Plastic parts in automobiles, electronics, medical, and precision engineering are important and require dimensional accuracy. A little bit of size variation can lead to assembly problems, malfunction, or expensive rework. This shrinkage of plastic is of a natural nature when the molten polymer cools down and becomes solid, and the extent of such shrinkage is different depending on the type of material, fillers, and the conditions of moulding. ASTM D955, standard test method of determining shrinkage of thermoplastics of dimensions (dimensions of molds), provides a sound model for determining and interpreting this behavior.
Understanding Plastic Shrinkage per ASTM D955
ASTM D955 states that the difference between the dimensions in the size of the mould cavity and the dimensions of the moulded part after conditioning is called shrinkage. The standard identifies controlled conditions of molding, specimen geometry, and timing of measurement to achieve consistency. The values of shrinkage determined using ASTM D955 are a guide to experimental outcomes of dimension changes and are necessary in determining calculations of mold designs and tolerance budgets.
Selecting Materials with Predictable Shrinkage
The first and most influential part size issue is material choice. Semi-crystalline polymers like polyethylene, polypropylene, and nylon exhibit higher, more unpredictable shrinkage than amorphous polymers like ABS, polycarbonate, and polystyrene. ASTM D955 shrinkage data allows designers an objective material comparison and to choose a resin with shrinkage characteristics that fit tight dimensional requirements. Reinforced and filled grades normally exhibit less and more directional shrinkage, which is beneficial in dimensional stability.
Applying Correct Shrinkage Allowance in Mold Design
The size of the moulds is made bigger to account for the anticipated shortening during cooling. Standardized data on shrinkage also avoids the problem of under-sizing or over-sizing the parts and saves money on expensive rework of the mould. Directional shrinkage data is of special importance to incorporate into reinforced plastics, where shrinkage varies both with direction (the flow path) and across the flow path.
Maintaining Consistent Molding Conditions
ASTM D955 focuses on the control of molding parameters since the processing conditions have a significant impact on the shrinkage. Polymer relaxation and crystallization depend on melt temperature, mould temperature, injection pressure, and cooling time. Having uniform processing conditions between the testing and production stages will maintain the predictable behaviour of shrinkage, making part dimensions the same across different production batches.
Accounting for Anisotropic Shrinkage
Plastic shrinkage does not normally occur equally in all directions. The molecular orientation due to flow results in parallel and perpendicular shrinkage rates to the flow direction. ASTM D955 permits measuring the shrinkage on both sides and gives essential information on parts that have long flow paths or awkward geometries.
Proper Conditioning Before Measurement
ASTM D955 states that molded specimens are conditioned after a prescribed time, at which they are finally. The process of post-mould shrinkage may last hours or days, particularly in semi-crystalline materials. Premature measurement of parts may give erroneous values of shrinkage and false dimensional forecasts. Adherence to conditioning requirements is used to ensure that measurements are taken of long-term and stable dimensions.
