It has been known in the plastic pipe industry for several decades that pipe couplings made by injection molding are inherently weaker than couplings made from extruded material. Further, accelerated life tests and experience with old pipe lines shows that these couplings have a life substantially less than that of extruded pipe which they join together. Part of the problem is due to the strength of the plastic materials. Plastic formulations for injection moldings are inherently weaker than the formulations that can be extruded. For example, polyvinyl chloride (PVC) that is extruded can easily have a design strength rating of 2,000 psi. However, to provide the necessary plasticity and flow characteristics for injection molding, it is difficult to use a PVC formulation that exceeds a design strength 1600 psi (design stress) for conventional injection molding machines.
Another part of the problem has to do with the injection molding process. The cavity of the mold is usually fed by a side inlet or gate, and the material flows around the inner core of the mold on each side of the gate. The liquid plastic being injected into the mold joins together approximately opposite the gate and more or less welds together along a line known as the weld line. This weld line can be a line of weakness where injection molded couplings fail upon being subjected to high internal pressures caused by the fluids in the pipe line. The various conditions of injection molding must be critically met in order to obtain a good weld line.
As a result, couplings have been largely replaced by bells or enlarged sockets formed on one end of each length of plastic pipe. A typical installation of a pipe line involves inserting the spigot or plain end of another section of pipe into an adjoining bell and the joint is sealed by a ring of rubber or rubber-like material held in an internal groove in the bell. The bells may be formed by properly expanding a pipe and thus the inherent strength of the extruded material and the extrusion process is preserved.
However, these enlarged bells give rise to difficulties in manufacturing, storing and shipping plastic pipe. In stacking the pipe, each adjoining length has to be reversed, and each layer has to alternate plain ends and bell ends. The stacks are thus longer than the pipe sections and the protruding bells receive much damage in handling and in transportation. Furthermore, each length of extruded pipe has to be subjected to the belling process which adds substantially to the cost. For these and other reasons the plastic pipe industry has long sought a satisfactory and economical coupling.
Various attempts have been made to provide extruded couplings. Oversize extruded pipe sections have been internally machined to form the seal grooves and tapers, but these have proved to be expensive. Also, oversize extruded pipe sections have been internally expanded to create seal grooves in each end, but because of thinning at the grooves, the material had to be grossly oversize to start with, resulting in unnecessary material and cost.