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.
The principle differences between metal and urethane machining are:
Modulus of Elasticity
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.
Lower Thermal Conductivity
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)|
|Cutting, Shearing, Slitting||Shears or belt-cutting machines with up to 1/8″ knife blade||Thin sheets (up to 1/2″)||Friction clamps|
|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|
|Tapping||Regular high speed taps||None||Vises or clamps|
|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|
|Grinding||Tool post grinder, 20-80 grit wheel||Lathe: 150 RPM or lower, Grinder: 2000-3500 RPM||Fixtures or friction mandrels|
|Knifing||Carbide or H.H.S. razor sharp and as thin as possible||600-1000 RPM||Fixtures or friction mandrels|
|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|