Injection-Molded Part Surface Finish

When designing a part for injection molding, it is important to keep in mind the relationships between surface finish, moldability, cost and manufacturing time.

Figure 1 contains the list of standard surface finishes available through Protomold's injection-molding service, listed in order from lowest to highest cost.

Figure 1: List of available standard finishes

Higher Cost
PM-F0: Non-cosmetic: finish to Protomold discretion
PM-F1: Low-cosmetic: most toolmarks removed
PM-F2: Non-cosmetic: Protomold discretion, EDM finish and/or toolmarks permissible
SPI-C1: 600 grit stone, 10-12 Ra
PM-T1: Protomold texture, SPI-C1 followed by light bead blast
PM-T2: Protomold texture, SPI-C1 followed by medium bead blast
SPI-B1: 600 grit paper, 2-3 Ra
SPI-A2: Grade #2 Diamond Buff, 1-2 Ra

“PM” in the table signifies a surface finish adjusted to fit the rapid injection-molding process, where SPI (The Society of the Plastics Industry) denotes an industry-standard finish.

The photographs shown in Figure 2 illustrate the difference in cosmetic appearance for a few of these options on some example parts.
Figure 2: Surface finish examples.

If the part will not be visible to the end user, you will probably choose to specify either PM-F0 or PM-F1 using the drop-down menus on your ProtoQuote (see sample). But many times your design will require a more cosmetic surface finish. In these cases, there are two key things to keep in mind:

  • Polishing: Smoother part surfaces are achieved using manual mold polishing techniques. Consider a part with tall, thin and curved ribs which need to have an SPI-A2 finish. In this case, you should expect a significant cost increase because it is very time consuming to polish deep, narrow slots in molds. Such lengthy polishing times may also affect the manufacturing time for your parts, potentially impacting Protomold's ability to accept your part(s) for quick turn orders.

  • Texturing: Given the line-of-sight nature of bead blasting, it may not be possible to texture the sides of minimally drafted ribs on a part because the mold surfaces may be inaccessible. In addition, if the walls of your part are textured, it may have an adverse effect on the ability of the part to release from the mold, potentially resulting in unsightly “drag marks.” For these reasons, we recommend that texture be specified only on areas of the part that are drafted at least 3 degrees. In fact, if your part does not have sufficient draft, your ProtoQuote may not even offer these textures as options for you to select. Contact your Customer Service Representative if this ever becomes an issue.