Threaded plastic parts for injection molding

Can You Add Threads to Plastic Parts?

By Ichiplas Design Engineering Team · Oct 2025 · Updated Nov 2026 · Reviewed by the Ichiplas engineering team

Threads are a fundamental part of mechanical design, but their helical, undercut nature seems to violate every rule of simple, pull-apart mold design. So, can you mold them? Absolutely.

In fact, threaded plastic components are produced by the billion every day.

The key is to understand that there are two main approaches. The first is a clever, cost-effective method for simple applications that avoids complex tooling. The second is a more robust but expensive solution for high-performance threads that require a perfect seal, like those on a bottle cap. Choosing the right approach early in your design process can have a huge impact on your tooling budget.

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. Our insert molding services offer another approach — embedding threaded metal inserts directly into plastic parts for high-strength connections. Unscrewing molds are a necessary investment for high-volume consumer packaging but can often be avoided for other applications.

Ways to Add Threads to Plastic Parts

Choose the method by how strong the thread must be, whether it must seal, and how often it will be assembled.

Comparison of methods for creating threads in molded plastic parts
Method	Best for	Tooling cost	Strength and reuse
External thread on the parting line	Simple external threads	Low	Good; leaves a small parting line across the thread
Interrupted (split) thread	Light-duty internal or external threads	Low	Moderate; no continuous seal
Unscrewing mold	Continuous sealing threads (caps, closures)	High	Excellent; full 360° thread
Collapsible core	Continuous internal threads	High	Excellent; full 360° thread
Molded-in threaded insert	High-load or frequently reused joints	Moderate	Excellent; metal thread
Heat-set or ultrasonic insert	Metal threads added after molding	Low	Very good; metal thread
Self-tapping screw into a boss	Low-cost assemblies	Low	Fair; thread wears after a few cycles

Frequently Asked Questions

Can you injection mold threads?

Yes. External threads can often be molded directly on the parting line or with split cavities, and internal threads can be formed with unscrewing molds or collapsible cores. For light-duty applications, interrupted threads avoid moving mold parts entirely, and threaded metal inserts are used where strength or repeated assembly matters.

What thread profiles work best in molded plastic?

Rounded or trapezoidal profiles such as buttress and ACME-style threads mold and perform better than sharp V-threads, because they have fewer stress concentrations and fill more reliably. Avoid very fine pitches — finer than roughly 0.8 mm (32 threads per inch) is difficult to mold reliably — and stop the thread about 0.8 mm short of each end so it doesn't create a feathered edge.

Are molded plastic threads strong enough?

For caps, closures and light-duty fasteners, molded threads in materials such as PP, POM or nylon are perfectly adequate. For joints that carry high torque, need a machine-screw thread or are opened repeatedly, a brass or stainless threaded insert is stronger and more durable.

Should I use a threaded insert or a self-tapping screw?

Self-tapping or thread-forming screws are the cheapest option and work well for joints assembled once or a handful of times. Threaded inserts cost more per part but give a durable metal thread that tolerates repeated assembly, higher torque and standard machine screws.

Why are unscrewing molds expensive?

An unscrewing mold contains a gear train, rack or motor that rotates the threaded core out of each part after it solidifies. That mechanism adds design, machining and maintenance cost and can lengthen the cycle, so it is usually justified only for high-volume parts that need a continuous, sealing thread.

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.

Getting your design ready for production can feel complex. Our team in Johor Bahru lives and breathes this process every day. If you want to ensure your design is optimized for manufacturing from the start, reach out to us. We're here to help.