Mold Shrinkage - ASTM D955 Plastic Test Standard

Mold Shrinkage - ASTM D955

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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.

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.

Specimen Type	Shape	Dimensions	Shrinkage Measured
D2	Square Plaque	60x60x2 mm	Flow & Transverse
A	Bar	12.7x127x3.2 mm	Flow
B	Circular Disk	100 mm diameter, 3.2 mm thick	Flow & Transverse

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

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

Similar Standards:

• ISO 294-4

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