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PMMA (Acrylic) Overview
Polymethyl methacrylate (PMMA) is a durable, highly machinable thermoplastic with relatively low density. Due to its distinct properties, this transparent amorphous thermoplastic homopolymer is widely embraced across industries. Thanks to its glass-like structure and UV stability, acrylic exhibits good weatherability. It is suitable for specific outdoor applications requiring transparency and physical aesthetics, such as lighting fixtures, signage, and display products. PMMA retains its remarkable attributes even after reshaping and melting.
Product manufacturers and engineers often use PMMA as an ideal alternative to Polycarbonate (PC) and glass because it is cost-effective. Compared to other transparent plastics, machined PMMA parts exhibit improved resistance to wear and tear and do not discolor after extended exposure to sunlight. Since PMMA is tough, easy to handle, and machine, it is well suited for machining even complex parts.
Casting and extruding are the standard PMMA manufacturing methods that make the material compatible with CNC machining applications. However, your preferred manufacturing method and intended product application will determine the ideal choice between cast and extruded acrylic PMMA. Cast acrylic exhibits less brittleness than extruded ones since casting causes less internal stress. Hence, cast acrylic may be more suitable for CNC machining processes.
Properties of Acrylic PMMA
Here are some of the material properties that make PMMA a widely embraced machining raw material:
- Chemical Resistance/Stability: Acrylic exhibits exceptional resistance to chemicals like organic solvents, most acids, and bases. Hence, it is suitable for applications with chemical exposure.
- Transparency: Optical transparency is one of PMMA/acrylic’s exceptional properties. It boasts about 92% light transmission, which makes it an ideal substitute for glass.
- Thermal Insulation: Although PMMA is an alternative to glass, it exhibits better thermal insulation properties. It provides energy-saving benefits in applications such as construction.
- Machinability: PMMA is easy to handle and compatible with CNC machining processes (milling, turning, drilling, engraving, etc.), allowing the creation and customization of various precise machined parts.
- Low Weight: PMMA’s relatively low density makes it an ideal alternative to glass in various applications that require lightweight components, such as automotive, aerospace, and construction.
- UV Radiation Resistance: This CNC plastic exhibits excellent weather resistance, maintaining its physical properties after extended exposure to UV light and other weather conditions.
- Mechanical Strength: Acrylic/PMMA boasts good impact resistance, making it less likely to shatter or break than glass.
Common PMMA Machining Methods
Depending on the customers’ requirements and intended product design, PMMA is compatible with different machining techniques. Typical PMMA machining techniques include:
CNC Turning
CNC acrylic turning involves cutting material from a rounded or cylindrical blank. In this technique, the cutting tool moves in a straight line over the workpiece’s surface, working its way around and removing excess material to achieve the desired shape. Standard CNC PMMA turning capabilities include thread cutting, boring facing, and grooving.
CNC Milling
This CNC machining process ensures excellent chip control and gradually removes PMMA material to achieve the desired design. The CNC machine tool moves along multiple axes to deliver shallow, deep, or peripheral cuts onto the stationary workpiece’s surface. CNC milling PMMA sheets allows for the creation of plastic parts with intricate geometries. Operators often use cutting tools with edge rake angles and rotating multi-points to attain the best outcomes.
CNC Drilling
This CNC drilling process uses multipoint drill bits to create cylindrical holes of varying depths and diameters in PMMA blocks or sheets. These tools are perpendicularly fed into the PMMA material’s surface to make vertical holes that match the bits’ diameters. Tapping, reaming, counterboring, and countersinking are typical CNC drilling capabilities. It is worth mentioning that CNC drilling sometimes offers less-than-ideal finishes even though it is efficient and straightforward.
CNC Engraving
Custom manufacturers sometimes need to personalize or add certain details to machined PMMA parts. CNC engraving is the proper technique to achieve this feature. This precision machining technology employs computer-controlled tools to mechanically create artwork, patterns, or symbols onto PMMA materials. Although it requires expensive machinery, it is efficient and widely embraced in both commercial settings and the art world. Precision CNC machines allow the engraving of complex patterns into acrylic workpieces.
Industry Applications of PMMA Machined Parts
Various industries employ PMMA as an ideal raw material for several machined components due to its superior finish and optical clarity, amongst other attributes. Here are typical applications of PMMA material:
- Automotive: Acrylic exhibits surface hardness and optical characteristics, making it a compatible material for automotive applications. Automotive manufacturers use PMMA for different applications, including interior and exterior trim, car windows, motorcycle windshields, taillights, headlights, and other lighting components.
