Flexible Resin 3D Printing: How Soft Photopolymers Actually Behave
A working explanation of elastomeric resin printing for engineers who need rubber-like parts, not display models. We cover the cure mechanism, the hardness and elongation limits you can expect, and the point where a printed part should be replaced by CNC-machined or molded rubber.
What flexible resin 3D printing changes about the part
Flexible resin 3D printing uses the same light-curing workflow as standard stereolithography, but the photopolymer is formulated with a long-chain oligomer network instead of a stiff, densely crosslinked one. The result is a part that bends, compresses, and recovers instead of cracking. Hardness typically lands between Shore 40A and Shore 90A, which puts these resins in the same neighborhood as soft TPU, silicone gaskets, and rubber bumpers.
The key difference from rigid resin is not just softness. It is strain at break. A standard rigid resin may reach 5–8% elongation before fracture. An elastomeric resin can hold 100–300% depending on grade and wall thickness. That gap decides which parts are printable and which are not.
Users often assume a soft resin part behaves like molded rubber. It does not. Printed flexibility is directional. Layers bond in the Z axis, so tension along the build direction behaves differently from tension across it. Design the load path with that in mind.
These resins also creep. Leave a compressed gasket under load for weeks and it will take a set. For static seals in low-pressure assemblies that is usually fine. For dynamic seals running at speed, it is not.
- 1Hardness rangeShore 40A to 90A, grade dependent
- 2Elongation100–300% at break on most elastomeric grades
- 3AnisotropyZ-axis bonding weakens tension along the build
- 4CreepSustained load causes permanent set over time
How the cure works, layer by layer
An SLA or DLP printer cures resin with UV light, usually 385–405 nm. SLA draws each layer with a laser spot; DLP flashes a full layer through a digital projector mask. Both build the part upside down on a platform that lifts out of a vat, one layer at a time. Typical layer heights run 25–100 μm. Thinner layers give smoother surfaces and better resolution on small features.
The chemistry matters more here than with rigid resin. Elastomeric formulations blend a soft oligomer with a crosslinker and a photoinitiator. UV light breaks the photoinitiator into radicals, which start chain growth and lock the oligomers into a loose network. Fewer crosslink points mean more stretch; more crosslink points mean a harder, stiffer part.
Because the network is loose, flexible parts absorb solvent and moisture differently than rigid ones. Post-curing under UV floods the part with extra radicals and raises hardness, sometimes by 5–10 Shore A. If your drawing calls for Shore 70A, cure the test coupon the same way you will cure production parts.
Oxygen inhibits cure at the surface. That is why flexible prints often feel tacky straight off the platform. A wash in IPA followed by a controlled UV post-cure removes the uncured skin. Skip the wash and the surface stays sticky for days.
Choosing between SLA and DLP for soft parts
SLA produces a finer spot and handles thin walls and small holes better. A 0.1 mm wall in a flexible bellows is realistic on SLA. DLP cures a whole layer in one flash, so it is faster on parts with a large cross-section, but projector pixel size sets the minimum feature, often 35–50 μm per pixel.
For flexible work, resolution is not the only variable. Peel forces matter. Soft resins stick to the film and stretch during separation, so a large flat cross-section pulls harder than a small one. Orienting the part to minimize cross-section per layer reduces peel stress and the risk of layer delamination.
Support placement is trickier than with rigid resin. Soft supports deform under their own weight and can leave witness marks on a rubbery surface. Put supports on faces that will be trimmed or hidden, and keep contact tips small.
If your part is a thin-walled bladder or a fine lattice, SLA is usually the safer route. If it is a chunky bumper or a pad with a thick section, DLP will finish faster with acceptable detail.
- 1SLAFiner features, slower on thick sections
- 2DLPFaster per layer, pixel size limits detail
- 3Peel controlSmaller cross-section per layer, fewer delaminations
- 4SupportsPlace on hidden or trimmed faces
Wall thickness, draft, and holes that print clean
Flexible parts need thicker walls than rigid ones. A 0.8 mm wall in rigid resin is stiff enough to hold shape. In Shore 60A resin, the same wall feels like a wet noodle and may sag during printing. Start at 1.5–2.0 mm for structural walls and go thicker where the part carries load.
Draft angle helps. Vertical walls in soft resin can slump under their own weight as layers stack. A 2–5° draft on tall faces keeps the part from leaning. For tall thin ribs, add a slight taper instead of a constant section.
Holes shrink. Soft resin relaxes after cure, so a printed Ø 5.00 mm hole may measure Ø 4.80–4.90 mm. If the hole is a locating feature, print undersize and ream, or design a slot that tolerates the shift. Threads in flexible resin strip easily; use a metal insert instead.
