Acrylic CNC Machining | Material, Process, Application

Acrylic or Polymethyl methacrylate(PEEK) is a versatile thermoplastic polymer widely used in industrial manufacturing due to its high optical clarity, surface hardness, and weather resistance and is often used as a lightweight, shatter-resistant alternative to glass. Achieving precision in acrylic components requires a specific understanding of its properties and how they interact with different machining and forming processes. This article provides an overview of acrylic plastic and fabrication methods used in production.

What is Acrylic?

Polymethyl methacrylate, commonly known as acrylic, is formed through the addition polymerization of repeating methyl methacrylate monomers by opening the C=C double bonds. Its molecular formula is (C₅H₈O₂)_n, where n represents the degree of polymerization, typically resulting in a molecular weight between 10,000 and 12,000. It has a density of 1.18–1.19 g/cm³, a small refractive index of approximately 1.49, a light transmittance of 92%, and a haze of no more than 2%. It is an organic transparent material.

Types of Acrylic

Acrylic sheets can be divided into cast sheets and extruded sheets based on the production process.

Cast Acrylic

These have a high molecular weight, providing excellent stiffness, strength, and superior chemical resistance. Therefore, they are more suitable for processing large-scale signage and plaques, though they require a slightly longer time during the softening process. These sheets are characterized by small-batch processing, unparalleled flexibility in color systems and surface textures, and a complete range of product specifications suitable for various special purposes.

Extruded Acrylic

Compared to cast acrylic sheets, extruded acrylic sheets have a lower molecular weight, slightly weaker mechanical properties, and higher flexibility. However, this characteristic is beneficial for bending and thermoforming processes, with shorter softening times. When handling large sheets, it facilitates various rapid vacuum suction forming processes. At the same time, the thickness tolerance of extruded acrylic sheets is smaller than that of cast acrylic sheets. Since extruded acrylic sheets are produced through high-volume automated production, adjusting colors and specifications is inconvenient, leading to certain limitations in product variety.

Properties of Acrylic

1. Physical Properties: PMMA exhibits excellent optical properties, with a light transmittance of up to 92%, which is 10% higher than that of inorganic glass. It is colorless, absorbs almost no visible light, can transmit ultraviolet light at 270 nm, and has good colorability. It shows almost no discoloration or fading under heat. It has a refractive index of 1.49, a surface reflectivity of no more than 4%, and high surface gloss. The relative density of PMMA is small (1.17–1.20), only about half that of inorganic glass.
2. Mechanical Properties: PMMA has high mechanical strength at room temperature and is minimally affected by temperature. However, the strength drops sharply as it approaches the softening point and glass transition temperature. PMMA has poor surface hardness and scratch resistance, and its impact toughness is also low, often requiring rubber modification. It has high water absorption and significant dimensional shrinkage.
3. Thermal Properties: PMMA has a large coefficient of thermal expansion, leading to significant dimensional changes caused by temperature.
4. Electrical Properties: Over a wide frequency range, the power factor of PMMA decreases as frequency increases, making it suitable for long-term outdoor electrical appliances. It has good arc resistance and tracking resistance, high surface resistance, and high electrical insulation.
5. Chemical Resistance: PMMA is resistant to strong acids, strong bases, inorganic salts, oils, and aliphatic hydrocarbons.
6. Aging Resistance: PMMA has excellent weather resistance. Even after long-term outdoor exposure, its transparency and gloss change very little.

CNC Acrylic Machining

PMMA plastic can be mechanically machined, including sawing, milling, drilling, and reaming.

CNC Machining Acrylic Example

The drawing shown below is an Acrylic component with a hollow, thin-walled structure. It requires high dimensional accuracy and must maintain the material’s original light transmittance after processing to meet measurement range requirements.

This part can be processed using CNC lathes, but if the process parameters and sequence are improperly selected, cracks can easily appear, leading to scrap. If the surface treatment method is chosen incorrectly, the surface will appear torn, losing the original transparency of the PMMA and failing to meet usage requirements.

Impact of cutting heat on Acrylic Machining

The thermal conductivity of PMMA is very poor, only 1/450 to 1/175 of that of common metal materials. Heat generated during cutting cannot diffuse quickly, accumulating at the contact point between the tool and the workpiece, leading to accelerated tool wear. Simultaneously, the thermal expansion coefficient of PMMA is 1.5 to 2 times larger than that of general metals, and the glass transition temperature is approximately 100°C. Excessive cutting heat causes volume changes and vitrification, reducing processing accuracy. The volume expansion also intensifies friction between the tool and workpiece, creating a vicious cycle of rising heat.

