Question

A phone housing case is to be manufactured from Polyester Based - Thermoplastic Polyurethanes (TPU) with the manufacturing process of Injection Moulding

**Problem** :

a. Describe how TIME is involved in each stage if the injection moulding process

b. How TIME in the injection moulding process influences the internal structure of the material

c. The defects that could likely be introduced due to TIME in the injection moulding process

d. The mechanical property issues that could be caused if TIME is not correctly controlled during the injection moulding process.

Answer 1

a) There are four main stages in Injection Moulding process and the whole injection moulding process usually takes 2 seconds to 2 minutes depending on the product to be manufactured.

i) Clamping: Both the female half and the male half of the mould has to be clamped before the material is injected into the mould. The molten material is then fed into the mould. Machines which have higher power consumption usually take longer time to clamp.

ii) Injection: The injection unit feeds the material through the heated barrel into the mould.The heat surrounding the barrel and the injection pressure helps in melting the material. The injection time is dynamic and can't be easily predicted as it depends on injection pressure, power and shot volume.

iii) Cooling : The heated material cools down when it comes into the contact of the walls of the mould. The material may shrink slightly during the cooling process. Several thermodynamic parameters like temperature of the material, wall thickness etc. decide the cooling time in the mould.

iv) Ejection: The mould is opened and the Ejection System forces the part out of the mould. The part is then processed further before finishing. The ejection time usually lasts for a few seconds.

b) The cooling time of the material in the mould affects the structure of the part:

* More cooling time is preffered for thicker walls of the part and thinner walls take lesser time.

* More cooling time results in coarse grain structure and fine grain structure results if the cooling time is less.

* The strength of the part increases if the grain structure is fine and ductility increases.

c) The defects which can occur are:

*Warping : The permanent bending of a part that occurs when certain section of the part shrink faster than others, as results if the cooling rate is not uniform.

*Sink Marks : When molten material is injected into a mold, voids can occur if the cooling rate varies at different sectrions.The remaining material will fill these voids as it continues to cool and shrink. This shrinkage causes marks on the part where the material sunk into the void.

* Unfilled sections: If the flow rate or the time taken for the material to flow into the part is slow then there might be unfilled sections in the mould.

*Bubbles: Air might get trapped inside the part and give rise to bubbles if the cooling rate is low.

d) The mechanical property defects which can occure are:

* Coarse Grain structure: If the cooling time is slow then the grain structure will be coarse and the part will possess less mechanical strength.

* Low surface finsh due to insufficient cooling time.

* Burn marks may occur. These are degraded plastic material due to improper injection rate.