The diversity of plastic material in the industry today makes it almost impossible to avoid some kind of machining problem. Plastic varies greatly through the manufacturing process, and these differences, combined with a multitude of applications, can serve as a precursor for problems. The focus of this discussion will be the most common of these plastic routing problems. However, before tackling the problems, a few basic premises should be reviewed.
All plastic is not created equal. Many times, a user will know the trade or generic name of a plastic being routed, but will fail to recognize there are physical properties of the material germane to the machining process. It is important to understand that even a change in color can drastically alter the way a plastic material reacts to a cutting tool. Luckily, there are resources, which will be addressed later, to sort through these differences. However, as a beginning point for the machining and the tool selection purpose, plastic can be categorized as either soft or hard plastic. This can be determined easily by the flexibility or rigidity of the material or the type of chip it produces in the routing process. When proper routing tools with plastic relevant geometry are utilized, the soft plastic will "curl" a chip, while hard plastic produces a "splintered wedge" chip. However, sometimes there can be soft and hard plastic characteristics within a generic plastic group. For instance, cast acrylic is classified as a hard plastic, while extruded plastic falls on the softer side. They require different tooling considerations because of the way they machine in the routing process. The point is, all plastics are not the same and ignoring this fact can create problems from the beginning. Cutting tool geometry is paramount. With the diversity of plastic in mind, it is equally important to acknowledge router tool geometry is the key to success. From the beginning, it was self-evident that plastic machined much differently that of other materials and special considerations were required. Through years of testing, tools specifically toleranced for plastic routing have been developed for hand fed as well as CNC applications. The common denominator in the success of these router tools is the presence of high rake and low clearance in the geometry of the tool. |
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Common Routing Problem #1: Welding of plastic material. Welding of chip during the routing of plastic parts is probably the most irritating problem encountered in the industry. It is costly in terms of time and scrap rate, but is also avoidable. Besides inappropriate tool selection, chip welding can occur because of improper chipload, small tool diameter size, influencing the chip improperly, or the direction of the cut. Plastic is extremely sensitive to heat, and the act of routing at high feed and spindle speed rates creates a rather warm environment. Chipload, which is the thickness of the chip, is formed through the function of feed rate, spindle speed, and the number of cutting edges in the router tool. The chip is the mechanism by which heat is transferred away from the tool and the part, thus maximizing it is critical. The secret in plastic routing is producing an adequate sized chip to remove heat, while accommodating finish requirements. The chipload formula is as follows: chipload = feed rate/ (RPM x # of cutting edges). As the formula indicates, there are several ways to adjust chipload. Merely raising feed rate to achieve maximum chip thickness is not always the best approach. In the case of small parts for instance, where feed rate is limited, the other part of the equation, namely spindle speed should be utilized to maximize chipload. Welding can also occur because of small tool diameter size, direction of cut and the way the chip is influenced. Small diameter tools because of limited chip clearance capability can cause welding. Selecting the right geometry for the router tool will fail miserably, if the chip is influenced incorrectly. For example, using a downcut spiral in a blind slot will serve to recut chips and thus weld. Lastly, direction of cut may be the culprit. In most cases, conventional cutting direction is recommended. |
Common Routing Problem #2: Poor finish. Probably the most important consideration in the plastics industry is the surface finish of the final product. This is especially evident in plastic products such as exhibits, signs, or P-O-P displays, where the public constantly views the finished edge of the product. As with all plastic routing applications, the selection of the proper tool is essential to good edge finish. However, finish is heavily influenced by the chipload. In plastic routing, the continuous generation of a properly sized chip will eliminate excessive knife marks in soft plastic and a cratered finish on hard plastic. The range of chipload for outstanding finish seems to occur between 0.004 and 0.012.
Besides chipload, the other areas of concern with finish lies with improperly holding parts and the condition of the CNC machine itself. CNC routers incorporate two different spoilboard systems, which utilize vacuum to hold parts. The dedicated spoilboard is most prevalent in thermoforming, while the flow-through system is more popular in sheet fabrication. In both cases, poorly designed spoilboard systems lead to inadequate part hold-down and subsequently unacceptable part finish. Lastly, the condition of the machine and the tool holding system is critical to excellent edges finish. The best plastic cutting tool invented will not perform properly if the machine and the collet system are not maintained to industry standard. The tool must run in a concentric or true circle to function at optimum levels. If not, the finish will deteriorate and the scrap rate will accelerate.
Resources. This information serves as a starting point to identify and correct the most common problems associated with routing plastic. However, there are many resources to further study these areas of concern and receive more detailed data. These would include the plastics material manufacturer or supplier, the CNC machinery manufacturer, www.plasticrouting.com, and The IAPD Magazine article archives at www.theiapdmagazine.com, where specific articles regarding tool selection, collet maintenance, and spoilboard techniques can be found and researched. The most common problems of welding and poor finish are not insurmountable. The key is to understand the material being machined, to select router tools with geometry specific to plastic, and to apply them with proper chipload recommendations. Rigidly holding parts and maintaining the integrity of the router machine through proper maintenance procedures will further enhance the process.
About the Author Van Niser is director of plastic application engineering for LMT Onsrud , Waukegan, IL. For more information about the company’s cutting tools, call (800) 234-1560 or visit www.onsrud.com or |
