September 2003 Design Tip

Start with the right finish!

When designing a part for injection moulding, it is important to keep in mind the relationships between surface finish, mouldability, cost and lead time.

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

PM-F0: Non-cosmetic - finish to Protomold discretion
PM-F1: Low-cosmetic - most toolmarks removed

SPI-C1: 600 grit stone, 10-12 Ra
SPI-B1: 600 grit paper, 2-3 Ra

PM-T1: Protomold texture, SPI-C1 followed by light bead blast
PM-T2: Protomold texture, SPI-C1 followed by medium bead blast

SPI-A2: Grade #2 Diamond Buff, 1-2 Ra
Table 1 - Protomold's standard surface finishes.

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

Figure 1 - Surface finish examples

The photographs shown in Figure 1 illustrate the difference in cosmetic appearance for a few of these options on some example parts.

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 mould 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 moulds. And such lengthy polishing times may also affect the lead time for your parts, potentially making it impossible for Protomold to accept your order for our famous 5-day turn.
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 mould 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 mould, 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 2-3 degrees. And 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).

See the Protomold Design Guide for other helpful design information.