Plastic CNC machining is a subtractive manufacturing method that involves placing a solid block of plastic against a moving cutting tool that removes material from the block. In order to shape the final product, the cutting tool follows a predetermined path directed by a digital design file. Plastic CNC machining is a popular way of producing plastic components. However, if you ignore essential aspects such as selecting the correct type of plastic, selecting the best machining method, maintaining an effective surface finish, and avoiding common flaws, it can be difficult to achieve effective machining results.
This article will cover the complete CNC machining process.
Common Plastics Used for Machining:
The most prevalent plastic polymers utilized in CNC plastic machining are thermoplastics. They retain their original properties after reforming and remodeling during melting. Check out our list of CNC materials at Prototool for a better idea and comprehension of what plastic polymer to use. However, the following are some common plastics that can be used in various items.
· Nylon or Polyamide:
Nylon is a pure, high-performance CNC plastic known for its exceptional thermal, mechanical, and chemical resistance. CNC machining nylon is simple because of the material’s physical characteristics and dimensional stability. It also behaves like metal, as evidenced by the idiom “machining nylon is like machining brass.” Tungsten carbide alloy CNC tools should be utilized while working with nylon.
· Delrin or POM:
Delrin is a thermoplastic with excellent stiffness and strength. It is an excellent plastic CNC machining material because it performs well under high tensile stress conditions. It is heat resistant (but cannot sustain temperatures above or equivalent to 121°C (250°F).
Acrylic is a good plastic CNC material because of its lightweight, high tensile strength, wear resistance, and stiffness. It has high machinability. However, there are several conditions to consider. For example, to achieve a perfect surface finish during CNC cutting, the cutting tool must be a carbide tool with an aggressive and razor-sharp edge.
PEEK, or Polyether ether ketone, is a class of high-performance plastics distinguished by its high melting point, corrosion resistance, and superior mechanical qualities. PEEK is widely used in 3D printing. However, its intrinsic qualities, accuracy, repeatability, tight tolerances, and other benefits make it an ideal material for CNC machining. While PEEK is an ordinary plastic CNC machining material, you need to be aware of its qualities to minimize breaking caused by heat and internal stress.
Teflon, also known as PTFE (Polytetrafluoroethylene), is a heat, water, electrical, and chemical-resistant material with an unusually high tensile strength. Because of its solidity and suppleness, it is a famous CNC plastic. However, it is challenging to attain tight tolerances because of its high coefficient of friction and stress creep qualities. As a result, before machining, you or the quick prototyping service you choose should know the implications of its qualities and the necessary tools.
Another CNC plastic machining material with great tensile strength, corrosion resistance, and other excellent mechanical features is UHMW Ultra-high molecular weight polyethylene. These characteristics make it a desirable material for quick prototyping. However, due to its instability, machining UHMW is a complex process. Because the material cannot be molded, machining is still the better alternative. As a result, it is preferable to outsource to a business with extensive experience working with the material.
How Are Plastic Machined?
CNC Machining of Plastics:
CNC milling is a type of computer-controlled milling in which a milling cutter, a cylindrical revolving tool, helps remove material from a solid workpiece to expose a finished component. Computer-controlled milling machines are classified into two types: those with three axes and multiple axes. Three-axis milling machines can move the cutting tool or workpiece in all three linear directions, i.e., back and forth, left to right, up and down, etc., making them excellent for generating designs with fewer complex features. Milling machines with four or more axes and multi-axis milling machines are the most effective instruments for manufacturing plastic components with complex geometries.
The following are some specific criteria for creating plastic parts using CNC milling:
- Carbon-based tools are preferred when machining thermoplastic materials bonded with glass or carbon fiber.
- You can attain higher spindle speeds by using clamps.
- One method for reducing the likelihood of stress concentration is to use end mills to create rounded interior corners and pockets.
