Learn & Understand

Why Plastics Shrink - and Why Not Evenly in Every Direction

A single shrinkage percentage assumes a part shrinks the same amount in every direction. For some materials that's a reasonable approximation - for others, it's a simplification that can leave a "corrected" part still out of tolerance in one specific axis.

Crystalline vs. Amorphous: The Real Reason Shrinkage Varies So Much by Material

Not all thermoplastics cool the same way at a molecular level. Amorphous polymers like PLA cool with their long polymer chains staying in a disordered, tangled arrangement, similar to how they existed in the melt - this produces relatively small, predictable shrinkage as the material simply contracts slightly with reduced thermal motion. Semi-crystalline polymers like nylon behave very differently: as they cool, sections of the polymer chains actively reorganize into tightly packed, ordered crystal structures, and that crystallization process pulls the material inward far more than simple thermal contraction alone would - which is exactly why nylon's typical shrinkage (1.5-3%) dwarfs PLA's (0.2-0.5%) by roughly an order of magnitude.

Why Shrinkage Isn't Always the Same in X, Y, and Z

A single scale-up percentage assumes uniform, "isotropic" shrinkage - equal in every direction. In reality, FDM parts can shrink anisotropically (unevenly by direction), for a layer-structure reason specific to 3D printing: the Z axis is built from discrete, separately-cooled layers bonded together, while X and Y are continuous within each layer. Some materials and print conditions produce measurably more shrinkage along Z than in the X/Y plane as a result, meaning a single "safe" scale-up percentage applied uniformly to all three axes can slightly overcorrect one axis while undercorrecting another, particularly on tall, tight-tolerance parts.

Open-Frame ABS: A Special Case Worth Calling Out

The reference table on the calculator page lists ABS shrinkage as notably higher when printed "open frame" (without an enclosure) - this isn't really a different material property, it's a side effect of uneven cooling. An open, unenclosed printer allows cool room air to hit the outer layers of an ABS print faster than the interior can cool, causing internal stresses and warping that effectively behaves like extra shrinkage and distortion beyond what a fully enclosed, evenly-cooled print of the identical material would show. This is exactly why enclosures are commonly recommended for ABS - the shrinkage compensation number changes not because the plastic itself changed, but because the printing environment did.

When to Go Beyond a Single Scale Percentage

For PLA and most PETG prints, a single uniform scale-up percentage is a reasonable, low-effort correction. For nylon parts with genuinely tight tolerances, or any material printed on a part where the Z-axis dimension is functionally critical (a precise press-fit height, for example), consider measuring a test print's shrinkage separately in the horizontal plane versus the vertical (Z) axis, rather than assuming one percentage figure applies equally to both.