A list of rim molding manufacturers with an explanation of the rim molding process
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Reaction injection molding, often referred to as RIM molding, is a specialized molding technique that utilizes two highly reactive, low molecular weight chemicals. These chemicals are combined and blended before being injected into a closed mold. High-pressure pumps are used to circulate isocyanate and polyol from their respective storage tanks into multi-stream mixing heads, operating in a continuous cycle. When these elements meet in the mixing head, impingement mixing occurs. This interaction produces turbulence and shear forces, creating a uniform liquid that is then injected into a mold to form polymer components.
Upon entering the mold, the materials initiate a curing process, driven by the exothermic heat and pressure, which can take a few minutes, depending on the complexity, size, and thickness of the part. The RIM molding method requires less pressure and operates at lower temperatures, enhancing its efficiency, reducing costs, and allowing the use of lightweight aluminum molds.
The term “reaction” distinctly sets reaction injection molding apart from standard injection molding, despite the common confusion among some engineers and designers. This technique allows for adjusting the raw materials and polyurethane reaction to create parts with specific attributes such as weight, strength, density, and hardness. Although the production timeframe with reaction injection molding is extended, the advantages include its versatility, cost efficiency, accommodating a vast range of part sizes, and enabling the creation of uniquely intricate designs.
The RIM process involves the mixing of highly reactive liquids and injecting them into a mold to form complex plastic components. Traditional Injection molding creates plastic parts using a wide range of thermoplastic polymers and involves melting thermoplastic pellets, injecting them into a mold, and allowing them to harden and cool.
Although thermosetting polymers are ideal for the traditional injection molding, they are not applicable for thermosets that have to be cured and hardened through the use of heat, radiation, or mixing them with a catalyst. Reaction injection molding was developed in the 1960s for the production of impact resistant auto parts. Instead of melting solid pellets, reaction injection molding combines liquid thermosetting polymers, which causes a reaction that quickly cures and hardens the mixture.
With typical injection molding, melted polymers are injected into a mold under pressure and held in the mold until they solidify. The process is quick and can produce hundreds or thousands of parts. As effective as the process may be for melted polymers, it is ineffective for thermosets, which has led to the development of reaction injection molding. The key to any discussion of reaction injection molding is the word “reaction”, a factor that is also essential for the process. Reaction injection molding is named after the chemical reaction that occurs within the mixture after being impinged and injected in a mold.
The plastic used for RIM molding is polyurethane (PUR), which is an organic polymer that is a highly resistant elastomer used to produce intricate designs. Polyurethane is an old plastic material that was invented prior to World War Two and was used extensively during the war as a replacement for airplane components. It is a plastic polymer that is produced by combining diisocyanates (TDI and MDI) and polyols.
A unique quality of polyurethane is its many varieties that look, feel, and are used in different applications. Polyurethane is one of the most widely used plastics and can be found in every aspect of life. The use of polyurethane is due to its affordability and sustainability. It is adaptable, recyclable, and available.
The mechanical properties of polyurethane can be chemically changed, manipulated, and isolated to alter performance characteristics. It is ideal for RIM molding due to its high load capacity when compressed. In addition, polyurethane can produce any size component from ones of a couple grams to ones weighing 1500 lbs. (680.39 kg).
In the RIM molding process, the ingredients for the formation of polyurethane are two monomers or prepolymers in a liquid state that positively react to form polyurethane. Known as reactants, polyol and isocyanate are chosen for the RIM molding process due to their low viscosity and their ability to expand and thicken as part of a chemical reaction.
After injection into the mold, polyol and isocyanate transform into polyurethane and expand to fill the mold. Their chemical reaction brings the polymers to the correct temperature and cures the PUR.
Polyol – Polyols are organic compounds that are composed of multiple hydroxyl groups. The term “polyol” is a basic reference to the presence of those groups. They can be highly viscous. Polyols have two or more reaction groups that react with isocyanate to form polyurethane. The chemical, physical, and mechanical properties of polyurethane directly relate to the use of polyols.
