Originally published April 1996
Jim Vance, Jr.
Medical Polymers, Inc., Spencer, IN
Insert molding is an injection molding process whereby plastic is injected into a cavity and around an insert piece placed into the same cavity just prior to molding. The result is a single piece with the insert encapsulated by the plastic. The insert can be made of metal or another plastic. The technique was initially developed to place threaded inserts in molded parts and to encapsulate the wire-plug connection on electrical cords. Today insert molding is used quite extensively in the manufacture of medical devices. Typical applications include insert-molded needle hubs and luer fittings and bifurcations, as well as encapsulated electrical components and threaded fasteners. Generally, there are few design limitations or restrictions on material combinations.
There are two types of bonding that occur in insert molding, molecular and mechanical. Molecular bonding can occur when the insert material is the same as or similar to the encapsulating resin. This will yield the best results from the joint, both for physical strength and leak resistance. An example would be molding a polyurethane bifurcation to a polyurethane catheter. Mechanical bonding can take place in two ways, by the shrinking of the encapsulating resin around the insert as the resin cools, or by the surrounding of irregularities in the surface of the insert by the encapsulating resin. Although shrinkage always occurs, it is rarely sufficient to produce adequate physical strength or leak resistance of the joint. In general, when insert molding dissimilar materials, the insert should offer some means of mechanical retention such as a sandblasted, flared, or knurled surface.
Equipment. Although insert molding can be performed using a standard injection molding press, doing so can make the critical step of loading and retaining the insert in the cavity a difficult operation, and can thus place restrictions on the design of the part. However, there are some specific molding machine designs that are better suited for insert molding and offer much greater flexibility and productivity. Rotary and shuttle-table-type injection molding machines are excellent for this purpose, because they allow operators to load and unload inserts in the bottom half of one mold while actual molding takes place in another. These machines also lend themselves well to automation of the loading and unloading of inserts and parts. Other machines that offer vertical mold clamping can work well for low-volume insert molding, but are generally less productive than rotary or shuttle-table machines.
With a few exceptions, molds for insert molding are generally designed in the same fashion as are molds for injection molding. Molds for a shuttle-table press have two ejector halves, and molds for a rotary press have two to four ejector halves. Tooling costs will be higher due to the additional mold halves. The actual molding-cycle time is the determining factor for establishing the number of cavities and mold halves required. For optimum productivity the time it takes to load the inserts and unload the parts should not exceed the molding-cycle time. This consideration is also important when molding resins such as polycarbonate or PVC, for which residence time in the heated barrel of the machine is an important factor. Because the molten plastic is typically injected into the cavity and around the insert at pressures exceeding 1000 psi, when the mold is designed it is important to determine the exact location of injection gates and how the insert is to be held in place.
Materials. Like injection molding in general, insert molding can be accomplished with a wide variety of materials, including polyethylene, polystyrene, polypropylene, polyvinyl chloride, thermoplastic elastomers, and many engineering plastics. The primary factors that restrict the use of insert molding are not process related, but are determined by the strength and other properties required for the molded product.
Processing Parameters. One of the chief causes of failure in an insert-molded part is the cleanliness of the insert. It is absolutely imperative that the insert be as clean as possible prior to molding. When molding with large metal inserts, the inserts may need to be preheated to minimize the stresses caused by differential thermal expansion and contraction. When inserts are manually loaded it is important that the operator maintain a consistent cycle time.
Design Considerations. In general, the basic design rules for insert molding are the same as those that apply to injection-molded parts. However, designers should also be aware of the following elements that may affect the design of their parts:
* The material from which the insert is made.
* Pull- and compression-strength requirements of the insert from the plastic.
* Leak test requirements.
* Torque or axial forces to which the insert will be subjected.
* Voltage requirements for electrical applications.
Any or all of these elements may establish parameters that can help the designer determine what encapsulating resins will work for the application. They may also create requirements for the type of material preparation that must be performed on the insert in order to ensure proper performance of the finished part.