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SINK MARK in MOULDING

pvT diagram represents the behavior of plastic melt when its temperature and pressure applied varied.

Temperature dependent change in volume - 29% in crystalline and 8% in amorphous-.

Copressibility of melt under pressure is 10-15%.

On falling temperature of melt in the mould decrease in volume is more than the increase in volume on relaxation of pressure.

Therefore void can not be perfectly filled in. Hence sink mark is inevitable.

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

Variation in Wall Thickness causes sink mark.

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