Shear Joint

The shear joint or interface joint shown in Figure 7 is generally recommended for high-strength hermetic seals on parts with square corners or rectangular designs, especially with crystalline resins.

Initial contact is limited to a small area which is usually a recess or step in either of the parts. The contacting surfaces melt first. As the parts telescope together, they continue to melt along the vertical walls. The smearing action of these two melt surfaces eliminate leaks and voids, making this the best joint for strong hermetic seals.

Several important aspects of the shear joint should be considered: 1) the top part should be as shallow as possible, 2) the outer walls should be well supported by a holding fixture, 3) the design should allow for a clearance fit, and 4) a lead-in (A) should be incorporated.

The shear joint requires weld times in the range of 3-4 times that of other joint designs because larger amounts of resin are being welded. In addition, a certain amount of flash will be visible on the surface after welding.

When flash cannot be tolerated for aesthetic or functional reasons, a well similar to the ones shown in Figure 8 should be incorporated.

Fig. 9 Shear Joint Variations

Modified joints, such as those shown in Figure 10, should be considered for large parts or for parts where the top piece is deep and flexible.