ASTM D955 Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics

    ASTM D955 Introduction

    ASTM D955 is based on the basic principle of comparing the dimensions of the mold cavity with those of the specimen and reporting the percentage differences of each dimension. The ASTM D955 test method is widely applicable in terms of quality control, cost-effectiveness, material selection, and compliance with industry standards and regulations. 

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    The ASTM D955 test method provides comparable data based on standard specimens. It can not predict absolute values in actual molded parts with varying flow paths, wall thicknesses, pressure and temperature gradients, and process conditions. Differences in mold shrinkage may also be observed among the three specimen geometries described in the ASTM D955 test method.

    Get Certified ASTM D955 for Uncompromising Quality and Project Success

    ASTM D955 certification proves that manufacturing and laboratories can determine the true thermoplastic shrinkage of products with a standard test specimen and controlled molding conditions. Precision in building the mold is achieved by compliance to ensure that dimensional variations are minimized, with defects being eliminated, such as warping or gaps in the bonding or assembly being misaligned. Certified testing promotes predictability of the product and tooling design, as well as new resin or process change qualification. Following ASTM D955 procedures allows companies to gain a high level of customer confidence, less scrap in the production process, higher consistency of mold performance, and enhanced part-quality checks in controlled and high-tolerance markets.

    Scope of ASTM D955

    • Develops a uniform system of testing the shrinkage of thermoplastic materials under moulding.
    • Applicable to molded test specimens that are made under specified conditions. 
    • Helps in the comparison of materials, optimization of the mold design, and even control of quality. 
    • Helps foresee the dimensional stability of molded parts under the actual manufacturing conditions. 
    • Applied in industries where the accuracy of dimensions and precision are of great importance.

    ASTM D955 Equipment and Sample Preparation

    Specimen PreparationASTM D955 specifies that before moulding, some specimens need special treatment. For instance, thermoplastics must be dried before molding since they absorb moisture
    Specimen DimensionsASTM D955 indicates that injection mold specimens should be molded in two cavity molds with dimensions 2 60 × 60 mm for plaque specimens, 12.7 × 127 mm for bar specimens, and 100mm × 3.2 mm for disc specimens. While compression moulding involves a single-cavity positive mould having cavity dimensions of 60 × 60 mm for a plaque, 12.7 × 127 mm for a bar specimen, and 100 mm × 3.2 mm for a disc specimen.
    Specimen ConditioningConditioning of moulded specimens must be done in the Standard Laboratory Atmosphere at 23 ± 2°C and 50% ± 5 RH.

    Use of ASTM D955

    • Assessment of shrinkage properties of thermoplastic materials in the design of moulds. Projecting part performance, dimensional accuracy, and assembly fit. 
    • Comparison of the shrinkage behaviour between materials, resin batches, or filler types. Enhancing the quality management of injection molding processes. 
    • Helping mould makers change tooling sizes to obtain the correct finished parts.
    • Getting baseline information to use in simulation software and optimizing the process.
    • Mold makers use this to supply their customers with data on which to apply accurate shrinkage allowances to get the desired final part dimensions
    • Material comparison and selection: Allows different thermoplastic resins or grades to be compared in terms of their shrinkage characteristics.
    • Process optimization: Assists in determining the effect of molding parameters on the shrinkage of the parts, including temperature, pressure, cooling rate, and packing.

    Common Challenges and Troubleshooting in ASTM D955

    The difficulties may be troubling because of uneven conditions of molding, wrong cooling speed, wear of molds, or wrong initial measurements. Changes in the material moisture content, melt temperature, or mold temperature may have a substantial impact on the shrinkage outcome. Troubleshooting involves close monitoring of the process, keeping of good mould, calibration with measuring instruments, conditioning of specimens in a uniform way, and repeatability to eliminate errors of operators. The imprecise specimen cooling can result in skewed or directional shrinkage to give erroneous measurements. This is minimized by the following measures in the proper design of molds, homogenous cooling patterns, and adequate cooling period of the part to be ejected.

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    ASTM D955 Testing Technique, Process, and Data Collection

    It is because the testing method is demanding in terms of molding, measuring immediately after ejection, controlled conditioning, and final measurements. Information received entails the dimensions of the cavity to which the mold is going to be cast, the initial and conditioned specimen, the shrinkage in both flow and transverse directions, the molding parameters, resin identification, and environmental factors. Full, mastered data is useful in material qualification, tooling, optimization of processes, and long-term dimensional analyses.

    ASTM D955 Testing Procedure and Requirements

    Test Setup and ExecutionASTM D955 is executed by measuring the length and width of the mold at 0.025 mm at 23°C and 50% relative humidity. The heating cylinder and mold temperature are kept within the material molding standard range. Flow for straightness, laminarity, and melt fracture is measured.

    Analysis Results and Interpretation for ASTM D955

    The results are in percentage of shrinkage or dimensional changes in millimeters. An increase in the value of shrinkage implies an increase in the dimensional change during the cooling process, and this may necessitate countermeasures during moulding or a change in the processing conditions. Plastics with a semi-crystalline structure normally exhibit higher extension compared to amorphous plastics. Interpretation entails comparison of actual shrinkage and anticipated material specifications to ascertain the accuracy of parts, compatibility of molds, and suitability of production. Correct interpretation ensures that there are no assembly failures, less scrap, and a high level of uniformity in the quality of the parts.

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    Problem & The Solution for ASTM D955

    Problem: Varying shrinkage due to inconsistent conditions in the molding, poor measurements of the cavity, improper conditioning of the specimen, or unstable processing of the material.
    Solution: Strict control of all molding parameters, proper verification of the dimensions of the molds, conditioning of specimens as per ASTM requirements, and calibrated measuring instruments. The repetition of measurements and monitoring of all the processing steps will guarantee reliable, repeatable values of shrinkage.

    Updated on September 17, 2026

    Divakar Shukla
    About Author
    Divakar Shukla
    Divakar Shukla is an Electrical and Electronics Engineer specializing in the convergence of embedded systems architecture, industrial automation, and applied artificial intelligence.
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