Plastic CNC Turning:
A plastic component is held on a lathe throughout the CNC turning process, and the lathe is spun or turned against the cutting tool. CNC turning can take many forms, and each has its own set of unique goals and outcomes. For example, straight or cylindrical CNC turning is best suited for generating significant cuts, whereas taper CNC turning generates a distinctive cone-like shape.
The following are some particular parameters for creating plastic parts with CNC turning machines:
- Check that the cutting edges have a negative back rake to limit the amount of rubbing.
- Using cutting blades with large relief angles
- Polishing the top surfaces to help reduce material buildup and give a superior surface quality
- Making use of fine C-2 grade carbide inserts
- When performing rough cuts, you should use a feed rate of 0.015 inches per minute. We recommend a feed rate of 0.005 inches per revolution (IPR) for more accurate final cuts. Furthermore, product teams must know that the clearance angles, rake angles, and side angles will vary depending on the materials used in CNC turning.
Drilling with a CNC Plastic Machine:
CNC drilling is placing drill bits into a plastic block to generate holes. After being lowered onto the block, a spindle that keeps the block and the CNC drill in position begins to drill the appropriate size holes. A CNC drilling machine can work with various drill presses, including radial CNC drill presses, upright CNC drill presses, and bench CNC drill presses.
When utilizing the CNC drill, keeping the CNC drill bits that you employ to sharpen plastic components is critical. Worn-out or misshapen drills can strain components, leading to tooling issues and poor performance. We recommend using a drill bit with a lip angle of 9 to 15 degrees and a range of 90 to 118 degrees for most thermoplastics. When drilling with acrylic, use a rake of 0 degrees.
Ejection of the chip from the CNC drill is particularly critical since poor chip ejection can cause friction and heat accumulation. To limit stress and damage, it is necessary to minimize heat generation during CNC drilling. This process generates more heat than any other plastic machining method. When attempting to remove a CNC drill safely and damage-freely, ensure that the drilling depth is no more than three or four times the diameter of the drill, and slow down the feed rate when the drill is almost entirely through the material.
How to Achieve the Perfect Finish on Plastic?
A faultless finish in CNC plastic machining is dependent on the use of specific procedures as well as an understanding of the properties of the material being machined. If you want your plastic product to have a pristine surface, you should follow the recommendations below to avoid this:
- Avoid using any cutting instruments with highly complex geometry.
- Stay within the maximum recommended feeding rate for the material being used.
- The plastic materials that will be machined must be given adequate support. The vibration will occur due to incorrect clamping of the materials, resulting in an uneven finish on the plastic.
- Furthermore, the cutting speed must be within the recommended range, which varies based on the type of plastic utilized.
Understanding the varied properties of plastic materials is required to account for these elements. However, you can further improve any plastic product’s surface roughness by using finishing methods such as annealing, sandblasting, powder coating, etc.
Common Issues to Avoid:
Even if you are incredibly cautious and well-trained in the plastic CNC machining process, there may be some issues that you need help to avoid. As a result, being aware of these issues and knowing how to handle them (if they arise) is critical. Let us go over these issues in greater depth:
- One issue frequently arises with CNC processing plastic quick prototypes is broken, cracked, chipped, or scratched products. So, if the knife speed is off, all the effort and work is for naught. This is because domestic plastic material providers are incredibly diverse. So, when processing, you must run a short trial time, and if you are not in a rush, different batches of material must be trialed! To avoid the difficulty, another option is to switch suppliers.
- Another common issue is the knife you choose. You should not use metal tools to process quick plastic prototypes during this process. Material hardness variations, like nylon versus metal, result in differing processing techniques, potentially causing debris and tool damage.
- The layout of processing methods should also be based on the coarse and fine principle. First, organize heavy cutting and rough machining to remove the machining allowance on the blank. Then, prioritize low processing accuracy requirements for the content.
- You may also need to employ a high flow of cooling to decrease the impact of a significant amount of heat created during the machining operation. This increases the tool’s durability and reduces the need to actively use the extensive flow cooling approach.
For more details and queries, contact our professionals at Prototool.