DESIGN GUIDE AIRBAG COVERS

Transcription

1 DESIGN GUIDE AIRBAG COVERS 1. Standard design rules Runners Injection points Venting Tear seam design Walls, ribs Cooling K airbag covers Skin thickness Tear seam Shrinkage 5 Design Guide - Airbag covers 1 Multibase - A Dow Corning Company

2 1. Standard design rules 1.1. Runners The most typical sections are discs (4 to 8 mm diameter) or trapeziums. In order to have a easy flow and ejection, the runner surface should be smooth, but not necessarily polished. Each runner must be exactly the same (flow length, dimensions, less pressure drops possible) for each cavity of the mould. Regarding hot runners, we need heating control to avoid overheating and freezing. The temperature should be uniform throughout the hot runner, as for the shear rate. Finally, the hot runner contours should be smooth to avoid material stagnation at high temperature and then degradation Injection points Location The gate should be located where uniform fill can occur (uniform heat dissipation), as well as located in the portion of the cover where you have the largest surface of material. Moreover, the gate should be located furthest away from the tear seam to avoid early pressure peak during the filling causing glossy tear seams. It is also preferable to locate them in the ribs whenever possible. In the case of a U or H tear line design, it is recommended to avoid injection points at 3 or 9 o'clock position. Indeed, in this case, the material needs to cross 2 tear lines, which slows down and freezes the material and creates pressure peaks. Number In the case of 1K painted and unpainted covers, an important number of injection points makes the injection easier ; with several injection points, the holding pressure, which causes glossy lines at the tear seams, can be decreased to minimal, without creating sink marks. Gate design We prefer cashew gates to fan gates. Please find below the different possibilities Cashew gate into a flange : this kind of gate should be round tapered down to 1,5 mm in the line of the flow. To avoid turbulence in the material before reaching the class A surface, avoid ribs placed in front of the cashew gate. The flange should be oriented in the line of the flow or no more than 90 to the class A surface. If possible, set the flange thickness to minimum (no more than 30-35% of Class A wall thickness) in order for to act as a runner before hitting class A Design Guide - Airbag covers 2 Multibase - A Dow Corning Company

3 surface. Some riblets can be added to increase the robustness of the flange, but not directly located in front of the cashew gate. Sub gate into a tab gate : it is a restricted gate which prevents jetting and minimises moulding strains. We need a minimum land of 4 mm to generate shear in the tab to lower the viscosity of the material. The tab thickness should be 15-25% of the part thickness at gate location. Edge / fan gate : its width should be at minimum to fill the part (between 20 and 40 mm). The thickness of the gate should be 15-25% of the part thickness at gate location. Half moon gate in ejector pin : it can be used but not recommended because it is a half moon runner, it is hard to obtain recommended shear through the gate, and the material flows into the class A surface at a 90 angle. Submarine gate : it should be a minimum of 60% of wall thickness to reduce sink marks. Pin / valve gate : it is only not recommended in the emblem zone because it can cause surface defects for moulded-in-color (MIC) applications. If this kind of injection point is needed for MIC covers, we have to follow the following steps : - make sure that there is a cold well in the emblem pocket to uniformly distribute heat into part ; - the wall thickness in the emblem should be as minimal as possible so that the emblem pocket acts like a runner before reaching class a surface ; - the side walls of the emblem also need to act as a gate therefore the wall thickness should be 25-30% of class A wall thickness in order to shear material before entering class A surface. Diameters We recommend 1,2-1,5 mm diameter injection points to avoid using too much holding pressure Venting A good venting allows air to escape during injection and prevents burning created on parts during the end of the injection. A optimal solution is to use a venting channel all around the part with little vents from this channel to the part cavity and vents from the channel to the edge of the mould. The exact location can also be found through mould flow analysis or by shooting "short shorts" Tear seam design The angle of the tear seam has to be the closest possible to reduce glossy lines ; we recommend an angle of 30 (Figure 1), which is a compromise between deployments and aspect. Design Guide - Airbag covers 3 Multibase - A Dow Corning Company

4 The thickness of the tear seam is around 0,5 mm, decreasing to serve as initiator. The bottom width of the tear line is generally between 0,4 and 1 mm. 0,4-1 mm 0,5 mm 30 Figure 1 : Tear line angle The ends of the tear lines are designed to stop the tearing ; that is why the tear line is generally thickened to reach a thickness of 1 mm. We can also add radius or U-turns. Finally, little walls can also be used. If the tear line has to be very thin to pass the deployments test, the stoppers has to be reinforced, but not to much otherwise the flap will not hinge correctly (less hinge function) and would probably fly away during deployment. In that case, we can create a small hinge between the stoppers to initiate the hinge of the flap Walls, ribs Ribs are generally added to increase the stiffness of the part. To prevent sinks marks corresponding to ribs, their thickness should be 40% of the class A surface thickens at maximum (Figure 2). The height should not exceed 150% of the wall thickness when numerous ribs in the part. d 40% of d (maxi) Figure 2 : Walls and ribs design If more robustness is required to flange to avoid tearing during deployment, it is possible to add riblets every 5cm. Walls and ribs should have a draft of 0,5-3 and be not polished in order to have a easy demoulding. We also can use permanent release agent coating such as Teflon coating, or roughen up the surface through sand-blasting Cooling It is recommended to have a controlled cooling of the part. Design Guide - Airbag covers 4 Multibase - A Dow Corning Company

5 2. 2K airbag covers 2.1. Skin thickness We recommend a skin thickness between 1,3 and 1,5 mm (Figure 3). A thickness bigger than 1,5 mm could engender aspect problems after heat ageing (see shrinkage). Moreover, as the skin material is very soft, we could have a "chewing gum" effect during deployment at +85 C Tear seam The tear seam should be designed in the 1 st shot material (Figure 3). Its thickness should between 1,3 mm and 2,3 mm (that is to say : between 0 mm of 1 st shot material + 1,3 mm of 2 nd shot material and 1,2 mm of 1 st shot material and 1,3 mm of 2 nd shot material). 1st shot 2 nd shot - skin 0,4 2,5 1,3 Draft angle Figure 3 : Tear seam of a 2 shots airbag cover 2.3. Shrinkage Because of the shrinkage differential between the 1 st shot material (TPO - 0,9%) and the 2 nd shot material (TPE - 2%), it is necessary to follow the rules below : - the 1 st shot tear line thickness should be higher than 0,4 mm (except for initiation points) ; - the 2 nd shot thickness should decrease on the edge of the part to avoid warping after heat ageing ; - add ribs to reinforce the 1 st shot core at the edge of the part (Figure 4); - design a slightly rounded cover to avoid warpage ; - logo reinforcement pins in cover need to be made out of 1 st shot material to avoid warpage of the part in the logo area. Design Guide - Airbag covers 5 Multibase - A Dow Corning Company

6 Emblem Tear seam Wall Wall Ribs Figure 4 : Standard 2K airbag cover LIMITED WARRANTY INFORMATION - PLEASE READ CAREFULLY The information contained herein is offered in good faith and is believed to be accurate. However, because conditions and methods of use of our products are beyond our control, this information should not be used in substitution for customer's tests to ensure that Multibase's products are safe, effective, and fully satisfactory for intended end use. Suggestions of use shall not be taken as inducements to infringe any patent. Multibase's sole warranty is that the product will meet the Multibase sales specifications in effect at the time of shipment. Your exclusive remedy for breach of such warranty is limited to refund of purchase price or replacement of any product shown to be other than as warranted. MULTIBASE SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY. DISCLAIMS LIABILITY FOR ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES. Design Guide - Airbag covers 6 Multibase - A Dow Corning Company

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