Storage tanks for RIM molding are an essential part of the process. Unlike other forms of storage tanks that simply hold materials until they are required, the storage tanks for RIM molding help maintain the proper consistency of the polyol and isocyanate components. The capacity of the tanks varies between 30 gallons up to 250 gallons (113.56 L up to 946.35 L).
The tanks are precision controlled for temperature using heat exchangers and equipped with agitators. Individual supply lines feed the ingredients from the tanks. Until the two liquids are released, high pressure pumps circulate them between the tanks using their supply and return lines. The agitators, attached to motors, continuously stir the ingredients to ensure homogeneity.
One of the necessities of RIM molding is precision measurement, which ensures the proper blending of polyol and isocyanate. As a foundational aspect of the reaction, the mixture has to meet the parameters for the RIM molding process. To achieve the necessary level of accuracy, hydraulic metering pumps, referred to as lances, are used that include pumps, valves, and the supply feed lines.
At the time of injection, the liquids pass through high- and low-pressure conversion to reach the correct pressure for injection. In the hydraulic metering pump, the liquids are quantitatively measured at an accuracy between ±1% and ±1.5%. The pumps dispense the perfect amounts and ratios of polyol and isocyanate for the injection phase of the process.
After the ingredients are dispensed, they are carefully mixed at a pressure between 10.34 MPa and 20.68 MPa, pressure rangers that produce the best mixing. Although each step of the RIM molding process is essential to its success, proper mixing determines the quality of a product. When the valves of the mix head open, the released liquids are impinged at pressures of 1500 psi up to 3000 psi, which is the initiation of the high velocity mixing process.
Impinging mixing is a process where the liquids are directed at each other at high velocity and meet at the impingement point. The meeting of the liquids creates turbulence and shearing forces, a factor that generates thorough homogeneous mixing.
The advantage of using impingement mixing for RIM molding is its ability to quickly mix viscous liquids. Flow rates can be as high as 17 lbs. per second with viscosities in the range of 500 centipoise, a measurement of the thickness or thinness of a liquid with water being one centipoise. A higher centipoise value indicates a thicker fluid.
The injection process for RIM molding is similar to the process used for traditional injection molding. The key difference between the processes is the chemical composition of the liquids. A piston or plunger inside the mix head retracts, breaking the continuous loop, allowing the impinged mixture to enter the mold. Since liquid polymers require less pressure and have lower temperatures, molds for RIM molding are made of lightweight aluminum. This reduces the cost of each part and removes the need for heavy tooling for shaping molds.
During injection, there is high velocity material flow. The viscosity of the mixture is about 0.1 Pascals per second or one poise. The temperature of the mixture is less than 194°F (90°C) with a flow rate of 0.5 meters per second. The thinness of the mixture decreases the amount of necessary clamping force, another cost saving factor.
An exothermic reaction occurs as the mixture enters the mold. The temperature of the mixture increases and reaches 350°F (176.67°C). The advent of the exothermic reaction is the beginning of the curing process. During this phase, depending on the component being made, clamping pressure significantly rises depending on the size, expansion rate, and density of the shape being formed. In some cases, molds are mounted on pneumatic or hydraulic presses to provide force sufficient to keep the mold tightly closed.
In order to maintain mold temperature, water lines are used as the thermosetting polyurethane polymer forms. The exothermic reaction is due to the chemical reaction between the polyols and isocyanates, which is necessary to trigger polymerization, moving the plastic to a solid form. The exothermic reaction makes it possible for designers to design parts with multiple wall thicknesses, sections, strength, and rib ratio.
As the exothermic reaction progresses and the polymerization process occurs, curing and solidification of the final completed part happens. These final steps in the process determine the mechanical properties of the molded piece. Due to the chemical reaction between the ingredients, the curing process immediately begins as the polyol and isocyanate are injected into the mold. The length of time for the curing process can be less than a few minutes or several minutes depending on a part’s size, geometry, function, and wall thickness.
