Injection molding for large parts

What counts as a large injection molded part?

There is no industry cutoff. A part is large when it needs one of the bigger machines in a plant: a lot of clamp force to hold the mold shut, a lot of plastic in each shot, and a mold heavy enough to need a crane. Think equipment housings, trays, panels and bins rather than clips and caps.

Large part molding follows the same steps as any other: melt the resin, inject it, pack it, cool it, open the mold. What changes is the scale of the machine, the mold and the handling. We run machines from 90 to 950 tons, so small and large parts run in the same plant on Herriman Boulevard.

How much clamp force does a part need?

Clamp force is measured in tons, and it depends mostly on the part's projected area: the outline you would see looking straight down into the mold. Plastics Technology suggests starting at about 3 tons per square inch of projected area, with real parts needing anywhere from 2 to 10 tons per square inch depending on the material and the part.

So a part with 100 square inches of projected area needs roughly 300 tons as a first estimate. The same article warns against simply using the biggest machine available: too much clamp force can crush vents and damage the mold. The right machine is the one that fits the part, not the largest one.

How big a part fits on a 950 ton machine?

As a rough rule only: at 3 tons per square inch, 950 tons holds about 300 square inches of projected area, a footprint of roughly 17 by 18 inches. An easy-flowing material can go larger; a hard-to-fill engineering resin may need more force per inch. The mold also has to fit between the machine's tie bars and open far enough to release the part. A real answer comes from your file, not from arithmetic.

What is shot size, and why does it matter?

Shot size is the most plastic a machine can inject in one cycle. The part, plus the runner that feeds it, has to fit inside it. Our shot sizes run from 140 to 6,803 grams across our machines; 6,803 grams is about 15 pounds of plastic in one shot. A heavy part that is too much for one machine may still run on another.

How are large molds handled?

Carefully, and with a crane. A mold for a large part can weigh several tons. Our overhead crane system is rated to 10 tons for setting molds, and our tool room builds, modifies and maintains molds on site. We also have hot and cold runners, thermolators, chillers and hydraulic core pulls at the machine, the equipment that controls how a big mold fills, cools and releases its part.

How should I design walls on a large part?

Keep walls even and moderate, and add ribs for stiffness instead of making sections solid. Thick plastic cools slowly, takes longer per cycle and is more likely to sink or warp. Our design work includes Moldflow and stress analysis, which shows how a large part will fill and where it may warp before any steel is cut.

What should I send you for a large part quote?

The part file, the material if you know it, the annual quantity, and whether a mold exists today. If it does, add the mold's size, weight and the tonnage it runs on now. The quote guide explains each item.