Ethan Clarke
Plastic is a versatile material that is being used more and more, especially with the rise of 3D printing technology. However, it presents unique challenges when it comes to fastening solutions. Regular screws are unsuitable for plastics, which has led to the development of specialised screws for use with plastics. When selecting a screw fastener for plastics, there are many considerations to take into account, such as the type of plastic, the application, and the performance requirements. Self-tapping screws are a popular choice for plastics, as they form their own threads when screwed into the plastic material. However, standard self-tapping screws can cause assembly problems with plastic, so self-tapping screws designed specifically for plastic should be used. Other types of screws for plastic include thread-forming screws and thread-cutting screws, which are more suitable for low-modulus plastics and medium- to high-modulus plastics, respectively.
| Characteristics | Values |
|---|---|
| Screw type | Self-tapping, self-forming, self-cutting, thread-forming, thread-cutting, machine screws, plastite screws, trilobular screws, Taptite screws, JCPlas screws |
| Screw considerations | Thread and head type, tensile force, thread engagement, screw length, safety, performance, space restrictions, assembly, disassembly, material type, hardness of plastic, torque level, pilot hole diameter, boss design, thermal expansion, clamp retention, tractile capability |
| Screw suitability | Self-tapping screws are suitable for injection-moulded parts, joining plastic to metal, or plastic to plastic. Self-forming screws are suitable for low-modulus plastics like polyolefins. Thread-cutting screws are suitable for medium- to high-modulus thermoplastics and thermosets. Thread-forming screws are suitable for thermoplastics and softer materials. |
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What You'll Learn
Self-tapping screws
The thread profile angle of self-tapping screws is smaller than that of standard screws, reducing internal radial stresses during plastic removal and deformation. The root diameter is also smaller, increasing the holding power on soft plastics. It is important to note that the pilot hole diameter and thread engagement are crucial factors in the performance of self-tapping screws. As a rule of thumb, 75%-80% thread engagement is used on stiff materials for a good joint.
When using self-tapping screws, it is possible to strip the threaded holes if enough tensile force is applied and thread engagement is minimised. Therefore, increasing the thread engagement can decrease stresses in the plastic and increase load-carrying capabilities. Additionally, screw length may be limited by space restrictions, so it is important to consider design modifications if needed.
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Thread-forming screws
These screws are designed to displace the plastic material without removing it, creating a secure fit that resists loosening caused by vibration. This is achieved through a unique trilobular tip that forms the thread, ensuring the plastic material flows around the screw threads. The trilobular tip also lessens the risk of burst out by minimising radial hoop stress.
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Thread-cutting screws
When selecting a thread-cutting screw, it is important to consider the application requirements, material type, and properties. The thread profile angle, which ranges from 30 to 45 to 48 degrees, should be chosen to suit the flexural modulus of the plastic. It is also crucial to achieve the optimal amount of thread engagement, which is the amount of thread flank depth filled by the material. As a general rule, 75%-80% thread engagement is used on stiff materials for a strong joint.
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Machine screws
When selecting a screw fastener for plastics, it is important to consider the application requirement, material type, and properties. For instance, self-forming screws are ideal for low-modulus plastics, while thread-cutting screws are better suited for medium- to high-modulus thermoplastics and thermosets.
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Insert screws
When selecting a screw for plastic levers, it is important to consider the specific type of plastic and the application requirements. The screw must be able to withstand the thermal expansion, clamp retention, and tractile capability of the plastic. Laboratory testing is often required to determine the most suitable screw for the material.
Self-tapping screws are a popular choice for plastic applications. They form or cut their own threads as they are screwed into the plastic, providing a strong and secure joint. Self-tapping screws can be further categorized into thread-forming and thread-cutting screws. Thread-forming screws deform the plastic material to create a secure fit, making them ideal for softer plastics. Thread-cutting screws, on the other hand, cut through the plastic and are better suited for harder plastics.
Thread-forming screws, also known as Tri-lobe or Plastite screws, have a trilobular tip that forms the thread. They are more effective with thermoplastic materials and do not generate waste during installation. The displaced plastic flows back into the thread root, providing a tight and secure fit. However, they can create high internal stresses in the plastic due to deformation.
Thread-cutting screws, such as Type 23 and Type 25, have a sharp point and a cutting slot. They are designed to cut their own hole as they are driven into the material, providing a secure fit without the need for a pre-tapped hole. Type 25 screws, also known as "thread-forming screws for plastic," have a wider angle thread and a blunt point, making them suitable for softer plastics.
In addition to self-tapping screws, other methods for fastening plastic include using inserts, socket cap screws, or trapped nuts. Inserts, such as heated inserts, provide the best longevity, while socket cap screws and trapped nuts are also options, although they may be less efficient.
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Frequently asked questions
The best type of screw for plastic levers depends on the type of plastic used and the application. Self-tapping screws are a good option for plastic levers as they are designed to tap their own threads into the plastic. They are available in two types: thread-forming and thread-cutting. Thread-forming screws are more suitable for softer plastics, while thread-cutting screws are better for harder plastics.
Self-tapping screws provide a strong and secure joint in plastics, which is essential for applications where safety and performance are critical. They also allow for easy assembly and disassembly, and can be an economical option for joining plastic to metal or plastic to plastic.
Yes, there are a few alternatives to self-tapping screws for plastic levers. One option is to use machine screws, which are often used in applications requiring repeated assembly and disassembly. Another option is to use plastic screws, which provide enhanced flexibility and enable quick assembly. Additionally, thread-forming or thread-cutting plastic screws can be used, as well as heated inserts and trapped nuts.
