Ethan Clarke
When it comes to selecting the right screw for plastic, there are several factors to consider, including the type of screw, size, length, and material. The most suitable screw type for plastics is the Type 25 self-cutting screw. Self-tapping screws, which can form unique threads when driven into plastic, are also a popular choice. They are often used in injection moulding due to their cost-effectiveness and ability to provide greater flexibility in assembly. Thread-forming screws are another option, known for their ability to slice through plastic with minimal stress and low installation torque. To ensure the best results, it is crucial to determine the specific requirements of your application, such as load-bearing needs, and select the appropriate screw type, size, and length accordingly.
| Characteristics | Values |
|---|---|
| Screw type | Self-tapping, self-forming, self-cutting, thread-forming, thread-cutting, Type 23, Type 25, Plastite, machine screws |
| Screw material | Metal screws are suitable for plastic, but plastic screws are better due to their flexibility |
| Screw size | Load-bearing requirements should be the first priority |
| Pilot hole diameter | Should be deeper than the screw engagement length to allow space for the removed material to move |
| Thread engagement | 75%-80% of thread engagement is used on stiff materials to achieve a good joint |
| Screw length | Limited by space restrictions in some cases |
| Boss design | Drafted holes are required for injection-moulded thermoplastics, but they reduce thread engagement |
| Applications | Thread-forming screws are for soft polymers with lower flexural modulus; thread-cutting screws are for stiffer plastic with higher flexural modulus |
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What You'll Learn
Self-tapping screws
Thread-cutting screws, also known as thread-tapping screws, have a sharp cutting edge that cuts into the plastic when screwed into the pilot hole, forming a threaded joint. They produce reduced internal stresses compared to thread-forming screws, making them suitable for stiffer plastic materials with higher flexural modulus. Type 25 self-cutting screws are the preferred screw type for plastics.
The selection of the screw type depends on the application requirements, material type, and properties. It is important to consider factors such as load-bearing requirements, thread engagement, pilot hole diameter, and length of engagement when using self-tapping screws for plastic.
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Thread-forming screws
When selecting a thread-forming screw, it is important to consider the pilot hole diameter and depth. The hole depth must be deeper than the screw engagement length to allow space for the displaced material to move. Additionally, the amount of thread flank depth filled by the material, referred to as thread engagement, is critical. As a rule of thumb, 75%-80% thread engagement is used on stiff materials for an optimal joint.
To prevent "burst out" when screwing into thinner materials, trilobular head thread-forming screws are available. These screws have a trilobular lobed cross-section that forms threads in pre-existing holes by pushing the material outward during installation. Thread-forming screws provide a reliable and secure fastening method for plastic and sheet metal applications.
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Thread-cutting screws
Additionally, thread-cutting screws provide increased thread engagement, which enhances their holding ability. This is particularly beneficial in plastic parts to improve load-carrying capabilities and ensure a secure fastening. However, it is important to note that thread-cutting screws can be stripped of their threaded holes if excessive tensile force is applied and thread engagement is minimised.
When selecting thread-cutting screws for plastics, it is crucial to consider the screw size, load-bearing requirements, and the specific characteristics of the plastic material being used. Standardised screw types and lengths can improve design efficiency and minimise assembly errors.
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Machine screws
Plastic machine screws are commonly made from Nylon, which has a very low coefficient of friction and is widely used due to its high strength, wear resistance, and corrosion, chemical, and electrical resistance. It is also lightweight and tough, offering excellent electrical insulation properties.
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Plastic screw inserts
When compared to standard fasteners, self-tapping screws are a more cost-effective option for injection-moulded parts. They have a narrow thread profile, increased pull-out resistance, and lower radial stress, which prevents boss damage.
Overall, plastic screw inserts offer versatility, durability, and cost-effectiveness, making them a popular choice for industrial applications.
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Frequently asked questions
Self-tapping screws are a good option for plastic as they can form a unique thread when driven into the material. Other options include thread-forming screws, thread-cutting screws, and machine screws.
Thread-forming screws, also known as PT® screws, slice through plastic with minimal stress and low installation torque. They are designed for use on thermoplastic materials and require pre-drilled or moulded holes.
Self-tapping screws are excellent for injection-moulded parts as they are cheaper than threaded holes or threaded inserts. They also have increased pull-out resistance and lower radial stress, which prevents boss damage.
Metal screws can be used for plastic, but screws designed specifically for plastic, such as plastic screws or self-tapping screws, are better due to their flexibility.
