This article takes an in depth look at Polyurethane Molding.
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Polyurethane molding involves creating plastic components by injecting a urethane polymer system into a mold, where it then solidifies. Much like other plastics, polyurethane is known for its excellent processability, making it highly effective for producing various consumer and industrial parts. This method easily attains precise tolerances and intricate shapes. It is acknowledged for crafting custom, high-quality components with impressive durability and performance attributes. Polyurethane, often used in its liquid or elastomeric state, can be tailored for specific characteristics throughout the molding phase.
Favored for its capability to form detailed shapes and complex designs, polyurethane molding serves numerous industries, such as automotive, aerospace, medical, and consumer products. Items produced via this technique include an array of applications like seals, gaskets, rollers, bumpers, wheels, and bespoke components. These parts are known for their significant resistance to abrasion, chemicals, and various environmental challenges, ensuring their reliability and longevity.
Polyurethane moldings find extensive use in:
The discovery of polyurethanes is attributed to Otto Bayer in 1937 when he worked with coworkers at the laboratories of I.G. Farbenindustrie A.G. in Germany. The first polyurethane was formed from the reaction between a diamine-forming polyurea and an aliphatic diisocyanate. The polyurea was later replaced by glycol due to the enhanced properties of the polyurethane created.
The early forms of polyurethane were used during World War II. The original intent was for polyurethane to act as a synthetic substitute for natural rubber, which at that time had an exceedingly high demand and was difficult to obtain. The versatility of polyurethane gave it potential applications in fiber, foam, and coating production. This stimulated the commercial use of polyurethanes in the manufacture of textiles, cushions, shock-absorbing pads, and metal finishing.
The availability of polyisocyanates in 1952 led to the widespread use of polyester-isocyanate urethane systems. A few years later, polyether polyols in the form of poly tetramethylene ether glycol (PTMEG) were introduced in 1956 by DuPont. The next year, BASF and Dow Chemicals developed polyalkylene glycols. These two developments paved the way for the commercial introduction of polyether-based urethane systems. They offered superior characteristics to those exhibited by polyester systems. Such characteristics are excellent processing, low-temperature stability, and resistance to water and humidity. Since then, they‘ve become the major polymer system in the polyurethane manufacturing industry.
Polyurethane is an excellent plastic molding material because of its many desirable characteristics. Some of these properties include hardness, resilience, and chemical resistance. Polyurethane materials can be engineered to have different mechanical and chemical properties by formulating the polymer system with different components blended in specific quantities.
Hardness is the relative resistance of a material to localized surface deformation. It is usually determined by measuring the depth of indentation on the material by a standard indenter, ball, or presser foot.
Materials are graded according to their hardness relative to one another. For elastomers, hardness is characterized by the Shore Hardness Number and measured by a durometer. The Shore hardness scale is categorized into 12 scales; each scale has its indenter configuration, profile, and force applied. The Shore scales used for polyurethanes are Shore A and D. The Shore A scale measures the hardness of soft, semi-rigid polyurethanes, while Shore D measures hard and rigid polyurethanes. However, remember that high hardness does not correspond directly to high rigidity or strength.
Abrasion can be classified into two types: sliding and impingement abrasion. Sliding abrasion occurs when a hard material like metal or ceramic slides or rubs into a softer material with or without contaminants between the surfaces. Impingement abrasion, in contrast, happens when particles impact the surface, causing erosion.
Polyurethanes with a low coefficient of friction and high tear strength offer good sliding abrasion resistance, while polyurethanes with good resilience are used for impingement abrasion. Resilient polyurethanes can yield elastically, allowing forces from the impacting particles to be distributed on the surface.
Abrasion resistance is achieved by the right composition of the polyurethane resin. Among the polyol compounds for making polyurethane, polyesters exhibit better tear and abrasion resistance.
This is the ability of polyurethanes to withstand the application of tensile forces that tend to rip the material apart and propagate the tear throughout the body of the material. Tear propagation can vary depending on how the force is applied and the microscopic structure of the material. Tear strength can sometimes be correlated to abrasion resistance. Polyurethanes are known to have both good tear strength and abrasion resistance.
As with abrasion resistance, polyurethanes possess good impact strength because of their excellent resilience. The polyurethane lining used in rollers can elastically deform to absorb the impact and return to its shape. All this while dissipating the energy throughout the structure of the roller.