The foundational difference between normal injection molding and RIM molding is in relation to the curing process where a part cures and solidifies due to the chemical reaction between the polyol and isocyanate. With traditional injection molding, melted plastic is injected into a mold and allowed to cool and solidify. With the RIM molding process, an exothermic reaction generates heat due to the chemical reaction, which is unlike the cooling process. This aspect of RIM molding radically differentiates it from traditional injection molding.
During curing, molded parts slightly shrink. This is a normal aspect of the process and included in the design parameters. After a few minutes of curing, a part is ejected from the mold and ready to use or sent on to post production processing, when necessary.
When the curing process is completed and a part is solidified, it is removed from the mold. As with most molds, RIM molds have an ejection system that includes ejector pins strategically placed in the mold cavity. During the demolding process, the pins are activated to push the molded piece out of the mold cavity. The types of pins used for the process vary in accordance with the type and size of the part. Step or shoulder pins are used for larger parts that require more force when being pushed out. Like other aspects of RIM molding, demolding is similar to traditional injection molding.
Although the process for removing a part from a mold is simple, special care is necessary to avoid damaging the molded part. Ejector pins come in several varieties. Their selection is important for ensuring proper removal. In molds, ejector pins are positioned such that they won’t make contact with areas where they might stick. The term ejector pin covers a wide swath of pins designed to push parts out of a mold. The key to their selection is to ensure the molded part will not be damaged.
The high quality and accuracy of the RIM molding process ensures that completed parts are ready for use after being demolded. In some instances, ejected parts may need certain post production processes performed such as trimming, surface finishing, cleaning, or coating. The nature of these aspects of the process are in accordance with design requirements for a part. As the complexity of parts increases, the post production processes increase in order to meet the parameters of a part’s design.
The types of RIM molding are divided by their types of additives and variations in the process. Some of the variances are manipulations of other forms of molding, such as rotary molding. The different methods and addition of other elements makes it possible to use the RIM molding method to create stronger, tougher, and more durable plastic parts. In addition, the inclusion of other RIM methods expands the potential for creating more aesthetically pleasing parts.
The steps for RRIM are essentially the same as those for regular RIM and is an alternative to standard RIM processing. RRIM is widely used by the auto industry to produce body panels, bumpers, spoilers, and floor panels. The factor that separates RRIM from regular RIM is the addition of fibers, such as glass or carbon, to polyol and isocyanate during the molding process. The fibers add to the strength and resilience of the thermoset polymer to increase impact resistance.
All steps of the RRIM process follow those of the RIM process with a significant change at the injection process where the fibers are added to the polyol and isocyanate mixture. The impingement method is still essential but includes the colliding of three elements instead of two. The parts produced by the process are lighter, more flexible, highly durable, and tough with exceptional strength and impact resistance.
The purpose of RRIM molding is to increase the strength, size, and durability of molded products. SRIM takes the process in a different direction with the goal of increasing the stiffness of the resultant products. Like RRIM, SRIM uses fibers that are more formed and not granular, such as mats, meshes, and preforms. Instead of injecting the fibers into the impingement mixture, with SRIM, the preforms are placed in the mold prior to the mixing and injection of the resin elements. As the mixture enters the mold, it soaks and saturates the fiber material creating a stiff and sturdy composite form. The low viscosity of the resin mixture enables it to slowly soak into the fiber material and provide complete uniform coverage.
The process for structural reaction injection molding eliminates the need for an additive storage unit since the fiber mats, forms, and mesh are already part of the mold. Aside from adding strength to the final product, the process is more efficient as it enhances molded products.
Unlike RRIM and SRIM, DCPD RIM involves the use of different chemicals than polyol and isocyanate. DCPD, known as C10H12, is a chemical made by polymerizing DPCD with monomers, such as styrene. It is normally used to produce paints, adhesives, and special forms of thermoset resins.
As a resin, it has low viscosity and resistance to heat, impact, and corrosion, which makes it possible to create larger, stronger, and lighter products. The special formulation of DCPD gives it higher filling capacity and exceptional mechanical strength. Common use of products produced using DCPD reaction injection molding are vehicle bodies, hoods, and various types of shields.
