Injection molding is a manufacturing process commonly used for fabricating products from plastic parts and toys to automotive cover components, cell phone cases, water bottles, and containers. Essentially many of the plastic products that we use in every-day life are made by injection molded. It is a quick process to create a mass amount of identical plastic products. The flexibility in shape and size achievable by using of injection molding has consistently widened the boundaries of design in plastics and be allowed substantial alternatives of traditional materials due to the design freedom and light weighting.
Do you know how does it work?
An injection machine is made by three primary components – the feed hopper, the screw and the heated barrel. Plastic granules for the part are fed into the heated barrel by a hopper. Plastic that is placed in the hopper is usually in powder or granular form although some resins, such as silicone rubber, may be a liquid and might not entail heating. The material is then melted utilizing the frictional action of a reciprocating screw accompanied with heater bands. The molten plastic is then injected through a nozzle and into a mold cavity – it may seem easy, however the injection molding is actually a very complex process. While in the mold cavity, the material cools and solidifies to the configuration of the cavity. When the product has hardened, the moveable platen that the mold is mounted opens and the part is ejected using ejector pins.
Injection molding machine sizes can run anywhere from 5 tons of clamping force to over 6000. The higher the tonnage, the larger the machine. As a matter of fact, the injection molding machines are classified based on tonnage, or more precisely the clamping force or pressure.
For instance, a machine that is capable of delivering 110 tons of clamping pressure is rated for 110 tons. This pressure ensures that the mold remains closed during the injection process. Too little or too much pressure can cause product quality issues as well as flashing issues, where excess material surfaces on the edge of the part. The viscosity of the plastic being used is also impacted by pressure. MFI or Melt Flow Index is a measure of the melt of a thermoplastic polymers ease of flow.
The greater the MFI, the higher the required tonnage. Such as, let’s say you have a 4 cavity mold and each part is a 5”x 5” x 0.2” square. First of all you must calculate the projected area of the product. With this injection tooling, the calculation would be 5” x 5” = 25 square inches x 4 cavities = 100 square inches of projected area. Projected area calculation is needed because it is the principal variable that affects clamp tonnage . If we use a 10 MFI Polypropylene, we would usually use a minimum of 2.5 tons per square inch of projected. So for this mold, the 100 square inches of projected area requires an injection machine sized to at least 250 tons.
The clamping unit’s main function is to close and open the injection tooling as well as ejecting the products. Two of the most common types of clamps are the toggle clamps and the hydraulic clamps. Toggle clamps are traversed by hydraulic cylinders. These clamps use mechanical connections to generate greater forces than a hydraulic cylinder of the same size that uses a direct connection.