Case Study: Design for Overmolding
Design for Injection Molding is one field within the Design for Manufacturability (DFM) body of knowledge. Design for Overmolding is an even more specific knowledge track within Design for Injection Molding. This tends to be a very tricky area because the rigid substrate essentially serves as an insert within the mold, and the overmold tends to be a flexible elastomer. This creates a plastic and steel parting line which has a tendency to flash. What is less commonly known is that the process of injecting the elastomer can alter the underlying substrate and cause issues that are best addressed during the design phase. As always, an example helps illustrate the point.
In this case, a medical diagnostics device required an overmold to serve as a fluid barrier for a plunger to press an onboard reagent through the device. The overmolded elastomer coats the inner diameter of the substrate’s well.
During the Design for Overmolding phase, the Natech Engineers identified the risk of collapse to the substrate. The issue arises when the substrate is being filled. In the illustration below, the gray substrate sits in the mold while the blue elastomer is overmolded under high pressure and high temperature. Due to the geometry of the substrate’s outer diameter, the original mold design would have caused the substrate to be unsupported during the overmold process. This lack of support on such a tall feature would allow the high pressure of the injecting elastomer to deflect the substrate outward.
In the extreme, this deflection could cause damage to the substrate. An even more likely scenario would be a flexing of the core pin relative to the part, which would result in an inconsistent wall thickness. Also, movement of the substrate would present gaps in the part and create overmold flash directly into the business area, rendering the part non-functional.
The Natech Engineers adjusted the mold design to add side cams along the entire outer surface of the substrate. In the image below, the red outer support of the long tower helped maintain the position of the substrate along the length of the inner diameter.
They then introduced interlocking features for the core / stripper sleeve insert to further fix the location from either end of the well. In the image of the modified mold design above, the green core above and below the gray part can be seen interlocking into the part itself to create a tight seal. If and when the elastomer starts to flash, it would flash away from the business area of the mating component eliminating the impact to the product’s functionality.
In this instance the Natech engineers delivered higher quality in less time for the client. With the mold design finalized, the mold was constructed and parts were run with a consistent wall thickness free of flash or deformation. The Design for Overmolding project phase required more time earlier in the project but resulted in greater time savings for the project during sampling because fewer iterations were required to achieve the desired part.
Any damage to a part during the molding process also increases the risk of damage to the mold itself. This risk increases during overmolding and insert-molding, so it must be considered with deep care during part design and mold design. Design for Manufacturability puts the principle of Quality by Design into action. Diligent forethought, deep experience, and open dialogue with clients and suppliers provide the ingredients for successful Quality by Design.