A repair putty’s published linear shrinkage figure looks reassuringly precise on a data sheet, right up until it’s applied to a defect shaped nothing like the small, uniform test coupon that figure was actually measured on.
Why a Published Shrinkage Percentage Doesn’t Transfer Directly
Manufacturer linear shrinkage data is typically measured on a small, controlled test casting — uniform thickness, unrestricted cure, standard ambient temperature. Real repair defects rarely match that geometry: a deep, narrow gouge cures differently than a shallow, wide depression, and a repair constrained on multiple sides by surrounding metal behaves differently than an open test coupon that’s free to contract in every direction. Treating a data-sheet shrinkage percentage as a direct prediction of how much a specific repair will recede is a common source of surprise gaps, because the number is real but the geometry it was measured under rarely matches the geometry you’re actually filling.
Calculating an Estimated Gap From Defect Depth
A rough working estimate is still useful, provided you understand what it does and doesn’t account for. Multiply the defect’s fill depth by the published linear shrinkage percentage to get a first-order estimate of how far the surface will recede as the material cures — a 12 mm deep fill with a putty rated at roughly 1.5% linear shrinkage suggests approximately 0.18 mm of recession under unconstrained conditions, a number small enough to matter on a precision-machined surface but negligible on a rough structural patch. This estimate assumes shrinkage occurs primarily perpendicular to the surface, which is a reasonable approximation for a shallow, wide fill but becomes less reliable as defect depth increases relative to its width, since deeper fills develop more complex internal stress patterns that a simple depth-times-percentage calculation doesn’t capture.
Running a Test Coupon Before a Dimensionally Critical Repair
For any repair where the calculated estimate above puts you close to your actual tolerance, don’t rely on the estimate alone — cast a test sample that matches the real defect’s approximate depth, width, and degree of lateral constraint, using the same batch of material and the same cure conditions you’ll use on the actual repair. Measure the sample’s dimensions before cure and again after full cure (not just after it feels solid to the touch), and use that measured figure rather than the data-sheet percentage for your actual repair planning. This single step — building a representative test sample rather than trusting a generic published number — is the difference between a dimensionally predictable repair and a guess that happens to work out or doesn’t.
How Filler Loading and Cure Temperature Shift the Actual Number
Two variables move the real shrinkage figure further from the data-sheet baseline more than any others. Filler loading matters because filler particles themselves don’t contract during cure — only the resin matrix between them does — so a heavily filled structural compound typically shrinks measurably less than a lightly filled or clear version of chemically similar resin, even when both carry a similar nominal rating. Cure temperature matters because accelerating a cure with external heat tends to increase shrinkage rather than reduce it, since volatile components are driven off faster than the polymer network can accommodate the resulting volume change; curing at the lower end of the manufacturer’s recommended ambient range, even though it takes longer, typically produces a more dimensionally predictable result. Email Us with your specific putty grade, defect geometry, and dimensional tolerance, and an Incure applications engineer can help you decide whether a calculated estimate is sufficient or a test coupon is warranted.
When to Skip the Calculation and Simply Overbuild
Not every repair justifies this level of measurement. A cosmetic patch, a non-critical structural fill, or any repair with generous tolerance for a slightly low or high final surface doesn’t need a test coupon — a straightforward overbuild-and-sand-back approach, filling slightly proud of the final surface and grinding back after full cure, handles ordinary shrinkage without the extra measurement step. Reserve the calculation-and-coupon approach for repairs where the final dimension genuinely matters: a machined mating surface, a sealing face, or any repair feeding into a downstream process with tight tolerance requirements.
Documenting Shrinkage Data for Repeat Repairs
If your shop repeats the same class of repair regularly — the same putty grade on a similar defect geometry — keep a record of the measured shrinkage from your test coupons rather than starting the estimation process over each time. A documented, measured shrinkage figure specific to your actual repair geometry and process becomes more reliable than the manufacturer’s data sheet the more repairs you log against it, since it captures your shop’s actual cure conditions and application technique rather than a generic lab measurement. For repairs where dimensional accuracy has to coexist with real mechanical load, our guide to adhesive strength for heavy-duty repairs is worth reviewing alongside your shrinkage data, and where a repair spans a joint with a differential-expansion concern on top of shrinkage, how CTE mismatch causes adhesive bond failure covers that separate but related mechanism. Our companion piece on counteracting cure shrinkage to prevent leaks in putty repairs covers the geometry and pressure techniques specific to sealing applications in more depth.
For help predicting shrinkage behavior on a dimensionally sensitive repair, Contact Our Team.
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