From Machining to Injection Molding

Successful product development depends upon successful transitions from one phase of the development process to the next.  Prototyping new components can require different methods of fabrication and manufacture along the way.  Constraints in one phase do not always exist in the next phase and vice versa, so the “rules” can seem to be constantly changing.  Add to this the fact that the process will be handed over from one engineer to another, and the challenges of each transition can compound the risks.

When a client asked Natech to review the output of a third-party Industrial Designer, the Natech Engineers had to take a critical eye to the design for manufacture beyond the typical wall thickness, draft, gating, parting line, etc.  The application included 13 injection molded components as a part of an electronic reader.  The CAD arrived at Natech designed specifically for the parts to be machined.  As such, the splitting out of the components was defined based on how they could be machined. This raised three issues, or opportunities, when transitioning to molded components.

Extra Components

The feature-rich internals of one component made machining two components much easier than a single component.  This was driven by the need for a bigger and bulkier material stock as well as the difficulty of machining those internal features.

DFM Component Reduction

By contrast, the injection molding process favored combining the two components into a single component.  The risks inherent in the machining process no longer applied to the mass production, injection molding process.  In fact, having two separate components would have increased costs, time, and risks due to the extra join and assembly step required.  Removing the join increased the strength of the component and removed a potential leak path.  This also brought the added economic benefit of requiring one less mold for manufacture.

Self-Threading Screws

The application included self-threading screws which were handled with through holes and blind holes as illustrated in the image on the left below.  As a machined component, small block features or small holes served as blind bosses.  However, with the injection molded component, these blocks posed sink risks because of the inconsistent wall thicknesses.  Using simple holes for the through bosses bore risks from stress, torque, and pull-out.

DFM Self Threading Screw Boss

The Natech Engineers redesigned the blind boss to maintain a consistent wall thickness which is represented by the above right cross section image.  This reduced the risk of aesthetic issues or potential functional impact on any neighboring features.  The through holes were replaced by through bosses to improve screw pull-out resistance, torque retention and vibration resistance, and hoop stress risks.

Thin Mold Steel Condition

The original component design included a long, narrow crevice along its bottom edge.  This posed no risk for the machined version, but the injection molded version would require a long, thin piece of steel to form the feature.  Over time, this steel area could be stressed by the flow of hot resin under high pressure with the potential to bend or break.

DFM Thin Steel Condition

A simple adjustment of the feature and the corresponding features on the mating components allowed for a more robust steel condition.  This adjustment brought a longer lasting mold with lower risk of mechanical failure.

Conclusion

Processing could sometimes be used to address or accept some risks, but addressing the issues in the design for manufacture phase broadened the processing window that would produce quality parts over the long term.  This type of design for manufacture work requires good communication and a collaborative working relationship.  Sometimes the knee-jerk reaction is for the Industrial Designer to get defensive when their design needs to be changed, but in some sense this is the way the process is supposed to happen.  As a design is developed, it works conceptually, then it works once, and finally it works over the long term.  Many decisions with tradeoffs must be made along the way, and if the cross-functional team works together to continually improve the design without falling into the finger-pointing trap, the process is successful.