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Why your 3D print comes out small
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A printed part cools and shrinks, so it comes out a little smaller than you modeled it, enough to matter for a press fit, a thread or two parts that have to mate. This gives you the scale factor to apply in your slicer so the part finishes at the right size, from a material's typical shrink or, better, from a test part you measured yourself.
Why parts come out small
Plastic is laid down hot and shrinks as it cools, and it keeps shrinking a little as it settles. The result is a part a touch smaller than the model, by a few tenths of a percent for PLA up to over a percent for nylon or polypropylene. For a loose bracket it never matters. For a press fit, a snap, a thread, or two printed parts that must mate, that fraction of a percent is the difference between a part that works and one that does not.
Scale up to cancel it out
The fix is to model, or scale, the part a little larger so that after it shrinks it lands on the size you wanted. If a material shrinks one percent, you scale to about 101 percent, and the shrink brings it back to 100. The exact factor is one divided by one minus the shrink as a fraction. Enter that scale in your slicer, or model at the larger size, and the finished part comes out on target.
Measure, do not trust the chart
Shrink is not a fixed property. It changes with the exact filament, the print and bed temperatures, the part size and even the geometry, so a chart value is only a rough start. The reliable way is to print a known size, a calibration cube or a long test bar, measure it with calipers, and let this work out your real shrink and the scale to correct it. Do that once for a filament and your prints land on size from then on.
Shrink is not always even
Parts often shrink a little differently along the build plate than up the Z axis, and large flat spans shrink more than small features, so a single scale factor is a good average rather than a perfect fix. For critical dimensions, calibrate against a part close in size and shape to the real one, and consider separate scaling per axis if your slicer allows it. For most work, one measured factor gets you well within tolerance.
More MakerKiln tools
- resizePrint Scaling CalculatorResize a model to a target size, or find the scale percent between two sizes.→
- filamentFilament Weight to LengthHow many meters are in a weight of filament, or the weight of a length.→
- steps per mmSteps per mm CalculatorSteps per mm for a belt or lead-screw axis, and E-steps calibration.→
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