Can You Add Threads to Plastic Parts?

The Cost-Effective Method: Interrupted Threads

For many internal components where a perfect 360-degree thread isn't required, you can design a “molding-friendly” thread. This typically involves splitting the thread profile into two halves that can be formed by the main A and B sides of the mold without any undercuts.

Imagine a nut. Instead of a continuous internal thread, you design two C-shaped threaded sections on opposite sides. The gaps between them allow the two mold halves to pull away cleanly. This is an extremely common and effective way to create a functional thread for light-duty applications while keeping your tooling simple and your costs down.

The High-Performance Method: Unscrewing and Collapsing Molds

When you need a continuous, leak-proof thread like the one on a soda bottle or a cosmetic jar, you can't have gaps. These features require specialized tooling with moving internal components.

• Unscrewing Molds: For external threads (like on a bottle neck), the mold contains a mechanism that literally rotates and unscrews the threaded steel core from the plastic part after it has solidified.
• Collapsing Cores: For internal threads (like in a cap), the solution is even more complex. A multi-part internal core mechanically collapses inward on itself after molding, reducing its diameter and allowing the part to be ejected without damaging the newly formed threads.

These solutions create perfect threads but add significant cost, complexity, and maintenance requirements to the mold. This is a necessary investment for high-volume consumer packaging but can often be avoided for other applications.

What's Next?

Understanding these different design strategies is the first step in managing your project's bottom line. In our next guide, we'll take a broader look at the financial side of a project: From Prototype to Production: How to Reduce Your Injection Molding Costs.