Project Description

Quality inspection report for a precision plastic part

Part description

This is the plastic housing for a pin connector, you can find more of its details here.

Analysis of main difficulties prior to production

Accurate reverse engineering from the original sample

This plastic housing fits with the metal pins, and it is most critical to have the right tightness of fit. Because of that, we need to accurately take the measurements of the plastic housing and metal pins.

  • metal pins of a pin connector picture 2
  • metal pins and plastic housing of a pin connector

What measuring equipments shall we use?

The Coordinate Measuring Machine (CMM) and the 3D scanner are more used for measuring larger components with complex shapes, so they are not the best types of equipment to be used. For this product, we used a video measuring machine (kind of like a profile projector) to do the work. We also need to create the 3D models for the metal pins, so as to get the right fit between the grooves and the metal pins.

video measuring machine reverse engineering
  • image measuring of plastic sample picture 1
  • image measuring of plastic sample picture 2
  • creating 3D model with image measuring

Difficulty in the mold machining

The grooves are only 0.33mm but 5mm deep, so not only the copper electrode need to be machined precisely, but also it needs to withstand the burning when it sparks its way down the path, it should not be burnt smaller and get bent.

drawing for the plastic housing of a pin connector

The succeed of the mold totally depends on choosing the right machines and tool material, but not the personal skills of the mechanics.

We asked several machines shops, only one accepted this job, the others do not want to give it a try. Because if something goes wrong, the whole piece of mold material will be scrapped, there is no way to manually fix it. This is basically a one-shot trial. If it does not succeed, then we will lose money. Our profit on this mold certainly will not be enough to make another mold.

Quality inspection report

Dimensional inspection report

Here is the dimensional inspection report.

  • QC report for the pin connector measurements record
  • the drawing for QC report pin connector

As you may see, we only checked the outside dimensions, these are all not the critical dimensions, so why should we check them?

Well, the outside dimensions are easy to be checked by a caliper, and they reflect the overall dimensional stability of the injection molding process.

In other words, it is quite difficult to check the internal dimensions, but it does not mean we can leave the dimensions unchecked.

Now let’s take a look at 2 dimensions so you can understand better

For dimension #1 (21mm), you will see all the measurements fall in 20.95 to 20.98mm, this means the contraction of plastic is even.

While for dimension #2 (8.31mm), the measurements range from 8.30 to 8.35mm, why it has poorer consistency, even it is smaller than 21mm? That is because some parts are a little bent, so the measurement gets a bit larger (if it is straighter then the reading will be smaller). This is fine for its application, but we need to know it does not get too bad.

Check the tightness of fit

As I have mentioned above, this is the most important thing to check for this part. All the dimensional tolerances designed earlier should lead to the right fit between the housing and the metal pins.

The tightness is checked manually. First, we press it in, then take it out to feel how tight it is. We need to check multiple samples to make sure all of them are good.

Visual inspection

This is to make sure the parts have good appearance and do not have visible injection molding flaws.