As with regular RIM molding, DCPD RIM begins with the chemicals that create DCPD and are injected into a closed mold with a catalyst, such as molybdenum or tungsten. The chemicals and catalyst with added heat set off a chemical reaction that converts the DCPD mixture into a solid thermoset. Parts normally cure in the mold in less than two minutes. The cycle for creating a part varies between 4 to 6 minutes.
Parts produced using DCPD RIM molding can have a surface area of 120 square feet (11.15 m²) with thicknesses up to 12 in (30 cm). Small DCPM RIM parts can have dimensions of 8 sq ft by 10 sq ft (80 cm² by 1 m²), a range that is higher than standard injection molding. Although DCPD RIM parts are larger, they still have the characteristic strength, flexibility, and lightweight that is expected of parts from the RIM method.
The quality of products produced by reaction injection molding and the durability of products has made the RIM molding the fastest growing manufacturing method. The low tooling costs and ability to produce complex parts has further enhanced its attractiveness and wide use.
The use and popularity of reaction injection molding is due to several factors including its ability to produce durable and resilient parts using a low-cost production process. Large scale parts produced by RIM molding have exceptional wall thicknesses, strength, durability, and endurance. The use of RIM molding is due to its dependability and repeatability.
One of the constantly repeated advantages of RIM molding is the lower tooling costs due to the less expensive molds used. Unlike other molding processes that require the use of steel tooled molds, molds for RIM molding are made of lightweight metals that are easy to produce. This makes the process applicable for prototyping, small batch production, and precision engineered parts.
The cost per part for RIM produced parts is higher than the cost of parts produced by normal injection molding. The lower tooling costs offsets the production costs. The tooling of steel for injection molding of large parts is several hundred thousand dollars. Since large parts are not produced at high volumes, the high cost of tooling is reflected in the cost of the product.
The flexibility of RIM molding makes it possible to produce complex and intricate parts with exceptional tolerances, avoiding post mold assembling. This aspect of the process makes it possible to produce multi cavity molds in a single cycle reducing production time and labor expenses. RIM molding is able to accurately produce the most intricate geometries with undercuts and internal features.
The deep draw capabilities of RIM molding are the main reason for RIM’s ability to produce intricate and complex parts. Designers and engineers can fabricate delicate and complex components that are molded with precision accuracy.
The RIM process makes it possible to place various materials and components into a mold, which helps in increasing the efficiency of the manufacturing process. As with SRIM, metal components, parts, and features can be placed in the RIM mold prior to the injection of the resin mixture. This helps increase efficiency and removes the need for post production assembly.
An aspect of modern industry that is constantly being emphasized is repeatability, the assurance of the uniform production of parts and components. This characteristic of RIM molding drastically lowers the rejection of poorly formed parts, which further lowers production costs. RIM always delivers consistent repeatable results through numerous production cycles. Such dependability ensures stability, uniformity, and reliability in regard to proper alignment with design specifications.
The advent of RIM molding has substantially changed the auto and aerospace industries due to its ability to produce streamlined, lightweight large parts with precise tolerances and dimensional accuracy. Single shot parts produced by RIM molding can be as large as 8 ft by 8 ft by 2 ft due the low pressure of the process and the speed of curing. Although other molding processes can produce the same size parts, tooling and machining costs make the final product too expensive.
The speed at which parts are formed, cured, and solidified ensures RIM’s ability to produce parts with beautiful textures, appearances, and smooth even finishes. The high-quality surface finishes remove the need for post production operations to bring products up to design specifications. The rapid rate of production and unequaled finishes streamline production and significantly lower costs.
RIM molding makes it possible for designers to envision aesthetically pleasing parts. The ability to design components with varying wall thicknesses, radical curves, and encapsulation is a feature that is characteristic of RIM molding. Highly technical equipment necessitates an appearance of professionalism and technological sophistication, which are features that can easily be designed into RIM molded parts.
The temperatures that are necessary for other forms of molding eliminate the possibility of encapsulating sensitive electronic components that would be damaged or destroyed by the process. The low temperatures, pressure, and viscosity of RIM molding makes it possible to encapsulate delicate precision devices during the molding process. The use of encapsulation for technical equipment protects instruments from harsh conditions, chemicals, vibrations, and extreme temperatures.
