The Best Way to Mold Undercuts in Plastic Parts

Rotational Molding Services | Undercuts
Rotational Molding Services | Undercuts

Undercuts are protrusions or indentations that are parallel to a mold’s parting line. Sometimes, undercuts are used to create screw threads, locking tabs, and slots for switches. Rotational molding isn’t the only plastics manufacturing process that allows for undercuts, but rotomolding can better accommodate parts with these features. If your molded part requires undercuts, it’s worth considering the advantages of rotational molding. You may also want to consider the disadvantages of injection molding, a common plastics molding process.

Undercuts: Rotational Molding Advantages

Rotational molding creates strong, hollow plastic parts in a wide variety of shapes, sizes, textures, and colors with features such as undercuts, multiple walls, and molded-in hardware and graphics. With its low tooling costs and quick tooling modifications, rotomolding is a good way to prototype a part that will be manufactured with another plastics molding process. For example, some companies will use rotational molding to prototype a part and then use blow molding when higher volumes justify greater tooling expenses. Rotational molding is also used for both low and higher volume production runs, and is flexible enough for projects ranging from small, intricate designs to large, complex parts and assemblies.

In other plastics manufacturing processes (such as injection molding), demolding may require the bending or twisting of parts that have molded undercuts. This lengthens cycle times, adds costs, and can cause part damage. By contrast, rotational molding lets the molded plastic shrink away from mold walls. As Machine Design explains, it’s possible to design rotomolded parts that completely shrink away from the undercut for simpler parts removal. When you design rotomolded parts, an undercut should equal half the material shrinkage.

Undercuts: Injection Molding Disadvantages

Some injection molding guidelines recommend avoiding an undercut entirely because these features complicate part ejection. That’s because when an injection mold is released, undercuts may interfere with the separation of the core and the cavity. Injection molders can use additional tooling called side actions to facilitate this separation, but that adds costs to your plastics projects. Plus, injection molding has higher tooling costs already. Unless you need a high volume of parts, the cost of an injection mold may not be economical.

Do you need to design molded plastic parts with undercuts? Would you like to learn more about rotational molding, or discover how value-added services from design assistance and tooling to quality control and assembly can help you save time and money? Gregstrom, a rotational molding with over 70 years of experience, is ready to help. To request a quote or learn more about our company, contact us.