CAD Shrink Compensation

Polymer Behavior

Thermal contraction occurs as the long chain molecules of a plastic resin lose kinetic energy and pack more closely together during the solidification process. Cad shrink compensation must account for the fact that semi crystalline materials like polypropylene generally exhibit higher and more complex shrinkage than amorphous resins such as polycarbonate. The rate of cooling also influences the final size, as faster cooling often traps the molecules in a less dense state.

Designers must also consider the orientation of the polymer flow, as shrinkage often differs between the direction of the flow and the transverse direction. This anisotropy can lead to warping if the geometric adjustments do not account for the uneven stresses within the part. A thorough understanding of how specific resins react to temperature changes is the foundation for any successful mold design.

Tooling Adjustment

Software tools used in the engineering process allow for the application of global or local scaling factors to the part geometry to implement the necessary changes. Cad shrink compensation is typically applied by multiplying the part dimensions by a factor such as one point zero zero five for a half percent shrinkage rate. While global scaling is common for simple shapes, complex parts may require non uniform scaling to address specific thick or thin sections.

The tool designer must also account for the thermal expansion of the mold steel itself during operation, although this effect is often much smaller than the polymer contraction. Adjustments are made to the cavity and core dimensions while the runner system and gates remain relatively unchanged in their initial design logic. Precise execution of these modifications ensures that the metal tool produces plastic parts that are consistently within the specified engineering limits.

Manufacturing Tolerance

Validation of the shrinkage assumptions occurs during the first article inspection when the initial parts from the tool are measured against the original design specifications. If the parts are outside the allowed tolerance, the cad shrink compensation may need to be refined and the tool modified accordingly. In some cases, steel safe design practices are used, where the mold is built so that metal can be removed later to enlarge the cavity if the plastic shrinks less than expected.

Adding metal back to a tool is significantly more expensive and difficult, so conservative estimates are often preferred. The final dimensions are also affected by processing parameters such as injection pressure, melt temperature, and hold time. Maintaining tight control over these variables is just as important as the initial geometric adjustment for achieving consistent product quality.

Accurate compensation reduces the need for costly tool rework and speeds up the time to market for new products.