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What Is Acrylic (PMMA)?
Acrylic or Polymethyl Methacrylate (PMMA) is one of the most versatile plastics and popular CNC machining materials. The engineered plastic polymer has been around since the mid-twentieth century. Acrylic (PMMA) is a solid, unbreakable, colorless, clear and 100% recyclable thermoplastic that comes in many forms such as small granules, pellets or sheets. It has an extremely high molecular weight, which accounts for its strength.
Properties of Acrylic (PMMA)
Here are some of the unique combination of properties that makes acrylic a widely used CNC machining material for prototyping and full production runs. Let’s examine these qualities closely:
- Optical Clarity and Light Transmission: Very clear with a refractive index of around 1.49, and light transmission of around 92%, higher than glass and other plastics like polycarbonate. Due to this, clear acrylic is an excellent substitute for glass where optical clarity is required, i.e., in lenses, lighting, and windows. You can even obtain custom acrylic parts without any compromise on optical clarity.
- UV Resistance: Acrylic differs from other materials, which turn yellow due to prolonged exposure to ultraviolet light. Acrylic has UV stability, which makes it suitable for most outdoor applications and can remain optically clear over time.
- Surface Hardness/Impact Strength: This lightweight thermoplastic comes with a density of about 1.17 g/cm3 to 1.20 g/cm3. Its remarkable impact and flexural strength allow it to withstand more load than glass can endure. As such, acrylic maintains its form while ordinary glass shatters under stress. Acrylic exhibits greater dimensional stability than most transparent polymers.
- Chemical Resistance: CNC acrylic materials can resist different chemicals or aqueous solutions, including aliphatic hydrocarbons, diluted acids, alkalis, and alcohols. Hence, acrylic components are commonly used in medical facilities and laboratories.
Essential Acrylic Machining Techniques
Manufacturers in different sectors employ various processing and machining techniques to obtain desired outcomes in their projects. Each process offers unique flexibility that allows you to get quality and aesthetically pleasing acrylic parts.
CNC Machining
Computer numerical control machining is a cutting-edge technique with a high degree of accuracy and fewer errors, perfect for working with acrylic material. This automated machining process removes material from raw materials with the right tools to create finished parts with a smooth finish, precise dimensions, and tight tolerances.
After you input parameters for the operation into the CNC machine, the multi-point cutting tool moves among different axes to deliver precise cuts on acrylic materials from various angles, shaping precision parts with clean edges and accurate dimensions. Besides, CNC machining plastics ensures minimal waste and produces low or high-volume acrylic parts faster compared to manual methods.
Acrylic Welding
Acrylic welding refers to the permanent bonding of two or more acrylic pieces into a single strong unit with heat or solvent methods. This manufacturing process doesn’t require adhesives to join these components into a single piece. Instead, you rely on high-frequency vibration, contact welding, or hot gas emissions to bond the pieces together.
Compounding (Blending)
Compounding involves blending raw acrylic materials with additives to produce uniform granules or pellets with enhanced aesthetics and performance. This manufacturing process forms custom acrylic blends with the attributes of different types of acrylics, ensuring they meet the required quality standards. Most time, acrylic parts made through this procedure are more durable compared to the raw materials.
Acrylic Extrusion
Acrylic extrusion involves heating acrylic pellets into molten forced through a shaped die to create desired shapes. Manufacturers employ this technique to improve the processing of acrylic materials since it often precedes the bonding or lamination phases.
Acrylic extrusion is ideal for tubing, piping, and sheeting acrylic components. Sheet extrusion and profile extrusion are the two widely used forms of acrylic extrusion. Both methods are renowned for providing the desired results.
Injection Molding
Injection molding is a prominent technique for forming complex, intricate acrylic components. It involves heating the acrylic pellets in a barrel and putting the molten acrylic in a metal mold where it cools and hardens evenly before being ejected. Injection molding is suitable for the mass production of complex, high-precision molded acrylic components.
Practical Applications of CNC–Machined Acrylic Parts
We’ll discuss some of the common uses of CNC-machined acrylic parts in various industries below:
Architecture and Construction
CNC-machined acrylic components are prevalent for diverse applications in the construction industry due to the material’s formability, resistance to high impact, UV light, and environmental wear and tear.
Machined acrylic components help to boost curb appeal and add functionality to architectural structures and designs. Typical uses of acrylic in the construction industry include façade designs, lightweight panels, lighting fixtures, and canopies.
Aerospace
Acrylic, being a tough material with low density, makes it a highly valued choice in the aerospace sector. CNC machining of acrylic allows aerospace component manufacturers to create precise parts that meet exact specifications and stringent industry standards. Aerospace applications of acrylics include aircraft canopies.
Automotive
Automotive parts manufacturers rely heavily on acrylic resins in manufacturing different high-quality interior and exterior components, including interior panels, glove boxes, dashboards, brake lights, gear bearings, headlight covers, fenders, motorcycle windshields, and indicators.
Medical and Healthcare
Medical parts manufacturers utilize acrylic materials to produce clear enclosures, incubators, test kits, syringes, blood filters, fluid silos, and many other instruments and equipment used in the medical field. The different grades of Acrylic/PMMA are embraced in medical device manufacturing since they are pure, biocompatible and easy to sterilize. Hence, acrylic is a suitable material for making fillings for dental cavities.
Consumer Goods and Electronics
CNC acrylic parts are popular in consumer goods from control panels, protective housings, laptops and smartphones to LED displays and signage applications. The ability of acrylics to boost the light-emitting potential of LED lights makes them an ideal choice for lighting purposes such as lamp construction.
