Designer’s Guide to Acrylic & Plastics

This guide provides engineers, industrial designers, and architects with the practical information needed to design parts and assemblies for fabrication from acrylic (PMMA) and other plastics. It covers dimensional tolerances, file preparation requirements, design rules for cutting, bending, bonding, and finishing, as well as a comprehensive FAQ and pre-order checklist. Following these guidelines will reduce lead times, avoid costly revisions, and ensure that your finished product meets expectations.

Dimensional Tolerances

Achievable tolerances depend on the fabrication method, material type, and part geometry. The table below lists standard tolerances that PlexiSystem holds without additional cost. Tighter tolerances may be available upon request and may require secondary operations or fixturing.

Minimum Dimensions & Wall Thickness

Minimum wall thickness and feature size depend on the material, fabrication method, and functional requirements. The values below are practical minimums — thinner or smaller features may be possible but require case-by-case evaluation.

File Format Requirements

2D files (flat cutting, engraving)

The preferred format for 2D fabrication files is DXF (AutoCAD 2013 or earlier for maximum compatibility). Accepted alternatives include AI (Adobe Illustrator) and PDF with vector paths.

• Units: millimetres (mm). If your file uses other units, specify clearly in the order notes.
• Scale: 1:1 only. Do not scale drawings.
• Geometry: use polylines, lines, arcs, and circles only. Avoid splines where possible — they increase processing time and may be approximated.
• Layers: separate cutting lines, engraving lines, and fold/bend lines onto distinct layers. Use descriptive layer names (e.g., “CUT”, “ENGRAVE”, “BEND”).
• No duplicate lines: overlapping or duplicate geometry causes double cutting and material damage.
• Closed contours: all cut outlines must be closed polylines. Open paths cannot be processed as cut paths.
• Text: convert all text to outlines / curves before exporting.

3D files (CNC machining, complex assemblies)

For three-dimensional parts, provide STEP (.stp / .step) files. IGES is also accepted but STEP is preferred for its better surface and tolerance data. Include a 2D drawing (PDF) with critical dimensions and tolerances annotated.

Design Rules for Acrylic

Acrylic (PMMA) is an excellent material for transparent and decorative products, but it requires careful design to avoid cracking, crazing, and premature failure. The following rules apply specifically to acrylic; for other materials, consult the Material Guide.

Wall thickness

• Structural applications (load-bearing shelves, enclosures, containers): minimum 3 mm
• Decorative applications (signage, light diffusers, non-load-bearing panels): minimum 2 mm
• Uniform thickness: maintain consistent wall thickness wherever possible. Abrupt changes create stress concentrations.

Corner radii

• Internal corners: minimum 2 mm radius. Sharp internal corners are stress concentrators and will crack under mechanical or thermal load.
• External corners: minimum 0.5 mm radius (deburred). Laser and CNC cutting naturally produce slightly rounded external corners.
• CNC limitation: internal corner radius cannot be smaller than the tool radius (e.g., 3 mm radius minimum for a 6 mm end mill). If sharp internal corners are required, specify “dog-bone” relief cuts.

Screw holes and fasteners

• Thermal expansion clearance: PMMA has a coefficient of linear thermal expansion of approximately 0.07 mm/m/°C. A 1-metre panel experiencing a 30°C temperature swing will expand by ~2.1 mm. All screw holes must be slotted or oversized to accommodate this movement.
• Hole diameter: drill or cut screw holes 1.5–2 mm larger than the screw shaft diameter.
• Edge distance: minimum distance from hole centre to edge = 2.5 × material thickness (e.g., 12.5 mm for 5 mm acrylic).
• Never force screws: use rubber or nylon washers and do not overtighten. Acrylic will crack under concentrated compressive stress.

Slot and tab joints

• Slot width: material thickness + 0.1 mm for a friction fit, or + 0.2 mm for an easy-assembly fit.
• Tab length: minimum 2 × material thickness.
• Corner relief: add a 1 mm radius or 1 mm × 1 mm notch at the base of each tab to prevent stress cracking.

Designing for Bending

Acrylic and most thermoplastics can be line-bent (strip heating) or freely formed (oven heating). The minimum bend radius depends on material type, thickness, and temperature. Bending below the minimum radius causes whitening (crazing) or fracture.

Kerf (V-groove) bending

For materials that cannot be heat-bent (foamed PVC, Dibond) or for achieving sharp 90° folds, a V-shaped groove is cut along the bend line, leaving a thin hinge layer. The groove depth is typically 60–70% of material thickness. This technique produces a crisp fold but reduces strength at the bend line.

