Thursday, July 11, 2013

GATE, SPRUE AND RUNNER: BASIC GUIDE TO WHAT MAKES A PLASTIC KIT



During a chat with a group of model makers recently, it seemed there was some confusion as to what the items that components arrive on are called. So, an explanation…


In injection-moulded plastic kits, a sprue is the passage through which a hot, liquid material - typically polystyrene or polyvinyl chloride - flows into a precision ‘female’ mould, where it solidifies as it cools, to form detailed components that exactly match the shape, contour and detail within the mould.

Once released from the mould, sprue also refers to the actual ‘male’ item, a tube of solid plastic carrying various parts, typically forming a framework or tree, carrying various kit components.


The framework has three elements - gate, sprue, and runner.

A gate is the location at which molten plastic is injected into the mould cavity. You often see it as a small nub or projection - the ‘gate mark’ - at one end of each moulded sprue.

A sprue is the tubular channel through which molten plastic flows, along straight lines and around precisely curved corners.


A runner is the smaller channel that delivers plastic from its parent sprue to an individual part.

The sprue-and-runner system is similar to a mains water supply, where the main underground pipe (the sprue) runs along the street, with smaller pipes (runners) leading to individual buildings.

Most model kits are made from plastic - typically polystyrene or polyvinyl chloride - and it’s usually good practise to detach parts from their runners with a craft knife, razor saw, or other specialist tool, to avoid damaging components, especially small ones.

Once the components have been detached, left-over sprues can come in very handy as source material to heat and stretch, thus making spare parts such as antennas or railings.