Machining of K•Prene® urethane proves useful when the required tolerances cannot be obtained by conventional molding methods, or when low production runs do not justify the cost of molds. We maintain a complete inventory of sheets, round rods, and bars in six different hardnesses which can often be used to make prototypes or limited production quantities.

The table below details the general machining characteristics of K•Prene® urethane K-900A durometer and harder. For K•Prene® urethane below K-800A durometer, cutting tools are often not satisfactory and grinding may be required. In some cases, machinability can be improved by freezing K•Prene® urethane below K-800A durometer with dry ice or liquid nitrogen.

Machining of K•Prene® urethane is similar to that of metals, and in most cases the same tools can be used.

machined urethane parts

The principle differences between metal and urethane machining are:

Urethanes are resilient and can be easily distorted, so care must be taken to avoid distortion due to holding or cutting forces.

Gumming, poor finishes and poor dimensional control will readily occur if excess heat is generated and allowed to accumulate. Proper tool geometry, feed rates and cutting speed in conjunction with coolants can usually overcome these problems. Water-soluble cutting oils and/or light machine oils are good coolants for urethanes.

Heat generated by the cutting tools stays close to the tool and raises the urethane temperature rapidly. This heat must be controlled. Melting can occur above 350° F.

Elastic recovery occurs in urethanes both during and after machining, so the cutting tool must provide clearance to compensate accordingly. Without compensation, expansion of the urethane after cutting 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 or larger external diameters than were measured during cutting.

Machining Applications Recommended Tools Size Limitations Maximum Speeds Workholding Recommendations Suggestions
Bandsawing Sharp 2-4 hook carbide blade with raker set Max. 8″ thick, Min. 1/8″ thick sheets  1800 ft./min. (reduce to 1200 ft./min. below K-900A) Vises or clamps (not required with flats or sheets)
  • Rotate rounds when sawing.
  • Maintain high blade tension.
Cutting, Shearing, Slitting Shears or belt-cutting machines with up to 1/8″ knife blade  Thin sheets (up to 1/2″) Friction clamps
  • Use bevel edge blades (square degree, compress urethane, resulting in uneven cuts).
Drilling High speed 90º + twist  drills with blunt point Min. of 3/8″ (no minimum if clamped between steel) 600-800 RPM Vises or clamps
  • Good tolerances possible with K-750D, for other grades use over-size drills.
Tapping Regular high speed taps None Vises or clamps
  • Except for K-750D, use over-size tap to allow for shrinkage or urethane.
Turning, Boring H.S.S. toll bits with positive rake of 5º-10º No limits on dia. length restricted (long soft bars may flex) 800 surface ft./min. Angle of cut to work: 90º-120º
  • Form tools may be used (especially when machining K-750D).
Grinding Tool post grinder, 20-80 grit wheel Lathe: 150 RPM or lower, Grinder: 2000-3500 RPM Fixtures or friction mandrels
  • Air blast to eliminate build up.
Knifing Carbide or H.H.S. razor sharp and as thin as possible  600-1000 RPM Fixtures or friction mandrels
  • Use lubricant to help prevent dishing.
Milling Sharp single-blade flycutters with 10º back rake and good clearance Min. 3/8″ thick  Approx. 2000 surface ft./min.  Vise or double-face clamping tape
  • Flycutting at approx. 3″ dia. give best results.