Cast urethane is used by a diverse number of industries in many different applications due to the many shapes in which it can be molded. This liquid material can be molded, or poured, to create very large parts weighing several hundred pounds to smaller parts weighing less than a grain of rice. The size and complexity of the part determine which type of molding technique will be used to produce your part.
Below we will discuss the different types of molding processes used to produce a cast urethane part:
Open Cast Polyurethane Molding
Open cast molding is the most popular type of process employed to mold urethane due to its low part cost, low tooling cost, and the ability of this process to produce both large and small parts.
In open cast molding, a portion of the mold is purposely left “open” and polyurethane is poured into the mold through this opening. Normally, several cavities for one part are utilized, so many parts can be poured at one time from a single batch of material.
After the liquid material reacts to form a solid part, the parts are demolded from their molds and the cycle is repeated. The molds are not put into a press or spun as in some of the processes we will discuss later, so no other support equipment is needed. This helps keep production high and the cost of the parts low.
All parts using this technique have an open cast end, or an unfinished area. Depending upon the needs of the application, parts can be used as is or this unfinished area can be machined to give a more finished appearance.
While open cast molding is used on many different types of parts for the reasons above, this process can leave some minor bubbles or blemishes in some parts with complicated geometries. Parts that need to be blemish-free would benefit from using another type of molding process other than open cast molding. Such parts would be O-rings, lip-seals, and detailed washers.
Open cast polyurethane molding is a great process for urethane rollers, vertical bend restrictors, risers, rotating control device stripper rubber, pulsation dampener bladders, and many more.
Compression molding utilizes a two-piece mold design. Under this process, the mold is put into a heated press to bring the mold to its processing temperature. When the material is ready, both halves of the mold are pulled from the press and laid open. The liquid urethane is poured and over-filled in each half of the mold.
The liquid will then react and begin to gel. Once the urethane is in this semi-solid state, the 2 halves of the mold are put together and the mold is put back into the heated press. The mold is pressed together with several tons of force from the press.
This pressure forces the material to fill all locations of the mold and forces out imperfections, such as bubbles. The mold stays in the press until the material is solid enough to be handled without damaging the parts. At this point, the mold is brought out of the press, the parts are removed, and the cycle is repeated.
Compression molding is ideal for small parts with thin and/or complicated geometry and can be used on parts with metal inserts. This process is often used when manufacturing parts such as O-rings, washers, and valve inserts. Compression molding is not recommended on large parts or parts that can be made more cost-effective under other techniques.
In spin cast molding, or spin casting, molds are placed into a machine that spins the molds inside a heated oven, normally specially built for this process. While the molds are spinning, the liquid urethane is poured in the center portion of the machine.
The centrifugal force created by the spinning action forces the urethane material outward to fill the molds. This force also causes the air bubbles to vacate the mold and move toward the center of the machine and out of the mold/part.
This process works well for parts with intricate detail, threaded parts, and parts with unusual geometry that might otherwise trap air bubbles. Due to the special built equipment needed, this process is not suited for large parts or parts that can be produced more cost effectively under another process.
Injection molding utilizes a minimum of a two-part mold. The mold is installed into an injection molding machine that clamps the mold together and prepares the material to be injected. This machine will heat the material, that is normally in pellet form, until it reaches a semi-molten state.
At this consistency, the material is injected under high pressure into the mold. The mold is cooler than the material, and this causes the material to immediately solidify after filling the mold.
Injection molding is used to make high volume small to medium sized parts. The process of making parts under this process is highly automated, so part pricing is normally lower than many other processes. However, tooling costs under this process are normally 3-10 times that of other processes and part volumes needed normally restrict the use of this technique.
Blow molding, which is sometimes referred to as blow forming, is a process by which polyurethane is forced to the outer dimensions of the mold through air pressure from a preform, or tube-like piece of plastic with a hole in one end allowing for air to pass through. Through this process the air forces the urethane to match the mold, making it an ideal molding option for hollow parts. After the urethane has cooled and hardened, the mold can be opened and the preform can be removed.
Blow molding is only used with parts that are hollow, and these parts can be small to very large in size. Parts such as flotation devices and plastic bottles utilize this process. This process does not work on solid parts, so the use of this process is limited.
Unlike the common mold used within the other molding processes, extrusion molding utilizes a shaped die. In the extrusion molding process, a polyurethane blank is pushed through a shaped die producing a formed urethane part with excellent surface finish, which can be accomplished with hot or cold material. Advantages of this process include the ability to form complex shapes and the ability to mold a continuous long final product or semi-continuous smaller products.
If a long continuous profile is needed, this process is very cost effective relative to some of the other types of processes. Many wiper blade applications utilize this type of molding process.
ESCO Plastics has been committed to manufacturing high-quality, reliable, and affordable molded urethane products for manufacturers around the world. Our expert engineers and trained technicians focus on providing only the best and most consistent polyurethane parts. Talk to an ESCO Plastics engineer to discuss our urethane molding options at 281-443-3710.