AVAILABILITY

Acrylic is available as either cast or extruded. Many people use cast acrylic in CNC machining applications where it is stable and available in a wide range of sizes and configurations. For the usual stock plastics shapes, cast acrylic is readily available in all sheet sizes, including extra-thick material above 4 inches. It is also possible to source rods and tubes. Extruded acrylic is typically used in sheet applications and is only available in thinner gauges. By comparison, extruded acrylic is lower in cost and more impact resistant but softer than cast.

Many manufacturers create cast acrylic for machined plastic parts as both domestic-grade and import-grade. High-quality import-grade acrylic offers a significantly lower cost than domestic-grade; however, the dimensional stability and stress-cracking resistance of import-grade are lower than domestic-grade. Whether you’re sourcing domestic-grade or import-grade acrylic for your plastic machining needs, the basic cost of either is about 15 -25% higher than Delrin, in the lowest quartile of machined plastics.

Acrylic is also available in a wide range of colors, both opaque and translucent. Since many use this plastic as sheet material for signage, it has the broadest range of hues on the shelf.

Acrylic Clarity and Polishing

One of the primary benefits of choosing acrylic for a machined plastic part is clarity. Available in stock shapes such as rods, sheets, or tubes, this type of plastic is clear and has no tint. It offers light transmission of up to 92 percent.

After CNC machining, this type of plastic may only be translucent. Fortunately, there are post-machining techniques that will restore a plastic machined part’s original transparent quality.

A plastic machine shop can employ various approaches depending on the end product’s geometry and polishing requirements.

• Vapor polishing uses a solvent to polish the surface and works well with irregular outside geometry and internal features such as holes and threads.
• Mechanical buffing uses a cutting compound and suits large, flat acrylic parts well where rolled or rounded edges are desirable. Mechanical buffing is great for displays requiring physical contact.
• Flame polishing directs a hot flame onto the surface of the acrylic machined part to reflow it. A skilled practitioner can produce extremely clear parts with no tool marks. Typically, the corners and edges are rounded since those parts are heat concentrators.
• True optical acrylic machining can create lens-quality finishes on flat surfaces and turned shapes. This method requires specialized optical machining equipment.

ACRYLIC MACHINING

For machining considerations, acrylic has its place in the world of plastics. A harder material than most, it has an excellent ability to hold close tolerances. On smaller dimensions, tolerances of +/- 0.0005 inches are possible and can be even more precise. Finishes directly from a machine tool can be less than 2 micro inches Ra. A 32 Ra-max finish for an entire part is reasonable and should be standard from a machining vendor.

Proper tooling will produce a smooth, burr-free finish, even when the part requires a complex geometrical design. A knowledgeable practitioner can also produce cleanly drilled intersections.

One benefit of the plastic’s clarity is that it allows for visual inspection of the machined part’s internal surfaces. On the ease-of-machining scale (stability, abrasiveness, burr formation), acrylic machining is a 3.7, landing in the lowest one-third of stock shape plastics.

LIMITATIONS AND CONSIDERATIONS

Like all plastics, an acrylic machined part has limitations. Avoid using acrylic parts as structural members since the material is stress-sensitive. Bending stress and over-tightened fasteners can cause cracking.

Similarly, this type of plastic does not wear well. With a maximum working temperature of 150 degrees Fahrenheit, it can easily overheat. Frictional forces for CNC acrylics will quickly degrade the plastic.

Acrylic’s chemical resistance is weak. When exposed, acrylic machined parts will stress crack. Simply wiping an acrylic component with isopropanol will degrade a clear surface over time.

A word of caution: when using acrylic as a liquid manifold, stress cracking can occur over a period of time. But a failure point may not be revealed until the part is in service.

DESIGN SUGGESTIONS

Here are a few acrylic machined part design suggestions:

• Consider minimizing stress risers. Add corner radii where appropriate to control stress.
• For applications that require regular disassembly or have small threads, use staked metal inserts.
• Avoid pipe thread; if required, assemble carefully. Straight threads are preferred.
• Use a torque gauge when tightening fasteners.
• While you may love the look of clear acrylic parts, polishing costs money. Only add it if necessary.
• Ask for precision as necessary.
• Expect 32 Ra max for the entire part.
• Consider bonding of manifolds instead of drilling to reduce footprint size.
• Check for chemical compatibility. Chemical concentrations of trapped liquids may change when a device is not being run.
• Watch for surprise temperature increases, for example, during the high-duty cycle of a valve block.
• Don’t design an acrylic machined part with large structural loading or as a bearing material.
• Visually check for the quality of the machining. There should be no burned surfaces, and threads should be clear and burr-free. Bad surface finishes can cause stress cracking.
• Request post-machine annealing. If the shop cannot anneal, find a plastics specializing machine shop that can.
• Assemble fasteners with care. Acrylic is not aluminum and requires appropriate handling.