April 2004 Design Tip

Get a feel for texture

As illustrated in the September 2003 Design Tip, Protomold offers the following two texture options:

PM-T1: SPI-C1 (600 grit stone, 10-12 Ra) followed by light bead blast
PM-T2: SPI-C1 followed by medium bead blast

If you plan to specify either of these textures via the drop-down menus of your ProtoQuote® you need to be aware of the fact that Rapid Injection Moulding requires a different draft angle on vertical faces for each of these texture options: 3 degrees for PM-T1 and 5 degrees for PM-T2.

And also keep in mind that due to the line of sight nature of the mould texturing process, it may not be possible to texture rib-shaped areas of the part design such as those illustrated in Figure 1.

Figure 1 : Example "rib" geometries that cannot be textured.

Figure 2 : Examples of texture problems
caused by variations in part wall

Another thing to remember is the effect part geometry may have on the quality of the desired texture, even if the mould itself is textured perfectly. For example, a wall with greater than nominal thickness will pull away from the textured mould surface during solidification, resulting in an untextured area on the surface of the part. And a wall with less than nominal thickness will tend to adhere more intimately to the textured surface, which can often result in a flat, chalky appearance on the part. Figure 2 illustrates examples.

So there are even more reasons to pay attention to the guidance in the October 2003 Design Tip about using consistent wall thicknesses.

You can visit the Protomold Design Guide for other helpful Rapid Injection Moulding design information.