One Formula Drives Shrinkage Compensation

The cavity is always bigger than the part. How much bigger is the whole job.

Every mold dimension starts from the same calculation. You take the target part size and scale it up by the resin’s expected shrinkage.

Mold cavity dimension = Part target dimension ÷ (1 − S), where S is the shrinkage rate as a decimal.

A 100 mm part in a resin with 1.8% shrinkage needs a cavity of 100 ÷ 0.982, or about 101.8 mm. That is the entire trick. There is no second formula hiding behind it.

You can also run it backwards. Measure the cavity and the molded part, then S = (cavity − part) ÷ cavity. T1 sampling works this way. The first parts tell you the real S for that resin, that wall, and that process.

The formula is exact. The input is the problem. Everything below is about getting S right.

Why the Datasheet Range Is a Starting Point, Not an Answer

A datasheet range is a menu, not a measurement.

Suppliers publish shrinkage as a range because the number moves with conditions. ABS might list 0.4–0.7%. The midpoint is a reasonable first pick, but the edges are 0.3 points apart.

On a 100 mm dimension, that spread is 0.3 mm. Many engineering drawings call ±0.05 mm on the same feature. Pick the wrong end of the range and you miss the tolerance six times over.

So treat the range as a screening value. Then narrow it with conditions: packing level, melt and mold temperature, wall thickness, and gate behavior. A shrinkage control reference from a working mold shop shows how these inputs interact in practice.

If the drawing tolerance is tighter than half the datasheet spread, do not guess. Run a trial tool or insert test before cutting production steel. Our sink mark diagnostic flowchart covers the packing-side checks that also set your real shrinkage.

Material Shrinkage Rates You Will Actually Use

Crystalline resins shrink roughly two to five times more than amorphous ones.

The split is structural. Amorphous resins like ABS, PC, and PS only contract thermally. Semi-crystalline resins like PP, PE, and nylon also fold into crystal structures as they cool, which pulls the volume down further.

Two rows deserve a second look. PA66 with 30% glass fiber drops from roughly 1.5–2.0% to 0.3–0.6%, but the shrinkage turns anisotropic. A 15% glass-filled ABS shrinks about 0.18% along flow and 0.35% across it. One number per dimension is no longer enough.

HDPE is the wild card at 2.0–6.0%. Wide range, thick sections, and heavy post-mold contraction make it the resin where buyers most often get surprised. If your part is HDPE and the tolerance is tight, ask one question. How will the supplier prove the rate before you approve steel?

Process Levers That Move Shrinkage After Steel Is Cut

Packing is the only lever that adds material after the cavity is full.

Higher packing pressure and longer holding time push more plastic into the shrinking core. That is why a small gate can sabotage everything. The gate freezes early, the pressure never reaches the thick section, and adding more packing time does nothing.

Temperature works the other way. Hotter melt and hotter molds both raise shrinkage. The part starts cooling from a higher point, so it contracts more on the way down. Cooler molds freeze the skin faster and resist contraction, but they can hurt surface finish and flow.

Wall thickness multiplies all of it. Thicker sections cool slower and shrink more, and crystalline resins keep shrinking after ejection. This is one reason uniform walls matter so much. Our conformal cooling guide explains how even cooling keeps differential shrinkage from turning into warpage.

To see how these levers trade off, try the interactive shrinkage simulator. Watch how resin choice and process settings change the result.

One concrete anchor: ABS typically runs melt at 220–250°C with the mold at 50–80°C, and it must be dried two to four hours at 80–90°C. Wet ABS does not just cause splay. It makes the viscosity drift, and drifting viscosity makes shrinkage drift too.

The Steel-Safe Rule Every Mold Maker Follows

You can always cut steel off. Putting it back costs weeks.

Good mold makers bias the first cut toward safety. For the cavity, they pick the small end of the shrinkage range. For the core, they pick the large end. If the part comes out slightly big, they grind or polish steel away. If it comes out small, the tool needs welding, stress relief, and re-machining.

This is why experienced shops talk about keeping steel safe rather than about hitting the number on the first try. T1 tells the truth. The first sampling run measures the real S for your resin, your wall, and your process, and the tool is adjusted to it.

Resin batches move the number too. Lot-to-lot variation can shift dimensions by 0.02–0.05 mm on a typical part. Tight-tolerance programs sometimes qualify a specific resin lot for the whole run, or at least re-check dimensions after a material change.

The part design decides how much freedom the shop has. Uniform walls, generous draft, and cored-out thick sections are covered in our part design guide. A part that fights physics costs more in tooling adjustments than a part that works with it.

Where Compensation Usually Goes Wrong

Most bad shrinkage jobs share one cause: one number applied everywhere.

A single shrinkage value across all dimensions ignores wall differences. The 1.2 mm wall and the 4 mm boss do not shrink the same amount. Neither do flow direction and cross-flow in glass-filled grades. Uniform scaling of a non-uniform part gives non-uniform dimensions.

Another trap is lifting values from the standard test plaque. ASTM D955 plaques are measured under fixed lab conditions after 24 or 48 hours. Your part is not a plaque. The plaque number is a material index, not your steel allowance.

Post-mold shrinkage bites thick crystalline parts. A PP housing can keep moving for days after ejection as crystals finish growing. Measuring parts at the press and approving them there can mean approving parts that are out of tolerance by the time they reach your dock.

And resin grade matters more than resin family. PA6 at 0.5% and PA66 at 2.5% are both nylon. Swapping grades without re-cutting steel is how a running tool suddenly makes bad parts. The Plastics Industry Association publishes industry references that can help buyers sanity-check material claims.

Gate choice belongs on this list too. Hot runners hold pressure longer and shrink less than cold runners in many cases, which changes the S the tool was cut for. Our hot runner versus cold runner guide walks through the trade-offs.

Four Questions to Ask Your Supplier

The answers tell you whether shrinkage was engineered or guessed.

• Which shrinkage rate did you use for each resin and each major dimension, and why that value within the range?
• Did you use one rate for the whole part, or different rates for flow and cross-flow on filled grades?
• How much steel did you keep safe on the critical dimensions, and where will you adjust after T1?
• Can you show measured shrinkage from the last similar part you ran in this resin?

If the supplier cannot name the rate per dimension, the tool was cut on hope. Walk away or budget for a re-cut.