Q&A: How to think about insert molding for prototyping


Becky Cater is a product manager at Proto Labs, with specific expertise in injection molding, overmolding, and insert molding. She recently answered some of Medical Design and Outsourcing’s key questions about insert molding, and offered some insight on how rapid manufacturing is making the technology more accessible than ever before.

Becky Cater Proto Labs insert moldingMDO editor: What is insert molding? How is it different (or similar) to overmolding?

Cater: Many people view insert molding and overmolding as very similar processes. Both overmolding and insert molding are molding processes in which you mold material over a preformed part – either an injection molded preform (overmolding) or a metal preform (insert molding). The main difference between the two is the manufacturing method used to make the initial part. Another difference is that overmolding typically results in a part that contains two or more types of plastic. In contrast, most insert molded parts incorporate both metal and plastic into a single part.

MDO editor: What are the attributes of a medical device prototype that uses insert molding? Can you share some devices on the market that feature insert molding parts?

Cater: One of the common applications of insert molding is to include one or more threaded metal inserts in a plastic part when that part is intended to mate to another part in an assembly. Plastics alone may not stand up to the forces required to fasten two parts together. For example, threads in a plastic part can become worn or fail over repeated uses which can result in a failed part. Metal inserts help reinforce the properties of the plastic and ensure reliable fastening over repeated use of the part. This combination of plastic and metal allows designers to take advantage of the weight reduction of plastics and the strength of metal. Common examples include handheld medical devices where multiple components are joined into an assembly. Insert molding is also commonly used to create robust fixture points on housings that contain the controls for medical devices.

MDO editor: What are the benefits of Proto Labs’ insert molding process?

Cater: The use of molded-in inserts is a viable alternative to post-molded inserts which are placed in the plastic part after the molding process is complete. Incorporating the inserts in the part during the molding process itself eliminates the extra time, labor and cost of having a secondary insert installation process. In the past, it has been difficult to get parts with molded-in inserts quickly and in low-volumes. With the advancement of rapid prototyping and on-demand manufacturing, insert molding is now accessible to more designers than ever before.

MDO editor: Can you explain why design for manufacturing (DFM) is so important?

Cater: Understanding as early as possible if a design is suitable for insert molding is critical to ensuring you can bring your product to market quickly. Proto Labs provides detailed DFM feedback via our interactive quote within hours. This means that designers will know right away whether their part can be molded as-designed. If not, the DFM feedback will include recommendations for making the part manufacturable. This short feedback cycle allows designers to iterate quickly to their final design and have confidence that their part will be moldable.

MDO editor: What are some design tips you might advise medical device companies seeking to use insert molded prototypes to improve their product’s manufacturability? 

Cater: In addition to the usual moldability design guidelines, there are some unique considerations to take into account when designing a part for insert molding. For example, designing in undercut features can help enhance the retention strength of the inserts within the molded part. Because the inserts must be placed into the mold, it is also important to make sure the insert is located in an accessible part of the mold. Making sure the inserts are not too deep within the part and have a way to be held in place in the mold during the molding process will help ensure successful results.

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