Definition: Check Ring / Check Valve, 1. to permit flow of a fluid in one direction only.
The check ring typically is a removable part of the front of the screw within an injection molding press. The function of this ring / area is to act as a seal when the injection screw is injecting plastic out the nozzle, and to allow shot formation when the screw is rotating. By acting as a seal we then have a piston action which compresses the material in front of it so that it only moves in one direction (hopefully) out the nozzle and into our mold. Check rings are assemblies of parts which function as one unit, comprised of ring, body (tip and shaft),and seat, that are screw into the front of the screw.
The comment most often made is that there are really only two types of check rings; those that work and those that do not. Though this is somewhat simplistic it is true. There are many designs of check rings /seals on the market, the sliding check ring being the most prolific, ball checks, and the non existent smear tip.
The workings of the sliding check ring are as follows:
1- Upon injection the material in front of the ring forces the ring back into a seal position thus allowing the screw and ring combination to act as a piston transferring the injection pressure upon the plastic material and forcing it forward into our mold.
2- After filling the mold and as the screw is in the forward position it continues as a piston contributing to our packing and holding pressure transferred from screw to the material into our mold.
3- Upon screw rotation the material pumping action causes the ring to slide forward, thus allowing material to flow under and around the design into the area in front of the screw thus forcing the screw back as pressure exceeds our backpressure on the screw.
There are many concerns with a check ring, movement, clearances, wear, seat area, type, and even material compatibility for the design in use. The movement of the check ring from an open to a seated position is important. For example if the ring travels 5mm from open to close and suck back was used on the machine the screw in theory would have to travel 5mm or more to cause the ring to seal. If in fact no suck back was used and a high backpressure was set it is possible that when screw rotation stops that the added pressure inside the barrel aids in pushing the ring back, the ring then when injecting does not take 5mm of stroke to seat into position. This is a key piece of information especially when not using 30% or greater of the barrel capacity as consistency in shot injected could be impacted.
The ring itself has clearances on both sides of its diameter. The outer diameter must seal against the barrel of the machine while the ID of the ring has a clearance to the shaft it floats around in some cases, in others the ring rotates with the screw as it is keyed into position. The ID clearance to the shaft is critical for some material as we could see high shear of the material as it is pumped through this area. An example of a clearance issue was with a polycarbonate material that exhibited splay in the part. Though the throw of the ring was good the clearance between ID and shaft was tight creating a high shear and thus splay on the part. Decreasing shaft diameter resulted in good parts.
The ring also has to form a seal to the seat, these typically are angled and of various designs, too many to discuss here. The importance though all of this is to create a seal, thus when applying a cushion of material with a shot we are able to maintain that cushion. If in fact it is noticed that the screw always bottoms outs upon pack or hold this is an indication that some area of the seal is not being maintained. Is it a clearance issue with the barrel, or a seating issue with the ring/seat area. These would need to be investigated.
An example would be "The case of the variable cushion."
In this case the cushion sometimes would hold and sometimes would not hold. We could add cushion and watch the screw bottom out and sometimes not. Upon examination of the check ring we found the flutes on the tip of the check ring assembly had worn off, thus the ring was not captured so that in some cases the ring would slide completely off the screw assembly and in other cases would stay on.
The point to make here is to inspect occasionally the condition of your check ring and screws, too many times no maintenance is perform, until there is an issue and than the costs are too great, meaning that we have to replace broken parts, but we also lost our machine time and production on the machine while waiting for parts. Where as upon inspection it is noted that there is wear, the unit can be replaced than or reassembled and the correct part ordered so that it is on hand to be installed when it all breaks or better yet at the next maintenance point.
Without a properly working check ring we cannot maintain consistency to part production, and thus waste much time in producing bad parts along with all the materials used to make said parts.
