Automotive door trim panel plastic fasteners overview

The door trim panels of the common models on the market and the inner sheet metal are generally used with plastic fasteners of various shapes but similar in structure, in order to understand the commonality of application environment and performance requirements in the development of such plastic fasteners, Better guidance for development and use, based on the benchmark analysis of multiple models, summarizing the application environment requirements, common material selection and performance requirements of such fasteners, combined with CAE analysis means for its most common fracture failure mode. Analysis is performed to determine the shear force during installation and is the main reason for solving the material and structure. Speculation on the development direction of plastic fasteners for door trim panels.

The popularity of automobiles has led to an increase in the awareness of ordinary customers about cars. From the initial fuel consumption to the coordination of the interior, the focus has become more and more detailed. The combination of the door trim panel and the inner sheet metal has attracted attention as a customer-friendly area. In order to ensure the cooperation of this place, most OEMs usually use a plastic bird-shaped clip to connect.

The development of such plastic fasteners has commonality in terms of application environment and performance requirements. Understanding the principles of these commonalities is instructive for better development and application of such fasteners. The article will discuss the plastic panel of the door trim panel from the application environment, performance requirements, material selection and failure mode.

Application environment

FIG. 1 is a schematic view showing an application environment of a car door trim panel. After the installation is completed, the clip connects the snap seat to the sheet metal, the tail is fixed on the buckle seat, the head is inserted and clamped to the outside of the sheet metal, and the umbrella surface is fastened to the inside of the sheet metal.

Figure 1 Schematic diagram of the application environment of the car door trim panel

1) The connection between the clip and the snap seat. In order to ensure that the clip can be aligned with the sheet metal hole during the installation process of the door trim, a certain amount of activity needs to be present in the base. It is usually necessary to ensure that there is a gap between the middle column of the clip and the opening of the buckle seat to enhance the clamp and the clamp. The centering of the gold hole; the upper and lower directions between the clip and the buckle seat also need to have a gap to ensure that the clip can smoothly adjust the position of the relative buckle seat.

2) The connection between the clip and the sheet metal. The OEM will select different thickness of door trim sheet metal for the positioning of the model. It is commonly used in thin sheet metal (0.5-1.0mm), thick sheet metal (1.5-2.0mm) and spliced ​​sheet metal, so the door trim panel will be selected accordingly. Different clips, different clips are distinguished by different colors; the size of the sheet metal hole is determined by the clip, and the diameter of the sheet metal hole of the common clip is 6-9 mm; at the same time, the clip needs to satisfy the manufacturing error of the sheet metal hole ±0.1 mm.

Sheet metal is generally treated by electrophoresis and painting. The sheet metal hole is a punched hole, which also affects the insertion and extraction force of the clip.

2. Performance requirements

According to the application environment of the plastic plate of the door trim, the performance requirements are mainly in the three aspects of insertion and extraction force, assembly and water resistance.

2.1 Insertion and pull force requirements

The insertion force is the most critical requirement for screws that are primarily used for fastening. The insertion force affects the use perception. Excessive insertion force will cause the operator to complain; the pull-out force affects the repair, the pull-out force is too small, the door trim and the sheet metal are poorly matched, and the pull-out force is too large. It is difficult to return the door trim.

Different OEMs do not have the same insertion and extraction requirements for the clips, but the approximate range is basically the same, as shown in Figure 2. According to ergonomics and performance requirements, because the door trim is snapped with the palm or fist, the insertion force of the clip on the door trim assembly is no more than 80N, and the retention force is required to range from 180-300N.

In order to ensure that the insertion and extraction force of the clip in the door trim assembly is within the range suitable for ergonomics, the insertion force of the single clip must be smaller than the insertion force of the assembly. The general insertion force is not more than 60N, and the pull-out force is It is 150-270N.

Figure 2 Man-machine selection of the car door panel clip insertion force range

2.2 Assembly requirements

The assembly requirements of the clip are mainly error-proof and leak-proof. Since the sheet metal corresponding to the same door trim panel may be welded with two thin sheets of sheet metal, two different clips are installed on the door trim panel, which provides the error proof and leakproof requirements for the clip.

