There are various challenges involved with fastening screws into a plastic material. The challenges revolve around joint failure issues related to stress relaxation, loss of clamping, load, and stripping. At ITW Shakeproof Industrial, we are your source standard and metric thread forming screws for plastic substrates that meet the needs of various industrial applications.
Decrease in Clamping Load
Over the years, designers have struggled to solve the issue of the clamping load loss connected with screws fastened into plastic. The reason for the challenge is due to plastic’s natural visco-elastic properties – the feature of plastic in which it functions as both viscous fluid and elastic solid.
When under stress, a plastic material tends to relieve stress as fast as possible. This causes the material to undergo rapid stress relaxation. The challenge for designers occurs when there is not much clamp load remaining in the joint due to the level of stress present. This results in a loose joint susceptible to failure.
The efficient and effective design of a plastic thread forming screw depends on the feasibility of the assembly. During the design of a plastic joint, designers often observe the mechanism of stripping in the joint to determine the effort necessary to produce threads in the plastic and cause the joint to fail.
Relaxation of Material Stress
Although one aspect of determining joint strength and reliability is determining the required safe tightening torque, the issue of stress relaxation must also be examined. If a joint will still be on the verge of failure if it is over-stressed and relinquishes much or all of its clamping load as a result of stress relaxation.
Visco-elastic materials often undergo stress relaxation. Plastic is an example of a visco-elastic material that, when subjected to stress, tends to naturally relieve the stress – this decreases concentrations of stress, but also the clamping load.
When the stress relaxation in joint is high enough, the joint will eventually lose all of its clamping load. When this occurs, the joint may loosen, slip, or fracture – any of these scenarios can cause product failure or occasion customer complaints concerning the product.
To learn about the steel thread forming and steel thread rolling screw for plastic options we offer at ITW Shakeproof Industrial, call us today at 920.261.2652. Otherwise, you may leave us a message or request a quote through our contact form.
