There are basically four assembly methods for plastic parts. Along with encapsulation of individual parts by an additional component acting as a bonding element, designers can choose from rivets, threaded inserts and thread-forming screws. The last two methods offer high mechanical load capacity and low cost if specific conditions are fulfilled.
Plastic parts assembly using threaded fasteners has the advantage that the joints can be dismantled and reassembled repeatedly, unlike glued or welded joints. Furthermore, the assembly and disassembly operations are clearly defined, unlike some other methods such as snap-fit. Careful study of the construction is often necessary in order to dismantle a snap-fit joint without destroying the parts. Furthermore, threaded fastener joints require only modest investments. As an alternative to threaded inserts, thread-forming screws do not need a mating part in the joint. This effectively eliminates one operation in the manufacturing process.
A thread is formed in the plastic when the fastener is screwed into the hole, causing the plastic to be displaced and forced into the spaces between the threads. However, there are many more types of plastic than metal, and plastics are often combined with fibres to obtain suitable properties. Every type of material has its own characteristics with regard to friction, glide and flow, and these characteristics have a direct effect on the threaded fastener joint. As a rule, secure joints are only possible with a large depth of engagement between the screw thread and the plastic. A higher loss of preload than with metals must be taken into account in the joint design. This results from the specific creep and relaxation characteristics of the polymer material. Although this loss can be reduced by using low-stress designs, it cannot be avoided entirely. Hole design is also a decisive factor for the quality of threaded fastener joints in plastics. A remedy for this problem is provided by REMFORM® screws from Arnold Umformtechnik. They feature optimal thread engagement to provide joints with high mechanical load capacity. As the mechanical load capacity can only be determined experimentally, users benefit from a wealth of measurement data accumulated by the manufacturer based on extensive application experience. The asymmetric thread geometry of REMFORM® screws causes the forces generated during thread forming to be steered in the desired direction. The aim is to form a thread with maximum possible load-bearing capacity over the full length of the screw shank. After the relaxation phase, the plastic should penetrate the space between the threads as deeply as possible. The thread flank facing away from the screw head is radiused to improve material flow, allowing the plastic to penetrate close to the root diameter.
The steep flank facing the screw head accommodates the polymer material displaced in the axial direction. This thread geometry also generates low radial forces during thread forming, which allows hole posts to be designed with thinner walls. This is especially important because the thickness of plastic parts should be kept as small as possible for cost reasons. The thickness determines the setting time, which is approximately 70% of the cycle time and therefore a significant cost factor. The thread profile generates low thread-forming torque and effective material displacement. The high torsion strength of the thread profile ensures a high stripping torque, which is beneficial in situations where screws are prone to break from high torsion stress.
To prevent failure due to stripping of the mating thread, the steep load-bearing flank causes most of the force to be steered in the axial direction, resulting in corresponding preload forces. Optimised material flow of the plastic creates a large difference between the forming torque and the stripping torque. This translates into high process security for users with automated threaded fastener assembly, due to the large tightening torque tolerance range between the two values. This ensures that all fasteners are fully tightened and eliminates the risk of stripped threads. The combination of a radiused flank and a steep load-bearing flank reduces radial stresses by minimising radial forces during thread forming and screw tightening. The steep load-bearing flank transfers most of the force generated during tightening. Summary Special fasteners are most often used for bolted joints with thread-forming screws. Screws with optimised thread geometries, such as REMFORM® screws from Arnold Umformtechnik, are most often used in order to achieve optimal properties. As different types of plastic have widely different thread-in characteristics, depending on the customer part, tests using the customer part are necessary after preliminary design of the part in order to optimise the design of the bolted joint. Asymmetric thread profiles achieve especially high stripping torque with low thread-in torque. Optimised material flow ensures high pull-out force. Minimum fracture torque is increased by 30% compared to screws with conventional 30° flanks. Security with respect to dynamic stresses is also increased. REMFORM® is a licensed product of Reminc USA and is available worldwide.
The Arnold Group is a fully owned subsidiary of the internationally active Würth Group, which realises sales of more than 8 billion euros worldwide with 420 companies and about 65,000 employees.
ARNOLD UMFORMTECHNIK GmbH & Co. KG
Phone: +49 (0)7947/821-170
Fax: +49 (0)7947/821-195