a) What injection molding process parameters affect the part quality the most? D

Question

a) What injection molding process parameters affect the part quality the most? Discuss 2 of these parameters and their specific effects on part quality. Which is the most important for cycle time?

b) Why is it important to have uniform wall thickness for injection molded parts?

c) Your injection molded part has air bubbles, and excessive flash. What’s wrong, and how could you fix it? Give 2 parameter changes.

d) Thermoforming uses preprocessed plastic as the raw material. What does this say about your material cost and scrap?

Explanation / Answer

A) Parameters affecting part quality:

1. Die geometry

Shape of the molded object is determined by die geometry. It determines the internal shear plane and the dead zone of the molding cavity which is important for determining the amount of material wastage per stroke.

2. Molding Temperature

This has to be optimal. As setting a high temperature will increase the fluidity of the molten plastic which wil reduce friction but at the same time will affect the uniformity of wall thickness and rigidity.

3. Molding Speed (Most important for cycle time)

4. Friction & Lubrication

B)

Injection molding is generally used for creating hollow parts for which the inner surface uniformity and wall surface uniformity is required to be constant and uniform.

Injection molding is a mass production process therefore the wall thickness has to be controlled precisely as the whole lot can be affected negatively.

C)

Air bubbles and excessive flash are caused due to more than required gap between the dies where the gutter is present. This can be reduce by reducing the gap between the dies.

But, it has to be optimal so that there is some space for the excessive material to flow to the gutter by forming flash.

D)

Plastic used for thermoforming is generally recycled, therefore the cost of raw material is very low.

This fact also means that there is practically no scrap forming. Whatever scrap forming in one cycle can be re-fed into the system by recycling.

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