Error proofing is mainly distinguished by the snap size of the clip and the snap seat. By changing the size of the snap seat, only the correct clip is allowed to be installed. The anti-leakage of the door trim board clip is often photographed, so the clip can be designed in different colors to meet the requirements of error proof and leak proof detection.

2.3 Water resistance requirements

The general waterproof test method for fasteners is to install the fasteners on the water container (the container is pre-installed with the sheet metal fixture), and test the fasteners for water drops under a certain height of water column for a certain period of time. Or test the number of water droplets, as shown in Figure 3, the height of the water column can be controlled by adjusting the dimension A in Figure 3.

Figure 3: Waterproof test method for automotive door trim panel fasteners

The water resistance of fasteners is mainly achieved by relying on extra sponge washers or overmolded TPE lips. Figure 4 shows automotive door trim fasteners with different waterproof properties.

In Figure 4, the clip I is not waterproof, and the conventional requirement for the clip II and the clip III is that no water droplets leak out within 10 minutes of the water column of 30 mm height.

Figure 4 Car door panel fasteners with different waterproof properties

If the fastener is over-angled with the sheet metal, the size of the lip is unacceptable, or the lip is too hard, it will cause waterproof failure. In general, closed-cell EPDM foam sponge gaskets are more water-repellent than TPE's over-molded lips, but at a higher price.

3. Common materials

Common materials for plastic clips are PA66, PP, and POM.

1) PA66 has good strength, impact resistance and wear resistance. It can ensure strong stiffness and strength at higher temperatures, but it has strong hygroscopicity. After injection molding into parts, strong hygroscopicity will lead to its The strength is reduced and the dimensional stability is poor, which is very disadvantageous for fasteners.

2) PP is basically non-hygroscopic, has good impact resistance, and has good dyeability, but PP is soft, not rigid enough, wear resistance is poor, and it is easy to decompose and aging; PP clip is easy to scratch and difficult to use multiple times.

3) POM has high hardness and rigidity, excellent wear resistance, self-lubricating and fatigue resistance, repeated impact resistance, and a wide range of temperature applications. The POM clip does not absorb moisture, and the insertion and extraction force of the POM clip does not change much at high and low temperatures. However, POM is also called polyoxymethylene, and the spatter is slightly higher.

Since the amount of plastic clips on the door trim is not too much, the overall sporadic contribution to the interior of the car is limited. The POM clip has basically replaced PA66 and PP and is the main material for the clip.

4. Analysis of common failure modes of door trims

The common failure modes of door trim plastic clips are:

1) The operator complains that it is difficult to assemble and the insertion force is large. The reasons for such problems may be: the size of the sheet metal hole is too poor, the size of the paint is reduced after coating; the clip is not centered with the sheet metal hole; the amount of activity of the clip in the base is insufficient.

2) Loose noise. Possible reasons for such problems are: the size of the sheet metal hole is too poor; the improper clamp is selected; the design of the edge of the door trim and the sheet metal is unreasonable, which makes the clip difficult to assemble.

3) Broken during installation. Installation breakage is the main failure mode of the door trim clip. The misalignment of the clip and the brittleness of the POM at low temperatures are the main reasons for the breakage of the clip when it is installed. Structural optimization for the part where the clamp is subjected to a large force can solve such problems well.

5. CAE-based clip fracture analysis

For the most common installation failure failure modes, the paper applies CAE to simulate the process of inserting plastic clips into the sheet metal hole to explore the main causes of fracture failure.

A CAE analysis model was built using a door panel clip of a production model as a prototype. The analysis uses 2D implicit algorithm, regardless of the actual friction factor. The sheet metal is rigid body simulation, and the top of the clip is fixed. On this basis, the CAE analysis of the clip insertion process is carried out, as shown in Figure 5.

Figure 5 CAE analysis process of the card inserting the gold hole

When the clip is being inserted and removed from the sheet metal hole, the clip does not deform any abnormally. Figure 6 shows the CAE analysis results of the clip being inserted into the sheet metal hole.

It can be seen from Fig. 6 that the maximum insertion force of the CAE analysis is 41.5N, which is 2.5N deviation from the average insertion force (39N) measured by the actual physical experiment.

