Knowing how the molding process works is an important factor in ensuring that the final product is produced to the highest possible standard. Being able to discuss the process with a molding engineer can allow both parties to discover why a particular part may not have turned out as they might have expected.
Although molding machines are built and operated on generally the same lines, they can also be produced on a bespoke basis for the manufacturer who may require other parameters or features that are non-standard to be included. This being the case, it is likely that some companies are able to produce items that others are not, due to the differences in their machine type and throughput. Other factors that should be taken into account are the type of product that is being produced, the material that it is made from and the quantity of products required by the customer.
So, what exactly does an injection molding machine consist of and how does it work?
- The main machine unit, comprising the mechanics, electronics and operator panel.
- The hopper where the resin pellets are held prior to being melted.
- The injection unit, which pumps melted resin into the mold cavity.
- Mold holding clamp to hold the mold firmly in place whilst the resin is injected.
It is important to site the machine onto a flat surface, which is made of a stable material such as concrete. The machine must also obviously be under cover and situated close to available power and other utilities.
Resin pellets are fed into the hopper on the top of the machine. They are gravity-fed into a heated section of the machine which contains heated screws that feed materials through the machine, whilst at the same time allowing the pellets to melt – commonly known as “plasticizing”. The molten resin then flows into the injection section of the machine. A screw pushes the molten resin until a cushion is formed between the end of the screw and the injection seal. The pressure that this cushion builds to prior to the seal being opened and resin flowing into the mold depends on the type, size and construction of the mold. It also ensures that only the correct amount of molten resin will be released so as not to overpack the mold, which can lead to poor part strength and shrinkage upon cooling. The pressure from the nozzle is not, however, released too much, as if this were to occur prior to solidification of the resin upon cooling the part would become deformed.
Once the part has cooled and solidified, it is ejected from the mold and the process begins again with another part. The time that this process takes over the production run of a product is called the ‘tooling time’ and this the length of projects is often judged by the amount of tooling time that it will take to produce.