Plastic is a common type of material used in CNC machining. Machining with plastics is very different from metals. This article will discuss the common plastics applied to CNC machining and the tolerances and finishing available. We will also explore the differences between CNC plastic machining and other plastic manufacturing methods.
What Is CNC Machining?
CNC machining, or Computer Numerical Control machining, is a manufacturing process in which pre-programmed computer software dictates the movement of factory tools and machinery. This process can control a range of complex machinery, from mills and turners to lathes and grinders. CNC machining enables complete three-dimensional cutting tasks using a single set of programmed instructions.
Can Plastic Be CNC Machined?
Yes, plastic can be CNC machined. CNC machining is a versatile process that can work with a wide range of materials, including various plastics. This capability makes CNC machining an excellent choice for producing plastic parts with high precision and complex geometries.
Types of Plastics Used in CNC Machining
There are some common plastics used in CNC machining.
ABS (Acrylonitrile Butadiene Styrene)
ABS is known for its toughness, impact resistance, and good machinability. It has a glossy finish and is easy to machine, which enables its wide application in automotive parts, electronic housings, and LEGO bricks.
PC (Polycarbonate)
Polycarbonate is highly durable, transparent, and resistant to impact and heat. It is also lightweight. It is often used in eyewear lenses, bulletproof glass, CDs, and electronic components.
PA (Polyamide, commonly known as Nylon)
Nylon is strong, durable, and resistant to wear and abrasion. It has good chemical resistance, is elastic, and has widespread use in textiles, automotive parts, and mechanical components like gears and bearings.
PMMA (Polymethyl Methacrylate, also known as Acrylic)
PMMA is clear, lightweight, and shatter-resistant. It has excellent optical properties and is weather-resistant. So, it enjoys broad applications in lenses, aquariums, signs, and display cases.
POM (Polyoxymethylene, also known as Acetal)
POM is recognized for its high stiffness, low friction, and outstanding dimensional stability. It is also resistant to chemicals. POM is perfect for precision parts such as gears, bearings, and fasteners.
PP (Polypropylene)
Polypropylene is lightweight, flexible, and resistant to chemicals and fatigue. It has a high melting point. It is used in packaging, automotive parts, textiles, and reusable containers.
The most commonly used plastic for prototyping is ABS, the cheapest of all the above.
Tolerances of CNC Machining Plastic Components
Generally, CNC machining can achieve tighter tolerances with metals than with plastics due to the inherent material properties. The main reason is that plastics are much softer than metals. Despite this, CNC machining is still capable of producing precise plastic components.
Tolerances for CNC-machined plastic components depend on size, structure, and whether the part needs to be made in one piece or be split and glued together.
General tolerances of CNC machining plastic parts:
Made in one piece: +/- 0.2mm (0.007inch)
Split and glued together: +/- 0.5mm (0.02inch)
Tighter tolerances:
Milling: +/- 0.05mm (0.002inch)
Turning: +/- 0.08mm (0.003inch)
Hole tolerance: +/- 0.05mm (0.002inch)
The tolerance requirement for each project is different. You'd better confirm with your vendor the best tolerance they can hit for your specific parts.
Finishing Available for CNC Plastic Machining
Plastics can work with versatile surface treatment. The most common ones are as follows.
Deburring
Deburring is the process of removing imperfections or rough edges (burrs) from plastic, metal, or other materials after machining or cutting.
It improves the appearance, functionality, and safety of the part.
Sanding
Sanding involves using abrasive materials to smooth surfaces by removing some materials. It prepares surfaces for further finishing, such as painting or polishing, and can improve texture.
Polishing (PMMA)
Polishing, usually only available for PMMA, involves using fine abrasives and polishing compounds to create a smooth, glossy surface. It enhances the optical clarity and appearance of acrylic products.
Vapor Polishing (PC)
As polishing cannot be done on polycarbonate(PC) parts, to improve the optical clarity and surface finish of PC parts, vapor polishing is used to smooth and clarify PC surfaces by exposing them to solvent vapors.
Painting (Matte, Glossy, Texture)
Painting involves applying a liquid pigment to a surface. Matte paintings are non-reflective, glossy paintings are shiny and textured finishes have a distinct surface feel. It enhances the aesthetic appeal and can provide additional protection to the surface.
Electroplating
Electroplating is depositing a thin metal layer onto a surface using an electric current. It improves corrosion resistance, enhances appearance, and can provide electrical conductivity.
Sandblasting
Sandblasting involves propelling abrasive particles at high speed against a surface to clean or texture it. It removes surface contaminants and creates a uniform texture.
Silk Screening
Silk screening, or screen printing, applies ink through a mesh screen onto a surface to create designs or text. It is for printing graphics (such as logos and texts) on various materials, including textiles, plastics, and metals.
Laser Engraving
Laser engraving uses a focused laser beam to etch designs or text onto a material's surface. It provides precise, permanent markings for decorative or informational purposes.
These finishing techniques enhance the appearance, functionality, and durability of CNC machining plastics across various industries.
How Is CNC Plastic Machining Different From Other Plastic Manufacturing Methods?
Besides CNC machining, there are ways to produce plastic products. They are traditional injection molding, 3D printing, vacuum casting, and reaction injection molding(RIM). Let's figure out what their differences are.
