A popular headline today is “Protect our environment, use less plastic and keep our oceans clean!”. Plastics get a bad reputation in part due to the environmental problem of ocean pollution and that they are produced from fossil fuels. There are thousands of tons of plastic containers, fishing lines, straws, shopping bags and other materials floating globally. Of course, we encourage everyone to support the effort to keep our planet green and many countries mandate plastic recycling. Plastic is not the evil it is reported to be; in fact, it’s environmentally friendly in many cases. Most discussions about electric and autonomous driving vehicles have one common theme, reducing weight. Lightweight solutions are essential to improve the mileage of ICE vehicles and increase EV’s range. Technical plastics are one proven solution. Contrary to what one might think, an increased utilization of plastics is another approach to protecting the environment. Replacing aluminum with plastics in some applications can reduce the cost of manufacturing and assembly, as with plastic, more and more functionality can be integrated in one component, as evidenced in smart phones. However, integrated manufacturing has its limits and quite often adaptive methods become necessary to join one component to the other. To ensure reusability, screws are the most desirable option.
It is important to understand that there are several precautions that must be observed when utilizing plastic in order to ensure a good joint. Plastic material behaves totally differently than metal. The clamp force is not created by fastener elongation but instead by the plastic’s deformation. As a result, one should expect a significant drop in assembly clamp load. Plastic, in terms of a thermoplastic material, is viscoelastic. It tends to creep and relax under load, highly influenced by temperature. The mechanical properties of plastic depend upon the injection molding process. That’s why making calculations can be so difficult. Measurements of those properties have usually been done with standardized test strips under laboratory conditions. However, your application results can be more than 100% different from the ones found in the laboratory. Please note that even if two different components are made from the same batch of plastic, that is no guarantee that tightening parameters or reachable clamp loads will be the same. Following are some reasons why the results can be different- direction and distribution of the reinforcement fibers, weld lines, different wall thicknesses, number and locations of the injection points, cooling processes, etc. Therefore, one should think twice before specifying that a specific fastener being used in one application should be used in another, without further analysis. For self-tapping joints, this is the reason for making a separate performance validation with drive-to-fail tests for each application, to make sure each application will achieve the desired result. The joint connection should be maintained over the end-product lifetime. Fasteners should not become self-loosened.
Sounds difficult? Yes, working with plastic often requires more trial and error instead of relying on calculations. Even though many clamp loads may loosen due to relaxation of the material, thermoplastics are viscoelastic, therefore creating an excellent anchoring performance. When using the correct fastener for an application and the right parameters are met, some heat will be generated, which helps displace the plastic material and form the thread. Once the fastener is seated, plastic tends to creep but then slowly adhere to the thread surface, resulting in a satisfactory breakaway torque, one that is normally much higher than the assembly torque.
Don’t be afraid to use thread-forming fasteners in plastics. The benefits obtained by using plastic are much greater than any disadvantages. Understanding the characteristics of plastic, we improved our REMFORM® fastener performance by creating the latest REMFORM® II™ design, the best thread-forming screw for plastics we have ever offered. These two designs are components in millions of time proven plastic joints, which convinces us that they will provide the best value for your application. To learn more about thread-forming in plastics, we encourage you to contact the REMINC/CONTI technical staff.