Chloe Zhang
CNC milling is a popular manufacturing method for creating plastic components. CNC-STEP routers, for example, can mill all plastics with high precision and accuracy to size. CNC milling machines can be categorized into 3-axis mills and multi-axis mills. 3-axis mills are best for creating simpler designs, while multi-axis mills are better for creating plastic parts with complicated geometries. When milling plastic, it is important to consider the type of plastic, the desired precision, and the management of heat and chip ejection to ensure the final product meets the desired specifications.
| Characteristics | Values |
|---|---|
| Types of plastics | PEEK, Acetal, Polycarbonate, HDPE, ABS, PVC, duroplastics, thermoplastics, plexiglass, POM, PTFE, PE, PS, PA, UHMW |
| CNC machine types | 3-axis mills, multi-axis mills |
| CNC processes | CNC milling, CNC drilling, CNC turning |
| CNC applications | Medicine, automotive, aerospace, food and beverage, semiconductors |
| CNC machining considerations | Impact strength, wear resistance, dimensional stability, heat generation, cutting tools, material handling, recycling, moisture, chemical sensitivity, heat resistance, mill speeds, chip ejection, cutting tool path |
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What You'll Learn
Selecting the right plastic
Mechanical Properties
The mechanical properties of the plastic, including impact strength, wear resistance, and dimensional stability, are crucial factors in the selection process. These characteristics should endure across a range of temperatures and mechanical stresses to ensure the plastic's effectiveness and longevity.
Environment
Consider the environment in which the final product will be used. Will it need to withstand extreme temperatures, open flames, UV radiation, or exposure to chemicals or other corrosive factors? Selecting a plastic that can tolerate these conditions is essential.
Industry Regulations
Depending on the industry, federal, or local regulations, specific standards must be met. For example, in the medical, food, or beverage industries, using biocompatible or hygienic materials that meet regulations without environmental degradation is crucial.
Material Type
There are two main categories of plastic materials: engineering plastics and composite materials. Engineering plastics are the most common choice for CNC machining, offering close tolerances and varying complexity levels. Composite materials are more challenging to work with but are ideal for specialized productions requiring complex final components.
Heat Management
Plastics have a high coefficient of thermal expansion, meaning slight temperature changes can lead to dimensional shifts and stress cracking. Selecting a plastic with adequate heat resistance and using appropriate cutting end mills are essential to maintaining the integrity of the workpiece.
Cost and Availability
Consider the cost and availability of the plastic material. Plastics like ABS offer a cost-effective alternative for prototyping and general-purpose applications, ensuring projects stay within budget. The availability and lead times of the chosen plastic should also be considered to avoid delays in the manufacturing process.
By carefully considering these factors, you can select the right plastic for your CNC milling project, ensuring optimal results in terms of functionality, aesthetics, and durability.
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Choosing the correct cutting tools
CNC machining is a popular manufacturing method for plastic parts. It involves using a computer-controlled, guided cutting tool to cut plastic materials into the desired shape with precision and speed.
When choosing the correct cutting tools for milling thick plastic, there are several factors to consider. Firstly, the type of plastic being cut is important. Different plastics have distinct properties, such as melting point, hardness, and thickness, which will influence the choice of cutting tool. For example, CNC machining is suitable for most plastics, but not PVC, as it releases toxic fumes. Additionally, plastics with a low melting point may benefit from a Single Flute Plastic Cutting End Mill, as it allows for more heat transfer away from the material.
The design of the plastic part also plays a role in tool selection. For instance, rounded internal edges in the design allow the cutting tool to run more efficiently. Shallow cavities and thicker walls in the design can also facilitate faster cutting. Furthermore, the radius of a fillet must be at least half the diameter of the end mill; any smaller and a tool change may be required.
The desired finish is another consideration. If a smooth finish is required, a 3 Flute Plastic Finisher End Mill can be used, as it leaves minimal marks. For engraving, a 'V'-shaped engraving tool can provide very fine detail while maintaining strength. For machining three-dimensional contours, a ball nose end mill is ideal, as it produces smooth surfaces with minimal chipping.
Lastly, workholding options should be considered. Some plastic parts may be too delicate to be secured by clamps or vices, so alternative methods such as vacuum tables or double-sided tape may be used. However, these methods may lead to lifting issues if the wrong tool is chosen. Therefore, it is important to select a tool with minimal upward forces, such as a Downcut Plastic Cutting End Mill, to prevent lifting.
In summary, choosing the correct cutting tool for milling thick plastic involves considering the type of plastic, the design of the part, the desired finish, and workholding options to ensure efficient and effective machining.
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Drilling and milling techniques
Drilling Techniques:
When drilling holes in plastic, it is important to use sharp drills and appropriate drill bits, such as forstner or brad point bits, to ensure precise and easily aligned holes. The back side of the plastic part should be supported to prevent chipping, and drilling into unsupported areas should be avoided. For deep hole drilling, "peck drilling" is necessary for proper chip clearance and heat dissipation. The drilling depth should not exceed 3-4 times the drill diameter before withdrawal to prevent excessive heat build-up, which can cause chipping, rough surfaces, and inadequate tolerances.
Additionally, the feed rate should be reduced as the drill exits the material to prevent cracking and other issues associated with improper drilling techniques, such as poor finishes and dimensional changes. Managing heat is critical during drilling operations, especially when drilling holes with a depth greater than twice their diameter. Preheating the material before drilling can also be considered, depending on the plastic type.
