Thread-forming screw for high-performance and bioplastics
When it comes to future topics such as sustainability in the value chain, the focus is just as much on the overall products as it is on the individual components and their connection. However, in view of the increasing use of bioplastics in high-performance products, the connections for these new materials must be reconsidered and optimized. ARNOLD UMFORMTECHNIK GmbH & Co. KG from Forchtenberg-Ernsbach has taken up this challenge. In a joint project with Bond-Laminates GmbH, the company also investigated whether the Remform II HS (High Strength) thread-forming screw is 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 in high-performance plastics. It is used when a high preload force and high loosening resistance are required.
The design of the Remform II HS is characterized by an asymmetrical thread profile with a rounded flank tip and a curved load flank. This geometry is adapted to the flow properties of plastic and enables gentle material displacement.
Optimized thread core increases the breaking torque
The Remform II HS is characterized by an asymmetrical thread profile with a rounded flank tip and curved load flank. This geometry is adapted to the flow properties of plastic and enables gentle material displacement. The screw also has an optimized, enlarged thread core, which significantly increases both the breaking torque and the tensile breaking force of the screw. This results in a more stable connection between the joining partners, but also allows a higher assembly torque to be applied to high-strength plastics without the risk of the screw breaking. The combination of radius profile and steep load flank allows the plastic material to flow towards the load flank when screwing in and ensures very good flank coverage. By reducing the load flank angle to 10°, the radial stresses are significantly reduced. Despite the high thread overlap, a low screw-in torque is generated and the risk of cracking in the plastic is reduced due to the lower radial expansion.
Observe factors influencing the bolted joint
In order to reliably design and test bolted connections, it is important to know the factors that influence the connection. This is particularly important when carrying out basic tests. Important factors influencing the screw connection are the tube and screw geometry, the core hole diameter and the material and, in this context, the strength, conditioning or fiber content. It is also important to consider assembly influences such as the accuracy of the screwdriving system or the speed. And last but not least, the various types of load in use are also important, i.e. temperature and dynamics, as well as the load duration or ambient conditions. All of these factors influence the assembly parameters of a plastic direct fastening. Taking these factors into account, an exemplary, real screwdriving curve shows that the screwdriving values can be significantly improved when using the Remform II HS in a high-performance plastic. "Compared to the Remform screw, which is virtually the standard solution for engineering plastics, the Remform II HS can increase the failure force and the failure torque by around 20 percent," says Sinja Strobl, Product Engineer at ARNOLD UMFORMTECHNIK.
"Compared to the Remform screw, which is practically the standard solution for engineering plastics, the Remform II HS increases the failure force and failure torque by around 20 percent," says Sinja Strobl, Product Engineer at ARNOLD UMFORMTECHNIK.
Basic investigations into the screwing of bioplastics
One thing is certain: Plastic direct screw connections are a reliable and cost-optimized solution for connecting plastic components. But what about bioplastics? ARNOLD UMFORMTECHNIK and Bond-Laminates have jointly investigated the suitability of the Remform II HS for bolting Tepex, a registered trademark of the Lanxess Group. Tepex dynalite 813-F250 (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 stalk of the flax plant and are grown locally. Tepex materials consist of one or more layers of semi-finished textile products with continuous fibers (flax) embedded in a matrix of PLA. These are fully impregnated and consolidated. All fibers are therefore coated with plastic and the material contains no air pockets. According to the manufacturer, Tepex dynalite therefore offers maximum strength and rigidity with a very low density. The Tepex material samples were provided as plates for the basic tests on suitability with regard to screwability, which were then cut into small samples for the screw tests. Due to the sample thickness of around 6 mm, a Remform II HS with a diameter of 2.5 mm was used as a fastener. "The effective screw-in depth was set to 2 x d. In the first series of tests, screw-in and over-tightening tests with preload force measurement were carried out in core holes of different sizes. This means that the screw is screwed in and tightened until the connection fails," says Sinja Strobl, describing the tests. Based on the results of these initial tests, the appropriate core hole diameter and tightening torque for long-term tests were determined. During the long-term measurements, the samples were then screwed together with the previously defined tightening torque and the preload relaxation was measured using a load cell.
Results prove the suitability of the Remform II HS
The initial results of the screw-in and over-tightening tests indicate that the Remform II HS can be used to form a nut thread with maximum load-bearing capacity very well in the bio-based material. "There were no stress cracks or undesirable deformations in the sample body during screwing. The Tepex material really surprised us. Compared to the other materials tested, it achieved the best failure torques even higher than a PPS with glass fiber reinforcement," says Strobl, summarizing the results. The combination of Remform II HS and Tepex also scored highly in terms of process reliability. "The low screw-in torques and high overtorques result in a high delta torque, which opens up a large process window for assembly and gives the user a high level of safety," Strobl continues.
Support with the design of direct screw connections
The results of these investigations have subsequently been incorporated into the various tools in ARNOLD UMFORMTECHNIK's digital engineering portfolio. This means that, in future, customers can be supported even better in the design and conception of direct fastening also with regard to bio-based composite materials. ARNOLD UMFORMTECHNIK offers various services in the plastics sector. One of these is the core hole tool. It offers the possibility of determining an optimum core hole design for almost all plastics on the basis of characteristic values determined in the laboratory. ARNOLD customers then receive the recommendation as a PDF with all the relevant dimensions for the design. Another tool is the Fast Designer Plastics, a forecasting tool for direct plastic screw connections. Fast Designer Plastics can be used to make statements about various assembly and operating variables, such as assembly preload force and tightening torque. It is also possible to predict the relaxation-related drop in preload force after time and temperature exposure.