Polyurethane has high fatigue resistance because of its flexural strength. It can elastically deform under cyclic conditions without failing. This makes it suitable for high-speed applications such as printing and milling. The only problem with using polyurethanes in high-speed conditions is their low heat dissipation, especially for thicker roller linings. High heat can eventually accelerate creep; this weakens the material.
Thermal aging is the gradual degradation of elastomers under conditions of high temperature and an abundance of oxygen. It is characterized by a loss of strength and elasticity. This irreversible process creates operating temperature limits for the material.
Polyurethane exhibits good thermal aging resistance when formulated with certain compounds such as PPDI and CHDI. Typical polyurethanes have a maximum operating temperature of about 90 to 100 °F (32 to 38 °C). Special but more expensive formulations can reach 302 °F (150 °C).
Polyurethane‘s coefficient of friction (COF) correlates with its hardness. The two properties have an inverse relationship, meaning the COF increases while hardness decreases. Since the hardness of polyurethanes is easily manipulated through blending, the desired COF can also be attained.
Machinability is a property observed in hard polyurethanes. This property allows polyurethanes to be shaped into perfect geometries. This is particularly useful for finishing molded polyurethanes as it allows the product to be ground or lapped. These secondary processes remove imperfections on the surface of the product.
The chemical resistance of polyurethanes depends on the type of polyol used in their polymer system. Ether-based systems are more resistant to water, making them suitable for wet applications. Ester-based, on the other hand, is best against oils, solvents, and most petroleum compounds.
Molding is the process of forming products by pouring or forcing molten or liquified material into a hard tool or mold. The mold is in the shape or profile of the finished product. In polyurethane molding, the processes are modified based on the form and fluid properties of the raw material and the geometry of the product being produced.
Polyurethane is an exceptionally flexible material that can be shaped, molded, formed, and produced in a wide variety of configurations to meet the needs of various industrial and commercial applications. It can be used repeatedly without concern for fractures, wear, or deterioration. Polyurethane is used for its many strengths, including its ability to withstand the effects of electricity and extreme temperatures.
Injection molding is the traditional process for creating polyurethane moldings. It is applicable for both thermoset and thermoplastic polyurethanes. This process involves heating and melting pelletized or powdered polyurethane to allow it to flow. A ram-type, reciprocating screw extruder then injects the molten polyurethane polymer system into a hard tool or mold. High pressures are used to force the polyurethane into the cavities and crevices of the mold. The mold is machined to a profile following the negative image of the finished product.
While inside the mold, the polyurethane starts to undergo curing. The curing phase transforms the liquid polyurethane into its final form. This is accomplished by applying heat or by allowing the curatives to react with the prepolymer system. In some processes, the heat carried by the molten polyurethane is enough to carry out the curing process. After curing, the molded polyurethane is cooled and released. The finished product can be solid, semi-solid, or cellular.
This process is similar to injection molding. However, its main difference is the form of its raw materials. Reaction injection molding uses liquid prepolymer components, which are initially separated and contained in individual tanks. The components are then pumped into a mixing head, where they are blended. Immediately after the mixing head comes the mold, where the blended polymer is formed and cured. This process is limited to producing thermosetting polyurethanes.
This process eliminates the heating and melting of the polyurethane blend before injecting it into the mold. It only involves immediate mixing of the raw materials before molding. The liquid components used intrinsically have a low viscosity, making them easy to inject. This reduces tooling costs by allowing the use of less robust and expensive molds.
Several variations of reaction injection molding exist, such as reinforced reaction injection molding (RRIM) and structural reaction injection molding (SRIM). Both processes use reinforcing materials to strengthen the finished polyurethane molding. RRIM uses reinforcing agents such as fiberglass and carbon fiber. SRIM, on the other hand, uses fiber meshes.
Compression molding is a popular method for forming large thermosetting polyurethane products. This process uses a mold composed of an upper and lower half. The lower half receives a compounded polyurethane mass or charge with a predefined weight. The two halves of the mold are then compressed against each other to force the polymer mass to flow in the shape of the mold. The displaced gas is vented out of the mold to produce a homogenous product. After molding, the product is cured, cooled, and released from the mold.
