the plan which is perpendicular to the mold opening direction. Projection area plays a fundamental role in the designing phase: from it depends the opening force generated by molten metal on die walls. Depending on shape dimension orientation, stronger or weaker forces will be applied in mold filling phase. An excessively strong force may bring to a material overflow, and thus to the formation of burrs on the product profile. If the forces are too strong compared to the closing force of the press, the result could be a non-compliant product or, in the worst case, an unfeasible process.

In order to prevent this, the maximum force generated by molten metal in filling phase can be estimated. This force is equal to the product of the projection area, the maximum specific pressure of the machine and a pre-set safety factor.

This factor is based on metal dynamic push. Its main function is to offer a wide margin to counter the moment of maximum pressure at the end of the filling process, usually called water hammer. At the end of the process machine transfers both static and dynamic force, leading to a pressure pick that must be absorbed by the closing force of machine itself.

The closing force produced by the press depends on the press model and on the dimension of its stroke.

The push of metal and the projection area are the core elements to consider during mold design.

Volume and shape of the die

As stated above, the volume of the component is fundamental for the design of the mold. In addition to the total volume, it should be taken in account that massive components will suffer a greater shrinkage,  caused by shrinkage rate increase, due to longer cooling times. Therefore the mold cavities should be sized accordingly.

Moreover it is necessary to consider a number of variables that will determine the final size of the mold. Among these, the most prominent are the injection channels, the kind of die closing, the width of the runners and the presence of overflows.

• The plainest kind of mold closing is the open/close one, which has a single parting line placed in the simplest position possible. This system is ideal for those products with a clean, simple shape that can be easily ejected by the sole movement of the die’s halves and ejection system, but it cannot be used for structures with undercuts or complex geometries, which need transversal movements to be ejected. In these cases, the designer should add to the total die size the dimensions of slides and their movements.

  

• The size of the injection channels depends on the position of the piece and gates, and on the number of cavities. Their shape must respect some requirements of fluid dynamics: for example, it is characterized by a decreasing section in the movement direction of the molten metal, so as to improve adherence on the mold walls. This progressive shrinkage leads to an accelerated flux and to a reduced chance of detachments of the boundary layer from channel walls. The speed grows throughout the funnel up to the entrance of the mold cavity, where the metal gets sprayed in the cavity. A smoother external layer prevents the formation of turbulences that may cause trappings of air or material, and even the erosion of the die.

• Overflows are similar to small wells placed in strategic points of the mold and they serve two main functions: the first one is to collect first metal shot, which is usually colder than the following flux. This strategy helps to avoid formation of cold laps and other aesthetical defects of the die casted product, improving its compactness and its superficial quality. The second function involves die temperature: by collecting molten material, overflows are turned into a source of heat that increases die temperature in those areas critical for the final casted quality.
• Another element that influences the mold size is the placement of channels for air expulsion system. This complex tunnels, called vents, let air escape the mold, avoiding gas entrapments in the metal.

Once positions and dimensions of these elements are decided, it is possible to move on to the next phase.

Simulation through semi-empirical models

After completing the initial design of the mold, it is possible to proceed with a simulation of die filling by the use of semi-empirical models.

These simulations are used to calculate the ideal modality of mold filling: depending on the function of the casted piece, the filling process may vary. For components with a structural role it is better to enhance mechanical resistance and compactness, while for aesthetic components it will be necessary to obtain the best surface finishing possible, thus avoiding any superficial porosity and internal air trapping that may escape afterwards, damaging the external surface treatment.

By varying the filling time it is possible to alter these characteristics: the quicker the filling, the better the surface quality; on the contrary, a longer filling time at higher pressure will lead to a stronger, more resistant compound.

After completing these analysis it will be possible to spot in advance the eventual issues in casting phase and therefore to intervene on the design, before building the die.

To sum up, mold design for high pressure die casting begins from a manufacturability analysis, followed by a calculus of metal pushing forces and a study of channels position for injection, vents, and the necessary slides. Design and optimization of these channels can be done through the use of a simulation software to decide the ideal filling mode and to spot any possible issue. Once all of these phases are completed, it is possible to move on to the production of the designed mold.

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Typical defects in die casting production can be caused by many different factors: some of them can be avoided by using a simulation software, while others are linked to machineries’ issues or mold anomalies.

The second kind of defects can be solved thanks to an advanced maintenance department with complex analysis and tailored solutions: that is one of the reason why having a Tool Shop located in close proximity with the production area is a great advantage.

Specifically, the most common defects that can be solved by the maintenance department are:

• Burr
• Surface erosion
• Sticking
• Gripping
• Broken plugs

In the post below, each of these defect is going to be described, starting from their definition up to their solving.

Burr

Burrs are among  the most frequent defects in die casting. They are caused by a material overflow from the die casted shape borders. Burrs presence is both unaesthetic and dangerous, for example it can cause short-circuit in electronics, or it can cause slight injuries to the workers handling the metal pieces through the production process.

Usually burr formation is due to an irregular closure of the mold. This is caused by either a lack or an excess of material on one side of the mold, an imperfection linked to an error in mold building or, more often, it can be the result of a damage occurred during the casting.

In order to solve this critical issue the standard resolving method involves the use of a CNC machine, which enables a precise processing of the contact surfaces. Sometimes to identify the most critical zones an analysis with a mold-trial press needs to be run, by using a Prussian blue dye to highlight the . Thanks to this method it is possible to identify the contact point of the mold, and to proceed with the necessary adjustments by adding or removing material with a laser welding.

Surface Erosion

Surface erosion is the removal of steel particles from the impression surface, and it is mainly caused by excessive speed and excessive flux turbulence during the injection phase. The maintenance department usually solve this issue by welding or by using dowels on the problematic zone. This method allows for a rapid recovery of the productive process, since the molds can be fixed quickly.

