Injection Molding Explained

Almost any plastic part or component can be done efficiently and accurately with injection molding - from parts for kitchen utensils, electronics, automobiles, medical devices and other items too numerous to count.

Simple parts are easily made. But, complex parts can be quickly made using this method, too, because the high pressures of injection molding, force material into every nook and cranny of the tool (mold) This can't be done efficiently with other casting methods. Because injection molding is in fact high pressure casting, that is, adding casting materials under pressure, it is possible to cast a wider range of details into the design of the part over other casting methods

Another major advantage over other types of casting methods is its low-cost of operation during production. In addition, there is typically very little post production work required because the parts usually have a very finished look upon ejection.

The manufacturing  process of injection molding itself is fairly simple. Small plastic pellets are fed into a hopper. The pellets are then crushed and heated into a thick plastic slag. The hot melted slag is fed into a piston which injects the liquid under very high pressure into a metal die. The die is the negative shape of the finished part. As this space is filled, the plastic is cooled and the finished part is ejected with the process repeating thousands of times a day.

Of course with all manufacturing benefits, there are also limitations to injection molding.  The main disadvantage to injection molding is the cost of the machinery The cost could be anywhere from $7,500 to $75,000 and the size of the machine could fit in a 400 square foot garage. Newer machines range in cost from $75,00 to $400,000. There are part size limitations to consider as well. As an example, injection molding equipment can only take on jobs that fit within set parameters if the product is to be one complete piece. Similarly, different designs and materials could limit how thin the product wall will be.

Parts with large undercuts cannot be cast using this method either. The initial cost to create tooling (molds) is very expensive.That is because the tools must be precision machined out of aluminum or steel to hold up to the heat and to long production runs. So, though the production cost is low, in order to amortize the tooling cost, parts production must be high. Thus, injection molding for small productions is often cost prohibitive and the option is eliminated for more cost-effective methods.

However, for prototype work and for short product runs, a cost-effective injection molding process can be used, whereby a two-part or multi-part block mold is created out of silicone rubber and a polyurethane casting resin is then injected under pressure into the mold with a large plastic hypodermic needle. The resulting casting can be extremely thin-walled and the method can be used as proof of concept before expensive tools are ordered.