Molded-in inserts are used with plastic products such as electronic housings and instrument enclosures. With injection molding and compression molding, high-pressure forces can cause molded-in inserts to break or deform. Rotational molding is a low-pressure process that reduces these risks, but engineers still need to follow some design guidelines.
How Insert Molding Works
Molded-in inserts are mounted inside the mold cavity. The plastic is melted and forms around the insert, locking it into place. Ideally, the insert absorbs heat from the cavity’s surface and conducts it along the insert’s entire length. For best results, the insert is shaped to provide undercuts into which the plastic material can flow.
Molded-In Inserts and Material Selection
Rotational molding supports the use of molded-in inserts that are made of many different materials. There are two basic requirements, however. First, the insert material must be chemically compatible with the plastic material that will be molded. Second, the insert must have adequate temperature resistance to resist deformation at molding temperatures.
Insert Molding and Molded-In Insert Sizes
Engineers need to choose insert sizes with care. When rotomolded parts cool, the plastic shrinks. This can cause the plastic to drawn-down tightly on the molded-in insert, which may cause stress cracking in the plastic part. Larger inserts are more prone to cracking than smaller inserts. Inserts with sharp edges can cause stress cracking, too.
Part Design with Multiple Inserts
Rotomolded parts with multiple inserts require special considerations. As a rule, keep the distance between the inserts as small as possible. Otherwise, plastic shrinkage between widely-spaced inserts can cause high stress at the fitment between the insert and plastic. This condition can also complicate the removal of the finished plastic part from the cavity.
Rotational Molding from Gregstrom
Do you have questions about molded-in inserts, or other aspects of rotational molding design? Would you like to learn more about rotational molding’s advantages over injection molding and other manufacturing processes for plastic parts? Maybe you need more information about rotational molding materials, or the different types of molds that are used.