United States. Bioplastics are also increasingly used in the components of industrial products.
Examples of bioplastic components are found in the automotive, electronics, and construction industries. However, threaded connections in biopolymers require properly matched screws to achieve consistent fastening quality.
With a view to long-term goals, such as sustainability in the value chain, the product as a whole, as well as its individual components and connections, must be considered. And now that bioplastics are increasingly being used in high-performance products, fasteners for these new materials need to be updated and optimized.
Arnold Umformtechnik, based in Forchtenberg-Ernsbach, has researched this topic. In a joint project with Bond-Laminates GmbH, the company also investigated whether the Remform II HS (High Strength) thread-forming screw was suitable for use in bio-based composite materials.
The Remform II HS is a thread forming screw with a rounded thread profile that has been specially designed for applications with high-performance plastics. It is used when high preload force and high loosening resistance are required.
Optimized thread core increases breaking torque
Remform II HS screws have an asymmetrical thread profile with a rounded sidewall tip and a curved load-bearing sidewall.
This geometry is designed to match the flow properties of the plastic and allow the material to travel smoothly. The screw also features an optimised and enlarged thread core, which significantly increases both the breaking torque and the tensile breaking force of the screw. This leads to a more stable connection between the two parts of the joint, but also allows for higher mounting torque by joining high-strength plastics without the risk of screw breakage.
Thanks to the combination of radial profile and sloping load flank, when screwed the plastic material flows into the load flank and ensures very good flank coverage. By reducing the angle of the load flank to 10°, radial stresses are significantly reduced. This results in a low screwdriving torque despite the high thread overlap and reduces the risk of cracks in the plastic due to less radial expansion.
Study of the influencing factors of screw connections
To reliably design and test threaded connections, it is important to know the factors that influence this. This is particularly important when studying the basic principles. Important influencing factors in threaded joints are the geometry of the tube and screw, the diameter of the central hole and the material and its corresponding strength, conditioning or fibre content.
It is also important to consider the influence of the mounting method, such as the accuracy and speed of the screwdriving system. And last but not least, the different types of charging that come into play are also important, i.e. temperature and dynamics, as well as charging duration or environmental conditions. All of these factors influence the mounting characteristics of a direct-screw plastic gasket.
Taking all these factors into account, based on a real example of a screwdriving curve, it can be demonstrated that the screwdriving parameters can be significantly improved when using Remform II HS on high-performance plastics.
"Compared to the Remform screw, which is practically the standard solution for technical plastics, with the Remform II HS the breaking force and breaking torque can be increased by about 20 percent," says Sinja Strobl, Product Engineer at Arnold Umformtechnik.
Basic research on threaded fasteners for bioplastics
Plastic direct threaded joints have long proven to be a reliable and economical solution for joining plastic components. But what about bioplastics? Arnold Umformtechnik and Bond-Laminates have jointly undertaken an investigation into the suitability of Remform II HS, a set screw for Tepex, a registered trademark of the Lanxess Group.
Tepex dynalite 813-F250 (or Tepex for short) is a 100 percent bio-based composite material with a matrix of PLA (polylactic acid) reinforced with flax fibers. The fibers are obtained from the stem of the flax plant and grown locally. Tepex materials consist of one or more layers of semi-finished textile products with continuous fibers (linen) embedded in a PLA matrix.
These are fully impregnated and consolidated. Therefore, all fibers are coated with plastic and the material does not contain air pockets. According to the manufacturer, Tepex dynalite offers maximum strength and rigidity with a very low density.
Samples of Tepex material were provided in the form of sheets, which were then cut into small samples for basic testing to assess their screwability. Due to the sample thickness of approximately 6 mm, a Remform II HS with a diameter of 2.5 mm was used as a fastener.
"The effective depth of screwdriving was set to 2 x d. In the first series of tests, screwdriving and over-tightening tests were carried out with measurement of the preload force in central holes of different sizes. This means that the screw is screwed in and tightened until the connection fails," Sinja Strobl describes the tests.
Based on the results of these initial tests, the appropriate center hole diameter and torque for long-term testing were determined. Then, for long-term measurements, the samples were screwed together with the previously defined torque and the relaxation of the preload was measured by a load cell.
The results demonstrate the suitability of the Remform II HS
Initial results from screwdriving and over-tightening tests indicate that Remform II HS can be used very well to form a nut thread with maximum load-bearing capacity in the bio-based material.
"There were no stress cracks or unwanted deformations in the sample body during screwing. The Tepex material really surprised us. Compared to other materials we tested, it achieved the best breaking moments, even higher than a PPS with fiberglass reinforcement," says Strobl, summarizing the results.
The combination of Remform II HS and Tepex also scored high in terms of process safety. "Low torques and high excessive torques result in high delta torque, which opens a large process window for assembly and provides the user with a high level of safety," Strobl continues.
Support for the design of direct threaded connections
The results of this research were then incorporated into the various tools in Arnold Umformtechnik's digital engineering portfolio. This means that in the future customers will be able to obtain even better support for the conception and design of direct threaded fasteners, also when considering bio-based composite materials.
Arnold Umformtechnik offers a wide range of services for designing with plastics. One of them is the Core Hole tool. It offers the possibility of determining an optimal center hole design for almost all plastics based on characteristic values determined in the laboratory. Arnold's customers then receive the recommendation in PDF format with all the dimensions relevant to the design.
Another tool is Fast Designer Plastics, a forecasting tool for direct plastic threaded joints. Fast Designer Plastics can be used to draw conclusions from various assembly and operating variables, such as mounting preload force and tightening torque. It is also possible to predict the relaxation-related drop in preload force after exposure to time and temperature.