The reasons for the analysis of deviations are mainly manufacturing factors such as part manufacturing errors and surface treatment of sheet metal, but this result does not affect the reliability of CAE simulation.

Figure 6 CAE analysis results of the clamp inserting the gold hole

Since the relationship between the clip and the gold hole in the actual installation process is not necessarily positive, the clip insertion process is largely eccentric. CAE analysis was performed on the eccentric insertion of the clip with the gold hole. The analysis results are shown in Fig. 7.

Figure 7 CAE analysis process after the eccentricity of the clip and the gold hole is 0.5mm

Since the sheet metal in the CAE model is rigid and the tip of the clip is fixed, a stress mutation occurs under the eccentricity analysis of 0.5 mm. When the sheet metal is close to the thickest edge of the clip, the CAE grid tears. As shown in Figure 8, the force value curve also has a sudden change, and finally diverges.

The study of the CAE model found that the model had a mesh tear on the side where the beak was stressed. Comparing the extreme values ​​appearing around 0.8s in Figure 6, the mesh tear occurs at 0.5s in Figure 8, indicating that the clip broke before it was taken into the sheet metal hole.

Combined with the stress-strain curve, it is found that the widest part of the clip is easily torn by the shearing force, which causes the leg to break.

Figure 8 CAE analysis results after the eccentricity of the clip and the gold hole is 0.5mm

By collecting and analyzing multiple failed clips, the break of the clip is mainly above the root of the bird's beak, as shown in Figure 9, which coincides with the location of the divergence in the CAE analysis.

It is concluded that the main cause of the failure of the clip is that the shear force generated by the impact during the insertion of the clip will shear and fracture near the root of the clip.

Figure 9 Car door trim panel clip break diagram in actual use

According to the above analysis, the main way to solve the cracking of the clip installation is:

1) use high toughness, shear resistant materials;

2) Improve the structure of the root of the clip. Improving the shear resistance of materials through material properties and root structure optimization is the key to solving the problem.

6. The development direction of door trim plastic fasteners

With the improvement of the process of the surrounding environment parts such as door trim and door sill, plastic fasteners are also developing.

At present, its development direction mainly includes:

1) Improvement of the scope of application: It can be applied to sheet metal of different thickness. The larger the thickness range of the applicable sheet metal, the more practical it is for the OEM, the sharing of multiple models can be realized, and it becomes a shared vehicle; Will increase accordingly, the unit price will be correspondingly lower.

2) Clamps suitable for niobium alloy plates: The lightweight demand of automobiles promotes the application of aluminum alloy plates. The manufacturing process of aluminum alloy plates and the first use of sheet metal will be very different, so it is urgent to develop clamps suitable for aluminum alloy plates.

3) Application of low volatility materials: Currently widely used POM materials are volatile, and with the increase of volatility requirements for automotive interior materials, low volatility and high performance materials will be widely used in automotive parts including clips. on.

4) Enhanced shear resistance: The improvement of shear resistance will greatly reduce the complaints of OEMs and improve the utilization of clamps. In particular, the improvement of the fracture resistance at low temperatures will reduce the difficulty of assembly and repair of winter car door trim panels in northern China and reduce customer complaints.

5) Application of 2-level self-plugging structure: The plastic fasteners are designed to be level 2, the first level is the fastener between the fastener and the sheet metal part, the second level is the internal fastening of the fastener; when disassembling, the level 1 Partially separated from the second stage, the self-lubricating effect between the plastic materials of the fasteners is utilized to avoid failures such as breakage or loss of retention force of the sheet metal scraping fastener during disassembly, and the repeated use rate of the fastener is improved.

7. Conclusion

The article explores the selection and performance requirements of plastic fasteners for door trims from the application layer. It will be based on the selection basis of sheet metal thickness, the ergonomic insertion force requirement and the waterproof method of the application position. The principle of choice and application.

In order to prevent the installation of the clip from failing, the anti-shearing ability is proposed as one of the indicators for measuring the performance of the clip. However, there is currently no uniform test method to obtain the shear resistance of the clamp, which is the further research direction of this subject.

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