CHANGE IN VOLUME and DENSITY OF MATERIAL

Materials

Specific volume AT 20 degree C

Specific volume AT 200 degree C

% age change

cubic-cm / g

cubic-cm / g

HDPE (crystalline)

1.03

1.33

29 %

PS (amorphous)

0.97

1.05

8%

Density

Density

HDPE (crystalline)

0.97

0.75

22.7%

PS (amorphous)

1.03

0.952

7.8%

This pvT diagram can be used to determine the maximum shot weight of the polymer for a given machine where maximum swept volume is known

Thermoplastic melt is highly compressible. It can be compressed up to 15% under pressure. There for, if increase in volume due to raise in temperature is 29%, then 15% compressed volume can not replace the void created by shrinkage due to falling temperature of melt during follow-up pressure phase in injection moulding. Hence under such condition sink marks are unavoidable. It can be made acceptable by designing the part with out much variation in wall thickness and with out large mass of melt at any region in the part

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