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 temperature is involved in each stage of the injection moulding process

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

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

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

Answer 1

(a) The unprocessed plastic, generally in the form of small, rounded, compressed mass, is delivered into the injection mold, and progressed in the direction of the mold from the injection portion. During this operation, the material is liquefied utilising heat and pressure. This melt temperature is significant in each stage of the injection molding process.

Next is the mold temperature which is the final temperature of the plastic mold. This temperature is seen from the nozzle. When nozzle is used, excess precaution is required during the measurement to protect the ejection of the hot plastic.

The liquefied plastic that is in the interior of the mold then starts to cool immediately as it comes in touch with the inside of mold surfaces. During molding, resin temperature must be usually around 260?.

(b) As the plastic cools, it will freeze and harden to the shape of the needed component. But, throughout cooling little reduction in structure of the component will take place. The enveloping of material in the injection stage permits more material to drift into the mold and lessen the shrinkage. Because of the destruction of the internal structure of material, the mold must be blended with raw material in optimum to be used again in the injection molding operation. If the molded product is sparse, usually the mold temperature will be large and big molding machine will be utilised.

(c) Warping: The irreversible buckling of a component which takes place when particular segment of the component shrink quicker compare to rest, due to non-uniform cooling rate.

Sink Marks: When liqufied material is injected into a mold, internal cavity can appear if particular segments harden initially, due to very less injection pressure or varying wall thickness.

(d) The appearance of liquefied material trickling out of the mold cavity and hardening. As soon as the part is removed, a thin layer of material will have materialized on the part along the parting line. The existing material now will occupy these internal cavities as it progresses to chill and shrink. This reduction in internal structure generates impression on the component.