Mold Design - Injection Molding Threads

In the field of plastic injection molding technology, injection threading is an indispensable and challenging process. From the most common PET bottle caps in our daily lives to water pipe fittings for industrial use, and even the screw holes on various precision plastic parts, the application of plastic threads is ubiquitous. However, successfully manufacturing functional and aesthetically pleasing plastic threads requires a more complex and sophisticated process design than for general plastic products. The demolding method for the thread is a core issue, as the thread structure itself is an undercut shape, thus requiring corresponding methods in mold design. The following will introduce several methods for injection threading.

external thread of mold

External threads are simpler to design than internal threads. A common practice is to use a semi-sliding block to avoid undercutting. See the diagram below.

The tooth profile of the slider is usually formed by electrical discharge machining. For higher precision, a custom tooth cutter can be used in conjunction with an NC milling machine. The tooth root shape produced by the two methods will be different, but as long as the radius (R) is taken into account, there will be no interference from locking.

Since the split slider will create a parting line, to avoid material overflow at the parting line causing product locking problems, the tooth profile at the slider mold closing point will be designed to be recessed so that the parting line can be avoided.

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Further Reading :《Injection Molding - Short Shot Countermeasures》

Mold internal thread

There are several ways to create internal threads, including forced release, thread-locking mechanisms, or manual embedding and demolding.

Forced demolding

Forced demolding, as the name suggests, involves directly ejecting the product, and the mold structure is similar to a standard mold design. However, this method is only suitable for softer materials, such as PE. Furthermore, the following points must be considered in the design of the forced demolding teeth and the mold itself:

1. The wall thickness should not be too thick, because the teeth need to be stretched open for smooth demolding; if the wall thickness is too thick, the structure will be too strong and unable to stretch open. Conversely, it should not be too thin either, as it will crack.
2. A draft angle should be applied to one side of the tooth profile to facilitate demolding.
3. Because the product's teeth will be stretched open during demolding, it is essential to ensure that the parting line is below the teeth. If it is below the parting line, it means the teeth are enclosed by the mold base and cannot be stretched open during demolding.

As shown in the left image, several principles should be considered when designing the tooth profile for forced avulsion.

1. The wall thickness T is approximately 1.5 times the tooth depth W.
2. Extraction is required on the side of the tooth to be demolded, with an angle of less than 55 degrees.
3. Demolding radius > 1
4. Tooth depth should ideally be less than 1.

Coil mechanism

A thread-unscrewing mechanism refers to a mechanism where the thread insert is rotated and unscrewed by a motor or hydraulic cylinder driving gears and belts. This method maintains the thread profile design. However, this method requires accurate calculation of the number of turns needed for the thread-unscrewing mechanism to remove the threads. It must be ensured that the number of turns driven by the motor to remove the threads is greater than the number of threads on the product. If the threads are not completely removed, and a few threads on the product are not fully disengaged from the insert, there will be problems with ejection. Therefore, accurate calculation is essential.

Calculation of the number of teeth removed: (using the above diagram as an example)
Number of rotations of the motor drive shaft > Number of threaded turns of the product ÷ [(Number of teeth on the motor drive gear ÷ Number of teeth on the drive gear of the drive shaft)] × (Number of teeth on the driven gear of the toothed shaft ÷ Number of teeth on the planetary gear of the toothed shaft)

If the product is not properly positioned during mold forming, it can be pulled along with the toothed shaft during rotation. In a better scenario, the product will simply spin freely with the toothed shaft, but in a more serious case, the product may become misaligned and seize, causing the toothed shaft to seize as well. If the torque is too high, it can directly damage the product or even the mold. Therefore, the product must be recessed into the appropriate position in the mold. Furthermore, do not raise the holding strip too high to achieve a more complete tooth shape, as this will cause the product to tightly enclose the toothed shaft, forcing it to be pulled along. Paying attention to the size and position of the gate is key to forming a good tooth shape.

Manual embedding and demolding

The threaded core is embedded and ejected together with the injection molding process. The core is then manually unscrewed outside the machine. This method requires the creation of a replacement module (core/mold core), which is removed after injection, and the threaded core is then unscrewed. Therefore, this method is clearly only suitable for small-batch production, and two sets of cores/mold cores must be prepared for replacement to avoid production interruptions: one set for in-machine molding and one set for external core removal.

Embedded nut

Although threaded inserts are metal parts, they can still be manufactured using plastic injection molding. Embedded nuts are a very common practice in electronic components. Because many electronic components have very small screw holes, injection molding would be very difficult, and small threads would result in poor strength. These products require direct attachment of metal screws, so embedded nuts are necessary to meet the structural strength and durability requirements. However, embedded nuts also have their share of challenges. Basically, most metal embedded injection molding processes have similar problems, including positioning, overflow, and part breakage.

When plastic is injected and flows, it may displace the insert, which is more likely to occur with small-diameter nuts. Because the corresponding insert is also thin, it is prone to deformation and misalignment. Another issue is insufficient contact area with the nut's inner diameter, causing the nut to pop out or fail to hold, resulting in rotation. This poor positioning can cause varying degrees of overflow. Usually, this situation requires adding a structure to the insert to hold the nut in place. Another common problem is the difference in the coefficient of thermal expansion between the metal nut and the plastic, leading to different shrinkage rates within the mold. Besides tolerance control issues, excessive shrinkage causing stress release can also lead to part breakage.

Therefore, two points must be noted. First, the copper threaded rod must be preheated to reduce dimensional expansion caused by rapid temperature increases. Generally, the copper threaded rod can be heated to around 60-70 degrees Celsius for at least 10 minutes before being placed into the mold. Second, the outer diameter of the plastic threaded rod should not be designed to be too large. A larger outer diameter means a thicker wall, resulting in greater shrinkage. It is generally recommended that the outer diameter of the plastic threaded rod be 1.5 times the outer diameter of the copper threaded rod, but this can be adjusted according to the actual wall thickness.

Want to learn about mechanical design, sheet metal, and screw hole machining?
Further Reading :Four methods for creating screw holes in sheet metal