Molded-in threads in rotational molding provide a cost-effective way to assemble plastic products like water tanks. They eliminate the need for secondary machining or inserts and can improve the strength and durability of rotomolded products. For part designers, it’s important to understand what molded-in threads are and to follow best practices for manufacturability and performance.
What are Molded-in-Threads?
Molded-in threads are threaded features that are molded directly into a plastic product during rotational molding. They can be internal (female) or external (male) and are integrated into the mold design. This approach reduces production costs and ensures proper alignment and consistent thread dimensions. It also eliminates the need for post-molding operations like tapping, cutting, or inserting threaded hardware.
Thread Design
When designing molded-in threads, it’s important to adapt standard thread profiles to the rotational molding process. Apply a coarse pitch and generous tolerances to account for material shrinkage and slight dimensional variability. Polyethylene is a good choice, but it has a relatively high shrink rate. Regardless of the rotational molding material you select, fine threads are more susceptible to incomplete filling or deformation during part release.
Note: If your project requires tight threads, spin welding can be used to add them after rotational molding is complete.
Draft Angles
Apply adequate draft angles to facilitate part release from the rotational mold. Apply a minimum draft angle of 1° to 2° on thread flanks. Some applications may require a larger draft angle depending on wall thickness and part geometry. Also, remember that rotomolded parts are not forcibly ejected like injection molded components. Therefore, their threads must allow the part to exit the rotational mold with ease.
Rotational Molder Selection
Finally, chose an experienced rotational molder. Often, parts with threads require manual unscrewing. In many cases, the molds have removable cores or threaded sections that can be unscrewed before the part is released. Molds also require proper venting to allow trapped air to escape during heating and ensure even material distribution around the threaded form. If a mold surface contains significant imperfections, molded-in threads could become stuck.
Ask Gregstrom About Molded-In Threads
Gregstrom Corporation is a third-generation, family-owned business with over 75 years of plastics manufacturing experience. Ask us about molded-in threads for rotational molding and learn more about our manufacturing capabilities and value-added services. From design assistance to tooling, and from rotomolding through finishing and assembly, Gregstrom does more. Contact us for a quote.
