How To Choose The Correct Injection Mold Surface Finish

The plastic injection mold making process consists of many steps, but in the end, one of the most critical is the injection mold surface finish. The dimensions may be correct and the molding process may be efficient, but if the surface finish is wrong, the part is rejected.

One very common mistake is to waste time and money by having too fine of a surface finish. The cost increases almost exponentially as the surface finish increases.

One reason for this is that designers overlook the finish requirement and so the CAD system automatically defaults to a finish that may or may not be the best for the job.

Considerations for choosing an injection mold surface finish

• Resin requirements

• Lead time

• Budget

• Cosmetic requirements

• Texturing

Plastics respond quite differently to different surface finishes. Some resins will stick to a highly polished diamond surface, others will slip easily off during ejection.

On the other hand, a plastic such as polypropylene requires a slightly rougher finish in order to be processed. ABS, Lexan, Nylon and Peek all have unique reactions to the injection mold surface finish.

The cost of hand polishing deep ribs and highly detailed molding surfaces to achieve an SPI 2 or SPI1 finish can quickly turn a profitable job into a loser, if the quote fails to consider this expense.

Mold designers must take care to include the most effective injection mold surface finish specification in order to meet the demands of the job. It makes no sense to over polish the mold, just to be on the safe side.

Often, the project manager may be able to discuss changing the surface finish in order to keep costs at a minimum. The same tendency to over-compensate in order to be on the safe side can occur from the customer's end as well. For this reason it is well worth the effort to question the surface finish.

If the mold requires a diamond finish, consideration should be taken as to doing this task in-house or having it done by a mold polishing house. These professional polishers are quite good at what they do and doing it quickly.

If the mold is to be textured, there is no need to have a highly polished surface finish. However, keep in mind that milling marks and some grinding lines might still be visible after texturing, so some stoning is required.

Chrome plating is quite common and needs special consideration. Any injection mold surface finish defects present before plating will be visible after plating, and sometimes exaggerated. Care must be taken to have at least as good of a finish as will be required after plating.