Resin dimensional checks after curing: reading details and sizes correctly

What the Formlabs documents actually measure, and what they leave out

The Formlabs design specification page for the Form 4 generation holds two data sets you should never merge. The first lists recommended minimum feature sizes in your CAD file, then compares, in a table, the intended value to what Formlabs measured on Form 4 and Form 4B printers using Grey Resin V5 at 50 microns. The second gives typical XY-plane tolerances from a single test model printed on three printers, also in Grey Resin V5, but at 100 microns and post-cured for five minutes at room temperature.

The two protocols share neither layer height nor purpose. One is about whether details survive, the other about size deviation over a length. Formlabs notes you may see slight variations with other Formlabs resins, other printers, and other layer thicknesses. The Form 3 column in the table uses Grey Resin V4, so it mixes resin and hardware rather than isolating the machine.

Nothing in these documents quantifies a general dimensional change between a washed part and a post-cured part. The Form Cure V1 page talks about strength and stability, not dimensions. You have to measure for yourself and treat the Formlabs numbers as context, not as a target your hardware is supposed to hit.

Setting up a reference part and choosing what to measure

The protocol depends on a simple reference part you reprint under identical conditions every time. Include a few clean XY lengths, walls of several thicknesses, one or two holes, raised text, and engraved text. Borrow the feature families Formlabs lists: supported and unsupported walls, vertical wires, embossed detail, engraved detail, clearance between two parts, holes, and drain holes.

Record the CAD dimensions before printing. That is your intent reference. Then fix your measurement states: after washing and complete drying, and after the post-cure selected for the resin. Handle the part with the protection described in the resin manufacturer's safety data sheet and workflow. Never measure a part that is still wet with solvent.

Choose an instrument whose resolution and uncertainty suit the feature. Calipers can measure accessible lengths; they may not reliably measure a tiny raised feature or a narrow hole. Keep the location, direction and method consistent. Record resin/version, layer height, orientation, washing and post-curing, plus temperature when measuring. Repeat readings: a difference near measurement uncertainty does not establish a real dimensional change.

Three separate readings: visible detail, measured size, mechanical behavior

A part can pass one reading and fail the other two. Detail visibility asks whether a raised or engraved feature remains visible. For Grey Resin V5 on Form 4, Formlabs says an embossed detail under 0.1 mm in thickness AND height may not be visible; an engraved detail under 0.15 mm in depth OR width may fuse with the model. Those are design-guide thresholds, not measurements of a part you printed.

Measured size compares a CAD number with a caliper number. The Formlabs table shows the measured value can land below or above intent: a 0.2 mm supported wall measured at 0.17 mm, a 0.4 mm clearance measured at 0.34 mm, a 0.75 mm drain hole measured at 0.91 mm. These are the manufacturer's observations on its own bench, not an offset to apply to your files.

Mechanical behavior is a third category. According to the Form Cure V1 settings page, Grey Resin V5 has a default of five minutes with no heat and an enhanced mechanical properties setting of fifteen minutes at 60 °C. That property gain is not dimensional information. The resin post-processing guide covers the sequence of steps.

Washing and post-curing are variables to lock down, not constants

Formlabs notes that thin walls and vertical wires can absorb IPA during washing, and that extended immersion can deform a part. It recommends limiting immersion time. For an across-state comparison, let the part dry completely before measuring and record wash duration; the page does not quantify how much of any observed deviation is attributable to washing.

Post-curing settings are a condition to record, not an established dimensional cause. The Form Cure V1 page says optimal settings depend on equipment and part geometry, thick bulky parts heat more slowly and may need more time, and longer post-curing is acceptable unless the resin says otherwise.

Your protocol should therefore freeze one recipe per measurement series: same wash duration, same drying, same post-cure program taken from your resin manufacturer's documentation. Changing two parameters between two parts makes the comparison unreadable. For resins intended for biocompatible applications, Formlabs points to the material's own instructions for use; do not carry a setting from one resin over to another.

Applying the XY ranges as an illustration, not as a rule

The XY tolerances Formlabs publishes are read by size band: 1 to 30 mm at plus or minus 0.15 % with a floor of plus or minus 0.02 mm, 31 to 80 mm at plus or minus 0.2 % with a floor of plus or minus 0.06 mm, 81 to 150 mm at plus or minus 0.3 % with a floor of plus or minus 0.15 mm. The floor is a minimum tolerance width, not a uniform 0.02 mm accuracy on every dimension.

As an illustration of the formula only, a 20 mm feature at 0.15 % gives 0.03 mm, which is larger than the 0.02 mm floor, so the illustrative band would be plus or minus 0.03 mm. This is arithmetic, not a measurement, and it guarantees nothing on your machine. It also says nothing about the Z axis, which the source does not cover.

Keep the exact test conditions in mind: Grey Resin V5, three Form 4 printers, 100 microns, five minutes of room-temperature post-cure. If you run a different resin, a Formlabs Form 4 with other settings, or a printer from another brand, these bands only help you place your own results in a rough order of magnitude.

Comparing geometry, printing and post-processing

If the reference part shows a difference between the washed and post-cured states, record it and check whether it repeats on another print. If a deviation is already present between CAD and the washed part, compare geometry and print conditions: Formlabs says walls of 0.2 mm or less may warp under peel forces, horizontal overhangs beyond 5 mm may not form completely, and clearances under 0.4 mm may fuse. These are risks described by that guide, not a certain diagnosis of your defect.

A hole under 0.5 mm may close during printing, according to Formlabs. The guide alone cannot determine whether a particular observed hole should be enlarged or rule out an effect from a later step. Record its CAD diameter, orientation, and measurement state, then compare a new reference part while changing one parameter at a time.

For deviations that persist on clean lengths, the dimensional accuracy page covers general diagnosis. This page stays on the change between post-processing states. Keep your logs: a dated series of measurements on the same reference part is worth more than a correction factor borrowed from someone else's shop.

Sources and limits

• Design specifications for 3D models (Form 4 generation) | Formlabs
• Form Cure V1 time and temperature settings | Formlabs

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.