IndustrY Joining technology
Background
In the automotive industry in particular, economic and ecological reasons lead to an increasing demand for lightweight design. Many lightweight design strategies use a composite design to fulfil the ambitious criteria. The material selection does not need to be restricted to the components to be joined. Depending on the joint specifications, the fasteners could be made from several lightweight materials. The use of self-tapping screws for fitting components to thermoplastics has been established since the 1970s. Today’s fasteners were generated by the optimisation of the thread geometries, which can be found in the application guidelines [1]. Fasteners for automotive applications in particular have to fulfil high quality standards for automated assembly. Therefore, very high reproducibility of the fastening parameters is necessary in order to obtain process stability.
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In order to save fastener weight, it is necessary to investigate the fastening parameters that occur due to the various material combinations, as well as the corresponding system properties. Accordingly, in this study investigations were carried out on joints made with steel screws (according to strength 10.9), highstrength aluminium screws (AA 7075-T73) and high-strength plastic screws (made from Grivory PPA HTV 5H1 [2]). Parameters for direct assembly
Various parameters have to be considered if a very stable process for direct tapping into plastics is to be achieved. The geometry of the boss and the fastener play an important role. Recommendations for boss dimensioning will generally be given by the screw manufacturers, [3], for example. High process reliability can be achieved if there is a large difference
between the installation torque (TI) and the stripping torque (TS), 1. The stripping torque arises from joint failure, either a failure of the screw (for example, fracture) or a failure of the boss material (destruction of thread formed). This difference is used for the calculation of a recommended tightening torque TT [4]. It is well known that thermoplastics exhibit relaxation under load. This phenomenon influences the clamping load introduced by direct tapping, which approaches a boundary value [5, 6]. In the case of additional exposure to temperature, the difference in the thermal expansion coefficients of the various materials applied in a joint may lead to an increased reduction in the clamping load. This effect can be reduced by using a suitable design combined with the correct choice of screw material. 2 shows the properties of the materials and screws used in this study.
Lightweight Design for Direct Assembly into Plastics Lightweight design for direct assembly into plastics can be achieved by a suitable selection of the screw material. A study by Ejot aims to investigate the influence of the screw material on the fastening parameters and the joint properties. This leads to recommendations for suitable screw materials for an optimised combination of lightweight design and applicationrelated demands for the joints.
autHorS
PD Dr. Ralph J. Hellmig
is Head of Research and Development Screws at Ejot GmbH & Co. KG in Bad Berleburg (Germany).
Volker Dieckmann
is Product Manager for Thread-Forming Screws in Plastics at Ejot GmbH & Co. KG in Bad Berleburg (Germany).
❶ Torque curve for plastic direct tapping, fastening using tightening torque TT (blue), associated curve of clamping load (black) and stripping torque using TS (red) 02I2011
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IndustrY Joining technology
Fastening experiments
Steel 10.9
Aluminium 7075
PPA HTV 5H1
1030
500
250
12
23
40
7.8
2.7
1.7
210
72
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Material properties
In order to obtain a recommendation for the tightening torque TT for the different materials, installation experiments into various thermoplastics were carried out until damage to the joints occurred (TS). A standard hole diameter of 0.8 times the screw diameter was used for the sample bosses. During the fastening experiments, both the rotation angle and the corres ponding torque were recorded. The curves obtained were similar to the curve depicted in ①. This data was used to calculate the installation torques (TI) and the stripping torques (TS) for the different material combinations. The results are presented in 3 (screws made from steel and aluminium alloy 7075-T73) and 4 (Grivory PPA HTV 5H1). It can be seen that all material combinations tested yield parameters for a stable fastening process that has a suitably large difference between the installation torque and the stripping torque. Additionally, it was demonstrated that the ten-fold repetition of the fastening process required for the repair of screw joints was possible for all material combinations. Temperature behaviour
Generally, the clamping load relaxation under thermal exposure must be kept as small as possible. Investigations on the relaxation behaviour of direct assemblies into PA6GF30 were carried out on joints created under the same conditions as for aluminium and steel screws.