Grain direction in extruded acrylic

Extruded PMMA has a preferred grain direction (extrusion direction). Bending across the grain produces better results and lower stress than bending along the grain. When ordering extruded sheet for bending, specify the bend-line orientation relative to the sheet edge so that material can be cut with the correct grain alignment.

Multiple bends and spring-back

When designing parts with multiple bends, account for spring-back — the tendency of the material to partially return toward its original flat shape after bending. Spring-back is typically 2–5° for PMMA at standard bending temperatures. For critical angles, we overbend by the expected spring-back amount and verify with a gauge. Parts with more than three bends should include a tolerance note for cumulative angular deviation.

Designing for Adhesive Bonding

Adhesive bonding (solvent cementing) is the primary joining method for acrylic fabrication. The joint type, bonding area, and surface preparation determine the strength and appearance of the bond. See also Joining Techniques for a full overview of all methods.

Joint types

Minimum bonding area

For solvent-cemented PMMA joints, the minimum recommended bonding area depends on the expected load. As a rule of thumb:

• Decorative / light duty: bonding width ≥ 1 × material thickness
• Structural / medium duty: bonding width ≥ 2 × material thickness
• High load / vibration: bonding width ≥ 3 × material thickness, or use mechanical reinforcement

Surface preparation

• Mating surfaces must be flat and smooth — laser-cut edges work well; saw-cut edges need sanding to 400+ grit.
• Remove protective film at least 10 mm back from the bond line before applying solvent.
• Surfaces must be clean and dry — no fingerprints, dust, or moisture. Use isopropyl alcohol (IPA) for cleaning, then allow to dry fully.
• Apply solvent cement (e.g., Acrifix 1S 0116 for PMMA) using a capillary applicator. Avoid excess — it causes crazing.

Colors & Transparency

Acrylic is available in a vast range of colors, transparency levels, and surface finishes. Understanding the options helps you specify the right material for your design intent.

Opal (diffuse white)

Opal acrylic provides even light diffusion and is widely used in LED signage and lightboxes. Light transmission (LT) ranges from 30% to 70%, depending on the grade. Lower LT gives better uniformity but requires brighter LEDs. Typical specification: “opal PMMA, 3 mm, LT 45%.”

Transparent colored

Colored transparent acrylic filters specific wavelengths while transmitting others. Common uses include architectural features, decorative panels, and lighting filters. RAL color matching is possible for opaque and translucent grades but has limitations for transparent colors — the perceived color depends on viewing angle and backlighting conditions.

Mirror acrylic

Mirror acrylic has a vacuum-deposited reflective coating on the back surface. It is approximately 50% lighter than glass mirrors and shatter-resistant. Available in silver, gold, and various metallic tones. Important: mirror acrylic cannot be heat-bent without damaging the coating. All shaping must be done by cutting only.

Matte and satin finishes

Matte (frosted) acrylic reduces glare and fingerprint visibility. It can be produced by the sheet manufacturer (through-body matte) or achieved as a secondary operation via sandblasting or satin coating. Through-body matte is preferred for applications where the surface may be scratched, as the finish extends through the full thickness.

Technical FAQ

Pre-Order Checklist

Before submitting your design for quotation or production, verify the following items. Completing this checklist reduces back-and-forth communication and speeds up delivery.

1. Material specified: material type (PMMA cast, PMMA extruded, PC, PET-G, etc.), colour code, thickness, and finish.
2. Quantity confirmed: number of identical pieces. If ordering multiple variants, list each variant separately with its quantity.
3. File format correct: DXF for 2D, STEP for 3D, at 1:1 scale in millimetres. All text converted to outlines.
4. Contours closed: all cut paths are closed polylines with no gaps, overlaps, or duplicate lines.
5. Layers labelled: separate layers for cutting, engraving, bending, and reference geometry.
6. Tolerances annotated: critical dimensions marked with required tolerance. If no tolerances are specified, standard fabrication tolerances apply.
7. Corner radii defined: internal corners ≥2 mm radius for acrylic. If CNC-routed, internal corners ≥ tool radius.
8. Thermal expansion considered: screw holes oversized, mounting slots elongated, expansion gaps included for panels >500 mm.
9. Bend lines marked: bend locations, angles, and directions (up/down) clearly indicated on the drawing.
10. Bonding joints designed: joint type specified, mating surfaces identified, bonding area sufficient for expected loads.
11. Surface finish specified: polished edges, flame-polished, matte, or as-cut. Specify per edge or surface if different finishes are needed.
12. Certifications required: food contact, fire rating, ESD, or other standards and certifications needed for the application.