Effectiveness of High Conductivity Inserts
A common method of reducing cycle times is incorporating high conductivity inserts into the mold design. Materials such as beryllium copper or aluminum have thermal conductivities that are much higher than that of traditional tool steels. The use of these higher conductivity materials can allow for the mold surface temperature to remain lower and faster heat extraction from the part, if used properly. However, just having cooling channels run through the high conductivity inserts is not always enough to help reduce the temperature of the mold. Additionally, the price of these materials can be significantly higher and a balance between cost and cooling efficiency must be achieved.
By incorporating the higher conductivity cap it was anticipated that a cooler mold temperature could be maintained on the core side of the tool, which would help reduce the cycle time. An initial cooling analysis showed that with the current cooling layout the temperature would be reduced, but was not sufficiently reduced to justify the added cost of the cap. Therefore, the cooling circuit layout was modified within the cap to help maintain the lower mold temperature (see Figure 5). By increasing the surface area between the coolant and the high conductivity insert the mold temperature dropped by 20 F. This analysis quickly showed the moldmaker an issue with the proposed cooling layout and an acceptable solution was found that allowed the moldmaker to save machining time and the cost of reworking the material.
Cycle Time Limitations
Another benefit to simulating the cooling stage is to understand what is happening inside the mold. When asked to reduce the cycle time, it is difficult to determine where energy should be focused, since the limiting factor is rarely known. Is it the material, is it the geometry or is it the cooling layout? Using the results of the simulation one can determine what is limiting the cycle time.
Even with an optimized cooling layout and mold construction, the geometry and material of the part may not allow for the desired cycle time to be achieved. From basic injection molding theory we know that the cycle time is directly proportional to the square of the part thickness and is inversely proportional to the thermal diffusivity of the resin. This means that if the wall thickness of the part is doubled, the expected cycle time would be four times longer. This also means that the polymer chosen can influence the cycle time. While cycle time is not a high priority when deciding what material is best for the application, there are often several resins that meet the performance requirements. The addition of additives such as talc, glass fibers,or carbon fiber can influence thermal properties of the base resin and allow the resin to cool faster. Awareness of the thermal behavior of the different resins could mean the difference between meeting the quoted cycle time or not.
Returning to the can example presented earlier, the moldmaker was able to reduce the cycle time by incorporating the beryllium copper cap and modifying the cooling layout. However, the end customer still wanted to further reduce the cycle time. After running several additional iterations it became clear that the combination of the can geometry and the material selected would not allow further reduction for the cycle time. The geometry could not be modified due to performance requirements, so an alternative resin (Material 2) was simulated to see its effect on the cycle time. Figure 6 shows that changing the resin allowed the cycle time to be reduced to meet the desired cycle time.
As pricing pressures continue to force our industry to find further innovative ways to manufacture parts more economically without sacrificing performance, it becomes our responsibility to determine the viability of these demands and to find solutions. The use of simulation has been a long-standing tool to injection molders and toolmakers to determine how to optimize the fill and pack stages. However, analysis of the cooling stage has often been overlooked. Simulating the entire injection molding process provides insight into where energy should be focused to help our customers bring their product to market faster while improving the bottom line.
Edited by Leafly Mould Provides Injection Mold, Plastic Mold, Injection Molding, Die Casting Mold, Stamping Mold