Injection pressure in injection moulding is the pressure required to push the molten plastic (resin) into the mould cavity (the cavity in which the moulded product is shaped). This pressure is very important to ensure that the resin fills the mould firmly and that the shape and dimensions of the product are accurately made.
Role of injection pressure
The following roles are played in the injection moulding process
Ensuring resin flowability.
Even when the resin is melted, pressure is required to facilitate flow inside the mould. If the pressure is right, the resin can spread smoothly throughout the mould and even complex shapes can be moulded securely.
Dimensional accuracy of products
Insufficient injection pressure means that the resin does not fill the mould, which can lead to short shots. Insufficient pressure can also result in inaccurate dimensions and shape of the moulded product.
Effect on product strength and appearance
Injection pressure also affects the appearance and strength of the moulded product. If the pressure is too high, burrs may appear, and if it is too low, sink marks and voids (cavities) may occur, adversely affecting quality.
Injection pressure can be compared to the force exerted by a syringe when it is pushed from behind.
If this force is too high, the liquid will eject vigorously, but if it is too low, the liquid will eject slowly.
Appropriate setting of injection pressure
The optimum value for injection pressure depends on the shape of the moulded product, the mould design and the type of resin used. In general, if the pressure is too high, the mould and product will be adversely affected, while if it is too low, the product will not be sufficiently moulded. It is therefore important to find the optimum injection pressure by closely adjusting the moulding conditions.
▲Disadvantages of too high injection pressure
(a) Increased load on the mould
Excessively high injection pressure places an excessive load on the mould, which can lead to premature wear and tear of the mould. This risk is particularly high for moulds with complex shapes.
(b) Burring.
If the pressure is too high, the molten resin can leak out of the mould's mating surfaces, increasing the likelihood of burrs (flash). This increases production costs as additional treatment is required in the subsequent process.
(c) Worse weld lines.
When resin is filled at high pressure, the weld line tends to weaken and the strength of the product may be reduced. This is particularly noticeable with fibre-reinforced resins.
(d) Deterioration of resin
If the pressure is too high, there is a risk of excessive shear stress in the resin, resulting in decomposition and degradation of the resin. This can lead to reduced product properties and poor appearance (e.g. burnt or unevenly coloured products).
(e) Generation of gases and bubbles.
When resin is filled rapidly at high pressure, air in the mould may not be expelled properly and gas and air bubbles may become trapped inside the product. This may adversely affect appearance and strength. (gas burning)
▲Disadvantages of too low injection pressure
a) Underfilling.
If the injection pressure is too low, the resin is not spread to all corners of the mould, increasing the likelihood of underfilling (short shots). This can result in chipping and dimensional defects in the product.
(b) Dimensional defects in moulded products
If the pressure is low, the product dimensions may not reach the specified values as the resin does not replenish sufficiently as it cools and shrinks. This results in problems with the accuracy and finish of the product. (Dependence of moulding shrinkage on holding pressure)
(c) Voiding.
If the resin is not compressed sufficiently in the mould due to insufficient pressure, voids may form inside the product. This reduces strength and deteriorates mechanical properties.
(d) Reduced weld line strength.
If the pressure is low, weld lines (weld lines) may form insufficiently and appear as weak areas in the product. This may result in reduced resistance, particularly to impact and tension.
(e) Surface defects.
Low pressure increases the risk of surface sink marks (sink marks). In addition, the surface may become roughened and a uniform finish may not be achieved!
In this light, it is desirable to mould (mass produce) at the minimum injection pressure required, which does not cause moulding defects.
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