Inspection equipment for plastic injection molding

For most plastic injection molding inspections, digital calipers, micrometers and gage pins are adequate for dimensional readings. However, it is important to treat the inspection of plastic injection molding materials differently than that of metals or other materials.

Unlike metals, some plastic injection molding materials are soft, and some are even considered elastomers. When inspecting these parts, it is critical to not squeeze calipers and micrometers on the part. Squeezing may cause deflection in the part, and make measurements inconsistent. Gage pins may also appear to “go” when they are actually a “no go”, because a thin plastic part stretches. That is why our employee quality training program uses examples of different plastic parts to illustrate the point. By learning the proper technique, our employees make consistent measurements.

Also, one must keep in mind the unique properties of certain plastic injection molding materials. Some plastics, such as nylon, have a high rate of moisture absorption. When dealing with such materials, Rebling Plastics always dries the material before plastic injection molding. This keeps the dimensional stability under control. However, some materials will still absorb enough moisture in normal post-molding humidity conditions to change critical dimensions. For consistency, measurements are always taken in the “dry as molded” conditions, just after plastic injection molding. If there are concerns about dimensional stability, the inspection plan may also call for measurements days later.

Advanced Inspection Equipment

On more complex part designs, measurement with hand instruments proves to be difficult if not impossible. Rebling Plastics often designs custom fixtures and gages to enable measurement of critical part features. In addition, we use thread gages, radius gages, gage blocks, dial indicators, optical comparator, microscopes, surface profilometer, and test equipment such as a torque gages and force gauges to check customer specifications.

In 2005, Rebling Plastics purchased a RAM Optical Sprint 200 CNC Video Inspection and Measurement System. This unit gives us more precise measurement of complex parts and assemblies. By setting up a custom program, a whole series of critical measurements can be taken automatically. The software on this unit creates measurement reports and allows quick repeatable inspection of complex parts. We can easily compare historical data to the most recent production run to detect material or processing issues or even tooling wear.

Each piece of Rebling Plastics’ inspection equipment is regularly calibrated and traceable to the National Institute of Standards and Technology.