In some ways, RIM molding encapsulation is a method for preventing the theft of technically advanced and proprietary devices. Since devices are securely held without the confines of equipment, it is unlikely that they can be easily removed or stolen. In addition, the protective aspect of RIM molding ensures that the technology used to create a device will not be corrupted or diminished.
The speed of cycle types of RIM molding makes it ideal for rapid prototyping. Aluminum molds for RIM molding can be shaped, cut, and readied quickly. Prototypes can be molded, trimmed, and presented without slowing the development process. This aspect of the RIM molding offers an additional tool for designers and engineers to test the practicality of their ideas.
As can be ascertained from the list of benefits above, the many positive aspects of RIM molding far exceed any of its negative drawbacks. The two prominent negative arguments regarding the RIM process are in regard to its speed and cost. Each of these factors is far outweighed by the exceptional quality of the products produced and their strict adherence to design parameters.
With RIM molding, computer aided design (CAD) models are used by plastic production for the creation of molds. The quick and easy fabrication of the aluminum molds allows for faster initiation of production and cost-effective tooling. CAD designs used for RIM molding enable side action and hand load inserts, over molding, and insert molding. Electrical discharge machining (EDM) is used to adjust and improve mold features, including surface finishes. The quick availability and access of RIM molding make it possible to produce parts in weeks instead of months.
All of the features of RIM molding are the same as those found in traditional injection molding. The difference between them is the distinctive way RIM molding can adjust and adapt these features to precisely match mold designs.
Wall thickness is a major issue with plastic part molding. Thin walls can lead to disastrous consequences. Wall thickness is the most important part of high-quality molded parts. Uniform wall thickness minimizes possible warping or distortion of a component. Wall thicknesses for RIM molds can vary from 0.118 in (3 mm) or 0.236 in (6 mm) up to 2 in (50.8 mm). A unique characteristic of RIM molds is the ability to have varying wall thicknesses with isolated areas having thicknesses as small as 0.08 in (2 mm) or as thick as 1 in (25 mm).
Although RIM molding allows for different wall thicknesses, it is important to understand that the thickness of part walls affects the length of part molding. As the walls of a plastic part become thicker, molding and cycle times increase, which may make the manufacture of a part uneconomical.
Ribs for a mold are supports that are strategically placed to prevent sink marks, warp, and voids. They are thinner than primary walls and are placed perpendicular to part walls. Ribs help parts retain the structural integrity of a parts original design. They improve performance and can be decorative additives. Ribs are used in conjunction with part walls and make it possible to have parts with thinner walls. They decrease the cost related to having thicker walls and lower cycle times.
The draft angle of a part is a taper applied to the walls that assists in part ejection. Draft angles are added at the design phase of RIM molding. They make part removal from the mold easier, reduce deep draw, and prevent damage to a part when it is ejected. In most instances, draft angles are 1° on outside walls and 2° on inside walls. As parts get taller, draft angles increase. The main focus of draft angles is for the core side of the mold due to molded parts shrinking in the mold cavity.
Bosses are included in molds to allow for inserts and make it possible for air to escape during molding to decrease cycle times. They assist in part assembly by providing holes for screws, locator pins, and threaded inserts. Bosses are an open topped cylinder and have wall thicknesses that are 40% to 60% of the thickness of the mold walls. To increase the strength of bosses, gussets are added for extra support. Although it may be tempting to connect a boss to a wall to give it extra support, such a design feature would increase the thickness of the wall and affect cycle times. In order to avoid such a complication, ribs are added to support bosses.
The inclusion of holes, grooves and vents is an efficiency design feature that removes the need to have them drilled, cut, or grooved during post production. Their inclusion enhances injection by reducing stress, the threat of air entrapment, and potential knit lines.
Undercuts are a blessing and curse for RIM molding. They add functional features and increase part support. Undercuts have several positive aspects that enhance plastic molds but have the downside of interfering with a mold’s separation from its core. Special measures used to overcome this difficulty include sliders, lifters, and loaded inserts. The addition of such supportive measures increases the cost of the RIM process.
