Assembly Considerations
| Joint Design |
Adhesive joints are generally stronger in shear and tension than they are in cleavage and peel. Therefore, the joint should be designed to confront the loads in a way that utilizes their strengths. For strong joints, maximize shear areas to distribute the shear forces over a larger area and reduce the shear stress. Joint designs should lend themselves to be easily molded and should incorporate self-aligning features. Aesthetic requirements may sometimes limit the choices of joint designs that are aesthetically pleasing (e.g. a lap-shear joint with a strap may be aesthetically objectionable).
Provide adequate radius at all internal corners in a joint to minimize stress concentrations. Avoid placing gates close to the joint surfaces. Gate areas have higher molded-in stress. Chemicals from the adhesive or the surface treatment preparations may attack the stressed area, causing crazing or cracking.
Figure 1 shows a simple lap joint and some variations of lap joints. Lap joints provide good strength when placed in the assembly such that they do not experience high peel or cleavage stresses. A simple lap joint can be vulnerable to peel or cleavage stress. Adding a joggle to the lap joint gives it self aligning capability and makes it stronger to cleavage stress. Placing a strap on the joint further increases the area and the strength of the joint. A Double-strap configuration is even stronger.