Marine
The excellent dimensional stability, resistance to salt water and low water absorbing capacity of acrylic materials make it an optimal choice for marine applications. Machined acrylic parts, enclosures, console doors, boat windows, and windshields can resist the harsh elements of marine environments.
Expert Tips for Achieving the Desired Results from CNC Acrylic Machining
While acrylic is a popular plastic polymer valued for its several advantages, machinists encounter certain challenges when CNC machining PMMA. Here are factors and design tips to keep in mind to mitigate damages and ensure the best possible results:
Design Considerations
Parts machining begins with the design. When designing acrylic parts for CNC machining, you must take special care of tolerances, wall thickness and undercuts. For example, the standard minimum wall thickness for CNC-machined acrylic components ranges from 0.5 – 0.75 mm.
Structural weakness may set in if the wall thickness falls below the stipulated minimum wall thickness. Additionally, the depth of the undercut shouldn’t go beyond 10 times the cutter’s diameter. Machining experts advise against adding sharp internal corners and sharp edges in acrylic parts to mitigate the risk of cracks.
Choose Proper Work Holding Devices
Since acrylic materials are relatively soft and can vibrate if not clamped securely, it is best to use robust clamping systems to achieve optimal machining results.
Failure to keep the acrylic billet on the worktable properly during acrylic CNC machining can result in severe complications. It can lead to inferior/subpar products since excessive vibrations can dislodge the workpiece from the workholding or cause misalignment between the tool and workpiece surface. However, experts recommend using vacuum-hold systems for more intricate or larger components and double-sided tape for smaller, delicate parts.
Use the Appropriate Tool
Choosing the right cutter for acrylic machining is another fundamental practice when operating a CNC machine. Choose strong CNC tools with sharp teeth to obtain machined acrylic parts with a fine surface finish. It is unadvisable to use cutting tools that have recently worked on metals since they may have jagged edges. Therefore, cut your acrylic materials with tools with smooth and sharp edges to achieve cleaner cuts and improve quality.
In addition, the material of your cutter is important to your machining result. For instance, using high-speed steel tools to cut acrylic doesn’t guarantee a superior surface finish. However, diamond cutters can provide an improved surface finish, but can be costly. Carbide is a good alternative for machining acrylic with a fine surface finish.
Input the Correct Feed Rate and RPM
Machining parameters are independent factors crucial to the success of acrylic CNC machining since it can melt due to frictional heat. Factors, including chosen acrylic material and machining components’ complex geometries, determine the optimal machining parameters for your project.
Although faster revolutions per minute and feed rate can mitigate the risks of melting when cutting acrylic workpieces, it may increase cutting pressure and risks of workpiece breakage or deformation. Therefore, use a moderate feed rate and speed of cut during acrylic CNC machining to achieve optimal machining conditions and boost overall performance.
Use of the Right Cooling Mechanism
A compressed air system is an optimal cooling method when cutting acrylic since coolants with organic solvents can penetrate plastics and cause surface damage or cracks over time.
The vacuum or air systems cool the workpiece effectively and remove chips from the cutting zone. Maintaining a steady airflow keeps the plastic material clear and prevents tool burn for large parts or extended operations.
Supervise the Cut Direction and Pass Depth
The direction of the cutter is as crucial as the depth of cut when cutting acrylic sheets with a CNC machine. It is important to know the direction of the cutter’s rotation (whether it is clockwise or counterclockwise) in relation to the rotating cutter’s direction to ensure accuracy in your part’s design features.
Generally, half the router bit’s is the ideal pass depth for most machining materials. However, other materials may require a pass depth of approximately 0.0625” for desired results, especially when cutting with smaller bits.
Conclusion
Acrylic CNC machining is a reliable and efficient approach that overcomes complications such as cloudy finishes and uneven edges common to traditional methods. It helps to ensure your design comes out as desired without wasting resources or time. You can also follow the provided tips and guidelines for CNC machining acrylic materials. However, AT Machining is your one-stop CNC machine shop if you need the help of professionals.
AT-Machining is a top CNC machining service provider with the latest CNC machining technology and many years of acrylic machining experience. Our professionals pay attention to detail and can perform full-dimensional and first article inspection to ensure your machined acrylic parts meet industry standards. Contact us for expert advice, and we can deliver your custom acrylic parts within a reasonable lead time!
FAQs
What Type of Acrylic Is Best for CNC Machining?
The two types of acrylics include extruded and cast. Both have different production methods, which affect their mechanical properties. Extruded acrylic passes through the extrusion process, which makes the material brittle and anisotropic (its reaction differs in different cutting directions). On the other hand, cast acrylic passes through the casting process, which gives it its great strength and isotropic properties, making it the ideal choice for CNC machining.
What Are the Common Setbacks of Acrylic CNC machining?
Even though plastics such as acrylic offer excellent strength and hardness in different applications, challenges such as cracking, chipping, surface scratches, and low melting temperature affect the machining of acrylic parts.
Are Acrylic Materials Heat Resistant?
Acrylic materials generally maintain moderate heat resistance. However, it has limitations since it possesses a heat deflection temperature of 95°C and melts at about 160°C. For this reason, it is not the best choice for high-temperature applications or direct exposure to flames.
What Surface Finishing Options Are Best for Acrylic Parts?
Acrylics work well with a range of surface finish options, including flame polishing, vapor polishing or buffing. These finishes often boost the aesthetic appeal of machined acrylic parts or function as protective coatings against scratches or damages that may occur during handling.
