a cylindrical switch has one or more passages which open onto a lateral cylindrical surface of the switch . the switch is capable of being tightly received in a cylindrical hole in a hot plate and of being selectively orientated so that the passages are angularly in line with or offset from two or more channels in the hot plate which open onto the hole in order to selectively permit , interrupt or divert the flow of molten plastics material between the aforesaid channels . the switch incorporates a circuit for a cooling fluid .