1) Do you supply your plastic part supplier with a “blueprint” along with the 3D CAD file at the start of your projects?
2) If so, how many dimensions do you usually show on the print?
3) How many of those dimensions do you have tight tolerances shown on the print?

If you have answered:
1) Yes
2) 10 or more
3) 3 or more

Then you could benefit from this article. This is a common problem that leads to higher costs on some projects; so please let me explain.

To start, I can tell you from more than 20 years in custom plastic injection molding that the injection molding manufacturing process can be very consistent and robust…or erratic and problematic. For those of us who do the manufacturing, having high quality and consistent processes involve making sure we have a combination of the best equipment, the most capable people, the best quality control systems and most of all…making sure our company maintains a strong customer driven “culture”. Now, for the customer (you), the difference between you receiving your parts made from a high quality, consistent and robust manufacturing process…and you receiving your parts from an erratic and problematic manufacturing process… will be your choice of a parts supplier.

For the purpose of this article, first let us take a look at what happens in each possible combination of your “blueprint” specs and your choice of suppliers:

1) Multiple tight tolerance print + high quality supplier – The good news is there won’t be any big quality problems under this scenario…the bad news is that you will have to pay more for your parts. This supplier (like MSINC) will do all the homework for the project up front with you and make sure that your print and tolerances can be met consistently. But, if your requirements will be to perform “in-process” SPC measurements on multiple features, destructive testing…etc…these things take time and cost money which will be calculated into your parts quote for sure.

2) Multiple tight tolerance print + poor quality supplier – The bad news is you will have quality problems…the bad news is that you will have to pay more for your parts…that’s right, only bad news! Bad suppliers usually will not do any of the homework for the project up front with you and will “over promise” capability to meet your print specs. Their manufacturing processes will be different from batch to batch and so will your part quality. However, rest assured your parts quote still had additional quality inspection costs added.

3) Common sense print + high quality supplier – The good news is there won’t be any quality problems under this scenario either…the good news is that your part pricing will be lower…that’s right, only good news! Don’t forget that this supplier still uses all the company tools in place for maintaining consistent high quality production… and it really doesn’t matter if there are five “in-process” SPC measurements or if there is only one!

4) Common sense print + poor quality supplier – The bad news here again is that you will have quality problems…the good news is that you will be paying less for your bad parts!

In summary, to keep your parts costs down don’t make extra work for your supplier by trying to maintain part drawing tolerances that are unreasonable and unnecessary. In fact, focus on functional testing because that is what really matters anyway. Additionally, choose a good supplier for your parts that is willing to warranty that your parts will be consistent.

Mark Schwager – MSINC, www.msinconline.com