February 2007 Design Tip

Sizing: an in-depth examination

There's a Wall Street saying often quoted to those who see no limits to a favorite stock's prospects: "Trees don't grow to the sky." In other words, everything has its limits. And so it is with Protomold's moulding capabilities. We can deliver great parts incredibly fast and at amazing prices, but due to a number of factors related to our existing moulding equipment we do have size limitations. We are, of course, always striving to expand our capabilities.

Until recently, our production was limited to parts cut no more than 50.8mm deep into each mould half. In other words, the depth of a carefully designed part could be a full 101.6mm, but only if the depth of the part were divided equally between the two mould halves. (See Figure 1)

Figure 1

With the addition of new technology, we can now produce parts with a total depth of 152.4mm as long as neither mould half is cut more than 76.2mm deep. (See Figure 2)

Figure 2

Regardless of the depth of the part, its total volume cannot exceed 258,096 cubic mm. The reason is simple: that is the volume of resin our largest press can currently inject into a mould in a single "shot."

The next issue is maximum part outline. Imagine that you sat your part on a flat surface running parallel to the part's parting line. The shadow of the part projected downward onto the surface is the part outline or "projected area." (Light shining through holes in your part don't count toward the projected area.) For parts up to 50.8mm in depth in each mould half, the part outline must fit within a rectangle measuring 190.5mm x 355.6mm. For parts up to 76.2mm in depth in each mould half, the part outline must fit a rectangle measuring 152.4mm x 203.2mm. The reason for this limitation is the size of the raw mould stock we use for moulds of different depths.

The final issue is total mould area. This is the actual area of the opening where the two mould halves meet, and it cannot exceed 48,387 square mm. This limitation is based on the maximum closing force our moulding presses can exert. That force must exceed the injection pressure, typically measured in psi, of the resin multiplied by the total mould area or the press will be unable to hold the mould closed during injection.

To summarise the above data:

Maximum depth per mould half 50.8mm 76.2mm
Maximum part outline 190.5mm x 355.6mm 152.4mm x 203.2mm
Maximum projected part area 48,387mm2 48,387mm2
Maximum part volume 258,096mm3 258,096mm3

Finally, there is the issue of draft. A good rule of thumb is that parts should be drafted one degree for every 25.4mm of depth cut into the mould half. In other words, 25.4mm of depth requires one degree of draft; 50.8mm requires two degrees; 76.2mm of depth gets three degrees. Parts of 12.7mm or less require a minimum of half a degree of draft.