Chloe Zhang
Plastic is a versatile material that is being used more and more, especially with the rise of 3D printing. However, it presents unique technological challenges, such as finding suitable fastening solutions. Regular screws are unsuitable for plastic because the material is liable to burst due to the stresses that build up during assembly. Self-tapping screws are a good option for plastic, as they form or cut their own threads. The two types of self-tapping screws available for use on most common plastics are thread forming and thread cutting screws. Thread-forming screws are better for plastic because they push the plastic aside instead of cutting through it, reducing the risk of cracks or damage.
| Characteristics | Values |
|---|---|
| Screw Type | Self-tapping, self-forming, thread-forming, thread-cutting, Tri-lobe, Plastite, Hi-Lo, Taptite, plastic thread forming, rolling, PT, sheet metal, wood screws |
| Screw Angle | 30° thread angle is ideal for thread-forming screws, while 60° flank angle is used for harder plastics |
| Screw Design | Pilot hole diameter, thread engagement, length of engagement, and boss design are key design factors |
| Screw Performance | Plastic screws must account for ductility, thermal expansion, clamp retention, and tractile capability of the material |
| Screw Joint | Testing screw joints with different screw types and designs is recommended due to the variety of plastics |
| Screw Assembly | Self-tapping screws are easy to assemble and disassemble, while regular screws may cause assembly problems and damage the plastic |
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What You'll Learn
Self-tapping screws for plastic
Self-tapping screws are an excellent choice for injection-moulded parts, as it is much more economical to mould pilot holes than threaded holes or threaded inserts. They are also useful when joining two plastics or metal to plastic, as they allow for easy disassembly and reassembly.
Thread-cutting screws, on the other hand, have a sharp cutting edge that cuts plastic when screwed into the pilot hole, forming a threaded joint. They are suitable for stiffer plastic materials with a higher flexural modulus, such as medium- to high-modulus thermoplastics and thermosets. Thread-cutting screws are available in two types: Type 23 and Type 25. Type 25 is preferred for plastics as it has a wider angle thread and a blunt point, which allows it to work well within softer materials without causing stress or damage.
It is important to note that standard screws for metal and self-drilling screws are not suitable for plastic due to their wide flank angle and shallow threads. When choosing a screw for plastic, it is essential to consider the unique performance difficulties of each plastic type, such as ductility, thermal expansion, and clamp retention. Testing the screw joint with different screw types and designs is recommended to find the best match for your specific application.
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Thread-forming screws
When selecting the appropriate screw for plastic, it is important to consider the unique performance difficulties of each type of plastic, such as ductility, thermal expansion, and clamp retention. Testing different screws on sample plastic material is recommended to find the best screw type and design.
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Thread-cutting screws
When selecting the appropriate screw for plastic, it is important to consider the unique performance characteristics of each plastic type, such as ductility, thermal expansion, and clamp retention. The pilot hole diameter, thread engagement, length of engagement, and boss design are also key product design factors that influence the performance of the fastener. Testing on sample plastic materials with different screws is often necessary to determine the best screw type and design for a specific application.
In summary, thread-cutting screws are a versatile and reliable option for joining plastics, offering a secure fit and the ability to tap their own holes. They are well-suited for a range of plastic types and provide a strong joint, making them a popular choice for various applications.
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Plastite screws
The smooth burnishing action of Plastite screws eliminates the risk of damaging the molecular structure of the plastic, reducing the danger of material failure. This also reduces assembly time and costs. Plastite screws' high drive-to-strip ratio permits a wider span of torque settings to be safely used with automatic drivers, virtually eliminating strip out.
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Pilot hole diameter
The pilot hole diameter and thread engagement are key product design factors that influence a fastener's ability to perform satisfactorily. The pilot hole diameter should provide for a minimum thread engagement of two times the screw diameter. The amount of thread engagement desired can vary from 100% to 40%. The type of plastic and thread depth can also impact the pilot hole size.
When selecting the right screw hole size for plastic projects, it is important to choose the correct screw length and diameter to avoid plastic deformation and ensure a secure fit. The screw size is defined by the screw diameter and pitch. The screw diameter is the width of the screw at its widest point, usually at the start of the threaded section. The pitch is the distance between the threads of the screw.
For thread-forming screws, the thread profile for plastic screws is typically 30 degrees, which induces much lower stress into the plastic than a broader angle. The pilot hole diameter should match the screw's major diameter, which is the widest diameter of the screw's threads. This helps distribute the stress more evenly as the screw is tightened, reducing the risk of cracking.
The height of the boss, which is the raised area around the pilot hole, should be less than three times its outer diameter. A taller boss can create issues with cooling, cycle time, and dimensional accuracy. The diameter of the boss should be 2.5 to 3 times the diameter of the pilot hole to ensure a secure fit and proper thread engagement for the screw.
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Frequently asked questions
The best types of screws for plastic are self-tapping screws, which form or tap their threads when screwed into the plastic material. Self-tapping screws are available in two versions: thread-forming screws and thread-cutting screws.
Thread-forming screws are also known as "plastic" screws because they are predominantly used in soft materials like plastic, nylon, or other thermoplastics. They are designed to displace the material without removing it, creating a more secure fit that can resist loosening caused by vibration.
Thread-cutting screws are designed for use in hard materials such as metal or hardwood. They cut threads into the material as they are driven in, removing material as they go.
