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Machining Plexiglass and Lexan

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Tools for Success by Scott Burton
Scott Burton is the sales and marketing manager for Royce//Ayr Cutting Tools. For more info email: sburton@royceayr.com

Images 1, 2 and 3.

Global health challenges have dramatically increased demand for transparent plastic products 
to be used in production 
of safety devices and commercial furniture.
Personal shield devices and fixed separation barriers are being machined in large volume in various formats. Many companies who traditionally machine wood products, have expanded their scope of machining into plastic products. The two most common products being used are acrylic (plexiglass or Lucite) and polycarbonate (Lexan).
Although both materials are commonly used in similar end products, there are some important differences between the two materials, which affect machinability.
Image 1 shows both plastic products, which are very similar in appearance.

Acrylic (plexiglass 
or Lucite)
easier to crack  
greater resistance against scratching
greater UV resistance than polycarbonate
can be polished smooth if necessary
shinier and does not yellow over time
less expensive

Polycarbonate (Lexan)
less clarity, cannot be polished to restore clarity
high heat deflection  
low moisture absorption
high tensile, shear, and flexural strength, which is 30 times than that of acrylic
can withstand temperatures up to 
240 degrees Fahrenheit
can be “cold-bent” 
without applied heat
highly resistant to acids 
and other chemicals such 
as gasoline
more expensive

FOCUS ON 
CUTTING TOOLS ROUTER BITS:
There are a wide variety of tool geometries available for routing plastics, which can be confusing, given the amount of tooling options, and material variations. To simplify the topic for machining purposes, plastic types can be divided simply into soft and hard categories. Generally, a plastic name including “poly” will be soft and most acrylics are considered hard.

Challenges:

Heat generation when routing plastics is a concern that needs to be addressed, otherwise plastic chips will re-weld to the cutting tool resulting in poor cut quality and tool life. Material thickness and the melting point of each grade of plastics will also contribute to this problem. Lexan is easier to work with because it is softer, and more flexible, but most plastic compositions are more susceptible to melting, compared to wood products.
Cut quality on the edge of parts is another important factor, which is directly impacted by the heat generation challenge mentioned above. Depending on the final product being produced, the expectations for edge quality can be very different. However in most cases, manufacturers need to achieve a good finish off the CNC router to reduce secondary finishing processes.   

Solutions:

Selection of the proper cutting tool is critical to achieve optimal results on any grade of plastic. Generic router bits designed for solid wood or wood composites will NOT perform on plastic materials.  
Image 2 shows a single flute plastic router bit, commonly known as “O flute” geometry, due to the “O” shaped design of the flute. This geometry effectively allows the plastic chips to curl off the primary cutting edge, and extract with minimal heat generation. It is very important to note that specific geometries are available for soft and hard plastics.  Both designs will look similar, but will have varied cutting-edge clearances and hook angles. Therefore, it is very important to understand the composition of the material being machined and choose the proper geometry. Most cutting tool manufactures make it very simple by listing soft and hard in catalogues and websites.
Machine parameters (feeds & speeds) are also very important variables, which must be considered before cutting plastic on a CNC router. There is a delicate balance between acceptable cut quality and limiting heat generation in the cut. If chip load is too small, the material will melt, but a larger chip load will have a negative impact on surface finish.   
Feed rate for cutting plastic is typically 75-300 IPM, depending on the size of your bit. For example, a 1/8” bit will work better on the low end (75-100), while a 1/2” bit would work well at 200-300.
RPM (spindle speed) is generally set at 18,000 to 24,000 and can be adjusted depending on feedrate and desired finish quality.
SAW BLADES:
Most plastic materials can be cut on traditional woodworking saw equipment, with modified tooling as a necessary requirement to achieve optimal results. Most saw suppliers’ stock specific plastic cutting blades for large panel saws, table saws, and cross cut saw machinery.

Challenges:

Like router tooling, the greatest challenge is finding a balance between acceptable cut quality and minimal heat generation. Poor cut quality can be an issue depending on the product, and expectations of edge quality. Melting due to heat generation can be an issue, depending 
on plastic composition and material thickness.

Solutions:
Image 3 shows a 3D model of a saw blade designed for plastic cutting. Machining plastic with a saw blade also requires specific cutting-edge geometry. Depending on the grade of plastic, thickness of material, and design of machine, the geometry will vary slightly.
Tooth style is designed to apply minimal cutting pressure to brittle plastics. Hook angle of the saw blade is minimized to further decrease aggressive cutting pressures. Saw manufacturers can revise the side clearance of the sawtooth to provide different levels of cut quality on the edge of parts.
Tooth count on the blade can be changed according to feed and speed requirements, as well as maximum material thickness. Material thickness over half an inch will require fewer teeth to allow chips to evacuate from the cut and minimize heat generation.  Thinner materials will require more teeth to ensure premium cut quality on the edge of parts.   

CONCLUSION
Market volatility often forces manufacturers to adopt new processes and enter new markets. Plastic machining has been an emerging market for many years, and crosses over into the wood industry in many product lines.
Corian countertops and phenolic surfaces are all considered a form of plastic which can be machined on traditional CNC routers and woodworking equipment.
The investment in new cutting tools is relatively inexpensive, so the transition into plastic machining is not a large financial investment. Spending some time and resources to optimize plastic machining can be a worthwhile investment as our businesses adapt to 
new challenges.

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