Rotational Molding is a process used to produce seamless, hollow products. This molding technique is done by loading powdered polyurethane into a mold. The mold is then heated while being rotated to melt the powdered polymer, so the material coats the inside surface of the mold. Unlike injection molding, this process does not use high pressure for injecting or extruding the polymer. Instead, it forms the container by spreading the plastic melt through rotation. After a predetermined number of rotations, the product is cooled and ejected from the mold.
Blow molding is the process of creating hollow products by inflating a softened preform inside a mold. It commonly uses thermoplastic polyurethanes as raw materials. This process involves heating the polyurethane and extruding it into a tube called a parison or preform. The preform is then enclosed and clamped inside a mold. Compressed air is then introduced on one end of the preform; this inflates the polyurethane to the shape of the mold. After molding, the product is allowed to cool and is later ejected from the mold.
Urethane casting is the process of injecting polyurethane and additive resins into a soft mold usually made of silicone elastomer. The casting process is similar to injection molding; however, injection molding differs by the use of hard, metal molds. Urethane casting is usually applied on short-runs and low to medium-volume production. This is due to the quick wearing of the silicone mold. However, the preparation of the silicone mold is cheaper and faster than making a metal tool without sacrificing the final quality of the molded product.
Open cast molding is the simplest of the molding methods and is used for the manufacture of polyurethane products with high hardness in a variety of sizes. It is used when the number of required parts and the part design do not justify the use of the other molding methods. Open cast molding is quicker than other molding processes and can have completed products available in a few short weeks. The process involves pouring polyurethane into an open-top mold or having it flow up from the bottom of the mold.
As with other polyurethane molding methods, open cast molding begins with a liquid resin and curative that is heated to processing temperature. A key factor of open cast molding is ensuring the correct temperature of the resin and curative liquid, which has to be carefully and precisely weighed and mixed. During the mixing process, a chemical reaction occurs that initiates the change of the heated liquid to a solid.
While the mixture is still in liquid form, it is poured into molds that are heated to the same temperature as the resin and curative compound, which is about 220 °F (104 °C). Once placed in the mold, the polyurethane material quickly sets and is removed from the mold.
The benefits of open cast molding include:
There are numerous machines available for polyurethane molding in the United States and Canada. These machines are essential in today's society because they enable the efficient production of a wide range of products and components using polyurethane, a versatile and durable material with applications in industries such as automotive, construction, furniture, medical, and consumer goods. Let’s examine some of these top machines below.
This machine is known for its robust C-frame design, which provides excellent stability and precision during the compression molding process. It is widely used for manufacturing various polyurethane products, including automotive parts, industrial components, and consumer goods. The press's hydraulic system ensures even distribution of pressure, resulting in consistent and high-quality molded parts.
Hennecke's STREAMLINE CSM machines are known for their continuous stream mixing technology. This innovative system enables consistent and accurate mixing of polyurethane components, leading to improved part quality and reduced material waste. These machines are also equipped with advanced control systems that allow precise adjustments to achieve the desired properties of the final product. They are commonly used in various industries, including automotive, construction, and insulation.
Cannon A-Series machines are high-pressure foaming machines used for producing polyurethane foam products, such as mattresses, cushions, and insulation panels. The A205 model is popular for its advanced mixing and foaming capabilities, ensuring uniform cell structure and density in the foam. This machine's user-friendly interface and automation features make it efficient and easy to operate.
The RimStar Compact machine is widely used for reaction injection molding (RIM) processes. It offers a compact design suitable for small to medium-scale production runs. The machine is equipped with precise temperature and pressure controls, ensuring consistent mixing and dispensing of polyurethane components. It is often employed for manufacturing automotive parts, electronics enclosures, and other high-precision components.
The KraussMaffei RimStar Smart Molding Machine is a high-performance and versatile polyurethane reaction injection molding (RIM) machine. It is designed for producing complex and high-quality polyurethane parts with precision and efficiency. The RimStar Smart Molding Machine features advanced technology for precise mixing and dispensing of polyurethane components, resulting in uniform part properties and excellent surface finish. It also incorporates intelligent process control systems, making it easy to operate and optimize production parameters.
Please note that the specific models and features mentioned above are based on information available up to the creation of this article. It's essential to reach out to manufacturers or distributors directly for the most up-to-date information on available machines and their features.
