Injection moulding is the process of manufacturing highly accurate plastic components such as closures, overcaps and reducing plugs.
Plastic resin is fed into a heated barrel and screw which is then injected at high pressure into a water cooled mould which may consist of various moving parts. Due to the high injection pressures the mould will be held together by the use of a clamping unit. While the plastic is held inside the mould, the plastic will freeze off to the mould shape. After the plastic has cooled sufficiently the mould and clamping unit will open and allow the finished component to be ejected from the mould so that the process can be repeated.
The plastics that can be used in the blowmoulding process include:
PP - Polypropylene
Polypropylene is the one of most common resins used in injection moulding and can be divided into 3 sub categories:
- Homo Polymer - for general use of moulding closures
- Co Polymer (Random) - gives excellent clarity when used for overcaps and when closures are used with active hinges as with 'flip top' closures
- Co Polymer (Block) - these components will generally have poorer clarity and are therefore generally coloured components but offer greater impact resistance
Generally PP is advantageous when being used in conjunction with a dissimilar bottle material ie HDPE / PVC / PETG as it will improve any binding issues between the 2 components.
BDS - Butadiene Styrene Block Copolymer
- This resin can be used where excellent clarity is required
LDPE - Low Density Polyethylene
- Primarily used in moulding reducer plugs as they require being flexible and when being used in conjunction with closures and bottles of a dissimilar plastic
HDPE - High Density Polyethylene
- This material is used when additional moulded components are required ie base cups, as this will improve the recycling of the container as the component may be the same material as the container
- HDPE is also used when closures require more than one part ie moving mechanisms as the dissimilar plastics will aid operation