Molded urethane, technically referred to as molded polyurethane (or PU), is an elastic polymer, or elastomer, containing urethane carbamate links. From a molten liquid state, polyurethane may be fabricated or molded into low-density flexible foam, low-density rigid foam, soft solid elastomers (gel) and hard solid plastics. Valued primarily for its uses in rigid and flexible memory foams, molded urethane is extremely useful in solid plastic form as well.
Molded urethane combines many of the desirable qualities of plastic, rubber and metal with a longer service life than plastics, higher impact resistance than rubber, and better noise reduction and resilience than metals. In addition, molded urethane possesses several advantages over other elastomers and materials, such as higher abrasion, cut, tear, oil, ozone and radiation resistance, heavy load bearing ability and a broad hardness range. Molded urethane products include a wide variety of hydraulic and pneumatic seals, pressure pads, shock absorber bushes, machine couplings, conveyor belt scraper blades and parts for a broad spectrum of industries such as construction, automotive, mining, engineering, food processing and athletic equipment.
The raw materials of polyurethane exist in a liquid state, which allows for easy mixing and measuring in preparation for molding, which may be open casting or closed casting. Raw materials react with one another to form a pre-polymer, then during the urethane molding process, a curative is introduced to the pre-polymer in order to complete the polymeric transition. Accelerated by heat and/or pressure, the mixture is poured into a mold cavity and cured to form the final polymer. During open urethane molder processes, the pre-polymer and curative are heated and mixed together, poured into an open cavity and cured without the application of pressure. During closed urethane molder processes, the pre-polymer and curative are heated and mixed together, then injected into the closed cavity through small holes. An example of a type of molded urethane is foam molded urethane, which is commonly used to make soundproof insulation for the residential industry. Since urethane has thick section molding, low pressure tooling capabilities and a castable nature, the cost of molded urethanes is on par with the cost of rubber and typical polymer molding, giving urethane molders an economic advantage over rubber and plastic molders as well.
Molded Urethane - Gallagher Corporation
Molded Urethane - American Urethane, Inc.
Applications of Molded Urethane
Urethane, which technically is known as polyurethane, has many applications. However, for making urethane products, the polymer with variable stiffness and density is utilized. For example, a low-density and flexible urethane is used as a raw material to make foam upholstery and bedding with high resilience, whereas semi-rigid urethane has application in making packaging foam. Molded urethane products have a high density as high as 1,200 kg/m3 with variable stiffness. For example, a urethane wheel can be flexible and still have a density in the range of 800 to 1,200 kg/m3.
The typical products made from polyurethane are:
- The low-density and flexible urethane is typically fabricated into bedding, automotive seats, and upholstery. The same material recently is also used as a plant substrate in wall and roof gardens.
- The rigid foam with low density has application in thermal insulation
- Molded urethane wheels and gel pads are made with soft elastomers
- Footwear can be made with low-density molded urethane
- Electronic and structural components are made with molded urethane
Molded urethane products used in automotive industry
Automobile seats are commonly made by molded urethane; the process involves a mold in which the mixture of urethane is injected into the mold. The seat then is removed and upholstered. There is another method, called in-situ, where the assembled seat is placed in the mold and then the mixture in injected into the assembled seat.
Surfboards have solid molded urethane cores, which make surfboards light as well durable. Similarly, inflatable boats have parts made from urethane sheets. Moreover, grips of high-end tennis racquets are made with molded urethane; as the material is highly stretchable, it ensures the grip drapes the handle closely. The tires of skateboards nowadays are fabricated from molded urethane; it enhances a skater’s ability to perform difficult maneuvers.
The molded urethane conveyor wheels are more durable and have greater abrasion resistance; however, they are more expensive than their rubber counterparts.
The other machine part where molded urethane is used extensively is urethane bushing. Bushings made with molded urethane are replacing conventional rubber bushings, which, in salty and chemical environments, fail and wear out at an accelerated rate. Urethane bushings fare a lot better in situations where rubber fails, as the material is less permeable to oil and has high abrasion resistance, which increases its lifespan. Moreover, urethane bushings help in maintaining a right wheel alignment, which is important for handling and control.
For protecting electronic components from the effects of environment and mechanical distress, the components are molded in urethane. Urethane has a number of properties, such as abrasion resistance, apt adhesion, and high impact strength; however, it limits the use of electronic components at temperature in the range of 250 degrees Fahrenheit.
For making molded urethane electronic components, a manufacturer needs to buy resin and catalyst separately, which, at the time of fabrication is mixed and molded over the surface of the circuit. The method shields components, but they cannot be repaired once the material is cured.
Methods Used for Making Molded Urethane
Polyurethane is majorly used in the form of flexible and rigid foams; however, molded urethane is also used for making many durable urethane products such as pneumatic and hydraulic seals, urethane sheets, urethane bushings, urethane wheels, and conveyor wheels.
These different molded urethane products have application in an array of industries such as athletic equipment, automotive, mining, construction, engineering, and food processing.
A single molding technique cannot produce products for diverse applications; therefore, primarily, urethane is molded by four methods.
Reaction Injection Molding:
In this method, polyisocyanate and additives, such as surfactants, polyols, blowing agents, and catalysts, are mixed in a liquid form, and then the mixture is injected into a mold. To give the final shape, the mold is exposed to high pressure using an impinging mixer. Unlike other molding methods, the curing step occurs in the mold. Therefore, for a predetermined period, the mix is allowed to set in the mold; during this period, the curing reaction occurs. When additives, such as mica and milled glass, are added in the process for reinforcing the structural strength of molded urethane, the process is known as reinforced injection molding. In its other variation called structural injection molding, a glass mat is incorporated in the mold, which adds strength to the final product.
There are several advantages of this molding technique; the most apparent is the ability to make high strength and lightweight products. Since the mixture is in a liquid form, less clamping force is required that eliminates the need for larger equipment, which ultimately brings the upfront investment down.
However, there are a couple of disadvantages too, such as expensive raw material and higher cycle time per product compared to conventional injection molding.
In this method, instead of a liquid mixture, thermoplastic polyurethane pellets are used. The method is similar to conventional plastic injection molding. The pellets are placed into the screw column where, under high pressure and temperature, the pellets turn into liquid and then the material is injected into the mold.
The process is cost-effective and produces products with high tolerances and in high volume. Moreover, complex parts can be produced at high speed using dual injection technology. However, the tooling of a die is very expensive, and the finished product can have knit lines, and the product quality is low, as heat is involved in the process.
In this method, a heated mold is used in which polyurethane is placed. For curing and giving the shape to the product, the pressure is exerted on the mold. With this method, products with large cross sections can be made and the secondary processing steps can be cut. However, the method can be expensive.
This method differs from other methods, as a fixed molded is rotated at an axis to give shape to the raw material. As the mold rotates, the raw material is exposed to the centrifugal forces, and it adheres to the wall. By this method, products of a large cross section can be made easily; however, the cycle time is significantly more than other molding methods.