Polyurethanes are composed of different components. The main components are polyols and diisocyanates. Other components are the curatives and additives that give some of the unique properties of a specific polyurethane polymer formulation. Another set of components particular to polyurethane foams is blowing agents, surfactants, and catalysts, all of which aid in generating gases for creating the structure of the foam.
The polyurethane prepolymer system is made from two components: polyols and diisocyanates. These compounds react and form the polymer backbone of polyurethane. These two components are common to all types, whether they be castings, moldings, elastomers, coatings, or foams.
Polyols are compounds with multiple hydroxyl (-OH) groups. They react with isocyanates to form the polyurethane polymer. Polyols can be classified as either polyester polyols or polyether polyols, depending on their chemical structure.
Polyether polyols, a class of polyols, are produced through the polymerization of epoxides (oxiranes) using initiators and catalysts. These epoxides, typically ethylene oxide or propylene oxide, undergo this transformation, though other variations of epoxides can also be employed. What distinguishes polyether polyols is their outstanding attributes, such as remarkable water resistance, flexibility at low temperatures, and resistance to hydrolysis. These properties make them particularly suitable for applications that involve exposure to moisture. Within the polyurethane industry, polyether polyols find extensive use, often contributing to the production of flexible foams, elastomers, and coatings.
Polyester polyols are another crucial class of polyols formed by the reaction of diacids (such as adipic acid or phthalic acid) or their anhydrides with glycols (e.g., ethylene glycol, propylene glycol). This chemical process yields polyester polyols with either linear or branched molecular structures. Polyester polyols are highly regarded for their exceptional chemical resistance and high tensile strength, making them a preferred choice for applications where durability and resistance to chemicals are paramount. These versatile polyols are frequently employed in the production of rigid foams, coatings, adhesives, and various other polyurethane products, catering to a wide range of industrial needs.
Specialty polyols encompass a diverse class of synthetic polymers that find application in various industrial sectors due to their unique properties and versatility. Two notable examples within this category are polycarbonate and polycaprolactone. Polycarbonate, known for its exceptional transparency, high impact resistance, and heat resistance, is commonly used in the production of eyeglass lenses, automotive components, and durable consumer goods
Polycaprolactone is a biodegradable polyester known for its flexibility, low melting point, and excellent compatibility with other polymers. This makes it a valuable material in biomedical applications, such as drug delivery systems and tissue engineering scaffolds. These specialty polyols showcase the ability of synthetic chemistry to tailor materials to meet specific industry demands, whether for optical clarity and toughness in the case of polycarbonate or biodegradability and biocompatibility with polycaprolactone.
Like polyols, diisocyanate compounds form the resin side of the polyurethane system. There are two main types of diisocyanate: aliphatic and aromatic.
Aliphatic diisocyanates contain aliphatic (non-aromatic) carbon atoms in their molecular structure. These diisocyanates are often characterized by their relatively low reactivity compared to aromatic diisocyanates. They are more UV-resistant and less prone to discoloration when exposed to sunlight, making them suitable for outdoor applications. The most common ADIs are hexamethylene (HDI), hexamethylene (HMDI), and isophorone (IPDI).
Aromatic diisocyanates represent more than 90% of total diisocyanate consumption. Aromatic diisocyanates contain aromatic carbon rings, such as benzene rings, in their molecular structure. These diisocyanates are highly reactive and are commonly used in the production of rigid and flexible polyurethane foams, as well as coatings and adhesives. However, they are more susceptible to UV degradation and discoloration when exposed to sunlight. This type is further divided into NDI, TDI, and MDI.
This type is more extensively used in Europe than in the TDI and MDI-dominated American market. NDIs are known to offer superior performance and long service life for dynamic applications. One downside of using NDIs is their high melting point, which makes them difficult to process. Moreover, they are highly reactive, which results in lower storage stability. Thus, they are usually manufactured with special equipment at the custom molder.
In contrast to MDIs, this type is popularly used for high-hardness applications such as guide rollers. Typical forms of TDIs used on an industrial scale are the 2,4 and 2,6 isomers at an 80/20 blend. Producing proportions other than 80/20 requires an additional process.
MDIs are known for imparting high resilience and impact strength to urethane casts. That is why MDIs, paired with either polyethers or polyesters, are used in dynamic, high impingement applications such as wheels, construction panels, automotive bumpers, and the like. The most common isomer used in casting is purified 4,4 isomers.