Injection Molding
(Image from the Internet)
Molding Method
CNC plastic machining involves cutting away material from blocks or sheets of plastic to create the desired shape. It is good for making a small number of custom parts.
Injection molding involves melting plastic pellets and injecting them into a mold where they cool and harden. It is suitable for making a large number of standard parts.
Material Use
CNC machining uses more material and creates some waste. In contrast, injection molding uses material more efficiently and usually does not produce waste.
Surface Quality
The surface quality of CNC machining depends on the tools and settings used and can be very smooth. The surface quality of injection molding depends on the mold's surface and is usually not as good as CNC machining.
Shape Complexity
CNC machining can create complex shapes, including holes and grooves. Injection molding is limited by the mold design, making it harder to create complex shapes.
Production Volume
CNC machining is suitable for making small-series parts. So each part is more expensive.
Injection molding is appropriate for making high-volume parts. Each part is less costly.
Precision
CNC machining offers high precision and can meet strict size requirements.
Injection molding precision is affected by the mold and shrinkage, so it is less precise.
Overall, CNC machining is suitable for making a small number of custom and precise plastic parts, while injection molding is better for making standard plastic parts in mass production.
3D Printing
3D printing involves melting plastic powder or filament and building the shape layer by layer.
Material Use
CNC machining has lower material use efficiency and creates some waste.
3D printing uses material more efficiently and usually does not produce waste.
Surface Quality
The surface quality of CNC machining depends on the tools and settings used and can be very smooth.
3D printing surface quality depends on the printing resolution and is usually not as smooth as CNC machining.
Geometric Shapes
CNC machining can create complex shapes, including cavities and slots.
3D printing can make even more complex shapes, like hollow structures and internal features, which are hard to achieve with CNC machining.
Therefore, CNC machining is suitable for plastic parts that need high precision and smooth surfaces, while 3D printing is better for making plastic parts with complex shapes.
Vacuum Casting
Vacuum casting uses silicone molds to cast molten plastic into the silicone mold, which cools to form plastic parts. It is suitable for low-volume production and can quickly replicate parts with complex shapes.
Cost and Efficiency
A CNC machine requires a high initial investment and per-piece costs, especially for small batches, because each part needs to be machined individually. However, CNC machining has advantages in precision and detail handling.
Vacuum casting has lower costs in small-series production because the molds can be reused, and each casting cycle is fast, making it suitable for large-scale production of the same type of parts.
Material Utilization
CNC machining typically generates material waste as it processes parts by cutting away excess material.
Vacuum casting has higher material utilization, with almost no waste, as the plastic is fully formed in the mold.
Surface Quality and Detail
CNC machining can achieve very high surface smoothness and precision. It is suitable for parts with high requirements for appearance and functionality.
Vacuum casting surface quality is influenced by the quality of the mold. While it can achieve good results, it usually does not match the precision of CNC machining.
Applicability
CNC machining is suitable for various plastic materials and can process complex geometries, making it ideal for applications requiring high precision and customization.
Vacuum casting is suitable for producing parts with complex shapes and large quantities, especially when rapid production is needed. Besides plastic parts, vacuum casting can also manufacture rubber parts.
Reaction Injection Molding(RIM)
Reaction Injection molding (RIM) pours liquid plastic into a mold at low pressure. The plastic hardens to form the part. It fits for making low-volume and large-sized parts quickly, especially if they have complicated shapes.
Cost and Efficiency
CNC machining: It can be expensive to start and costs more per part, especially if you're making just a few. But it is accurate and detailed.
Reaction injection molding: Using the same mold many times makes it cheaper to produce parts and can be done quickly.
Material Waste
CNC machining wastes some material because it cuts away extra plastic to make the part.
Reaction injection molding uses almost all the plastic, so there's little waste.
Surface Quality and Details
CNC Machining: Parts made this way can be very smooth and detailed, which is great for parts that need to look or work well.
Reaction Injection Molding: The quality depends on the mold. It can be good, but usually not as precise as CNC machining. The surface can be made smoother with extra work.
Applicability
CNC Machining: Works with many types of plastic and can make complex shapes. It's well-suited for parts that need to be very precise or customized.
Reaction injection molding is best for making parts that might be complicated in shape, especially when you need them fast. Besides normal plastic components, RIM can create structural foam and rubber ones.
FAQs
1. What plastic is best for CNC milling?
ABS. It's the cheapest plastic material and very easy to do secondary treatment.
2. What is the general lead time of CNC plastic machining?
The average lead time is 5-7 days. CNC machining is a fast process. The lead time depends on the part's size, structure, and quantity.
3. What factors should be considered when choosing CNC plastic machining?
Key factors include the type of plastic material, part complexity, dimensional tolerances, surface finish requirements, and production volume.
4. Are there any limitations to CNC plastic machining?
While CNC plastic machining is versatile, it may not be cost-effective for large or highly complex parts that require extensive material removal. Additionally, some plastics may have limitations due to their thermal properties.
5. How can I ensure quality in CNC plastic machining?
It involves picking the right materials, designing the products that can be made easily, choosing a trustworthy machine shop, and carefully checking and testing the finished parts.
Work With X Rapid Technologies
As X Rapid Technologies has been in the CNC machining sector for over 15 years, our expertise can deliver your project on time and at affordable costs. Don't hesitate to contact us today to get your pending project rolling!