Because the melting point of PMMA is low (only 160–200°C), it is a thermoplastic material. High cutting heat easily causes the machined surface to melt. Specifically, when machining internal holes, poor heat dissipation can cause the surface to become “scuffed,” showing tiny tear-like cracks. This is often mistaken for material brittleness or internal defects. However, transparency or water-leakage checks show no leakage; rather, the part fails technical requirements for transparency, appearing cloudy and lacking the natural texture of the material.

Impact of machining parameters

At room temperature, PMMA is a hard and brittle material. For thin-walled structures, improper selection of cutting parameters leads to unstable cutting forces, causing the part to crack or even shatter.

Surface roughness

Due to its low hardness, the surface roughness of PMMA after machining is often poor. To ensure technical requirements are met, special polishing materials and processes must be employed after machining.

Acrylic CNC Machining Solution

To ensure dimensional accuracy, surface roughness, and technical requirements while improving the yield rate, the internal hole should be processed after machining the end face and the end face stop groove. To avoid material spring-back, the drilling feed rate is controlled at 0.05–0.08 mm. Simultaneously, the rate and delivery of cutting fluid are controlled to ensure consistent temperature during drilling. After the internal hole is finished, a stop-positioning auxiliary fixture is used. A tailstock center is used against the auxiliary center hole of the fixture to ensure axial limitation during external diameter machining. The 50mm outer diameter is processed first, using the clearance of the offset tool, followed by a single pass to form the 20mm outer diameter, completing the cutting process.

Other Acrylic Processing Technology

Other technologies for Acrylic forming include casting, injection molding, extrusion, and thermoforming.

Casting

Casting is used to form PMMA sheets, rods, and other profiles using the bulk polymerization method. Products after casting require post-treatment; typical conditions are 60°C for 2 hours and 120°C for 2 hours.

Injection Molding

Injection molding uses granular materials produced by suspension polymerization. The process is carried out on standard plunger or screw-type injection molding machines.

Extrusion

Polymethyl methacrylate can also be extrusion molded. Granular materials from suspension polymerization are used to prepare sheets, rods, pipes, and strips. However, profiles prepared this way, especially sheets, have inferior mechanical properties, heat resistance, and solvent resistance compared to cast profiles due to the lower molecular weight of the polymer. The advantage is high production efficiency, particularly for pipes and other profiles that are difficult to manufacture with casting molds. Extrusion molding can use single-stage or double-stage vented extruders, with a screw length-to-diameter ratio generally between 20 and 25.

Thermoforming

Thermoforming is the process of making PMMA sheets or strips into various sized and shaped products. The blank is cut to the required dimensions, clamped onto a mold frame, heated to soften it, and then pressurized to fit the mold surface to obtain the same shape. After cooling and setting, the edges are trimmed to obtain the product. Pressure can be applied via vacuum drawing or direct pressure with a male mold. When using rapid vacuum low-draw forming, a temperature near the lower limit is advisable, for complex, deep-draw products, a temperature near the upper limit should be used. Generally, normal temperatures are applied.

Heat Treatment and Surface Treatment

Acrylic products are prone to developing internal stresses during secondary operations like machining or thermoforming. If left untreated, these stresses can lead to micro-cracking or even structural failure when exposed to heat, solvents, or environmental stress. To mitigate this, acrylic requires a post-fabrication heat treatment known as annealing.

Annealing for CNC Machined Parts

For acrylic parts made by CNC machining processes, place them in an oven at room temperature and increase the temperature at a rate of approximately 15°C per hour. Once the target temperature of 90°C is reached, the soaking time (holding time) should be proportional to the material thickness:

• 3 mm sheet: 1 hour
• 6 mm sheet: 2 hours
• 12 mm sheet: 4 hours
• 20 mm sheet: 6 hours

After the soaking period is complete, cool the parts back to room temperature. The cooling rate should be strictly controlled at approximately 10°C per hour to prevent new stresses from forming due to rapid thermal contraction.

Annealing for Thermoformed Parts

For acrylic products that have been heat-bent or vacuum-formed, the annealing temperature is slightly lower than that for CNC machined parts, typically ranging between 70°C and 85°C. The soaking time requirements remain the same as those listed for machined sheets above.

Annealing for Injection Molded Parts

The parameters for injection molded components depend largely on the specific part design and wall thickness. Soaking temperature is generally between 60°C and 80°C. Soaking time is controlled between 2 and 4 hours.

