We’ve posted tips in the past regarding maximum part size, but since our mouldable dimensions have continued to increase, we thought it was time for an update. Some of the figures that follow are simple and absolute; others may vary based on multiple factors. If your design seems to be approaching the limits, the best way to determine whether we can mould it is to submit a 3D CAD model for a free online ProtoQuote®. Knowing the guidelines in advance, however, can help speed up the process.
The simplest and most absolute limit to part size is the total volume of your part, which cannot exceed 967cc. That is the maximum capacity of the barrel from which our largest presses inject resin into a mould. This maximum figure has increased significantly over time as we’ve added larger-capacity presses to our production floor, and the good news is that if you follow standard guidelines for wall thickness, 967cc of resin goes a long way.
This one is also simple: the Protomold process allows parts to be milled to a maximum depth of 100mm from the parting line. So, if the parting line falls exactly halfway between top and bottom, total part height can be up to 200mm.
Mould Area at the Parting Line
Because resin is injected into a mould under pressure, the two halves of a mould must be clamped together during injection to keep them from separating prematurely. The pressure that must be overcome equals injection pressure times projected part area (cross section of the part, at the parting line). Our presses can exert up to 650 tonnes of clamp pressure, which allows a projected part area at the parting line of up to 113,000mm².
Pressure generated by injection is exerted in all directions and does more than try to force open the mould. It also presses outward against the sides of the mould. If that force is exerted over a large enough area and the mould wall is too thin, the pressure of injection can actually bow the walls of the mould. To prevent this, as the part is milled deeper into the mould (increasing the area of mould wall exposed to pressure) the mould wall must be thicker. Thicker mould walls reduce the maximum rectangular outline into which the part must fit. In other words, the taller the part from the parting line, the smaller the maximum outline (as defined in Table 1).
Allowance for Cams
Because side action cams must fit within the allowable mould outline, they also reduce the maximum size of a part. The space a cam requires will vary with the stroke required to create the undercut, but it can be significant. The Protomold software will make appropriate allowances for side actions, but designers should be aware that parts with side actions must fit within a reduced outline.
Part Wall Thickness
Larger parts generally mean longer resin flow paths, which, in turn, require thicker walls; how thick depends on the resin. Minimum wall thickness for a long-fiber-filled resin can be nearly 2.5 times that for a nylon wall. Check out our information on range of thickness for various resins.
For more information on design guidelines, visit www.protomold.co.uk.