The Most Common Problems in Plastic Routing
The diversity of plastic materials in the industry today makes it almost impossible to avoid some kind of machining problem. Material differences, combined with a wide range of applications, can serve as a precursor for routing problems.
Before tackling the most common 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.
As a beginning point for machining and tool selection, plastic can be categorized as either soft or hard plastic. This can be determined by the flexibility or rigidity of the material or the type of chip it produces in the routing process.
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.
Cutting Tool Geometry Is Paramount
With the diversity of plastic in mind, it is equally important to acknowledge router tool geometry as a key to success. From the beginning, it was self-evident that plastic machined much differently than 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.
Today, there are literally thousands of tools at the disposal of the plastic fabricator. By utilizing the soft and hard plastic categorization, a general tool selection process can be developed.
Soft plastic utilizes “O” flute router tools in straight or spiral configurations. Hard plastic tools may use “V” flute straight tooling or “O” flute spiral tooling with hard-geometry considerations.
The decision regarding straight or spiral tooling hinges upon the direction the user wants to influence the chip or part. Straight tooling has a neutral effect, while spiral tooling can move chips in an upward or downward direction.
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.
Chipload = Feed Rate ÷ (RPM × Number of Cutting Edges)
The secret in plastic routing is producing an adequate-sized chip to remove heat while accommodating finish requirements. Merely raising feed rate to achieve maximum chip thickness is not always the best approach.
In the case of small parts, where feed rate is limited, 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 can cause welding because of limited chip-clearance capability. Selecting the right geometry for the router tool will fail 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.
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.
Besides chipload, other areas of concern with finish include improperly holding parts and the condition of the CNC machine itself.
CNC routers incorporate two different spoilboard systems that 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 edge finish. The best plastic cutting tool will not perform properly if the machine and collet system are not maintained to industry standards.
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. Additional resources include plastics material manufacturers or suppliers, CNC machinery manufacturers, and technical article archives.
The most common problems of welding and poor finish are not insurmountable. The key is to understand the material being machined, select router tools with geometry specific to plastic, and 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.