Surface Treatment

To enhance specific performance characteristics, such as improving wear resistance or increasing surface gloss, PMMA products can undergo various secondary operations. These treatments, including polishing, painting, or laser etching, can be used to meet the specific demands of different working environments.

Protective coatings can be applied to acrylic components via flow coating (for large, non-perforated sheets), spray coating (for complex or irregular shapes), or dip coating (for small parts). These chemical treatments create a dense, protective film on the acrylic surface to meet high hardness and wear-resistance requirements.

Flow coating is the most cost-effective method; sheets can be hardened first and then subjected to secondary processes like cutting.

Physical Vapor Deposition(PVD) can be used to deposit an ultra-hard film on the surface. This process minimizes impurity interference during production, resulting in superior coating density and uniformity. While PVD significantly boosts surface hardness and abrasion resistance, it is a high-cost process. It also has specific requirements regarding material thickness and is generally better suited for thin sheets.

Applications of Acrylic

Thanks to its superior optical properties, PMMA is used across various industries. General-purpose PMMA is primarily found in advertising light boxes, signage, lighting fixtures, bathtubs, instruments, consumer goods, and furniture. High-end PMMA is critical for applications such as LCD screens, radiation-shielded PMMA, optical fibers, solar photovoltaic cells, automotive lens covers, bulletproof glass, aircraft cockpit canopies, and biomedical polymer materials.

Biomedical

The biocompatibility and low toxicity of PMMA microspheres make them a preferred material for drug carriers, tissue engineering scaffolds, and cell labeling. Through surface modification, microspheres can carry drug molecules for targeted release, extending the drug’s half-life. As bone repair materials, their mechanical strength and plasticity aid fracture healing. In cell culture, microspheres provide a 3D support structure to promote cell proliferation. Microspheres with a 100nm particle size are easily engulfed by cells, making them suitable for tumor therapy and vaccine delivery.

Display Light Guide Plates

The backlight module of an LCD light guide plate primarily consists of a light source, a light guide plate, and optical films. The light guide plate is used in LCD backlight modules to uniformly guide the light emitted from the light source onto the display surface; the primary material used is PMMA.

LED Lighting

In the lighting field, LED lighting is a new type of green light source. Compared to traditional sources, it is energy-saving, environmentally friendly, long-lasting, and compact. It is widely used in indicators, displays, decoration, backlights, general lighting, and urban nightscapes.

LED panel lights use light guide plates to convert LED point or line light sources into area light sources, allowing light to emit uniformly from the front. LED panel lights mainly use PMMA light guide plates as raw materials due to their higher light transmittance.

PMMA Fiber Optic Materials

Plastic optical fibers made of PMMA are excellent media for short-distance data transmission and are considered the best solution for the “last mile” of Fiber to the Home (FTTH). Beyond communication, PMMA fibers are used in landscape lighting, such as the landscape projects for the opening and closing ceremonies of the 2012 London Olympics.

Automotive Lightweight Materials

With the implementation of new energy vehicle subsidies, increasing driving range is a major challenge. Lightweight technology is an effective way to improve range; for every 10% reduction in vehicle weight, energy consumption can decrease by 6%–8%.

PMMA is widely used in various vehicle positions due to its excellent optical properties, light weight, and weather resistance:

• Lights: Requires materials with good light transmission, impact resistance, and aging resistance. PMMA is widely used for taillight covers in new energy vehicles.
• Window Glass: Using low-density, lightweight materials for windows is a key weight-reduction strategy. PMMA satisfies requirements for light transmission, impact resistance, and low fragmentation while providing UV protection. It is used by many European manufacturers for side and rear windows, reducing weight by 40%–50% compared to traditional glass.
• Dashboards: PMMA has excellent toughness and high impact strength. It does not crack under vibration or pressure tests and provides high optical clarity.
• Bumpers: PMMA and ABS composites can be used for bumpers, combining the scratch resistance and environmental friendliness of PMMA with the impact resistance and heat resistance of ABS, all at a lower cost than magnesium-aluminum alloys.

How Getzshape Can Help

Getzshape delivers high-quality custom CNC machining, sheet metal fabrication, electrical discharge machining, die casting and more. Leveraging advanced equipment and strict quality control, we ensure accuracy and on-time delivery for prototypes to large production runs. As your end-to-end manufacturing partner, we streamline sourcing, machining, post-processing, and logistics.