The German company Weippert Kunststofftechnik GmbH & Co. KG has introduced screws to the market that are up to 80 percent lighter than their steel counterparts. By using short lengths of glass fiber in the plastic, the material is able to achieve a relatively high tensile strength for plastic products. Other advantages of using plastics in bonding applications include the high resistance to corrosion and the insulating properties of the material.
The weight reduction through WT screws is significant. A M6x40 size hexagon screw made of PA6GF30 plastics weighs 1.6 grams. The same screw made of PAGF60 plastic weighs in at 2.3 grams. The same screw made out of steel weighs in comparison a hefty 9.3 grams. A M8x40 size cylinder head screw with an Allen bold shows an even greater weight difference. Made out of PA6GF30, the screw weighs 3.2 gram. Made out of steel, the product weighs a whopping 17.2 grams.
Weippert uses granulates both with an without fillers. The glass fibers for example can contribute as much as 50 percent of the material mix. It is also possible to add substances such as glass pearls, wood fibers or carbon fibers. The glass fibers have a length of about three to five millimeters and are already included in the plastic granulate that is used in the injection molding process. Only thermoplastics that conform with the RoHS Directive are used and can be colored to the customer's specifications.
Tensile strength is sufficient for many applications
The tensile strength of the plastic screws is clearly sufficient for many applications, largely thanks to the fiberglass technology. Tests carried out in the laboratory at Heilbronn University showed that the screws could withstand forces between 2,000 and 4,500 N in tensile testing experiments. Efforts are being made to improve the torque performance. The test values here are suitable for many applications, but as expected, they fall far short of the performance of steel.
In order to improve mechanical properties such as tensile strength and tightening torque, Weippert has slightly modified the geometry of the screw heads. The WT washers have been adapted as well. The interior and exterior diameters have different heights giving the screw a concave shape. This helps to compensate for the penetrating properties of the plastic which in turn improves the sealing along the application surface.
There are many applications for the plastic screws. Currently the electrical and electronics industries are the primary consumers as well as water and waste treatment. Systems engineering applications, container manufacturing, robotics, the automotive sector, furniture builders and the medical industry as well as other sectors offer plenty of potential for the implementation of lightweight plastic screws.