Fastening screws into plastics can present multiple challenges. One wonder why due to the generally softer nature of plastic material compared to harder steel, but there are some joint failure considerations related to stripping, loss of clamping load, and stress relaxation to make with these joints. At ITW Shakeproof Industrial, we are your source for thread forming screws for plastic.
Loss of Clamping Load
The problem of the loss of clamping load as it relates to screws and plastic mating surfaces has often been a difficult one for designers to completely solve. This is due to the natural visco-elastic properties of plastic. This refers to the fact that plastic acts like both an elastic solid and a viscous fluid.
The plastic material, when under stress, wants to rid itself of the stress as quickly as possible. Due to this condition, the material experiences rapid stress relaxation. The problem for designers occurs when the value of this stress is great enough such that there is no much clamp load left in the joint. Such a joint is a loose joint and subject to failure.
The design of thread forming screws for plastic often revolves around the question of assembly. If proper assembly is not feasible, then problems will result. Designers must determine how much effort is required to develop threads in the plastic and fail the joint – this is usually done by stripping. When designing a plastic joint, this is often the sole consideration.
Although determining the safe tightening torque is necessary, the joint will still have problems in plastic material if it is overly stressed and loses a large percentage or all of its clamping load due to stress relaxation.
One of the common behaviors of visco-elastic materials is stress relaxation. A visco-elastic material such as plastic, when under stress attempts to relieve the stress which reduces stress concentrations but also reduces the clamping load.
With a high enough stress relaxation, the joint will lose all of its clamp load over a period of time. Once that happens, the joint may self-loosen, slip, or break – none of which are desirable and may cause the product to fail or prompt complaints about the product from customers.