Standard resin

What standard resin is

Standard resin is a liquid photopolymer: a blend of monomers, oligomers, and a photoinitiator that hardens when light of the right wavelength hits it. An SLA printer cures the resin one thin layer at a time, so parts come out with smooth surfaces and small features that FDM printing cannot reproduce as cleanly.

It is a general-purpose material, made for accuracy and appearance rather than load. Cured parts are rigid and a little stronger than printed PLA in tension, but they stretch little before breaking and crack under sharp impacts.

Typical properties

The ranges span two Formlabs formulations, its earlier standard resins and the current Grey Resin V5, printed at 0.1 mm layers, washed, and fully post-cured. Other brands differ, and parts that are not fully cured are weaker.

Why post-curing matters

Parts leave an SLA printer in a green state: solid, but not fully cured. They are washed in isopropyl alcohol to remove liquid resin, then post-cured with light and heat. That final step is where standard resin gets its strength.

Post-curing raises strength by about a third and stiffness by about a fifth, at the cost of some stretch. Formlabs' earlier standard resins showed a bigger jump, from 38 MPa green to 65 MPa post-cured.

Where standard resin works well

• Visual and presentation prototypes that need a smooth surface close to moulded plastic.
• Small parts with fine detail, such as text, textures, and sharp edges.
• Form and fit checks where accuracy matters more than toughness.
• Jigs and fixtures that see light loads at room temperature.
• Parts that meet water, oils, fuels, or isopropyl alcohol briefly. In Formlabs tests, cured cubes gained under 1% in weight after 24 hours in each.

Limits to design around

• Brittleness. Standard resin stretches only 6 to 13% before breaking, so it is a poor choice for snap fits, hinges, and parts that get dropped.
• Sunlight. Sunlight contains the same 405 nm light used to cure the resin, plus heat. Formlabs notes that parts left in the sun become more brittle and change colour, and advises keeping load-bearing parts out of direct UV for long periods.
• Heat under load. With 1.8 MPa of stress on it, post-cured resin starts to deflect at about 58 °C (136 °F).
• Solvents. Acetone attacks cured resin. A Formlabs test cube of its earlier standard resin cracked in acetone, and the current grey resin gains about 5% in weight after a day in it.
• Food contact. Treat printed resin parts as not food-safe.

Designing parts for standard resin

• Avoid thin pins, tabs, and clips, or make them thicker than usual. Brittle features break during handling and support removal.
• Supports leave small marks where they touch the part, so keep one face free of cosmetic detail to carry them.
• If a part is hollowed, add drain holes so trapped liquid resin can escape.
• Add fillets to internal corners. Sharp corners concentrate stress in a brittle material.
• Use standard resin for looks and fit. For parts that flex or take impact, print in PETG or nylon instead.