Molding 101
Polymer Viscosity
Properties of Plastics
Injection Molding
Screw and Check Ring
Commonly Used Calculations
Properties of Plastics

Following are some of the important properties of plastics.

a. Density:

Density of a material is the measure of the mass per unit volume and is generally expressed as grams per centimeter cube. Example: Density of General Purpose Polystyrene (GPPS) = 1.06 gms/cc

b. Specific Gravity:

Specific Gravity is the ratio of the density of a material to the density of water. Density of water = 1 gm/cc. Specific gravity of GPPS = (1.06/1) = 1.06 It is a ratio and therefore has no units.

c. Water Absorption:

Water Absorption is the percentage increase in the weight of a material due to absorption of water. Most plastics are hygroscopic (water absorbing). They absorb water and form a chemical bond with it. Example - Nylon, Polyester

This should not be confused with Water Adsorption. Adsorption is attraction of water molecules on the surface of the plastic. The phenomenon is similar to water condensing on a cold soda can. Example: Polyethylene fed into a hopper from a cold silo outside the molding facility

d. Shrinkage:

 Shrinkage is the ratio of the volume of the plastic in the melt to the volume of the plastic in its final useable state. When applied to molding, it is expressed as the ratio of the dimension of the mold to the dimension of the part that is molded. Shrinkage is expressed in inches per inches or in percentage

In case of crystalline materials, shrinkage is not the same in the cross flow and the parallel flow direction. Such materials are called anisotropic materials.


In case of amorphous materials, shrinkage is the same in the cross flow and the parallel flow direction. Such materials are called isotropic materials.

 e. Melt Flow Rate or Melt Flow Index:

 Melt Flow rate (MFR) or Melt Flow Index (MFI) is the amount of material extruded in 10 minutes under certain testing conditions. The plastic is fed into a heated vertical barrel and a known weight is kept on top of the plunger. The amount of plastic that is extruded in 10 minutes is called the MFR or MFI.

MFI and viscosity are inversely related.

High Melt Viscosity = Low MFI

Low Melt Viscosity = High MFI

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