Injection moulded parts can be produced with; ribs, varying thickness and good surface finishes using all thermoplastic materials. The orientation of molecules and reinforcement occurs during the process. High pressure, nonuniform polymer shrinkage and orientation can lead to warpage and shrinkage over ribs and bosses. Warpage is most apparent with crystalline materials and with large, flat parts. Methods of controlling these effects are described below.
Plastic granules are softened during injection moulding and forced under pressure into a cold mould through small orifices or gates. Pressure is maintained on the material after injection is complete so as to reduce shrinkage of the ribs and bosses as the material cools. Pressure is higher at the gates because it will not transfer effectively through the compressible and rapidly cooling melt. The additional packing pressure leads to a higher density of material near the gates and causes internal stresses. These stresses tend to be partially relieved when the part is removed from the tool, resulting in warpage.
The plastic melt must flow from the gates, through the narrow gap between cooled mould surfaces, to the edge of the tool. The gap becomes narrower as the material flows because some of the melt solidifies at the mould surface. The pressure, flow rate and distance between the mould faces must be great enough, and the material viscosity low enough, to fill the mould before the solidifying material closes off the flow path. For each material and part thickness, there is a maximum practical flow length from a gate.
High pressures and narrow flow paths increase the orientation, which becomes greater as the gap freezes off. Therefore, the orientation at the centre of the part wall is much higher than at the surface. For the same reason, orientation is highest near the gates. The gates should not be areas that are likely to suffer impact or other stresses, such as chemical attack.
The maximum practical thickness of the part is about four millimetres. Above this thickness, cooling time becomes excessive. The minimum normal thickness for injection moulding is about one millimetre. Below this level, the party cools before the tool is filled and orientation is excessive.
The largest readily available injection moulding machines have a 3000 tonf clamping force, which restricts part size to about one cubic metre or less, for more difficult and filled materials. The flow length of the plastics from any one gate is limited to about 500mm with a 3mm wall thickness. Therefore, multiple gates must be used for large parts. Gate design and position are very important for reducing part warpage and add to the complexity of the orientation effects.
The surface finish of injected moulded parts replicates the mould surface as it cools in contact with the surface, except over ribs and bosses. Part design must be aimed at keeping ribs and bosses away form the back side of visible surfaces, reducing material in the rib root. With filled or reinforced materials, the surface tends to be dull shows flow marks.
Cycle times vary from less than a minute to five minutes. Injection moulding is the most useful thermoplastic processing method. However, there are size limitations and a tendency towards warpage in flat parts. Shrin
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