Routing & Trimming PET

Tooling choices for trimming PET and PETG are dependent upon the materials' unique characteristics, such as whether it is reinforced or unreinforced.

BY VAN NISER

PET (polyethylene terephthalate) is a popular material for thermoforming and can be formulated several ways: unreinforced, glass fiber reinforced, other types of reinforced and PETG to name a few. Each of these formulated materials has unique characteristics which may require different cutting tools, spindle speeds, feed rates and fixturing to optimize finish and production rates.

PET is used for POP displays, drafting templates, window well covers, beverage centers, auto accessories and many types of clear or see-through covers. Unreinforced PET and PETG are flexible materials that will flutter when machined if not fixtured properly. All PET materials can be gummy, yet tough, and require the less violent cutting action of an O flute router bit. Do not use V or Z flute cutting tools with PET because all grades are subject to crack propagation which will occur more frequently with such geometry.

Routing the Products
Routing products such as displays, auto accessories and beverage centers on a CNC router is best accomplished with solid carbide single-edge O flute router bits. If a curved edge is required, a solid carbide O flute edge rounding bit is the best choice. Both tools are also available in a two-flute configuration which can be tested if finish is a problem with the single-flute tool.

If air routers are being used to rout or trim unreinforced PET or PETG, high-speed steel O flute router bits may be the most cost effective choice. The high-speed O flute bit is shown in Figure 3. Solid carbide O flute bits are also a good choice for air routers when cutting all types of PET.

When programming a CNC router for cutting PET, there are a few considerations that will dramatically affect production, scrap and finish. Entry into the cut should be a ramping and not plunging action. Outside or perimeter sharp corners should be programmed as a loop or exit ramp movement to prevent tool stops or dwells which can cause crack propagation, heat or burning. Spindle speed should be set at 18,000 rpm and feed rates between 200 to 350 inches per minute for 1Ú4 -inch and below diameter tools. PET should be cut as fast as possible with a one pass cut. There is reasonably little experimenting to be done other than finding the fastest speed rate when routing PET materials once the right cutting tool has been selected and proper programming techniques have been employed.

Van Niser is director of Plastic Application Engineering at Onsrud Cutter. Based in Libertyville, IL, Onsrud Cutter is a manufacturer of a wide range of cutting tools for the plastics industry.

If you would like to contribute a question or topic for a future article, please submit it to VanNiser@Onsrud.com or fax it to (847) 362-5028.

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