CNC Plastic Machining: What Is It, Creating Prototypes, Operations, Materials

What is CNC Plastic Machining?

It is necessary to understand what “CNC” means to appreciate the depth of knowledge we possess in CNC plastic machining. “CNC” refers to “computer numerical control,” a kind of machining in which a pre-programmed computer software provides instructions to tools and machines to make products. Because CNC machining is so efficient, it’s been used in various operations and procedures. CNC machines can manage multiple mechanisms: grinders, lathes, and mills.
Another advantage of CNC plastic machining is that it can do jobs requiring three-dimensional cutting with only one set of prompts. It offers a lot of creative possibilities. Because of this, it is favored for use in more complicated components. Machining by hand necessitates the presence of an operator, who uses controls such as levers and buttons to direct the machine or machines. Machining plastic with CNC eliminates the need for the additional worker previously required. This cutting-edge approach necessitates the provision of precise instructions to a machine for it to carry out a set of responsibilities successfully and automatically.

Creating Plastic Prototype with CNC Machining

Metal was traditionally the go-to material for operations until machinists began employing CNC machining for prototype production. However, this has since changed. It is because injection molding used to be the primary method of plastic machining in the past.
To create the mold for injection molding, you will need to pay a significant amount of money. During producing the mold, the machinist must also exercise extreme caution. It is the case since mistakes made throughout the procedure might result in extra expenses, which can even cause the project’s total cost to increase. Along similar lines, the following is an explanation of the procedures and some helpful advice for plastic CNC prototyping:

Tool Selection

Even though plastic and metal each possess unique characteristics, the kinds of tools required to work with each are the same. However, the difference between these two types of materials lies only in the ferocity of the cut. The following are some examples of plastic machining tools that may be used for creating plastic CNC prototypes:

End Mills

End Mill

They are among the most known types of milling cutters. They can cut materials in an axial direction and are relatively comparable to drills in their functionality. The CNC machine tool comes in a few different styles. They are differentiated by the number of flutes they possess and their different noses. Some could even have more than eight flutes in their collection. Flat noses, ball noses, and bull noses are the many types of noses.

Ball Cutters

Ball Cutter

You may easily recognize ball cutters by their hemispherical cutting tips, which create corner radiuses on perpendicular sides. These tips are what give the cutter its name.

Hollow Mills

Hollow Mill

This mill has the appearance of being upside down and has the shape of a pipe. Because hollow mills have a cutting edge on the inside of the mill, machinists may utilize these tools to manufacture complex forms such as total points and form radii.

Machine Setup

When machining plastic prototypes, it is as vital to properly set up the tools as to choose the tools. Plastic remains comparatively soft, making it more difficult to leave a mark on the surface of the plastic. When configuring tools used for plastic machining, there are a few things to keep in mind to prevent mistakes like this one and others throughout production, including the following:

  • Before machining, place a soft pad between the material and the cutting tool. It reduces the machine’s clamping force and material cutting marks.
  • The machinist must monitor the distance between the drill and fixtures. During machining, the drilling spot constantly pulls the workpiece up. The material will be removed if the drill is distant from the fixtures. It causes bending or cutting.
  • Finally, the machinist should cool the cutting tool via coolant to avoid plastic melting.

Surface Finish

Plastics are only available in a limited number of surface treatments, in contrast to the extensive range of options available for metal. However, if the machining is carried out accurately, surface finishes may not be necessary. The following are some examples of typical CNC plastic finishes:

  • Utilizing compressed air and an abrasive substance to bead blast includes blasting the surface of a plastic material with an abrasive material.
  • Spraying paints onto the plastic component surface is involved in the painting process. It contributes to improving the overall visual attractiveness of a plastic component. Additionally, it serves to protect the surface of the plastic.

Critical Instructions to Plastic CNC Machining

Engineers have to adhere to various guidelines depending on the technique of CNC machining that they use to manufacture high-quality plastic CNC machined components. In the following paragraphs, we will investigate each in further detail.

