Gate

Where it comes up

• Plastic injection moldingMolten thermoplastic injected into a steel or aluminum mold. Lowest piece price for plastic parts at 5,000+ a year.
• Zinc and aluminum die castingMolten zinc or aluminum injected into steel dies at high pressure. Thin walls and net shape at 2,500+ parts a year.

Related terms

Cold runner

A cold runner is an unheated channel system in an injection mold that carries molten plastic from the sprue to the gates. The runner solidifies with each shot and is ejected with the parts, then scrapped or reground. Cold runner molds cost less than hot runner molds but waste more material.

Hot runner

A hot runner is a heated manifold and nozzle system inside an injection mold that keeps plastic molten all the way to the gates. It eliminates runner scrap and often shortens cycle time, but it adds several thousand to tens of thousands of dollars to the mold, plus controller maintenance.

Knit line

A knit line is a visible line and potential weak spot where two flow fronts of molten plastic meet and fuse inside a mold, usually around holes or bosses. In glass-filled resins, knit line strength can drop to half of the base material or less. Gate location controls where knit lines fall.

Sink mark

A sink mark is a shallow depression on the surface of a molded part over a thick section, rib, or boss, caused by the thick area shrinking as it cools. Keeping ribs at 50–60% of the adjacent wall thickness is the usual design fix, along with coring out thick sections.