Mold design has been widely described as both an art and a science. An art because some level of creativity is required to visualize how the part gets formed in the mold and to actualize this through the right design parameters and features to ensure the part gets formed efficiently.
A science because mold design also requires good knowledge of the material chemistry, tribology and how factors such as temperature and pressure affect the behaviour of the material and the outcome of the molding and demolding process. The mold designer also needs to have good analytical skills.
Material Specific Mold Design
Indeed when designing a mold, the type of material is a key factor to consider. For example feeding PET into a mold intended for the injection molding of polyamide is very likely to lead to defects and losses. So one of the first questions your mold designer should ask is what material is being processed.
Where the gate is placed, the type of cooling system implemented or the design of the cooling system and ejection system are key factors all dependent on the polymer being molded. As will become evident by the end of the article, these factors are interrelated to an extent. Other factors also interwoven include the dimension of the part, mold material, the injection moulding process as a whole, and whether hot runners or cold runners are being used.
Polymers like PLA, being a biodegradable plastic, have become more important as they achieve wider and relatively advanced applications in fields such as biomedicine, robotics and tissue engineering amongst others. In these fields parts with complex shapes mimicking biological structures that allow little room for dimensional and geometrical errors are more in demand. It is therefore even more important that there is effective communication between the mold designer, the process engineers and the manufacturers.
Simulation of PLA Mold Design
While softwares like Autodesk Moldflow and Solidworks exist for designing and simulating mold filling processes. A significant level of theoretical knowledge is required to make good use of these tools. There could be hundreds of places to place a gate or ejector pins in a mold. It is not practical to simulate all these iterations.
A good mold designer with the right background knowledge and experience, can narrow this down to just a few ideals and then use these simulation tools to further iterate and narrow down to the optimized placement. Thus saving a lot of time and resources.
The key parts of a mold are; the mold core, the mold cavity, the cooling system and the ejector pins and other components of the ejection system. Previous blogs have discussed the different types of runner systems. In this article, we focus on the cooling, gate placement and part ejection. Looking at the important factors to consider when designing these parts of the mold.
Gate Placement
The optimum location for most mold is normal to the part of the cavity with the least thickness. This allows faster and more even solidification. The cooling of the product is significantly affected by the location of the gate. This is particularly important for PLA parts that are likely to have varied thickness and prone to dimensional instability.
The gate needs to be placed in such a way that the parts that take longer to cool get filled first, air traps are prevented, and the filling does not weld lines or similar defects. The gate placement should aim to fill all the cavity walls at the same time.
If solidification begins near the gate or in a part of the mold cavity that can obstruct the flow of the melt into the rest of the mold, then this is a bad mold design. A well placed gate should maintain smooth laminar flow without turbulence anywhere during the filling.
In some cases more than one gate is required. The number of gates per cavity depends on the size and shape of the part. A small part with a complex shape might require more gates than a larger part with simple geometry. So for PLA you are more likely to be molding complex parts which in more cases would require multiple gates strategically placed.
Typically gates are placed in the thick sections so that the flow thins out into the thinner sections of the mold cavity. In doing so, turbulence is avoided. The longer flow path also aids even heat removal. The gate should therefore be placed in such a way that the melt has sufficient distance to flow. This is also important for even pressure distribution. So hot spots and high stress spots are prevented.
Furthermore, the gate is likely to leave some marks or sprue on the part. The gate should ideally be placed in the least visible part of the gate. The mold designer should creatively blend the mark with the product surface and shape. Some do it so well that the sprue appears to be part of the product aesthetics or even functional feature.
It should also be noted that the gate placement may also vary for the different types of gates; edge, pin or hot gates.
Cooling
The conventional cooling channels are usually straight holes strategically drilled into the mould. However in many cases where PLA is being used, the parts to be molded have complex geometries such that the conventional cooling channel design just won’t work.
One of the goals of a cooling system is to maintain a minimal temperature gradient across the melt as it cools. This promotes uniformity of the part. To achieve this, the cooling channels must be well placed to synchronize with the mold filling and ensure even contact and hence even heat removal.
Effective cooling can significantly reduce cycle time. Design factors to consider for the cooling system include diameter of the cooling channels, the distance from the cooling fluid to the surface of the part being molded, and the spacing between the cooling channels.
Almost all plastics will experience a level of shrinkage upon cooling. This is influenced by the properties of the material, in particular, crystallinity. PLA can vary from amorphous to semicrystalline. With the semi-crystalline having higher shrinkage.
Cooling Fluids
The cooling fluid is typically either water or oil. Since these fluids are not in direct contact with the melt, the important consideration here is choosing the fluid that will not introduce any leakage problems and has sufficient heat transfer coefficient to effectively remove heat from the mold.
Ejection
The ejection of a part from the mold marks both the end of a cycle and the beginning of a new cycle. How easily this transition occurs affects cycle time and down time, hence productivity.
Having a part stuck in the mold can be the beginning of a nightmare or a very bad day if not well designed. It delays the entire process and it can cause costly damage to the mold. In a process meant to be churning out hundreds to thousands of parts per minute or so, every second counts. So ejection should be as fast and as seamless as possible.
Features like ejector pins, plates, sleeves, cores and lifters can aid demoulding process or part ejection. However the mold design should inherently facilitate ejection. Well polished surfaces with minimal friction between mold cavity surface and the part surface is ideal.
A well designed mold will ensure that it takes advantage of gravity to aid part ejection where possible. Better to have a mold design where the part falls out by itself than have one that needs additional features to be lifted against gravity before ejection.
PLA in particular has tendencies to get sticky compared to other more hydrophobic polymers with higher glass transition temperatures. The low glass transition temperature (Tg) and low hydrophobicity of PLA calls for more control over the mold design parameters to prevent part sticking.
The mold material needs to be compatible with PLA. The surface chemistry of PLA and the mold material should repel each other well. A well polished surface and the use of mold release agent and/or applying mold coating is often required for PLA. Optimal draft angles should also be included to aid ejection.
Other factors affecting the demolding or ejection of a PLA part include the temperature, product design (for example presence of undercuts), tribology of the PLA and mold.
Ejection should be fast and smooth, causing no damage to the part or the mold.
Shrinkage and Part Ejection
Some level of shrinkage is desirable to aid ejection. Shrinkage can always be accounted for in calculations of parameters like shot size and cavity volume and dimensions. What’s also important is to have even shrinkage without loss of dimensional stability.
Semi-crystalline PLA having more complex parts may be more prone to uneven shrinkage. So while the shrinkage might aid part removal, it could lead to loss in dimensional accuracy. Additional ejector pins and additional ejection system components like lifts, stripper plates to ensure effective demoulding of complex PLA parts.
Conclusion
Due to their relatively low Tg and degradation temperature, hydrophilicity, tribology and wider applications in fields demanding more complex geometry, PLA injection molding calls for strategic multiple gate placement, cooling channels that go beyond the conventional straight drilled holes and ejection systems that are relatively complex with multiple ejection pins and additional features such as lifters and sleeve plates.
In addition to designing a mold that results in a part well suited to the function and aesthetics, PLA molds should be designed for optimum productivity and manufacturability. The mold should be well suited to fit into the entire injection molding process seamlessly. From the gate receiving the melt from the injection unit to the part getting released by the ejection unit. This should be repeatable for several cycles creating identical parts without rejection or need for rework of parts.