Specifying the right bending radius for acrylic parts is one of the fastest ways to prevent stress whitening, edge cracks, springback surprises, and inconsistent fit during assembly. A clear radius spec also helps a manufacturer choose the correct process window, line-heater width, tooling, and finishing steps, so the first samples are much closer to your target.

At YUCHENGDINGSHANG, we commonly produce acrylic components that require controlled radii through precision line bending and thermoforming, then restore clarity through polishing and surface refinement.

What “Bending Radius” You Should Specify

Acrylic drawings often omit key radius details, which forces assumptions. For consistent results, specify:

• Inside radius at the bend

• Bend angle and whether it is the final angle after cooling

• Material type and thickness

• Bend length and bend location tolerance

• Cosmetic requirement for the bend zone, such as optical clarity, allowable haze, no whitening, no bubbles

• Edge condition at the bend line, such as laser-cut edge, CNC edge, routed V-groove, polished edge

Why inside radius matters most: acrylic failures usually start at the outer surface of the bend because that surface sees the highest tensile strain during bending.

Why Radius Control Is Critical for Acrylic

Acrylic, also called PMMA, is stiff and dimensionally stable but has limited elongation before it cracks under tensile strain. Typical PMMA data sheets report elongation at break around 4 to 5 percent and tensile modulus around 3200 to 3300 MPa under ISO 527.

Because “at break” is not a safe design target, practical bending specifications use a much lower allowable strain to protect appearance and long-term durability.

A Practical Engineering Method: Convert Radius to Surface Strain

A simple and widely used approximation for bending strain is:

• Outer surface strain is approximately thickness divided by two times the inside radius

In symbols: strain ≈ t ÷ 2R where t is sheet thickness and R is inside radius.

This gives you a way to move from “I want a sharp corner” to “what strain will that impose,” and then decide whether the part should be heat-bent, V-grooved, or redesigned.

Recommended strain targets for specification

Use conservative targets unless you will validate with trials:

• Cold bending: keep strain very low, often below 0.3 to 0.5 percent for cosmetic parts

• Line bending with proper heating: you can allow higher strain because the material is softened during forming, but you still want to avoid creating locked-in stress that later crazes or cracks

• Thermoforming: strain depends on draw ratio and forming method, so radii should be tied to forming trials and tooling design

For quality control and material comparison, flexural behavior is commonly evaluated using ASTM D790.

Process Choice Drives the Minimum Feasible Radius

Cold bending, free bending without heat

Cold bending acrylic is highly stress-sensitive. A widely cited fabrication rule for acrylic sheet is:

• Minimum radius about 200 times the sheet thickness to avoid stress cracking in a cold bend

This guideline appears in acrylic fabrication documentation for extruded acrylic sheet.

Cold bending is best treated as an exception for gentle arcs, not as the default for precision parts.

Line bending with controlled heating

With line bending, the bend radius is strongly influenced by:

• Heater width and temperature profile

• Heating time and whether both sides are heated evenly for thicker sheets

• Use of a forming jig to hold angle and radius through cooling

• Any pre-machining such as V-grooving for sharper internal corners

This aligns with how we produce clean geometric forms and controlled radii by managing the heated zone width and temperature, and using V-grooving when ultra-sharp corners are required.

Thermoforming, vacuum or pressure forming

Thermoforming is used for complex 3D shapes and smooth radii across larger surfaces. Forming temperature ranges vary by acrylic type. Published guidance for acrylic sheet shows typical forming ranges such as:

• Cast acrylic sheet: about 340 to 380°F, roughly 171 to 193°C

• Extruded acrylic sheet: about 290 to 320°F, roughly 145 to 160°C

These ranges are stated in acrylic fabrication references.

For curved parts, thermoforming often yields the best optical consistency because the strain is distributed rather than concentrated at a single sharp fold.

A Specification Table You Can Use as a Starting Point

The numbers below are practical starting points for quoting and prototyping. Final values should be validated with a forming trial, especially for tight cosmetic requirements.

Cold bend guidance is based on the 200 times thickness guideline for acrylic sheet. Line-bend ranges reflect common manufacturable outcomes when the heated zone is controlled and a forming jig is used, then confirmed by sample trials.

What to Put on the Drawing to Prevent Misinterpretation

Add these notes directly into the bend feature callout:

1. R inside at final condition after cooling

2. Angle tolerance and whether it is measured inside or outside

3. Allowed whitening level, or explicitly “no whitening allowed” if cosmetic-critical

4. Preferred method if you have one, such as line bending or thermoforming

5. Finish requirement around the bend, such as flame polish not allowed, vapor polish allowed, edge must be optical clear

If your part requires a controlled radius plus a high-clarity edge, it helps to specify whether the part will be polished after forming. Our process commonly includes edge and surface polishing to remove micro-scratches and restore acrylic clarity.

Common Design Mistakes That Force Radius Problems

Calling out “sharp corner” on thick acrylic

A sharp corner implies near-zero radius, which implies extremely high strain. For thick sheets, that either becomes a crack risk or forces a process change such as V-grooving, bonding, or redesign.

Specifying radius but not specifying thickness or material grade

PMMA grades differ. Extruded and cast sheet behave differently in heating response and stress retention. Without the exact sheet spec, radius feasibility and springback prediction become guesswork.

Ignoring cooling and stress relief

Even if a tight radius is achieved, trapped stress can lead to delayed crazing. Clear process notes and acceptance criteria help prevent this.

How We Turn Your Radius Spec Into Stable Production

A stable acrylic bend is not only “bend to angle.” It is process control:

• Set the heated zone width to drive the radius outcome

• Heat uniformly and avoid overheating that can blister or mark the sheet, following best-practice line bending guidance

• Use jigs to hold geometry through cooling

• Apply finishing that matches the visual requirement, including mechanical or vapor polishing when specified

If your project includes line bending plus finishing, you can reference our process page here: Acrylic Heat Bending.

Conclusion

A strong acrylic bending radius specification connects three things: the inside radius value, the material thickness and grade, and the intended forming method. When you translate radius into strain and document cosmetic requirements, you eliminate most bend-related failures before they reach prototyping.

Share your thickness, material callout, bend angle, and inside radius targets, and we can recommend a manufacturable radius window, confirm it with samples, and lock in a repeatable process plan for production.