Plastics and bioplastics are widely used in various industries, such as automotive, electronics, and construction, due to their advantages of light weight, durability, and environmental friendliness. However, joining plastic parts can be challenging, especially when using high-performance plastics and bioplastics that have high strength, stiffness, and temperature resistance. One of the most common methods of joining plastic parts is using thread-forming screws, which create their own threads in the plastic material by displacing it. However, not all thread-forming screws are suitable for all types of plastics and bioplastics. Choosing the wrong screw can lead to poor connection quality, reduced performance, and increased risk of failure. Therefore, it is important to consider the following factors when selecting thread-forming screws for high-performance plastics and bioplastics:
- Material properties: The properties of the plastic or bioplastic material, such as strength, hardness, elasticity, ductility, thermal expansion, and fiber content, affect the behavior of the material when the screw is inserted. For example, harder and stiffer materials require more force to form the threads, while softer and more ductile materials tend to flow more easily. Additionally, some materials may have additives or fillers, such as glass or carbon fibers, that can increase the strength and stiffness of the material, but also increase the abrasiveness and wear on the screw. Therefore, the screw should be designed to match the flow and deformation characteristics of the material, as well as to withstand the friction and wear caused by the material.
- Screw geometry: The geometry of the screw, such as the thread profile, the thread pitch, the thread core diameter, the head shape, and the drive type, affect the performance and quality of the connection. For example, the thread profile determines how the material is displaced and how the screw engages with the material. A rounded thread profile with a curved load flank and a reduced load flank angle can reduce the radial stress and the screw-in torque, while increasing the flank coverage and the preload force. The thread pitch and the thread core diameter affect the thread overlap and the breaking torque of the screw. A larger thread pitch and a larger thread core diameter can increase the thread overlap and the breaking torque, but also increase the radial expansion and the risk of cracking. The head shape and the drive type affect the head seating and the drive torque of the screw. A flat head shape and a hexalobular drive type can provide a flush head seating and a high drive torque, while reducing the risk of stripping or cam-out.
- Hole geometry: The geometry of the hole, such as the hole diameter, the hole depth, the hole shape, and the hole location, affect the ease and accuracy of the screw insertion. For example, the hole diameter determines the clearance between the screw and the hole, which affects the thread formation and the connection strength. A smaller hole diameter can increase the thread formation and the connection strength, but also increase the screw-in torque and the radial stress. The hole depth determines the length of engagement between the screw and the material, which affects the tensile strength and the fatigue resistance of the connection. A deeper hole can increase the length of engagement and the tensile strength, but also increase the material displacement and the risk of cracking. The hole shape and the hole location affect the alignment and the stability of the screw. A tapered hole shape and a central hole location can improve the alignment and the stability of the screw, while reducing the risk of cross-threading or misalignment.
The table below summarizes the recommended values for some of the key parameters of the screw and the hole geometry for different types of plastics and bioplastics, based on the guidelines from Arnold Umformtechnik, a leading manufacturer of thread-forming screws.
| Material type | Hole diameter (% of nominal screw diameter) | Thread pitch (% of nominal screw diameter) | Thread core diameter (% of nominal screw diameter) | Hole depth (times nominal screw diameter) |
|---|---|---|---|---|
| Thermoplastics (soft) | 75-85 | 50-70 | 65-75 | 1.5-2.0 |
| Thermoplastics (hard) | 85-95 | 70-90 | 75-85 | 2.0-2.5 |
| Thermosets | 95-105 | 90-110 | 85-95 | 2.5-3.0 |
| Bioplastics (soft) | 80-90 | 60-80 | 70-80 | 1.5-2.0 |
| Bioplastics (hard) | 90-100 | 80-100 | 80-90 | 2.0-2.5 |
- Assembly method: The assembly method, such as the assembly speed, the assembly torque, the assembly angle, and the assembly sequence, affect the reliability and consistency of the connection. For example, the assembly speed determines the rate of heat generation and dissipation in the material, which affects the material viscosity and the thread formation. A higher assembly speed can increase the heat generation and the material viscosity, while decreasing the heat dissipation and the thread formation. The assembly torque determines the force applied to the screw and the material, which affects the preload force and the connection strength. A higher assembly torque can increase the preload force and the connection strength, but also increase the screw-in torque and the risk of over-tightening or screw breakage. The assembly angle and the assembly sequence affect the alignment and the stress distribution of the screw and the material. A perpendicular assembly angle and a symmetrical assembly sequence can improve the alignment and the stress distribution of the screw and the material, while reducing the risk of misalignment or uneven loading.
