Mold Shrinkage – ASTM D955

This standard covers the measurement of specimen shrinkage for injection and compression molding. Data for mold shrinkage should be used for material comparison. Actual mold shrinkage values are highly dependant on part geometry, mold configuration, and processing conditions.

Mold shrinkage for many materials differs for flow and transverse (or across flow) directions. Flow direction is taken as the direction the molten material is traveling when it exits the gate and enters the mold.

Shrinkage Direction

Measurement Orientation

Three specimen types may be used to determine mold shrinkage, Type A, Type B, and Type D2. Mold shrinkage is expressed as a percent change in dimension of a specimen in relation to mold dimensions.

Mold shrinkage in the flow direction is calculated by:
S =100*(L -L)/L
where L is the length of the test section of the mold cavity and L is the corresponding length of the test specimen after it has cooled.

Mold shrinkage in the transverse direction is calculated by:
S =100*(W -W)/W is the width of the test section of the mold cavity and W is the corresponding width of the test specimen after it has cooled.

Calculating Shrinkage & Cavity Sizing

Shrinkage = (Cavity – Part length)
Cavity dimension
Cavity Dim = Finished part length
(1 – Shrink rate)

 

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