Injection Molding – Injection Molding Process

During the injection molding process, issues arise with part quality, part cosmetics and part dimensions.  These issues are resolved through the injection molding process parameters and/or modifications with the injection mold. During the initial sampling, once the injection mold is complete, there are a number of problems that can occur.

injection molding process

First, it must be determined if the problem is with the injection mold or with the injection molding process. The injection molding set-up person, processor, or the floor supervisor should have the ability to have a large processing window to be able to produce qualifying samples. One of the most common problems during sampling is the part sticking and not being able to come out of the mold.  This is always caused by the mold itself and must be fixed by the mold maker. Most of the time this is caused by the lack of a good polish on the mold.  This can also be caused by not enough draft or the presence of an undercut. Another issue is lack of venting which will not allow the part to completely fill due to burning on the filling stage. The gating and runners could also add to problems when producing parts.

Once the initial injection mold problems have been resolved, then the injection molding processor must use his knowledge and expertise in producing finished parts.  You must first have the right melt temperature and the material must be dried to the data sheet recommendation.  However, not all materials need to be dried. Next the mold temperature must be set to the recommended setting. The initial injection phase needs to start out on the low end of speed and pressure to avoid part sticking and flashing. The cooling phase is also important.  The time must be set long enough so the part has enough time to set up and is able to be ejected out of the mold and the ejector pins do not poke through the part.

If the injection molded part is coming out of the mold and there are no problems with ejecting it, the next concern is the part cosmetics. The part must be visibly filled, no sink marks, no warpage and no gas burn marks. If one or all of these appear, the settings must be changed. Once a part is cycled in to run acceptable parts, then that setup will be used for future production runs. I will discuss molding defects in more detail for injection molding in future blogs.