Here at Mearthane, we frequently get asked a lot of questions about the process of machining cast polyurethanes, from the steps involved to the traits of the finished product, and everything in between. As urethane experts, we’re always happy to take some time to sit down and share helpful information with our valued customers so that they may better understand what we do, and how much care we put into the custom urethanes we produce. Today we’ll be discussing machining cast urethanes, including the differences in how urethanes of different durometers involve different types of machining, the different tools involved, and so much more. Curious about how the urethane experts machine cast polyurethanes to create custom products? Let’s begin!
The Hard Facts about Durometer
First, it’s important to address how the level of hardness or “durometer” factor into the machining process. Generally the harder polyurethanes (70A durometer and greater) are easier to machine, like a harder wood being easier to carve a smooth sculpture from. As far as tooling methods, lathe turning, cutting, milling, grinding, contouring and more are all possible on conventional metal working equipment. Cast urethanes can also be easily turned, sawed, drilled, ground, and milled! Softer urethanes (70A and lower) are typically machined by grinding, sanding, knifing and also by freezing using dry ice or liquid nitrogen which the components to be machined just like the harder polyurethanes. This secondary operation does add cost, but it’s worthwhile because it allows the softer urethane to be machined to your specifications.
Now that we’ve established the difference between how harder and softer urethanes are machined, we can get into more detail about the machining process. Because of urethane’s low thermal conductivity, the machining tools tend to heat up. This can significantly and rapidly raise the temperature of the urethane. Heat generated during the machining process causes the thermal expansion of the component, so when it finally returns to its ambient temperature, it could shrink and measure undersize. That’s where a principle called “elastic memory” comes into play.
Elastic memory is the elastic dimensional recovery that occurs in urethane during and after the machining process. The machining tools must provide clearance to compensate for this, otherwise the expansion of the polyurethane as it passes the tool will result in increased friction between the cutting surface and the machining tool. Elastic recovery after machining can result in smaller internal diameters and larger external diameters than were measured during machining, especially on components with softer durometers. The opposite may occur on harder durometer polyurethanes, so being mindful of this principle is equally important for both hard and soft urethanes.
Precision Makes Perfect
Urethanes are pliable and can be distorted by too much clamping force, resulting in the final machined shape to be distorted after the machining has been completed and the fixture pressure has been released. For this reason, it’s important to grasp components securely without distorting them while machining. In addition to this, there are several other factors that contribute to a precisely machined result. Proper machine tool geometry, feed and speed rates, and use of coolants are necessary to avoid gumming up, poor finishes, and non-conforming dimensional control caused by excess heat. Cool water-soluble machining oils or other light machining lubricants are an optimal choice for urethanes and ensure the best results.
Get Familiar with Fixturing
Fixturing is an extremely important consideration when machining polyurethane. In fact, machining can’t even begin until the fixture is tested for safety and security. Wondering why it matters? Think of the fixture like a seatbelt in your car, if you’re in an accident without the seatbelt, you could be ejected from the car. If the fixture isn’t secure, the urethane component being machined can come loose and be propelled at the operator at an extreme velocity, causing serious injury! Improper machining practices can result in excessive heat build up in the component being machined, damaging it and requiring the component to be machined again, wasting both time and materials. For this reason, if the operator notices any smoke during the machining process they should immediately stop and correct the process. Additionally, for increased safety, operators should refrain from breathing smoke or grinding dust while machining urethanes.
Better Safe Than Sorry
Once urethane has been chosen, and all machining details have been considered, safety is the final trick in producing a high quality product. Proper safety equipment and personal protective gear should be worn at all times while machining urethanes. Most importantly, eye protection is integral. There is no substitute for approved safety glasses with side shields, since they protect all parts of the eye and prevent small bits of urethane from getting into the eye area. For improved protection, goggles or a full face guard should be used to avoid the urethane chips, dust particles, and other foreign matter from hitting the operator in the eyes or face. With high quality protective coverings, the operator can confidently machine the urethane, increasing the chance of a precise end product, and reducing the chance of injury. Here at Mearthane, we take every precaution to stay safe to ensure that we can keep producing the high quality cast polyurethane components our customers rely on. Now that you understand our methods for machining, why not contact us so we can get started on your project today?!