Meaning
Dimensional offsets applied to the cavities and cores of a molding tool during the design phase compensate for the predictable contraction of the polymer as it cools from melt temperature to room temperature. Specifying the correct tool steel allowance ensures that the ejected parts meet the required nominal dimensions and tolerances. This calculation relies on material shrinkage data, part geometry, and processing conditions.
Shrinkage Compensation
Polymers shrink as they cool, with semi-crystalline resins such as polyoxymethylene and polyamide shrinking significantly more than amorphous materials like polycarbonate. The tool steel allowance adds material to the core and subtracts it from the cavity so that the mold is slightly larger than the target part. If this allowance is calculated incorrectly, the finished molded parts will be either too large or too small, requiring expensive re-machining of the steel surfaces.
Machining Strategy
Mold makers often apply a steel-safe approach when calculating these dimensions for critical part tolerances. This strategy involves leaving extra steel on the tool, which results in a smaller molded cavity or a larger molded core. This conservative approach allows the tool to be adjusted by simple grinding or milling if the initial trial molding runs yield parts that are slightly out of specification.
Shrinkage Variance
Material shrinkages are not uniform and vary based on flow direction, gate location, and packing pressure. High-fiber reinforcements reduce shrinkage along the flow direction while increasing it transverse to the flow, creating anisotropic behavior. A precise tool steel allowance must account for these directional differences, using advanced flow simulation to predict how the part will contract in three dimensions during cooling.
Moulders who skip this detailed analysis risk building tools that produce warped or out-of-round parts, necessitating complex and costly tooling rebuilds.