Sharp internal corners concentrate stress and tear first. A 0.5–1.0 mm fillet at every inside corner spreads the load. This single change removes most field failures we see in printed gaskets and seals.
- 1Minimum wall1.5–2.0 mm for load-bearing sections
- 2Draft2–5° on tall vertical faces
- 3HolesExpect 0.1–0.2 mm shrinkage; ream if critical
- 4Fillets0.5–1.0 mm at internal corners
Where flexible resin stops making sense
Flexible resin is not a rubber replacement across the board. It has poor tear resistance compared with molded silicone or EPDM. A thin edge that sees repeated abrasion will rip. If the part is a wiper blade, a dynamic O-ring, or a seal running against a rotating shaft, molded rubber is the right answer.
Temperature is another boundary. Most elastomeric photopolymers soften above 60–80 °C and lose recovery. Under a hot engine bay or near a heater, the part deforms and stays deformed. Silicone holds to roughly 200 °C and keeps its elasticity.
UV exposure degrades these resins over time. Parts left outdoors yellow and embrittle within months. For indoor use behind a housing, that is irrelevant. For an outdoor gasket, plan on a different material.
Compression set is the quiet failure. A printed pad compressed to 30% for a month may recover only part of its thickness. Measure recovery after the same load and time the part will see in service, not after a two-minute bench test.
- 1Poor tear strengthThin abraded edges rip; use molded rubber
- 2Temperature ceilingSoftens above 60–80 °C
- 3UV agingOutdoor parts yellow and embrittle
- 4Compression setTest recovery under real service load
Flexible resin vs TPU vs molded rubber vs CNC
Match the process to the part, not to the printer you already own.
| Factor | Flexible resin (SLA/DLP) | TPU (FDM) | Molded rubber / silicone | CNC-machined elastomer |
|---|---|---|---|---|
| Typical hardness | Shore 40A–90A | Shore 85A–95A | Shore 20A–80A | Shore 40A–90A |
| Feature detail | 25–100 μm layers | 0.1–0.2 mm nozzle lines | Draft-limited, tooling cost | Tool marks, sharp corners |
| Tear resistance | Low | Medium | High | Medium to high |
| Heat limit | 60–80 °C | 80–100 °C | Up to 200 °C silicone | Material dependent |
| Tooling needed | None | None | Yes, mold and setup | None |
| Best for | Prototypes, small seals, gaskets | Wearables, jigs, grips | Production seals, high cycle | Low-volume dense pads, one-offs |
| Worst for | Dynamic seals, hot zones | Fine lattices, thin walls | One-off prototypes | Thin flexible walls |
When to print, when to machine
Print flexible resin when you need a soft prototype, a low-volume seal, or a complex lattice in days without tooling. Switch to molded silicone or EPDM when the part sees heat above 80 °C, continuous abrasion, or hundreds of thousands of cycles. For dense elastomer pads and one-off fixtures where print resolution is not the point, CNC-machined elastomer gives more consistent hardness and no layer anisotropy.
Questions engineers ask before printing soft parts
Can flexible resin parts be painted or glued?
Painting works poorly. The surface flexes and most paints crack within a few cycles. If you need color, tint the resin before printing or accept the natural amber to gray tone of the cured part.
Gluing is possible with cyanoacrylate, but the joint stays rigid while the part flexes, so it peels. For a permanent bond, design a mechanical interlock or a press-fit insert instead of relying on adhesive.
How do I measure hardness on a printed part?
Use a Shore A durometer on a flat section at least 6 mm thick. Thinner sections read high because the platen bottoms out on the build plate.
Measure at least 24 hours after post-cure, and measure the same orientation you will use in service. Z-axis faces often read 3–5 Shore A harder than XY faces on the same part.
Does post-cure time change the final stiffness?
Yes, and by more than most people expect. Extending UV post-cure from 10 to 30 minutes can raise hardness by 5–10 Shore A on some grades.
Fix the cure recipe before you validate the design, and keep it identical between prototype and production runs. A different cure time makes a different part.
What is the largest flexible part I can print?
It depends on the machine build volume and the peel forces during printing. Large flat cross-sections pull hard when the platform lifts.
For a soft part with a big footprint, split it into sections and bond with a mechanical joint, or orient it so each layer has a small cross-section. A tall thin part prints more reliably than a wide flat one.
Can I use flexible resin for a food-contact or medical part?
Not without checking the specific resin data sheet. Most standard elastomeric photopolymers are not rated for food contact or prolonged skin contact.
Some grades carry biocompatibility ratings for short-term use. Ask for the resin documentation before you design a medical or food-handling part around it.
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