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
HOME Contact-to buy this book Things to know- Process Things to know- Polymers Things to know- Part Design Things to know- Mould
Things to know- Machine Injection Moulding Process Screw Plasticating Moulding Key factors influencing proces Shear strain curve
pvt diagram

Thermal stability Balancing melt Flow Balancing Heat Freezing Quality Problems
Three Balances

Moulded-in Stress Shrinkage Inconsistent Dimension Sink-mark Weld-lines
Moulding Problems

CYCLE TIME Plastics Part Design Flow Analysis .. ..
Engineering Basics in Machine Glossary of technical terms- Physics Glossary of technical terms- Polymer process Myth Understanding Mould Cooling Channels Ideal Moulding Shop