Press fits work on a somewhat different principle than snap fits. In a snap fit, the goal is to temporarily deflect a detail and have it return to rest after capturing a matching detail in another part. Press fits work by permanently deflecting a detail and using the surface friction and surface affinity of the plastic material to hold the assembly together. The classic press fit is a pin engaging a slightly smaller hole. The technique is not limited to round pins in round holes; a common variant is a round pin in a hexagonal hole. The interference fit between pin and hole and the wall thickness of the boss are very important in making this type of assembly work. Sloppy molding practice that embrittles the bosses or leaves a weak weld line in the boss can sabotage the best press fit design. The optimal interference dimensions and length of engagement also depend on material properties including stiffness and lubricity, and creep can be an issue as assemblies age. Some years ago a technology was introduced that executed the press fit at high velocity and claimed to actually weld the pin in the hole through heat produced by surface friction. As with all other techniques, success depends on good design and careful testing to get the details right. Press fit assemblies generally cannot be disassembled, which may be either an advantage or a disadvantage compared to snap fits. At one manufacturer, a war broke out between engineers who favored press fits and engineers who favored ultrasonic welding. Recollection is that good points were made on both sides, and it is possible that the conflict my still be unresolved more than a decade later.