Ejector pin marks (mould ejection marks), circular indentations in injection moulded plastic parts. The photo shows ejector pin marks on the rear wheel of ESCI’s Opel Blitz cargo truck. The damaged wheel rim and tire sidewall will have to be repaired with modelling wax or putty.
Thermoplastic parts are injection moulded in two- or multi-part aluminium or steel moulds, into which molten plastic is injected under high pressure (typically between 500 and 2000 bar). Demoulding begins as soon as the finished product has solidified in the mould. For this purpose, the ejector (B) plate of the mould, with the moulded part inside it, is withdrawn hydraulically or electromechanically from the injection (A) plate; in the course of this retrograde movement, the ejector pins located behind the ejector plate penetrate the corresponding holes in the ejector plate, hitting the moulded part and ejecting it.
If possible, ejector pins are placed where they cause the least damage to the moulded part. However, unsightly ejector pin marks cannot be avoided completely, and modellers are well advised to check model parts for pin marks which will have to be removed prior to assembly. Repairing an individual part first is more cost effective than stumbling across and fixing the damage later, on a fully assembled or even painted model.
In the meantime, air pressure ejection systems are being developed which no longer require ejector pins for successful demoulding.