Major Considerations in the Routing of Plastics

Before routing or machining plastic, the user must first evaluate the capabilities of the machinery, fixturing, tooling, and cutting process involved.

Capability

The first consideration is machinery, which can vary widely throughout plastic fabrication. Common machines include air and electric routers, pin routers, and CNC machinery. These machines are used extensively in sheet fabrication, thermoforming, rotational molding, and blow molding operations.

Hand-Fed Routing

Air, electric, and pin routers fall into the category of hand-fed applications and present a different set of circumstances than CNC machining.

Because these machines are heavily influenced by operator skill, high-speed steel or carbide-tipped tooling with a steel shank is generally preferred. These tool materials are more forgiving in hand-fed applications and are less likely to fail than solid carbide, which performs best in the controlled environment of a CNC machine.

Air Router Tooling

One of the more prevalent machines in hand-fed applications is the air router. Its construction has a direct effect on tooling requirements.

Air routers typically incorporate a nose or support bearing, guard, and guide bushing. These components perform important functions but require router tooling designed specifically for the machine.

The tool must have sufficient overall length to bottom out properly in the collet while extending beyond the guide bushing and reaching the workpiece.

The tool must also be slightly undersized along the appropriate portion of its length so that it passes smoothly through the support or nose bearing.

CNC Routing

CNC routers are extremely popular among plastic fabricators, with both 3-axis and 5-axis machines filling different needs.

Flat-sheet fabricators generally use 3-axis machines, while thermoformers machining formed or multi-shaped parts at various angles account for much of the 5-axis equipment.

Solid carbide is widely used in CNC routing because of its toughness and cutting-edge longevity when chipload is properly maximized.

Solid carbide router tools are available in a broad range of geometries and styles. Sheet fabricators often prefer upcut spirals because they assist in evacuating potentially soft plastic chips.

Thermoformers using formed fixtures frequently use straight-edge tooling because it has a neutral effect on the part.

Downcut spirals can be used in some 5-axis applications, but the fixture must allow chips to fall freely away from the workpiece. If chips remain trapped near the cut, they can be recut and cause welding that damages both the part and the router tool.

Part Hold-Down

Regardless of the machinery being used, securely holding the part is critical.

The three primary hold-down methods are mechanical clamping, dedicated spoilboard systems, and flow-through spoilboard systems.

Dedicated and flow-through systems are the most common in CNC routing. Flow-through systems have become popular because of their ease of setup, while a properly designed dedicated spoilboard generally provides the most rigid part holding.

Machine Maintenance

Machinery is only as effective as its maintenance schedule. One of the most critical maintenance areas affecting router tooling is the collet system.

Cutter concentricity can only be maintained with a clean and properly maintained collet system.

Tool Selection

Once machine capability, maintenance, and part rigidity have been established, tool selection becomes paramount.

Router tools for plastic cutting are highly dependent on both the application and the material. In most cases, a single cutting tool should not be expected to perform equally well across a wide variety of plastics.

Soft vs. Hard Plastics

As a general starting point, plastics can be categorized as either soft or hard.

Soft plastics tend to curl a chip during machining. Hard plastics tend to produce a splintered wedge that breaks away during the cut.

Cratering in hard plastics occurs when the shear strength of the material is exceeded during routing.

These tooling recommendations provide useful starting points, but they remain general in nature. Tool geometry should ultimately be selected for the specific plastic material and application.

Chipload

Once tool selection has been finalized, chipload becomes another critical consideration.

Chipload is the actual thickness of the chip removed by each cutting edge. It is determined by spindle speed, travel speed, and the number of cutting edges on the tool.

Finish requirements are ultimately application-specific. Some applications may justify a larger chipload and a slight reduction in finish quality in exchange for increased productivity.

The recommended range should therefore be treated as a guide rather than an absolute limitation.

Conclusion

Plastic is increasingly being machined in routing and machine shops, but it cannot simply be treated like metal or wood.

A systematic process that considers machine capability, part hold-down, tool selection, maintenance, and functional chipload is critical to successful plastic machining.

Once these factors have been addressed, the routing process can be optimized for improved finish, productivity, and tool life.