The Cutting Edge of Plastics: Cutting Tools and Routing
by Chris Corona
In today’s plastics industry, being a knowledgeable professional not only means understanding the properties and application of hundreds of different types and grades of materials, but also having a thorough understanding of production and fabrication methods. With today’s ubiquity of CNC routing machines, a basic understanding of cutting tools and routing is essential in providing a superior customer service experience and increasing your bottom line.
Today’s plastic tooling generally comes in three basic varieties: solid carbide, high-speed steel (HSS) and carbide tipped. For most CNC production routing applications, solid carbide is the tool material of choice as its low deflection allows for the fastest feed rates and keenest edge finishes. High-speed steel tooling is most often used in hand routing of plastics, where inconsistent hand feed rates can lead to breakage of solid carbide tools. Carbide tipped tooling is often used in hand routing as well, particularly in hand routing of tougher composites where the edge finish carbide provides would be of benefit, while providing the core strength of high-speed steel to avoid breakage.
Basic tool geometry
Tool geometries for plastic routing generally fall into two main categories: O-flutes and V-flutes. The appropriate choice of geometry comes down to the type of plastic being routed. “Soft plastics,” such as ABS, polycarbonate, extruded acrylic and olefins, generally require the use of O-flute geometries, while “hard plastics,” such as cast acrylic, melamine, nylon and PVC may require the use of V-flute geometries. O-flute geometries create a “scooping” action in the substrate, aiding in the creation of small curly chips, whereas V-flute geometries create a “chopping” action that aids in the creation of dust-like chips. A helpful analogy is to think of hard plastics as a block of ice and soft plastics as a tub of ice cream. A block of ice would generally be chopped, while a tub of ice cream would be scooped. Using the incorrect tool geometry can cause poor edge finishes and tool breakages, as the tool is essentially working against the natural chipping tendencies of the material.
Helical direction determines which direction chips are evacuated from the substrate, either downwards (downcut), upwards (upcut) or neutral (straight). Evacuating chips upwards is generally ideal as the potential of welding plastic chips back onto the substrate being routed is greatly reduced. The most consistent edge finishes are most often attained with upcut tooling. However, occasionally because of poor table vacuum, substrate thinness (.093" or thinner), or because of the small size of some parts, upcut tooling may cause the substrate to be pulled up slightly, reducing edge finish quality. In these cases where proper table vacuum cannot be attained, a downcut or straight tool may be appropriate. Your tooling manufacturer representative should be able to help you determine which helical direction is appropriate for your material and routing application.
The number of flutes, or cutting edges, is generally dictated by the size of the part being routed and the attainable feed rate. Cutters with two or more cutting edges can and must be run at higher feed rates than cutters with just a single cutting edge. As there is not enough time to “ramp up” to higher feed rates, small parts or parts requiring intricate detail are forced to lower feed rates, requiring the use of single flute cutters. The highest feed rates and best productivity can be attained with large parts using cutters having two or more cutting edges.
Speeds and feed
If the appropriate tool material, geometry, helical direction and flute count are being used, poor edge finishes and tool breakages can often be traced back to incorrect speed (the number of rotations per minute the routing head makes) and feed settings (how fast the routing head moves relative to the material being routed, measured in inches per minute). The speed and feed rates being used, along with the type of tool being used, determine what is called the “chip load,” or size of the chip being evacuated. For a given material and tool, there is an ideal chip size that will yield the best edge finish and tool life. Your tooling manufacturer should be able to provide a “chip load chart,” which will give a benchmark speed and feed for the tool being used. A reputable tooling manufacturer should also be able to provide on-site aid to help determine the proper speed and feed for the application.
While the basic considerations mentioned previously will help you determine the proper tooling and speed/feed recommendations for the majority of routing applications, certain applications may require the use of specialty tooling. These specialty tools may be coated to impart better tool life or enhance edge finishes while routing exotic materials. Other specialty tooling may incorporate a diamond edge to polish acrylic, eliminating the need to flame polish edges. Your cutting tool manufacturer can aid you in the selection of tooling in these and other specialty applications.
As with any production method, properly maintained equipment is essential. One area of potential concern is in properly maintained collets (the fixtures that hold the cutter). Old collets can become loose, causing vibrations that can lead to poor edge finishes and tool breakage. Improperly maintained vacuum tables can also create these vibrations. Any routing job should begin with a thorough inspection and, if necessary, repair or replacement of these components.
Being familiar with the basics of cutting tools will not only help you be a more knowledgeable and helpful plastics professional, but can also increase your bottom line by reducing material returns and complaints. In today’s competitive plastics sales environment, the next large bid you encounter may just be won by the sales professional that is able to enhance part quality and production efficiencies through the proper recommendation of cutting tools.
Online resources such as www.plasticrouting.com (a website offered by LMT Onsrud), provide real-world speed and feed recommendations for a variety of material types and manufacturers.
Originally published in the International Association of Plastics Distribution Performance Plastics Magazine, February/March 2015 publication.
Chris Corona is the Principal of CartierWilson LLC. For more information, contact CartierWilson LLC, 34194 Aurora Road, Suite 231, Solon, OH 44139 USA, phone (770) 644-0000, email@example.com, www.cartierwilson.com.