ROUTING: Routing Polyethylene

Polyethylene is a widely utilized plastic in the thermoforming and sheet fabrication industry. These manufacturing processes involve secondary routing operations to produce the end product. Part of the popularity of this material can be attributed to how easily it is routed when proper cutting tools and procedures are a part of the manufacturing process.

Plastic is generally classified as soft or hard for routing purposes. Polyethylene falls on the soft side of the equation and mostly utilizes “O” flute geometry routing tools. These tools are available in straight and spiral configurations with single- and double-edge cutting capability. (See Figures 1-4). The choice of straight versus spiral tooling becomes a function of how the chip needs to be influenced. Straight tooling has a neutral effect, while spirals can move chips in an upward or downward direction.

In most instances, the thermoformershould choose straight “O” flutes, while the sheet fabricator should rely on spiral upcuts.

When the part is held solidly, these tooling choices accommodate the removal of chip without adversely moving the part being routed. Most “O” flutes in the marketplace are single-edge because they provide increased chip clearance capability without sacrificing edge finish.

Double-edge tools usually become a part of the routing process when more refined edge finish is required or tool balance is an issue.

Beside the type of tool selection, tool diameter is always a critical issue. Plastic fabricators, particularly sign makers, tend to use small tool diameters because of small radii and material waste considerations. However, with a softer material like polyethylene, larger tool diameters can play a positive role. The larger the diameter, the more flute space is available to remove gummy chips. Also, the larger diameter provides more stability and increased benefits fits in the area of better edge finish. Larger diameters are beneficial, but the end-user should be cautious in this area, as well. Single-edge tools should never be applied in a CNC machine in diameters over 3/8 of an inch. These tools in larger sizes can create out-of-balance issues. When larger than 3/8 is required, double-edge tools should be utilized.

These tool selection ideas are general in nature and provide a good starting point for decision making in this area. In order to enhance this process, the end user can visit www.plasticrouting.com or any number of tooling or manufacturing sites to get specific tool recommendations on polyethylene and a variety of other plastic materials.

Polyethylene, like most soft plastics, will produce a curled chip. The range of chipload to produce and maintain a properly curled chip is somewhere between .004 and .009. Chipload is actually the thickness of the chip, and is a function of the feed rate and the spindle speed of the CNC routing machine. The size of the chip has a direct correlation on the edge finish and the ability to adequately remove soft chips (See Figure 5). When chiploads get too far outside the recommended range, the tool or knife marks are more prevalent, thus adversely affecting edge finish (See Figure 6). Conversely, when chiploads become too small, the ability to clear chips without melting becomes problematic. Consequently, it is imperative to maintain a fairly narrow range of chipload to maintain both of these parameters.

Polyethylene is a mainstay in the plastic fabrication industry. It is one of the easiest plastics to cut, but only when the proper tool selections and chipload requirements are met.

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