Designing Molded-In Threads

You can find molded-in threads on any variety of items – from nuts to screws to bottle caps to medical devices and more. Molded threads are a critical feature on many parts, but that doesn’t mean they’re easy to design. With a few exceptions, threaded parts won’t just pop out of the mold without special design considerations.

Here are some tips to help you incorporate in-mold threads into your mold design.

Types of Force Relevant to Threads

With molded threads, there are two important types of force to consider: pull-out force and torque-out force. These forces are best thought of in the context of a screw in a hole.

Pull-Out Force

Pull-out is the force that would be required to pull the screw out of the hole without unscrewing it first. You want the pull-out force to be as high as possible so the screw can’t be pulled out by brute force rather than being unscrewed.

Torque-Out Force

Torque-out force is the torque required to remove a screw by turning it once it has been tightened. You want this force to be as low as possible while still letting the screw get tightened to the point where the pull-out force is as high as possible.

Injection Molding Thread Types

There are two different types of injection molding thread types: external and internal. Each thread type requires very different mold designs to accomplish.

Injection Mold External Threads

External threads are much easier to incorporate into a mold design than internal threads. To create external threads, you simply need to add them to both the core and cavity sides of the mold. The only downside is that you will have parting lines on the threads, but UPM has the experience to keep the parting lines as small as possible.

Injection Mold Internal Threads

Internal molded-in threads are much more difficult to incorporate into your mold design because you need an additional core or cavity specifically for creating the threads, often called an

unwinding core or unwinder. While you won’t have parting lines on the threads, your mold design will be more complicated and more expensive, and it may slow your cycle times.

Another way to achieve this is to put a threaded core into the mold each time it opens. The

operator then removes the part from the machine with the core still in the part, and uses a hand

tool to remove the threaded core from the part by unscrewing it from the part. In order to do this

process, multiple hand loads need to be made to allow time for them to cool before being

reinstalled in the mold.

Injection Molding Thread Release Methods

There are three common methods for releasing finished parts that have internal threads: force release, manual insert, and fully automatic. 

Force Release – Rarely Used

If you have a small enough thread size, pitch, and depth with an injection material that is flexible enough to avoid cracking issues, you may be able to use a force-release method to pop your finished part out of the mold. You must incorporate draft and radii into the threads, and you must have consistent wall thickness that isn’t too thick and doesn’t have sharp corners.

Manual Insert – Sometimes Used

For low-volume production runs, a manual insert is a common method for removing the thread core or cavity. With this method, a thread core or cavity is placed into the mold before the cycle and ejected out with the part, then a person manually removes it from the part.

Fully Automatic – Most Common

For high-volume applications, it may be worth the cost to create automatic unscrew molds. These molds have an unscrewing action using a hydraulic or electric motor. Cycle times are much shorter than for manual inserts, but the additional tooling and complexity of a fully automatic unscrew mold makes it cost-prohibitive for some applications.

Factors to Consider When Designing Molded Threads

Here are some things to keep in mind when designing a plastic injection mold incorporating molded threads.

Thread Size and Pitch

Plastic threads strip more easily than metal threads, so they have different design rules. Try to keep internal threads at least 0.3 inches in diameter and use the coarsest pitch possible.

Undercuts

Any kind of undercuts make mold design trickier, and threads are a complicated type of undercut. One way to eliminate undercuts is to use cam-activated side-actions. However, this does increase the initial tooling cost and complexity, so be sure to ask UPM whether the cost is worth it for your particular project.

Materials

Some materials work better for creating internal molded threads than others. Try to stick with ABS, POM (Delrin), or nylon. For external molded threads, the material is much less important than it is for internal threads.

Best Practices

Always follow injection molding best practices, such as incorporating draft angles, consistent wall thickness, and using a radius design rather than sharp corners whenever possible.

Alternatives to Molded Threads

You may be able to find alternatives to molded threads that are more cost-effective, such as:

• Machining threads in a secondary operation
• Self-tapping screws
• Overmolded thread insert

Contact UPM to Learn More or Get a Quote

Molded threads are one of the trickiest parts of mold design, which is why you should trust Universal Plastic Mold. We have 50 years of experience with plastic injection molding, so we have the expert knowledge to help you get the molded threads you need in your part. To get a quote or learn more, call 1-888-893-1587 or click here.