Plastic CNC Turning

In CNC turning, a plastic component is held in position on a lathe. The workpiece is turned against the cutting tool in a revolving or turning action. CNC turning may take various forms, and each of these forms has its own set of goals and outcomes exclusive to it. For instance, straight or cylindrical CNC turning is especially suitable for making massive cuts. In contrast, taper CNC turning produces a characteristic cone-like appearance.
The following are the guidelines for creating plastic components using CNC turning machines:

  • To reduce the amount of rubbing, be sure that the cutting edges get a negative back rake.
  • Implementing sizable relief angles onto the cutting edges.
  • The top surfaces are polished, which helps minimize the accumulation of material and produces a superior surface finish or quality.
  • Utilizing delicate C-2 grade carbide inserts.

We advise utilizing a feed rate of 0.015 inches per minute (IPR) while doing rough cuts. We suggest using a feed rate of 0.005 inches per revolution (IPR) for more accurate final cuts. Additionally, product teams must be aware that the clearance angles, rake angles, and side angles will alter based on the material utilized in CNC turning.

Plastic CNC Milling

The technique of CNC milling is a kind of computer-controlled milling in which a milling cutter, which is a revolving tool with a cylindrical shape, is used to remove material from a solid substrate to expose a finalized component. Milling machines controlled by a computer may be divided into two primary groups: those with three axes and those with multiple axes. Milling machines with three axes can move the cutting tool or the workpiece in all three linear directions (left to right, back and forth, and up and down), making them ideal for producing designs with less complex elements. Milling machines with four axes or more and multi-axis milling machines are the most effective tools for making plastic components with intricate geometries.
The following are the particular guidelines for the creation of plastic components by CNC milling:

  • When machining with a thermoplastic reinforced with glass or carbon fibers, it would help if you used tools made of carbon (carbon tooling).
  • By using clamps, more incredible spindle speeds may be attained.
  • Using end mills for rounded interior corners and pockets can reduce stress concentration.

Plastic CNC Drilling

CNC drilling includes inserting drill bits into a plastic block to create holes. After being lowered onto the block, a spindle holding block and CNC drill will begin to drill holes of the proper size. A CNC drilling machine is compatible with various drill presses, including radial CNC drill presses, upright CNC drill presses, and bench CNC drill presses.

It is indispensable to maintain the CNC drill bits sharp, which you use to create plastic components. Worn-out or wrong-shaped drills might stress the element excessively, leading to tooling difficulties and performance concerns. We suggest using a drill bit with a lip angle of 9 to 15 degrees for most thermoplastics, and it should range from 90 to 118 degrees. When drilling with acrylic, it would be best to use a rake that is 0 degrees.

Chip ejection from the CNC drill is critical, as improper chip ejection may lead to friction and the buildup of heat. Reducing the amount of heat produced by CNC drilling, which creates more heat than almost any CNC machining technique, is essential to limiting the effects of stress and damage. When trying to remove a CNC drill securely without damaging the component, you should make sure that the drilling depth is no more than 3-4 times the diameter of the drill. It would help slow down the feed rate when the drill has almost left the material.

Selection Guide to CNC Plastics

In this section, we will walk you through matching the requirements of your project with the appropriate material for CNC plastic machining. So, here we go.

ABS Plastic CNC Machining

Description

The abbreviation for the plastic known as ABS stands for “acrylonitrile butadiene styrene.” It provides excellent impact strength, toughness, and resistance to electrical current at a cheap cost. Additionally, finishing it is simple since it is simple to paint, glue, or weld it together, depending on your preference. When machined, ABS will have a matte surface. However, the finish may be relatively sparkly depending on the specifics of the machining process. A CNC milling machine is required for the cutting or engraving stages of the ABS CNC machining process.

Strengths and Weaknesses

ABS is an excellent material for prototyping with a broad function, for pre-molding prototypes, for components that will be subjected to impact and need robustness, and when it is desirable to keep costs low. On the other hand, ABS does not have a strong resistance to abrasion or chemicals and will melt when exposed to acetone. In addition to that, it is not an exceptionally sturdy plastic.

Applications

The most typical uses of ABS include making pre-injection molding prototypes, enclosures for electrical components, home appliances, and even the recognizable Lego bricks.

Nylon CNC Machining

Description

Nylon 6/6 is a versatile material that is both strong and long-lasting. It may be used for a broad range of different purposes. It possesses a reasonably strong resistance to both heat and chemicals, the requisite rigidity to keep its shape, and the strength to resist deformation even when subjected to loading circumstances. Glass-filled nylon is another standard option, and it is an excellent material for CNC machine shops to deal with.

