The inspection process for injection mold making is not such a straight-forward matter. The problem with precision inspection is that the inspector needs to know and understand the fit and function of the mold components, and few people outside of the actual mold maker truly understand this.
To some this may sound condescending, but it is just a simple matter of experience, not an indication of the skill and ability of the inspector. As precision measurement becomes less subjective and the tools become easier to use, the process has become more reliable.
Old school precision measurement
Most toolmakers pride themselves on their ability to perform highly accurate measurements of the mold components they are manufacturing. Over time, they learn the feel of their personal micrometer, know that their dial indicator reads slightly different from one direction to the other, and can just tell when something is wrong.
This works wonderfully when you are making one-off parts or custom molds that are only worked on by a few toolmakers. It is relatively easy to communicate between two mold makers who understand where to adjust for a discrepancy in order to make the mold operate properly.
This does not work at all when the mold has multiple cavities, such as 16, 32, or 64. What worked on a single cavity quickly becomes a nightmare on high cavitation tools.
Obviously, the need for skilled precision measurement in the hands of toolmakers is still required, but the moldmaking process has become one of specialists and near perfect fit components. This production like process demands that standards be adapted and adhered to in order for everything to function as it should.
It is common to outsource various components, even on a global scale. This requires that everyone use the same standards and that the inspection process is at least similar wherever the parts are manufactured. In some ways it does not matter if the components are made across the shop or across the ocean.
Precision inspection tools
- Precision micrometer
- Dial indicator
- Granite surface plate
- Depth micrometer
- Digital caliper
- Gage pins
- Gage blocks
- Optical comparator
- Sine plates
- Transfer stand or surface gage
- Height gage
- Vision systems
These are a few of the common precision measurement tools used in a plastic injection mold making inspection department. In reality, most of these tools are found throughout the shop and are used daily by the toolmakers.
Having a dedicated, in-house inspector is a very good idea, for several reasons. One, it keeps the mold makers honest and two, it puts everyone on the same page. With an unbiased inspector, the toolmaker should be more alert to quality and obtaining the exact dimension.
The typical problem with an in-house inspector is that of understanding what to measure and what is important. Mold designs often have very close tolerances on meaningless dimensions, such as the secondary vent location or depth on a core pin.
The inspector may not know or understand that the primary land can be within .005 in and work just fine, yet the tolerance on the print may be +- .0002 in. There are many such cases that need to be understood by all in order for the process to succeed.
Over time and with training the dedicated inspector has a positive effect on the quality of work, as well as causing better communication between every department in the shop.
The CNC milling department will need to insure that their work is done accurately, the surface grinder will have to hold the close tolerances required and the EDM operators must make sure their work is done per design. Then, when the mold maker assembles the tool, each component fits perfectly with the other and hand-fitting becomes a very small part of the process.