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Components can be produced with ribs, varying thickness, and superb surfaces, using almost all thermoplastics materials.
Orientation of molecules and reinforcement occurs during the process.
High pressures, nonuniform polymer shrinkage, and orientation can lead to warpage and sinkage over ribs and bosses.
Warpage is most apparent with crystalline materials and with large, rather flat parts.
In as9100 accredited mouldings, plastics granules are softened 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 sinkage 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 plastics melt must flow from the gates, through the narrow gap between cooled mould surfaces, to the edge of the tool.
As the material flows, the gap becomes narrower as 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.
The higher the pressure and the narrower the flow path, the greater the orientation.
As the gap freezes off, the orientation becomes greater.
Therefore, the orientation at the centre of the component wall is much higher than that at the surface.
For the same reason, orientation is highest near the gates.
The gates should not be in areas that are likely to suffer impact or other stresses.
such as chemical attack.
Technical mouldings.
The maximum practical thickness of components is about 4 mm (0.
16 in.
); above this thickness, cooling time becomes excessive.
The minimum normal thickness for injection moulding is about 1 mm (0.
04 in.
); below this level, the part cools before the tool filled, and orientation is excessive.
Polystyrene drinking glasses, for example, will always split in the direction of flow when squeezed.
The largest readily available presses have about a 27 MN (3000 tonf) clamping force, which restricts part size to about l m2 (10 ft2) or less for more difficult and filled materials.
The flow length of the plastics from any one gate is limited to about 500 mm (20 in.
) with a 3 mm 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 orientation effects.
The strength and modulus values of parts when plastic component moulding as9100 accredited mouldings are limited by the inability of the process to handle reinforcement longer than a few millimeters without breaking the fibers or blocking me injection system.
Although fillers and short fiber reinforcements can be added.
this tends to produce stiffer components having greater resistance to load at elevated temperate but much lower impact resistance.
Some specially formulated materials have been produced that contain glass approximately 10 mm (0.
4 in.
) in length.
These materials can be used to a limited extent with injection moulding, but are better suited to injection/compression processes.
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