Milling Techniques:
Milling involves using a high-speed cylindrical cutting tool on a stationary plastic shape to make subtractions from the shape in different directions. Climb milling, or down milling, is recommended over conventional milling to improve accuracy and reduce vibrations that can lead to chatter marks. Proper stabilization of the part on the worktable is crucial to minimizing vibrations and maintaining accuracy. The shape should not be fixed too tightly to avoid deformation or springing of the material.
When milling plastic, it is important to select the appropriate cutting end mills based on the plastic's melting point and hardness. Single-flute end mills are suitable for low melting point plastics, while multi-flute end mills are used for plastics with higher heat tolerance. Polished top surfaces and generous relief angles on the cutting edges can help reduce material build-up and improve surface finishes. Fine, C-2 grade carbide inserts are recommended for turning operations.
Furthermore, managing mill speeds and feed rates is essential to prevent excessive heat generation and potential stress in the finished part. HSS tooling works well for most thermoplastics, while carbide tooling is recommended for reinforced materials.
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CNC machine types
CNC, or Computer Numerical Control, machines are programmable machines that can autonomously perform CNC machining operations. CNC machines are available in a wide range of types, each with its own unique mode of operation, cutting tools, materials, and number of axes they can cut simultaneously.
One of the most common CNC machine types is the CNC mill or CNC milling machine. These machines use cutting tools to remove material from a workpiece and shape it accurately to the required specifications. The workpiece is usually fixed in location while the high-speed rotating cutting tool removes material from it. CNC mills can have a wide range of cutting tools, each with a different purpose, such as end mills, reamers, face mills, taps, and drills. They can also be horizontally or vertically oriented, with basic mills having three-axis movements and more advanced models accommodating additional axes.
Another type of CNC machine is the CNC router, which is similar to a CNC mill but is generally used for machining softer materials and is typically less precise. CNC routers are characterised by their ability to use computer numerical control to route spindle and machine tool paths to design and shape materials like wood, steel, foam, composites, aluminium, and plastic. They consist of stepper motors, stepper drivers, a mechanical base, a spindle, controllers, and a power supply. CNC routers reduce waste, increase productivity and accuracy, and produce products faster.
CNC lathes or turning machines are another type of CNC machine. These machines rotate the material of the workpiece about a primary axis, and the lathe cutting tools remove and shape the material as needed. Examples of CNC lathes include turret lathes, engine lathes, and special-purpose lathes.
Drilling CNC machines are also common, especially in automobile manufacturing, astronautics, shipbuilding, and furniture making. These machines use rotating drill bits to produce cylindrical holes in the workpiece, with the drill bit design allowing waste metal to fall away. The dimensions of the hole that can be created are limited, and different diameters of drill holes require specific-sized drill bits.
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CNC machining applications
CNC machining is used in a wide range of applications across major industries, including construction, automotive, aerospace, medicine, and advertising. CNC machining is used to mill plastics like acrylic glass, thermoplastics, plexiglass, POM, and polycarbonate. CNC-STEP routers, for example, can mill all plastics with high precision and accuracy.
CNC machining is often used for prototyping and general-purpose applications. Plastics like ABS are cost-effective alternatives to metals, offering lightweight and dense parts that are more suitable for machining than 3D printing or injection moulding. CNC plastics are also used for engineering models and end-use components.
CNC machining is useful for milling soft plastics (thermoplastics) that are easy to shape under heat, solid plastics (thermosetting plastics), and cutting flexible plastics (elastomers) like rubber. The material is not only milled but also drilled, cut, and engraved.
The choice of plastic for CNC machining depends on the desired mechanical properties of the final product, such as impact strength, wear resistance, and dimensional stability. The best plastic materials for milling are said to be Acetal, PEEK, and PVC, as they offer good dimensional stability and superior machining characteristics. PEEK is also used in aerospace, medical, and automotive industries for its ability to withstand heavy loads, high temperatures, and harsh chemicals.
To achieve high-quality results, it is important to manage heat generation during the CNC machining process, select the appropriate cutting tools, and ensure proper material handling.
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Frequently asked questions
CNC milling is a computer-controlled process that uses a cylindrical rotating tool to remove material from a solid workpiece and create precise and uniform plastic parts.
Millable plastics are selected based on their mechanical properties, such as impact strength, wear resistance, and dimensional stability. Common plastics used include PEEK, Acetal, Polycarbonate, HDPE, and ABS.
CNC milling machines can be categorized into 3-axis mills and multi-axis mills. 3-axis mills are suitable for simpler designs, while multi-axis mills are used for creating complex plastic parts. It's important to manage heat generation effectively, select the appropriate cutting tools, and ensure proper material handling.
When milling thick plastic, it's important to use a high chip load and a low rpm with a high feed rate to create thick chips that don't ball up. Managing the ejection of the CNC drill is crucial to prevent friction and heat build-up. Additionally, drilling entry holes can improve the process.
Millable plastics offer superior characteristics such as high-temperature resistance, impact strength, and electrical insulation. They provide precision, performance, and cost advantages, making them suitable for industries like medicine, automotive, and aerospace. Plastics are also adaptable, durable, and lightweight, contributing to lower freight costs.