- Construction: Thanks to its UV radiation and high impact resistance, acrylic is compatible with construction applications such as canopies, façade designs, building doors, window glazing, and skylights.
- Medical Devices: The biocompatibility, low toxicity, and reliability of PMMA material make it a widely used material in medical parts CNC machining. Manufacturers in the medical industry use PMMA for components such as contact lenses, incubators, dental cavity fillings, prosthetics, fasteners, bone cement, and artificial joints.
- Displays and Signage: PMMA plastic polymer’s transparency and resistance to dust, moisture, and damage make it suitable for producing point-of-sale displays, as well as other retail display types. The high transparency or optical clarity properties of PMMA make it suitable for finding signs and other signage.
- Lighting: Manufacturers in different industries, including automotive and lighting, use PMMA for light diffusers, lenses, and other lighting components due to its excellent optical clarity.
Surface Finish Options for CNC Machined PMMA Parts
The excellent transparency of acrylic PMMA prevents the need for the surface finishing of machined PMMA components to maintain visibility. However, these post-processing operations improve machined acrylic parts’ mechanical properties and functionality, having achieved the required precision and shape with CNC machining techniques.
Surface finish options compatible with machined acrylic parts include:
As-Machined Finish
The as-machined finish is the default finish achieved directly from the CNC machining process. This finish is typically rough and may show visible tool marks. While it may not be suitable for applications where appearance is crucial, it can be sufficient for functional prototypes or parts that will undergo additional processing.
Chemical Polishing
Chemical polishing involves using a chemical solution to dissolve the surface of the PMMA part, resulting in a smooth, glossy finish. This finish is ideal for applications requiring a high-luster finish, such as in optical components or display parts.
Sanded Finish
A sanded finish involves sanding the PMMA part to smooth out the surface and remove any tool marks. This finish is available in various grits, ranging from coarse (80-100 grit) to fine (220-240 grit). A sanded finish is ideal for applications where a smooth surface is necessary. Examples include medical devices or optical components.
Laser Polishing
Laser polishing uses a high-precision laser beam to melt and resolidify the surface of the PMMA part, achieving a smooth, polished finish. This finish is perfect for applications where a high-precision finish is necessary, such as in medical devices or microfluidic components.
Helpful Consideration for PMMA CNC Machining
PMMA CNC machining starts with a 3D model creation using computer-aided design (CAD) software. This CAD file guides the CNC machine’s operation. Then, the workpiece is loaded into the CNC machine and shaped into different shapes and sizes using machine tools like end mills and drills guided by a computer program. We’ll explore key tips to consider for effective CNC machining of PMMA:
Tool Selection
Experts recommend machining PMMA/acrylic with carefully selected sharp tools since it is a relatively soft material. It helps to prevent rapid tool wear and breakage. Also, carbide and diamond-coated tools and high-speed steel (HSS) tools are typical tools operators use to cut PMMA into desired designs.
AT-Machining experts advise using CNC tools specified for plastic machining to shape PMMA. Use a cutter with a high-shear flute geometry (1 – 2) flute tools) and about 5 degrees edge rake angle and 2 degrees clearance angle is a good rule of thumb.
Secure the PMMA and Optimize Cutting Parameters
When CNC machining PMMA, it is crucial to secure the workpiece properly to avoid extreme vibration and chattering that could affect the quality and finish of the final product. Set the appropriate feed rate to prevent the workpiece from melting.
Due to its relatively low melting point, professional machinists recommend employing moderate-to-low cutting speeds when CNC machining PMMA parts. Conversely, using high cutting speeds can lead to deformation or melting, resulting in tool breaks and poor surface finish. Also, it would help if you matched the feed rates with the RPM to achieve optimal results.
Coolant
Machining PMMA with coolant would be a good idea since it exhibits a relatively low coefficient of thermal expansion. Coolant prevents the material from rising in temperature and improves the surface finish of machined PMMA parts. Besides, choosing a suitable machine coolant isn’t difficult since PMMA works well with extensive solutions.
Conclusion
PMMA (acrylic) is a versatile engineering plastic alternative to glass and a highly machinable material. CNC machining PMMA allows the production of highly accurate parts with the tightest tolerances for different applications due to its high accuracy and repeatability. However, it is paramount to understand the basics of PMMA machining to make informed decisions about your products.
AT-Machining is the right CNC machining service provider for your custom PMMA machined parts. We leverage our state-of-the-art CNC machines and years of experience to deliver CNC machining solutions and meet the finishing needs that your projects require. Our experts have an in-depth understanding of machining a wide variety of materials. Submit your CAD file now to get an instant quote!
