The reciprocating screw apparatus is attached to the screw motor drive in the above illustration. The reciprocating screw offers the advantage of being able to inject a smaller percentage of the total shot (amount of melted resin in the barrel). The ram injector must typically inject at least 20% of the total shot while a screw injector can inject as little as 5% of the total shot. Essentially, the screw injector is better suited for producing smaller parts.
The mold is the part of the machine that receives the plastic and shapes it appropriately. (see image, The left end)The mold is cooled constantly to a temperature that allows the resin to solidify and be cool to the touch. The mold plates are held together by hydraulic or mechanical force. The clamping force is defined as the injection pressure multiplied by the total cavity projected area. Each resin compound has a calculated shrinkage value associated with it.
Compared to other forms of plastic parts production, injection molding is a cost effective manufacturing method. It can be used for high production runs, and in fact the more parts that are made from a mold, the more cost-effective it is. Once a mold has been made for a certain product, it can be used again and again to produce the same object with close tolerances. In injection molding, there is little or no change in the manufacturing process between cycles, and so often the plastic part extricated from the mold will need little finishing. Furthermore, because the molten plastic is forced only into the space of the cavity, there is very little material waste apart from joining lines and negligible other amounts. The production of minimal scrap saves further money and is a responsible manufacturing practice. Furthermore, what plastic is wasted and considered scrap can often be melted and recycled. Labor costs for the injection molding process are minimal as there is little need for human interference in the injection process.