tensile strength [MPa] thermal expansion [10E-6/K] Density [g/cm3] Young‘s modulus [GPa] Screw properties Break torque [Nm] Tensile breaking stress [kN]
DOI: 10.1365/s38312-011-0011-2
3.8
1.4
10
5.4
1.2
2 Properties of materials/screws selected (thread diameter 5 mm)
It was observed that a higher clamping load could be retained after a thermal cycle due to the smaller difference in thermal expansion between the aluminium screw and the thermoplastic boss compared to the steel screw, ❺. Similar effects can be expected in the case of direct tapping using a screw made of Grivory PPA HTC 5H1 when compared to a steel screw. Application limits
Thread-forming fasteners made from steel can be used in all common thermoplastics. A stable fastening process is achieved due to the high stability of the screw and thread flanks. In the case of direct assembly into thermoset materials, screws containing a special cutting geometry can be used. This geometry can be applied either by a milling process or by thread rolling [7]. A cutting geometry is necessary in order to avoid boss cracking due to the high brittleness of the boss materials. Thread-forming fasteners made from high-strength aluminium can be used for
3 Installation torque, stripping torque and clamping load at stripping using a self-tapping screw made of steel and AA7075-T73 in different thermoplastics
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7.1
direct tapping into soft and medium strength thermoplastics due to their strength. The thread flanks are strong enough to form an internal thread into glass fibre reinforced materials containing a fibre content of up to 30 %. The high-strength alloy 7075, which is generally used for this kind of screw, is protected against stress corrosion cracking due to an overageing heat treatment (called T73). As the unavoidable relax ation of the boss material leads to a reduction in the clamping load, an uncritical stress level that avoids stress corrosion cracking is generally achieved for aluminium screws. There is a restriction according to the application temperature for this aluminium alloy, as the typical ageing temperature is in the range of 120 °C. Therefore, a corresponding thermal load will lead to further ageing of this alloy. Practical experiments suggest a possible application limit near 150 °C, as the boss mater ials will have a much lower strength compared to 7075 even after such a tempera-
4 Installation torque, stripping torque and clamping load at stripping using a self-tapping screw made of Grivory PPA HTV 5H1 in different thermoplastics
5 Clamping load reduction under thermal load, boss material: PA6GF30
Thanks The authors would like to thank Marion Bätzel for her kind support in the experiments.
ture exposure. The high strength of the aluminium screw is mainly required for the thread forming itself. For a screw made of Grivory PPA HTV 5H1, the result of this study demonstrates that it can be used in soft and non-reinforced thermoplastic materials. In the tapping process, a slight filleting of the thread flanks, which did not affect the functionality of the screw, was sometimes observed. However, this kind of screw cannot be used in reinforced thermoplastics due to its low strength. Specific applications
Compared to universally applicable steel screws, the other screw materials tested can be beneficial for certain specific applications. In these applications, screws made from aluminium 7075-T73 or Grivory PPA HTV 5H1 can be superior to steel screws for direct fastening into plastics, 6.
Screws made of aluminium alloy 7075T73 exhibit a very low weight combined with a high strength. They are non-magnetic. Aluminium screws exhibit a higher resistance against clamping load losses due to thermal exposure compared to steel screws if the temperature limitations are not exceeded. As anodised surfaces can be used, lots of design vari ations are possible. Screws made of Grivory PPA HTV 5H1 exhibit an attractive potential for direct fastening into soft and non-reinforced thermoplastic materials, despite their low strength. Like aluminium screws, they combine lightweight design with a reduced clamping load relaxation. Additionally, they have some unique properties: they are electric insulators, corrosion resistant and do not produce any metallic debris during fastening. They may also offer some advantages for recycling due to the similarity between
Steel 10.9
Application fields Highly reinforced thermoplastics Reinforced themoplastics (up to 30 %) Non-reinforced thermoplastics high clamp load low clamp load losses under temperature
Aluminium 7075
( )
no metal debris non-magnetic no corrosion electric insulation
lightweight design
PPA HTV 5H1
the screw and the boss material. Due to its low strength, this kind of screw can be used if the joint is not required to transfer high loads and if lightweight parts are securely fixed. Summary
A comparison of different screw mater ials shows some advantages over conventional steel screws in special appli cations if aluminium or high-strength reinforced PPA is used as a material for self-tapping screws. Therefore, lightweight design and unique properties can be achieved by using these kinds of screws for direct assembly into plastics. However, due to its broad application range, the classic steel screw will still be used for most applications. References
[1] DVS-Richtlinie 2241-1, Direktverschraubung von Formteilen aus Kunststoffen [2] Jeltsch, T.: Bis an die Grenze des Machbaren. In: Kunststoffe 8 (2007), pp. 144 – 147 [3] Produktinformation Delta PT, Ejot GmbH & Co KG [4] Dieckmann, V.; König, G.; Weitzel, S. P.: Direktverschraubung dynamisch und thermisch beanspruchter Bauteile mit einer neu entwickelten Gewindegeometrie. In: Ejot-Forum 6, 2003 [5] Tome, A.: Vorspannkraftrelaxation von Kunst stoffdirektverschraubungen. Erlangen, Universität, Dissertation, 2000 [6] Dratschmidt, F.: Zur Verbindungstechnik von glasfaserverstärktem Polyamid. Erlangen, Universität, Dissertation, 1999 [7] Hellmig, R. J.; Achenbach, M.: Intelligente Leichtbaulösungen durch Verbindungselemente – Spezielle Anforderungen an die Direktverschraubung in duroplastische Werkstoffe. In: Tagungsband zur 9. Schraubentagung des Deutschen Schraubenverbands, Düsseldorf, 2009
6 Possible application fields for self-tapping screws made from different materials 02I2011
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