Strengths and Weaknesses

Nylon offers excellent electrical insulation, chemical and wear resistance, and high strength and rigidity maintained across a broad temperature range. Nylon 6/6 is a perfect material option for situations in which low-cost components that are nevertheless robust and durable are required. The dimensions of glass-filled nylon remain constant despite its rigidity, strength, hardness, and toughness. But nylon 6/6 can absorb water, resulting in the material swelling and becoming less accurate in its dimensions. As a result of the internal stresses inherent in the material, it may also distort if there is a significant degree of asymmetric material removal while machining.

Applications

The most typical applications for nylon are medical hardware, mounting hardware for circuit boards, electrical insulation, components for the engine compartment of automobiles, and aeronautical components. It is utilized instead of metals since it is a more cost-effective material.

Acrylic CNC Machining

Description

The chemical makeup of acrylic is characterized as PMMA (Poly Methyl Methacrylate) plastic. Acrylic is also known by the trademarks: Plexiglass or Lucite. It is scratch-resistant, impact-resistant, tough, and can be readily fixed using acrylic cement.

Strengths and Weaknesses

It is an excellent choice for applications needing optical transparency or translucence and a substitute for polycarbonate that is less durable but more cost-effective. Because of its resistance to scratching, it is an excellent choice for locations that are just somewhat abrasive.
On the other hand, acrylic is a brittle plastic that is more likely to fracture or shatter than stretch. When working with acrylic components, pay attention to the wall thickness since thinner walls tend to be more brittle. The machined surfaces will lose transparency and appear frosted and opaque when machining. Hence, it is ideal to check whether or not a component made of acrylic should be kept at its stock thickness to maintain its transparency. Polishing a machined surface is an extra post-processing step that may be done if there is a need for the surface to be transparent.

Applications

In its machined state, acrylic is transparent. It is most often used as a lightweight alternative to glass or for the construction of light pipes. Components for car light systems, light tubes, tanks, panels, and parts for optical engineering are a few examples of further widespread usage.

HDPE CNC Machining

Description

High-density polyethylene, or HDPE, has a crystalline structure and a naturally opaque appearance with a waxy finish. However, it is also available with a black dyed finish.

Strengths and Weaknesses

In addition to its exceptional resistance to chemicals and electrical insulation, its surface is also quite slippery. It also provides excellent impact resistance at low temperatures and has a low coefficient of friction. Its inexpensiveness and durability make it perfect for CNC machined plastic components. But the most significant drawback of HDPE is that it has a low strength, especially when subjected to tension and flexing. Thus, it is vulnerable to cracking under stress.

Applications

HDPE is often utilized in fluid applications, such as fuel tanks, plastic containers, and pipes. Due to its resistance to chemicals and its slipperiness, it is excellent for weight-sensitive or electrically sensitive applications. In addition, it is perfect for creating plugs and seals.

Polycarbonate CNC Machining

Description

Polycarbonate, also known as PC, is the most long-lasting kind of plastic. PC remains a transparent thermoplastic material. In addition to that, it is one of the plastics machined with a CNC the most often and is also one of the plastics recycled the most frequently worldwide. In its natural state, it has the appearance of a clear milky blue, but in the marketplace, it may be purchased in black color. Glossiness is an inherent quality of both the black and milky blue colors.

Strengths and Weaknesses

Polycarbonate has extreme resistance to impact, as well as very high stiffness. It maintains its functionality throughout a broad temperature range.  Applications that need a highly tough or robust plastic and those that require polycarbonate’s optical clarity are the most significant candidates for using this material.  It is common practice to use polycarbonate instead of glass since it is 250 times more resistant to impact than glass and has a better resilience than acrylic.
The unmodified form of polycarbonate does not have much wear resistance and is easily scratched. Further post-processing steps (vapor polishing and anti-scratch coatings) may be applied to increase the material’s wear resistance or optical clarity. As it is not easily accessible in sections with a thickness of over two inches, the part size fabricated from polycarbonate is restricted.

Applications

Because of its durability and purity, polycarbonate is used for a wide variety of products: optical disks, light pipes, safety glasses, CDs & DVDs, mobile phones, and even impenetrable glass.