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Cast polyurethanes can be readily turned, sawed, drilled,
ground, or milled. These and other secondary operations present
many similarities to the machining of metal, but there are
also some important differences. This paper is intended to
provide some general guidelines for machining urethanes, and
also focus on the most common group of machining operations
and discuss some specific tools and techniques.
It is important to note the material
presented here is a starting point. The wide variety of urethane
compounds and their respective physical properties and characteristics
creates a wide range of machining situations. Experimentation
and experience will tell you what speeds, what feed rates,
and what types of tools will work best for the urethanes you
machine regularly.
Harder urethanes – 90A and up
– have a high degree of machinability. Lathe turning,
fly-cutting, grinding, contouring, and more are easily accomplished
on conventional metal-working equipment by machinists who
are familiar with procedures for handing plastics.
Some different tools and techniques
are required for compounds of 80A durometer and lower. These
lower modulus compounds are typically machined by knifing,
grinding, and sanding. In some cases, however, they can be
worked like higher modulus materials by “freezing”
them in dry ice or liquid nitrogen environments.
SOME KEY POINTS TO REMEMBER
IN MACHINING URETHANE
| 1. |
Urethanes have much lower thermal conductively
than metals, so heat generated by cutting tools stays
close to the tool and raises the urethane temperature
rapidly. This heat must be controlled.
Melting can occur above 400 degrees F.
In addition to possible melting, heat generated by machining
causes the part to expand. When that part cools, it
shrinks down and can end up undersize. |
2. |
Elastic memory - Elastic recovery
occurs in urethane both during and after machining.
The cutting tool must provide clearance to compensate
for this. With compensation, expansion of the urethane
as it passes the tool will result in increased friction
between the cut surface and the cutting tool. Excess
heat build-up will result.
Elastic recovery after machining can result in smaller
internal diameters and larger external diameters than
were measured during cutting. |
3. |
Modulus of elasticity - Urethanes are
resilient and can easily be distorted. It is possible
to alter the shape of a urethane part by clamping or chucking
it with too much force. This would cause the final machined
shape to be distorted after the cut had been made and
the fixturing pressure was released. Care must be taken
to hold parts securely, but avoid distortion due to holding
or cutting. |
4. |
Softening point - Gumming, poor finishes,
and poor dimensional control will occur if excess heat
is generated and allowed to accumulate. Proper tool geometry,
feed rates, and cutting speed, in conjunction with coolants
usually overcome these problems. Water soluble cutting
oils and/or light machining oils are good coolants for
urethanes. |
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Die-Thane Tooling 
