CNC Machining Acrylic for Rapid Prototyping

Acrylic, chemically known as Polymethyl Methacrylate (PMMA), is an exceptionally popular material for rapid prototyping via CNC machining. It is prized for its optical clarity, excellent stiffness, good chemical resistance, and ease of machining and finishing. It is often used for prototypes requiring transparent parts, lenses, light pipes, signage, and aesthetic models.

I. Types and Colors of Acrylic for CNC Machining

Acrylic is typically supplied in cast or extruded sheets, rods, and tubes.

• Types:

• Less expensive and more readily available in a wide variety of sizes.

• Softer and more prone to melting during machining if not careful.

• Can contain internal stresses that may lead to cracking during machining or when exposed to solvents.

• Excellent for machining: It produces tighter, cleaner chips and is less prone to melting.

• Superior for polishing: It anneals better and achieves a higher optical clarity finish.

• Has higher chemical resistance.

• Cast Acrylic: This is the preferred grade for CNC machining and finishing. It is produced by pouring a liquid monomer into a mold where it cures. This process creates a material that is:

• Extruded Acrylic: Created by forcing heated acrylic through a die. It is:

• Colors: Acrylic is available in a vast spectrum of colors and transparencies.

• Transparent/Clear: The most common type, used for windows, lenses, and displays.

• Tinted Transparent: Colors like smoke, bronze, red, blue, etc., that retain see-through properties.

• Opaque: Solid colors throughout (white, black, red, etc.).

• Translucent: Milky or semi-transparent colors that diffuse light.

• Fluorescent and Metallic: Special effect sheets are also available.

II. Surface Finishing Processes for CNC-Machined Acrylic

CNC machining leaves behind fine tool marks. Acrylic is unique because it can be finished to a perfectly transparent, glass-like surface.

1. Polishing (To Achieve Optical Clarity)

This is the most critical finishing process for clear acrylic. The goal is to remove all tool marks and restore perfect transparency.

• Process:

1. Sanding: Start with wet sanding using progressively finer grits of silicon carbide (SiC) sandpaper. A common progression is 320 -> 400 -> 600 -> 800 -> 1000 -> 1200 -> 1500 -> 2000 grit. Always use water as a lubricant to prevent clogging the paper and washing away debris.

2. Buffing: After the finest sanding, a buffing wheel with a specialized plastic polishing compound (e.g., NOVUS, Tripoli, or a dedicated acrylic polish) is used. This compound contains very fine abrasives that remove the fine scratches from sanding.

3. Final Polishing: A final pass with a clean, soft wheel and a lighter compound (or even a flame polish for edges, see note below) can achieve a water-clear finish.

• Important Notes:

• Flame Polishing: The edge of a clear acrylic part can be polished by briefly passing it through a flame from a torch. This melts the surface layer, making it perfectly clear. This requires high skill to avoid creating bubbles, waves, or burning the material. It is not suitable for large surfaces.

• Chemical Polishing: Dipping acrylic in a heated chemical bath can produce a brilliant finish. However, this is an industrial process involving dangerous chemicals (e.g., dichloroethylene) and is not recommended for typical workshops.

• Heat Management: Sanding and buffing generate friction heat. Too much heat can melt or warp the acrylic. Keep the surface cool and use light pressure.

2. Painting

Acrylic is often painted for decorative effects or to create opaque, colored prototypes.

• Process:

1. Cleaning: Clean the part thoroughly with isopropyl alcohol to remove all fingerprints, oil, and dust.

2. Sanding: Lightly sand the surface with 400-600 grit sandpaper to provide a mechanical "tooth" for the paint to adhere to.

3. Priming: Use a plastic primer spray designed for non-porous surfaces. This is crucial for good adhesion. Apply multiple light, misting coats to avoid runs.

4. Topcoat: Once the primer is dry, apply the color paint. Acrylic lacquer or enamel sprays are ideal. Apply in several light coats.

5. Clear Coat (Optional): A clear protective coat can be applied for added durability and gloss.

• Important Notes:

• Paint Compatibility: NEVER use paints containing strong solvents (like ketones or acetones). These solvents will chemically attack and craze (create tiny cracks in) the acrylic surface. Always test on a scrap piece first.

• Adhesion: Without a proper primer, paint can easily chip or peel off the smooth acrylic surface.

3. Chrome Plating

Achieving a chrome-like finish on acrylic is almost always done using a vacuum metallization process, not traditional electroplating (which is for conductive metals).

• Process (Vacuum Metallization):

1. Part Preparation: The acrylic part must be polished to an extremely high gloss, as any defect will be visible under the metal layer.

2. Applying Base Coat: A special conductive lacquer is applied to the surface to prepare it for the metal layer.

3. The Vacuum Chamber: The part is placed in a vacuum chamber. A small amount of aluminum (or chrome alloy) is heated until it vaporizes.

4. Deposition: In the vacuum, the vaporized metal particles flow evenly and condense onto the surface of the part, creating a thin, mirror-like metallic layer.

5. Applying Top Coat: A clear protective top coat is applied to protect the soft metal layer from abrasion and tarnishing.

• Important Notes:

• It's Not Real Chrome: The result is a thin decorative layer, not a durable electroplated chrome finish.

• Cost and Expertise: This process requires specialized equipment and is always outsourced to finishing specialists.

• Design Limitations: Complex geometries with deep recesses may have uneven coating distribution.

4. Silk Screening (Screen Printing)

This is used to add logos, text, or graphics onto the acrylic surface.

• Process:

1. Surface Preparation: The surface must be clean and free of dust. For optimal adhesion on smooth acrylic, a very light sanding or an adhesion promoter may be used.

2. Ink Selection: Use inks specifically formulated for plastics and outdoor durability if required. UV-curable inks are a common and excellent choice.

3. Printing: The ink is forced through a fine mesh screen onto the part.

4. Curing: The ink is cured. For UV inks, this is done instantly with a UV light source. For other inks, air drying or heat curing is used.

• Important Notes:

• Ink Solvents: Similar to painting, the ink must be free of solvents that attack acrylic. Test first.

• Curing on Clear Acrylic: When printing on transparent acrylic, curing must be controlled to prevent the UV light from curing the ink through the material from behind, which can cause adhesion issues.

• Surface Geometry: Best suited for flat or mildly curved surfaces.

Summary and Selection Guide

General Machining Tip for Acrylic: To achieve the best finish straight from the CNC machining, use sharp, single-flute O-flute carbide cutters. They are designed for plastics and produce a clean shearing cut, minimizing melting and leaving a much smoother surface that requires less post-processing. Always use compressed air instead of coolant to evacuate chips and cool the cut, as coolant can attack the material.