Every plastic injection molded part made has a parting line on it somewhere...the question is, can you see it? In every injection molding process the part's shape and features are formed by a mold with all the features of your part machined into the surfaces of the mold and various components of the mold that will contact the plastic as it is injected into the mold. For now, we will only talk about the (2) halves of the mold and what their surfaces mean to the parting line quality of your parts.
As you can now determine, a basic plastic injection mold is made up of (2) halves. One side is traditionally called the "cavity" (the stationary non-moving side during the molding process) and the "core" side (the side that is fixed to the moving platen of the molding machine during the molding process). The surfaces on each side that come together, touch and then get held together by the clamping forces of the molding machine are called the parting line surfaces of the mold. Since the injection molded part is often formed with 1/2 of it's shape in the cavity side of the mold, and the other 1/2 of it's shape in the core side of the mold, it is extremely important for the parting line surfaces to line up perfectly where they meet to form the part's shape, right?
Right! Now that you can visualize the mold halves working together and a part being formed 1/2 on each side of the mold (not all parts are formed 1/2 on each side of the mold), what would happen if each half of the mold came together and the parting line surfaces of the mold did not match up perfectly around the shape of the part...i.e..1/2 of the mold is offset in one direction? That condition would cause parting line mis-match and be very apparent on the parts and is a sign of poor tooling workmanship or an old worn out mold.
Now, how many other factors besides mis-match affect the quality of the parting lines on your parts? A lot! The edges of the mold that create parting lines need to be a clean sharp edge for the best quality (least noticeable) parting line. If the parting line edges get nicked by tools during cleaning or maintenance of the mold, it will leave a witness mark in the plastic part at the parting line. Also, certain abrasive plastic materials will just naturally wear out the steel edges at the parting line over time during the injection molding process. For those very reasons, I always recommend hard tools steels for molds that require a long production life over thousands or millions of parts. Lastly, sometimes even the nicest molds with the cleanest parting line surfaces and edges can leave an undesireable parting line if the molding process requires high plastic pressures inside the mold to meet part dimensions or desired surface finishes.
OK, now we understand that a parting line is the visible line along a plastic part that shows where the mold halves and components meet to form the shape of the part. If you can't see it or feel it, it's perfect! If you can see it, but can't feel it, it's about as good as it gets. If you can see it and feel it slightly, it's normal. And if it's alarmingly bad, please consider the many possible causes to make an improvement.
Mark Schwager - MSINC, www.msinconline.com |
