Preparing for Plastic Routing - Part 2

As companies transition from routing wood or aluminum to machining plastics, planning around material selection, tooling, and programming can reduce startup problems and improve consistency.

Factors that may be secondary when routing wood or aluminum can become critical in plastics machining. Good preparation can help reduce the costs and difficulties associated with starting a new machining process.

This is the second article in a two-part series. Part 1 focused on the CNC router and its associated hardware. Part 2 focuses on material selection, tooling selection, and programming considerations.

Material Selection

Depending on customer requirements, fabricators may have some flexibility in specifying the exact plastic material to be routed. When possible, this flexibility should be used to reduce machining problems later in the process.

Some plastic sheet manufacturers offer versions of common materials that are specifically marketed for improved machinability. These materials can produce better chips, reduce melting or scarring, and sometimes lessen chip wrapping during plunging operations.

Different grades of the same basic plastic can also behave very differently during routing.

Some grades are also formulated as copolymers, meaning a sheet sold under one general material name may contain another plastic within the formulation.

New grades should therefore be test-cut before estimating tooling costs and cycle times.

Extrusion Direction

Another characteristic worth verifying with the sheet manufacturer is whether machinability changes depending on the extrusion direction.

This can be thought of similarly to cutting with or across the grain. Most plastics show little difference, but some materials can machine differently depending on direction.

Tooling Selection

Tooling selection should be considered early in the preparation process.

It may be difficult to predict tool life accurately when working with a new process or material, but the selected tooling can have a major effect on cycle time, fixture design, programming, and material waste.

There are many dedicated plastic-cutting tool styles available. Narrowing the selection to one or two likely tools or tool families can make it easier to determine material-removal rates, chip direction, and appropriate cutter diameters.

Programming

Cut Direction

One of the first programming considerations is cutting direction.

For many plastics used by fabricators, conventional cutting produces a better edge finish than climb cutting. Exceptions can occur with composites and combined materials.

Nesting software may prioritize material yield and cycle time by reducing the total number of cuts, which can force the cutter into a climb-cut condition.

If nesting software is used this way, sample cuts should be made to determine whether the climb-cut edge quality is acceptable. If not, material usage and cycle time may need to be reconsidered.

Plunging and Chip Wrap

Plunging is another important programming consideration.

Soft plastics can develop severe chip wrap after repeated plunges. Hard plastics can craze or spiderweb depending on tool geometry, plunge speed, spindle speed, and fixture support.

Ramped entry requires careful consideration of entry-hole location and nearby cutting paths. Other approaches include pre-drilling entry areas and using router tools with specialized point geometries.

Hard-Plastic Plunging

Poor plunge finish in hard plastics can also be addressed with ramped entry, although other options are available.

Craze-resistant grades of common plastics may better withstand the pressures created by standard router-bit plunging.

Rigid fixturing with support directly beneath the entry point can also improve plunge consistency.

Specialized point geometry may be useful when other methods are insufficient.

Fixturing Through Programming

Programming can also improve the effectiveness of part fixturing.

In 3-axis routing, one common technique is to leave the paper masking on the bottom of sheet goods intact. The cutter passes through the plastic without cutting through the masking.

This helps preserve vacuum and provides additional vertical and lateral holding force on small parts.

This method requires a very flat spoilboard and may require frequent resurfacing as humidity and temperature cause the board to change shape.

Five-Axis Routing

Five-axis routing programs can sometimes benefit from higher spindle speeds, which reduce chip size and may lower the cutting force applied to fixtures.

Multi-flute tooling can also reduce cutting force, but it can increase the likelihood of melting and accelerate cutter wear.

Order of Operations

The sequence of machining operations should also be reviewed for opportunities to improve part hold-down.

Small cuts and pockets should generally be made first while the surrounding scrap material still provides a large vacuum-holding area.

Large part cutouts should be completed later in the program so the remaining sheet continues to support and stabilize both finished parts and scrap.

Conclusion

Advanced planning can prevent many of the problems associated with starting new plastic-routing jobs or processes.

Material selection, tooling, programming, runout, collets, vacuum, dust collection, and cooling should all be considered before production begins.

Addressing these areas helps ensure that tooling, equipment, fixtures, and programs are ready for production and that actual machining costs remain reasonably aligned with estimates.