Short answer: Acrylic can take almost no press-fit interference: about 0.5% of the pin diameter, or roughly 0.001 in. on a 1/4 in. pin, before the hoop stress around the hole passes the 1,500 psi that Acrylite says sustained loads should stay under to avoid crazing. That's smaller than normal laser and sheet tolerances, so press fits in acrylic crack unpredictably. Design a slip fit and solvent-bond or epoxy it, or use a mechanical fastener.
Key facts
- Elongation at break: 4.2% for cast acrylic (Acrylite, ASTM D638).
- Sustained stress limit: Acrylite advises keeping continuous design loads under 1,500 psi (10.4 MPa); crazing can occur well below the 10,000 psi tensile strength.
- Interference budget: about 0.5% of the hole diameter by our estimate: 0.0006 in. at 1/8 in., 0.0013 in. at 1/4 in., 0.0025 in. at 1/2 in.
- Tolerances working against you: laser ±0.005 in., kerf variation, and sheet thickness variation are all larger than that budget.
- Laser edges: carry high stress from cutting (Acrylite), which adds to the press-fit stress.
Why press fits are risky in acrylic
A press fit makes the hole slightly smaller than the part going in. The hole stretches, and the stretched material squeezes back, holding the part. Metal and nylon stretch a lot before failing, so that's fine. Acrylic breaks after about 4% stretch, and it's sensitive to stress that never goes away.
The stress around a round press fit is hoop stress: the ring of material around the hole is pulled in tension all the way around, permanently. Acrylite's data sheet warns that continuous loads far below the breaking strength can cause stress crazing, and recommends keeping them under 1,500 psi. Add a laser-cut edge (which the same data sheet says holds high stress), a cold day, or a wipe with alcohol, and you get crazing and cracks (why acrylic cracks later). A press fit that seems perfect today can split next month.
How much interference can acrylic take?
For a stiff pin (metal, or another acrylic part) in a hole with plenty of material around it, the hoop stress at the hole is roughly the modulus times the interference divided by the diameter, reduced slightly by Poisson's ratio. Using Acrylite's published modulus (400,000 psi) and its 1,500 psi sustained-load limit, that works out to a total (diametral) interference of only about 0.5% of the diameter:
| Pin or hole diameter | Interference that reaches about 1,500 psi |
|---|---|
| 1/8 in. (3 mm) | About 0.0006 in. (0.015 mm) |
| 1/4 in. (6 mm) | About 0.0013 in. (0.03 mm) |
| 1/2 in. (12 mm) | About 0.0025 in. (0.06 mm) |
| 1 in. (25 mm) | About 0.005 in. (0.13 mm) |
This is our engineering estimate from the maker's numbers, not a published spec. Real parts do worse: holes near an edge or with thin walls around them see higher stress, and the stress concentrates at any nick in a cut edge. Flat tabs in slots are a little more forgiving because the slot walls can bend, but slots near an edge split easily.
Why you can't hit those numbers reliably
- Laser tolerance is about ±0.005 in., several times the whole interference budget for a small pin.
- Kerf varies by a few thousandths with thickness and focus (acrylic laser kerf).
- Sheet thickness varies by about 10% on cast acrylic, which matters for tabs in slots (actual sheet thickness).
- Thick cuts taper, so a hole is slightly wider at the top than the bottom.
- Temperature changes the fit. Acrylite says acrylic expands about 3 times as much as metals, so a metal pin in a 1/4 in. hole gains roughly 0.0004 in. of interference if the part cools 50 °F, about a third of the budget.
In practice some parts come out loose, some tight, and some crack. That's why we don't recommend press fits for production parts in acrylic.
What about snap fits?
Snap fits use a hook or latch that bends and springs back. They can work in acrylic only if the arm is long and thin enough that the strain stays small, and only for a few cycles. With a 4% breaking strain and a much lower safe sustained level, short stubby hooks crack. Give the base of the hook a generous radius, since the sharp inside corner concentrates stress. For a lid you open often, use magnets, a hinge, or a friction lip instead.
Better alternatives
- Slip fit plus solvent cement. Size the slot or hole about 0.004 to 0.008 in. larger than the part, then glue it. The joint is strong and carries no built-in stress. See how to glue acrylic.
- Epoxy for metal pins and inserts. A slightly oversized hole and epoxy holds metal in acrylic without hoop stress.
- Mechanical fasteners. Pan-head screws with washers, T-slot nuts, and standoffs (tab and slot design).
- Something soft takes the squeeze. A rubber grommet, O-ring, or nylon bushing in a clearance hole.
- Magnets set into pockets for removable lids and panels.
If you still want a press fit, test it
- Cut a test comb: a row of holes or slots in 0.001 in. steps around your target.
- Use the same sheet the final parts will come from.
- Press the parts in, then wait a few days, ideally through a cold night, and check for crazing or cracks before committing.
- Keep holes at least two diameters from edges.
- Never clean the assembly with alcohol or ammonia.
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Questions
Can I press a bearing into acrylic?
It's risky: bearing fits usually call for more interference than acrylic can hold without crazing. A slip-fit bore with epoxy, or a retaining compound labeled safe for acrylic, is the safer choice.
How do I press fit magnets into acrylic?
Don't press them. Cut the pocket slightly larger than the magnet and hold it with a drop of epoxy, or a little cyanoacrylate glue with good ventilation, since its fumes can fog clear acrylic.
Does cast or extruded handle press fits better?
Cast acrylic is somewhat more resistant to solvent stress cracking, so it's the better of the two, but neither is a forgiving press-fit material.
Does annealing make a press fit safe?
Annealing before assembly removes the stress from cutting, which helps, but it can't remove the hoop stress the press fit itself creates once the pin goes in.