ASTM D955 Shrinkage From Mold Dimensions

ASTM D955 is a standard test method of measuring shrinkage from mold dimensions of thermoplastics & thermosetting polymers and plastics. Mold shrinkage is the contraction of the polymer as it cools after the molding process.

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Overview
Scope, Applications, and Benefits
Test Process
Specifications
Instrumentation
Results and Deliverables

Overview

ASTM D955 is a standard test method for determining mold shrinkage, which is the contraction that a molded plastic part experiences relative to its mold cavity under a set of molding conditions.

This test is vital for understanding the behaviour that occurs as a thermoplastic cools and solidifies during a compression or injection molding process. The test results for mold shrinkage allow for the tailoring of mold designs and processing conditions to ensure that parts are manufactured with exact dimensions. The test is run on samples at both ends and at the middle of a processing range because mold shrinkage can vary with molding conditions.

Scope, Applications, and Benefits

Scope

ASTM D955 includes thermoplastic materials and test pieces made by injection molding and compression molding. ASTM D955 defines how much molded plastics shrink in size compared to the mold cavity, under controlled processing conditions, to see how the material responds to cooling and solidification. ISO standards for this test are ISO 294-4 for thermoplastics and ISO 2577 for thermosetting materials.

Applications

Mold design optimization
Injection molding process control
Compression molding process validation
Dimensional stability evaluation
Material comparison and selection
Quality control in plastic manufacturing
Tooling adjustment and calibration
Research and development of polymer processing

Benefits

Ensures accurate final part dimensions
Helps compensate for material shrinkage
Supports mold cavity design corrections
Improves dimensional consistency
Enhances manufacturing repeatability
Provides standardized shrinkage values
Assists in material comparison

Test Process

Mold Measurement and Specimen Molding

Mold cavity dimensions are measured, and specimens are molded under specified conditions.

Cooling

The molded specimens are allowed to cool for a defined period before further evaluation.

Dimensional Measurement

After 48 hours, the stabilized dimensions of the specimens are measured.

Shrinkage Calculation

Dimensional shrinkage is calculated by comparing the mold cavity dimensions with the final specimen dimensions.

Technical Specifications

Parameter	Details
Applicable Materials	Thermoplastics
Specimen Geometries	Plaque: 5 × 5 × 0.125 in (128 × 128 × 3.2 mm) Disk: 4 × 0.125 in (114 × 3.2 mm) End-gated flame bar: 5 × 0.5 × 0.125 in (128 × 12.7 × 3.2 mm)
Measurement Timing	48 hours after molding
Output Units	Inches per inch, percent (%), or mm per mm

Instrumentation Used for Testing

Injection or compression molding machine
Standardized mold cavities
Precision calipers or micrometers
3-D measuring device
Temperature and process control equipment
Data analysis software

Results and Deliverables

Mold shrinkage values (in/in, %, or mm/mm)
Dimensional comparison report
Process parameter documentation
Material shrinkage evaluation

Frequently Asked Questions

ASTM D955 is standard test method for determining linear shrinkage of molded thermoplastic materials by comparing mold cavity dimensions to the final molded specimen dimensions.

Shrinkage affects dimensional accuracy, part fit, and tooling design. Understanding shrinkage helps manufacturers compensate in mold design to achieve desired final product dimensions.

The standard defines a number of specimen geometries, depending on the material and the desired use: Type A: 12.7 X 127 mm bar. Type B:100 mm diameter disk. Type D2: 60 x 60 mm square plaque.

Material type, filler content, processing temperature, cooling rate, mold temperature, and part geometry significantly influence shrinkage results.

The test measures mold shrinkage, expressed as a percentage of the original mold dimension, indicating dimensional change after cooling and solidification.