The 3D drawings only give geometrical information but leave key requirements unspecified

Very often, we only have the 3d drawings to work from for the manufacturing of the plastic parts and their molds. However, this is like a shot in the dark, there are many important specifications that may be missing:

  • Key dimensions and their tolerances. We need to choose just the appropriate tolerances for the right engineering fit, too loose will result in underperformance, while too tight will lead to unnecessary cost increase.
  • The aesthetic requirements: type of surface finishes, which area need to be nice looking (and to what level), which area are hidden in the assembly then they are less demanding.

Usually these kind of requirements are specified on the 2D CAD drawings, but many times, our customers do not provide them, either they want to save time, or some customers just are not capable of doing so.

More information of the assembly will help us to define product specifications

Defining all quality requirements is a demanding and time consuming job, so, is there an easier approach?

There certainly is one, that is you provide us full details of the product (assembly), including:

  • What the parts are used for (their application), so we understand what level of aesthetical requirements it should be, what type of surface finish it hsould be, what loads the parts are subjected to, and so forth.
  • How the parts are assembled together, so we will figure out what dimensional and geometric tolerances (like flatness, roundness) are required.

An easy to understand example

Let me give an example here.

2 plastic parts as an example for this post

These 2 parts are somewhat like Lego bricks, they are quite simple in geometry, which make them a great example for this topic.

At first when asking, the customer said he would be happy with the general tolerance of +/-0.10mm (+/-0.004 inch), but we were suspicious that some key dimensions missed their tolerances, because these parts look like Lego bricks and should function in the same way.

What are the matching components and how they are assembled

Just before we were about to start on the molds, we requested again for more information of the full assembly. The customer sent us the drawing of the matching components, they proved our guess is correct, they do resemble Lego bricks, the parts will be inserted into the array of round extrusions on a board:

And they also need to be pressed into a cavity by hands with appropriate pressing forces.

You can send us realĀ  samples of the matching components, or tell us the measurements of key dimensions

You can send us the samples, so we know what the measments are for the key dimensions. It is always adviced to send multiple samples, because the measurements can not be exactly the same between them. There is always an allowance for dimensions in manufacturing.

If you want to save the time and money, you can just send us the drawing, or the pictures and samples, and take measurements by yourself. With the datas you send, we can make inspection gauges just as the same way as the matching components.

We chose the tolerance for key dimension to be +/-0.02mm

Now we know a much tighter tolerance is needed. Since shrinkage always comes with plastic parts, which leads to poorer uniformity in geometry, we will usually choose a looser fit compared to CNC machined metal parts. We made it 29.80 -0.02/-0.06mm (or 29.86 +/-0.02mm). To further prove this is the appropriate tolerance, we made a inspection gauge, with the key dimensions made exactly the same as the real matching components.

from drawing to machined inspection gauge

This is how it looks like when the inspection gauge is CNC machined:

cnc machined inspection gauge for plastic bricksanimation for inspection gauge for plastic bricks

We control the part dimensions by adjusting molding parameters

Maybe you have a question, once the injection mold is made, can we still control the dimensions of the molded parts?

The answer is yes. The part dimensions do not only have to do with the sizes of machined sizes on the molds, but can also be fine-tuned by adjusting molding parameters. The most significant parameters are the mold temperature and the packing pressure/time.

When we are producing the parts, we need to check the dimensions from time to time, and adjust the parameters to make sure the parts are within the tolerances, however this is a quite time consuming process and requires extra attention.

We check the key dimensions with both a caliper and the inspection gauge

When we are injection molding the parts, we first check them with the inspection gauge, see if they can be pressed into the cavities by hands, and be taken out with the right amount of tightness.

We then checked the key dimensions with 15 samples. They are picked out from the 300 or so pieces we produced (that is 1 in every 20 pieces produced). The measurements showed good consistency and they all fall in the tolerances.

using a caliper to check the plastic bricksinspection result for the plastic bricks