Definition: Shrinkage 1: drawback 2: reduction in size

Shrinkage is the end result of thermal expansion within the mold and or molten polymer extrusion cross section.  What that really means is that all tools and most dies are cut oversized so as to allow for the filling and flowing of molten material and upon losing its temperature the molten material shrinks or basically gets smaller in cross sectional shape and size. Actual mold shrinkage values are highly dependent on part geometry, mold configuration, and processing conditions.

In molding this is where we cut the tool oversize so that when packed with material and ejected the part than cools and the resulting measurements are what were predicted.  The material manufactures supply generic data on shrinkage, based on plaques and ideal processing conditions and wall thickness of 0.125" (3.2mm) in ASTM D955 and ISO 294-4is using a 2 mm thick plaque. In addition there are set rules for process conditions to mold the parts.  Though all the preceding are important to note the key is that these parts molded, then measured are molded in what may be a called a loose fit meaning there is nothing to restrict the material from shrinking in the mold cavity that forms it. Think ribs, holes, bosses, side walls etc.

The information taken from the material data sheets is only a guideline and should be used as such.

The other important point to take in this is that materials (parts) well continue to shrink outside the mold for an extended period of time. There is also the fact that materials of the amorphous classification have low shrinkages, while those of the semi-crystalline have high shrinkages and can readily be effected by processing conditions in the mold, namely mold temperature which than effects the crystallization they see, colder  the mold the less the crystallization the less the shrink.  The other extreme is shrinkage in end use, given that the part was molded in a cold tool but the actual use temperature is elevated, at which point semi-crystalline parts shall seek further crystallization and shrink further.

The calculation of shrink in both the ASTM and ISO testing is based on plaques, disc and bars, cut to specified dimensions, with specified gates, molded under certain processing conditions.  The steel is cut and measured to defined dimensions so that it becomes standard and the parts produced after cooled for a proper time frame are measured. By knowing the difference in steel versus finished part size and multiplying by 100 a percentage for shrinkage can be calculated.

The ending shrinkage valves are given in a percentage, based on a formula such as follows:

S_Flow = 100 * (L_M - L_S) / L_M   for flow direction

S_Transverse = 100 * (W_M - W_S) / W_{M M}   in the transverse direction.

L = length            W= width            M= mold              S= plastic part after shrinkage

It is important to note that there may be two shrinkage valves given, one in direction of flow and another transverse direction of flow.  This is again dependent on the material being processed, as certain fillers in material well effect the shrinkage of materials and in what direction they shrink.

SL Silvey

silveysplastics@hotmail.com

360-882-3183