You can work iteratively with the shrinkage analysis functionality to discover the required mold cavity dimensions.
If you are unsure about how much your part can shrink, run a shrinkage analysis. This analysis reveals how much the software estimates the part will shrink in the X, Y, and Z directions. This value is expressed as a percentage of your required nominal dimensions. The shrinkage analysis also explains how the shrinkage percentage equates to real mold cavity dimensions.
For example, Inventor calculates the (nominal) dimensions of your part as the bounding box that contains the original part dimensions. In the Suggest tab they appear in the Nominal Dimensions column, as shown in the following table. If you do not enter values in the Shrinkage Allowance column because you are unsure what a reasonable shrinkage value is, and you then run a shrinkage analysis, the result is the following values displayed in the Suggest tab:
| Nominal Dimensions | Shrinkage Allowance | Mold Cavity Dimensions | Estimated Shrinkage | Estimated Dimensions | |
|---|---|---|---|---|---|
| X | 81.29 mm | 0 % | 81.29 mm | 0.94 % | 80.52 mm |
| Y | 50.52 mm | 0 % | 50.52 mm | 1.19 % | 49.92 mm |
| Z | 25.40 mm | 0 % | 25.40 mm | 0.41 % | 25.30 mm |
The Mold Cavity Dimensions values are the Nominal Dimensions plus the Shrinkage Allowance percentage, expressed in mm. As you did not enter a shrinkage allowance, the Mold Cavity Dimensions have not changed after the shrinkage analysis. However, after analyzing your part, the software returned an estimated shrinkage percentage shown in the Estimated Shrinkage column. This value represents how much (as a percentage) the software estimates that the part shrinks in each principle direction from the Nominal Dimension value. The software then subtracts that Estimated Shrinkage percentage from the Mold Cavity Dimensions to arrive at the Estimated Dimensions value, expressed in mm. If you accept the shrinkage allowance that is specified, these values are the dimensions of the finished part. If the estimated dimensions are not within an acceptable tolerance of the Nominal Dimensions, modify the Shrinkage Allowance. Then run another iteration of the shrinkage analysis. You can use the Estimated Shrinkage values as the Shrinkage Allowance in this new analysis.
To move into that next iteration of the shrinkage analysis, click OK on the Summary tab while it displays the values shown in the sample table. The values from the Estimated Shrinkage column are displayed in the Shrink Percentage values on the Set tab. If you now rerun the shrinkage analysis from the Suggest tab, the table appears as follows:
| Nominal Dimensions | Shrinkage Allowance | Mold Cavity Dimensions | Estimated Shrinkage | Estimated Dimensions | |
|---|---|---|---|---|---|
| X | 81.29 mm | 0.94 % | 82.06 mm | 1.08 % | 81.17 mm |
| Y | 50.52 mm | 1.19 % | 51.12 mm | 0.78 % | 50.72 mm |
| Z | 25.40 mm | 0.41 % | 25.50 mm | 0.43 % | 25.40 mm |
For this iteration, the Estimated Dimensions of the final product are closer to your Nominal Dimensions. These values can be within your required critical dimensions. If so, you have identified the required size of your mold cavity. If the Estimated Dimensions are still not acceptable, click OK and repeat the process until you achieve Estimated Dimensions that are acceptable.