Reaction Injection Molding (RIM)
Injection molding is a process that has been used for many years to form plastic products. In essence, it is a method for forming plastic components by injecting, at great force, melted plastic into a mold cavity. It is a quick and effective method for creating exceptionally high tolerance products. Many industries rely on injection molding to produce their plastic components and parts.
Manufacturing engineers are constantly looking for different ways to improve and enhance production methods, which results in better products. Although injection molding has been very successful, engineers investigated different parameters and methods to enhance components produced by the process. The results of their efforts have led to the development of reaction injection molding (RIM) that is similar to the injection molding process but with radical twists.
Injection molding begins with plastic resin pellets that are melted using ovens, pots, or other heating processes. When the melted resin has reached the appropriate melting point, it is forcefully injected into a mold cavity where it remains until it solidifies. The time required for a molded shape to solidify varies according to the type of plastic and the part being formed. When a material is sufficiently cooled, it is ejected, and the process repeats. It is a quick, easy, simple, and highly productive manufacturing process.
The process for reaction injection molding retains the injection part of the process, with its force, but has changed the preparation of the plastic. Unlike injection molding that begins with solid pellets of plastic resin, reaction injection molding begins with the chemicals that are used to produce the plastics. This change speeds up the molding process and produces high tolerance parts.
Chemicals for Reaction Injection Molding
What is normally noticed when examining reaction injection molding is how much more complicated it is compared to traditional injection molding. Although the two processes are sometimes referred to in the same sentence as being injection molding, reaction injection molding involves far more than melting plastic and injecting it into a die cavity. RIM molding is a chemical process that uses several aspects of mixing and combining of the base chemicals of a plastic material.
The complexity of RIM molding begins with two storage tanks that contain the base chemicals of a plastic, which is normally polyurethane. For the chemicals to be properly prepared, the tanks are heated by heat exchangers and have agitators. These measures are necessary to prevent the chemicals from settling. Aside from the heaters and agitators, the chemicals are constantly circulated through supply and return lines until they are needed. Each tank, with a capacity of 30 gallons to 250 gallons, contains one of the chemicals for producing polyurethane, which are polyol and isocyanate.
Polyurethane, the most widely used plastic in the world, has several properties that make it ideal for the RIM molding process and the manufacture of plastic products. It is affordable, adaptable, recyclable, and readily available. The mechanical properties of polyurethane can be chemically changed, manipulated, and altered for performance purposes. This factor makes it ideal for the RIM manufacturing.
Measurement
As can be assumed from the nature of the RIM process, precision and accuracy are a necessity to ensure proper mixing and blending of the two chemicals. The measuring process for RIM molding involves the use of hydraulic meter pumps, or lances, that have a pump with feed lines and valves. The measurement of hydraulic pumps is in regard to its flow rate, which can be adjusted for a particular number of units per minute and pounds per second. These factors are crucial for chemical measurements for RIM molding.
Hydraulic pumps, also known as positive displacement pumps, deliver the same quantity for each rotational cycle. They create a vacuum that pulls in the exact quantity for the process. This ensures a constant flow of chemicals in the right proportions. Errors in the measurements of hydraulic pumps are minimal and not sufficient to cause concern. It is their accuracy that makes them ideal for RIM molding.
Mixing
Proper mixing determines the quality of the product being produced. In the case of RIM molding, an impingement jet mixer is used. When the mix heads open, the released quantities of chemicals are impinged at 1500 psi up to 3000 psi at high velocity. With impingement mixing, materials to be mixed are forced at each other such that they crash together creating substantial turbulence and shearing forces. The combination of these factors ensures a homogeneous mixture prior to being injected into the mold.
Impingement mixing is critical to RIM molding in that it quickly blends, combines, and mixes highly viscous substances. Flow rates can be as high as 17 lbs. per second with materials having viscosities as high as 500 centipoise, which is the measurement of the thickness or thinness of a liquid. A higher centipoise value indicates a very thick fluid.
Injection
The injection stage of RIM molding is very much like traditional injection molding but with the melted resin replaced by a chemical mixture. A plunger extracts the mixture from the mixing head, which breaks the continuous looping of the chemicals and makes it possible for the impinged mixture to enter the mold. A major benefit of RIM molding is the lower temperatures and pressures required to complete the process. In addition, molds are made of lighter metals due to the lower pressure and temperature, a factor that decreases the cost of the process.
Injection happens rapidly at high velocity. The temperature of the mixture is a little over 190o F, which is far lower than traditional injection molding. The mixture has a viscosity of 0.1 pascals per second or one poise. The increased flow rate of 0.5 meters per second fills the mold rapidly and evenly, another factor that makes RIM molding attractive. Since the mixture is thin, the amount of clamping force necessary to contain the mixture is minimal.
The consistency of the mixture ensures that every crack, crevice, and cranny of the mold is filled. This aspect of the process makes it possible to produce intricate and detailed plastics.
Exothermic Reaction
As the mixture enters the mold, an exothermic reaction occurs and the temperature of the mixture increases to 350o F. The initiation of the exothermic reaction is the beginning of the curing process. Clamping pressure rises in accordance with the size of the part being produced, its expansion rate, and the density of the shape. Depending on the size of the mold, hydraulic or pneumatic force may be necessary to keep the mold closed.
The rise in temperature requires the use of a cooling mechanism to assist in forming the thermosetting polyurethane. The exothermic reaction is the chemical reaction between the polyol and isocyanate, a factor that produces the polymerization of the polyurethane. Many plastic molding processes, regardless of the method, have to have consistent wall thicknesses for every part of a mold. The exothermic reaction makes it possible to have varying wall thicknesses such that sections can have very thick walls next to thin walls. This gives designers more freedom.
Polymerization
Polymerization occurs as the exothermic reaction progresses. As the mixture is polymerized, it cures, solidifies, and takes the form of the mold cavity. The curing process is carefully monitored, the length of time of which can be a few minutes up to several minutes depending on mold size, geometry, function, and wall thicknesses.
It is the chemical reaction of the polyol and isocyanate that cures and solidifies a part. This is unlike melted resin cooling. The chemical reaction of the exothermic reaction is what creates the heat, not some oven or cauldron. As with many parts of RIM molding, the heat derived from the chemicals is crucial to the forming of a part.
Like traditional injection molding, cooling of the mold causes shrinkage in a completed part, which is calculated into its design. Once a part is sufficiently 1480 cooled, it is ejected from the mold.
The Final Step
A great deal of care is taken when removing a RIM molded part. As with traditional injection molding, carefully selected pins are used to detach the molded part from the mold cavity. Pins come in a wide variety of forms to fit the requirements of different molds and parts. RIM molding is capable of producing large truck bumpers and very small components for complex equipment. Each of these types of parts requires a different type of ejection pin.
Conclusion
Since its introduction in the middle of the 20th century, RIM molding has become a very popular form of manufacturing. The versatility of the process provides engineers with a more flexible method for designing a wide assortment of parts of any size. The accuracy and precision of RIM molding makes it possible to produce large parts with exceptional tolerances. One drawback to the process is its inability to produce components and parts in high volume.
