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KEY FACTS:
- Transparent and clear available
- Lightweight
- Shatter resistant
- Excellent impact resistance compared to glass
- Weather and UV resistant
- Is recyclable depending on local and regional guidelines
Acrylic (aka, polymethyl methacrylate (PMMA)) is one of the most popular plastics for CNC machined parts, particularly in medical and diagnostic applications. Known as a transparent thermoplastic, it machines well, holds tight tolerances, is relatively low cost, and offers optical clarity if needed.
AVAILABILITY, COST, & AESTHETIC OPTIONS
Acrylic material comes as either cast or extruded. Many use cast acrylic in CNC machining applications because 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.
Cast acrylic for machined plastic parts come as both domestic-grade and import-grade. Import-grade acrylic is significantly lower in cost than domestic-grade. However, be warned 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. See all material costs comparisons here.
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.
EASE OF ACRYLIC MACHINING
For machining considerations, acrylic has its place in the world of plastics. As it's a harder material than most, it has an excellent ability to hold close tolerances. On smaller dimensions, tolerances of +/- 0.0005 inches are possible, but machines can be even more precise than that. Finishes directly from a machine tool can measure less than 2 micro inches Ra. A 32 Ra-max finish for an entire part. For most machining vendors, this should be standard.
Controlled Fluidics' experienced machinists can produce a smooth, burr-free finish, even when the part requires a complex geometrical design with cleanly drilled intersections.
Lastly, 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.
One of the primary benefits of choosing acrylic for a machined plastic part is clarity. Designers have often referred to optical acrylic as "plexiglass" or "acrylic glass" when such specifications are crucial. In its native, tint-free state, its ability to make optical grade components make this material versatile in its use and availability. With raw materials available in stock shapes such as rods, sheets, or tubes, this type of plastic retains that clarity while offering light transmission of up to 92% post manufacturing.
However sometimes after CNC machining, this type of plastic may only be translucent. Fortunately, there are post-machining techniques (e.g. polishing) that will restore a plastic machined part’s original transparent quality.
Controlled Fluidics employs 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.
ABOUT UV RESISTANCE & POLISHING
Although general-purpose grade cast acrylic polished parts will block UV wavelengths up to 360 NM, consider UVT grade for any application requiring transmittance of 280 NM and above. Polishing of acrylic allows for many novel edge lighting approaches, including the possibility of total internal reflection.
LIMITATIONS & CONSIDERATIONS
When deciding on materials, its best to know their pros and cons. And like every material, 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°F, it can easily overheat. Also, frictional forces for CNC acrylics will quickly degrade the plastic.
Moreover, acrylic has a weak chemical resistance. When exposed, acrylic machined parts will stress crack. Simply wiping an acrylic component with isopropanol will degrade a clear surface over time. For a list of which chemicals work against acrylic, visit our chart.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
When designing something using acrylic, we have a few tips:
- 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.
Plastic Machining with Acrylic Plastic transcript