Wall thickness decides your cycle, and your unit cost

On most jobs, the press spends more of the cycle waiting for the part to cool than doing anything else. That waiting time is governed by the thickest section in the part, and it does not scale in a straight line: cooling time is roughly proportional to the square of the wall thickness. A part with a 4 mm wall takes on the order of 1.8 times as long to cool as the same part at 3 mm.

Thick does not mean strong

Designers coming from machined metal often add material to add stiffness. In a moulded part, a thick section is a liability. It cools from the outside in, so the centre solidifies last and pulls material away from the surface as it does. That gives you a sink mark on the show face and a void inside, and it lengthens the cycle you are paying for. Stiffness comes from geometry — ribs, gussets, and curvature — not from bulk.

• Keep the nominal wall uniform through the part wherever the function allows.
• Where a wall must change, blend it over a distance of at least three times the wall thickness rather than stepping it.
• Core out heavy sections instead of leaving them solid.
• Set rib thickness at 50 to 60 per cent of the wall it stands on; a thicker rib prints a sink mark on the opposite face.
• Keep rib height under about three times the wall, and put draft on both faces of the rib.

The same rule works in reverse. If a part is running slower than it should, the first question is not press speed or resin temperature. It is where the thick section is, and whether the cooling in the tool reaches it.