Sticking

The term “sticking” identifies an alloy deposit on the die walls due to an electrochemical reaction between the casted Zamak and the steel walls of the mold, which leaves clear traces on the pieces, such as stripes or excessive surface roughness. In this case it is necessary to work directly on the mold, restoring it to its original status through an ultrasonic cleaning of the matrices or, more frequently, through a mechanical removal of the alloy deposit. Independently from the chosen solution, having a Tool Shop close to the foundry accelerates the repair process and helps avoiding long machines stops.

Gripping

Gripping is the stall of two solid surfaces that scrape one against the other, creating friction between two pieces. In die casting, gripping concerns mainly the extractors and more generally all the moving parts of the mold. Usually the moving parts are made of materials with a different hardness than their sliding rails so as to avoid gripping.

Nevertheless, wrong tolerances in the passing holes, excessive temperatures or a lack of lubrication of the mold in production phase can cause gripping. To sum up, this problem can show up in case of a mistake in mold design, in mold production or bad upkeep in production or storage. In these cases the tool shop can intervene and fix the issue by restoring the damaged surfaces, by improving the hardness with nitriding or rebuilding the deteriorated parts.

Broken Plugs

In die casting industry, the term plug identifies small-sized cylindrical shapes that are used for making small cavities in the casting. Plugs are fundamental mold components and in some cases they can be very fragile, given their size, and are subjected to many strains such as overheating, compression, traction and other deformations that can damage their integrity. For some projects an extraordinary maintenance can be planned, so as to avoid the breakage of these small elements and avoid delays in products delivery.

The previously exposed issues are among the most common deriving from anomalies in molds. Although this is not a complete list, it clearly shows how an up-to-date and experienced maintenance department can prove essential in accelerating the fixing process. Whether the problem is a common one or an unusual one, the Tool Shop along with the maintenance department can help solve them and gain new experiences from them.

The cases analyzed in the Tool Shop can produce important data. In fact, this process is very useful not only to find a quicker solution for the defects in die casting, but sometimes they also offer inputs for the prevention or the removal of the issue in mold designing phase. Recurrent problems often have the same cause, which can be prevented and dealt with in mold design phase. This kind of solution can increase the whole production capacity.

All of this increases the knowledge of the product and the processes both in the maintenance department and in the whole plant, making the production process faster and avoiding wastes of time and resources by reducing occurrences of defects in die casting.

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Optimizing the mold guarantees a better satisfaction of the designer, therefore the client, that obtains a product as demands and, in economic terms, a better optimization of the mold assures a considerable reduction of the maintenance costs, improving also the productivity with a consequent impact in terms of lead time as time to market or time to order.

To optimize the mold, in terms of die casting mold design, is necessary to operate on 3 variables.

• Product
• Technology
• Production

How to optimize mould design in zinc die casting?

These 3 variables need to be lead to produce the best performance maintaining the equilibrium of factors, being interconnected among them.

In terms of product design it is necessary a co-design activity with the client and/or the designer to develop a more compatible system between mold and component. In terms of technology it will be necessary to produce a mold that guarantees the best performances possible and, regarding the production, it will be necessary to find the best solutions for the mold regarding the productive lines of the company.

Shown below, in details, the principal actions of optimization for every variable.

Product: Co-Design Phase

The first step to optimize the mold is understanding the needs of the client and/or designer, in order to produce a mold that is able to maximize the product demand and properties.

The second step will be to understand the functional or aesthetical features of the products, searching for a better optimization reducing the stylistic excess, such as not relevant aesthetic details, physical excesses as non functional material masses that can be replaced with ribs or adequate structures. In this phase is necessary to define the interaction of the product with other components: it is important to work together with the client, trying to understand the use of the part as a component of a complex project.

These two phases are the right steps to choice the ways of fall down of the piece. In fact the part can come out of the mold to fall or can fall inside an automation system: the choice of this step, well defined with the client, depends on the aim of the die cast.

Technology: maximise the materials

It is used alloy steel to create better mold for zinc die casting, in particular alloy of chromium molybdenum vanadium: many treatments have been accomplished to increase the main characteristics of the material and specific treatments to improve the usury resistance.

About the consolidation of the casted products inside the mold are fundamental implement solutions to develop a circuit conditioning, using water, oil or air. This step is very important because it allows the product a better solidification phase.

In terms of mechanical function, during the mold planning phase and its optimization, it is necessary to choose high level materials for standard mechanics organs, to guarantee a better performance without extra costs.

Now, that the mold has been defined, it is necessary to implement actions to guarantee the best fill up for the mold: so it is essential to compose a gate and a runner to achieve the client’s expectations. As said before, the material flows liquid in the mold, then it cools down, so the injection part is fundamental for the good success of the process.

To deal with this phase calculation tools and simulations are necessary for helping in the choice of the best solutions of fill the mold cavity, in terms of gate and overflow.

Software tools and specialists’ experience can make the difference.

Production

In terms of production the mold need to be suitable to the productive lines of the company: in this way the production time is optimized with a lot of advantages for the client.

It is a tailor made process, that has to factor in the plant characteristics.

It is a important step because the mold has to work on a precise machine in the productive plant, with fixed characteristics of tonnage, that guarantee the die cast quality. In this case the outcome of the product is fundamental, as above described, but it is also important not to underestimate the conditioning of the die cast itself that is related to company availabilities.

Depending on the conditioning system it will be necessary to define all the conditioning process.

In the last step, after the outcome of the piece out of the mold, in the production phase, it will be necessary to define the flow marks process, that can have different grades in automation.

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