There are many factors that have to be considered during the design process that may increase the cost of RIM molding. Most of these factors are part of traditional injection molding but are adjusted for the speed and accuracy of RIM molding.
Radii – Radii are the rounded corners of molds or fillet radius that improve mold quality and increase load bearing ability and strength. Fillet radii are found on inside corners or at the bottom of a mold and are used between ribs, bosses, and gussets to connect them to mold walls.
Over Molding – RIM molding is an ideal process for over molding since the process operates at low temperature and pressure, neither of which damages the piece being over molded. With RIM molding, over molding is completed in a single cycle and does not require multiple resin injections.
Although polyurethane is one of the more popular types of plastics used in RIM molding, other types of plastics are also used due to their specific properties and characteristics. The increasing popularity of RIM molding has led to the introduction of other resins that have the qualities and features that fit the RIM molding process.
Reaction injection molding is a thermoset process that uses low viscosity liquid polymers. It is designed to produce tight tolerances, intricate designs, and molded in distinctive features. Reaction molding is a low pressure, low temperature process that produces exceptionally accurate and resilient parts using various materials.
Polyurea – Polyurea is produced by the reaction between isocyanate and amines and has many of the same properties as polyurethane. The difference between them is the use of amines instead of a hydroxyl to create the reaction.
Polyisocyanurate – Polyisocyanurate is a thermoset plastic with the same starting materials as polyurethane but uses polyester polyol for the reaction instead of polyether polyol. The variation produces a highly complex polymeric structure.
Polyesters – Polyester RIM molding involves the use of a polyester polyol instead of a polyol hydroxyl. Polyester polyol assists in enhancing the complexity of RIM molded products.
Polyoxide – Polyol is the most common form of chemical used in reaction injection molding.
Nylon 6 – Nylon 6, in its liquid form, rapidly polymerizes when mixed with a curing agent, which makes it ideal for RIM molding.
Dicyclopentadiene (DCPD) – Dicyclopentadiene is a thermoset resin that is known for its impact resistance. It is used to produce large RIM molded parts.
The range of products produced by RIM molding is ever expanding as new and innovative methods are being introduced. Unlike other plastic molding processes, RIM molding is not designed to produce products in volume and is best used for product runs of 60 to 100. Aside from this one restriction, the quality, tolerances, and accuracy of products produced by RIM molding is far beyond any other injection molding process.
The automotive industry uses RIM molding to produce bumpers, quarter panels, trim, and wheel arch liners. RIM produced plastics are ideal for automobile production due to their durability, strength, lightweight, and flexibility, which are key and necessary features for modern automotive manufacturing. In addition to the many positive properties, RIM molding is able to produce large automotive parts quickly and efficiently. Design changes can be rapidly produced due to RIM’s low-cost tooling.
For heavy equipment, RIM molding produces heaters, mobile generators, and light towers. The polyurethane material is resistant to corrosion, wear and tear, and impacts. Parts and components are tougher and require less maintenance. Included in RIM molding is B-side geometry, in mold paint, and inserts, which gives designers more latitude and control.
The healthcare industry relies on RIM molding for its accuracy and ability to overmold and encapsulate certain aspects of medical instruments. The high quality and tolerances of products makes the use of medical equipment easy and convenient. In special cases, RIM molding is used to produce complete devices, such as electronic spray systems, DNA analyzers, and full body examination equipment. Polyurethane’s antibacterial properties and its reputation for being hygienic enhances the use of RIM molding for medical instruments.
The requirement of lightweight super structures for aircraft necessitates the use of RIM molding for the manufacture of interior elements, cabin components, seating, and other structural factors. The stability and durability of RIM molded products makes it a perfect choice for airplane and aircraft implements and components.
The list of industries above is an example of the many industries that rely on RIM molding to produce parts with the properties and characteristics that benefit a wide range of products. From consumer products to roofing for construction projects, RIM molding is used by manufacturers to produce high tolerance, dimensionally accurate, and high quality plastic molded products.
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