These are components unique to polyurethane foams. For the prepolymer system, in addition to polyols and diisocyanates, blowing agents, surfactants, and in some blends catalysts are essential components for making polyurethane foams. These additional raw materials generate foaming gases and control them to acquire the right foam structure.
Blowing agents are used to generate gas to produce the foam‘s cellular structure. Gas can be introduced into the polymer system through chemical and physical means. The first blowing agent used was CFC-11, or trichlorofluoromethane. This was considered an ideal blowing agent due to its non-combustibility, appropriate boiling point, good compatibility with polyurethane, and non-toxicity. However, the chemical, along with other hydrochlorofluorocarbons, was banned through the Montreal Protocol in 1987 because it tends to cause ozone layer depletion. Today, CFCs are being replaced by water, pentane, methylene chloride hydrocarbons, halogen-free azeotropes, and other zero ozone depletion-potential blends.
Chemical blowing agents generate gas by adding compounds that react with the isocyanate groups to form carbon dioxide gas. A common chemical blowing agent is water. Using water alone poses several problems, such as higher temperatures from the exothermic reaction, high polymer system viscosity, high consumption of isocyanates, and inefficient mixing. Thus, it is usually paired with a physical blowing agent. Other chemical blowing agents include enolizable organic compounds, polycarboxylic acid, and boric acid.
Physical blowing agents, on the other hand, operate by vaporizing a volatile compound from the heat generated by the exothermic polymerization reaction. Since the ban on CFCs and HCFCs, new physical blowing agents have been developed, such as cyclopentane, n-pentanes, liquified CO2, methyl chloride hydrocarbons, and halogen-free azeotropes.
Surfactants are additives that help form, stabilize, and set polyurethane foams. The most widely used are silicone-based surfactants. Silicone surfactants perform important functions such as reducing surface tension, preventing foam collapse until cross-linking, controlling cell size, preventing cell shrinkage after cure, and counteracting any deformities induced by adding solids into the system.
Catalysts are used to control the rate of reaction of the isocyanate and hydroxyl groups and the rate of gas generation. These polymerization and gas generation processes usually need to occur simultaneously. If the polymerization process proceeds faster than the gas generation, the cells tend to remain close; this causes the foam to shrink as it cures and cools. Consequently, if the gas generation is faster, the cells expand before the polymer can cure and provide support. The rates of these two reactions must be balanced to produce uniform open cells.
Curatives and chain extenders are used to cross-link the long, chained molecules formed by the reaction of the polyol and diisocyanate. They are mixed with the polyol and diisocyanate prepolymer system to form a solid or semi-solid elastomer. They are added in most thermosetting polyurethane formulations. There are two basic types: hydroxyls and amines.
These curatives have hydroxyl groups (OH) at the molecule terminals that link prepolymers. The standard hydroxyl curative is 1,4-butanediol (BDO), commonly used in MDI prepolymer systems at room temperature.
Aside from hydroxyl groups, amine groups (NH2) can also bond on the terminals of the prepolymer. The widely used amine curative was 4,4-methylenebis (2-chloroaniline) or MOCA as the base curative for TDI prepolymer systems. However, this type was identified as a carcinogen by OSHA. Other amine chain extenders are now being used, such as 4,4-methylenebis (3-chloro-2,6-diethylaniline) (MCDEA).
Additives are used to give polyurethane products additional properties and characteristics. The type of additive and its amount is selected based on the formulation of the polymer system and the intended application. Some polyurethane additives are:
Fillers are typically used to enhance the mechanical properties of the product. A less popular intention is to decrease the manufacturing cost by using less expensive raw materials without affecting the final characteristics of the product.
Plasticizers are additives that enhance the flexibility and softness of polyurethane materials. They are particularly useful when creating elastomeric or flexible polyurethane products, like foams, seals, and gaskets. Plasticizers improve the material's ability to withstand bending, stretching, and compression, making it more suitable for specific applications.
Stabilizer is a general term used to describe additives that prevent the degradation of the final polyurethane product. Common polyurethane degradation mechanisms include oxidation, thermal aging, and photo-oxidation. Stabilizer covers additives such as:
Antioxidants are additives that inhibit the oxidation of polyurethane materials when exposed to air and heat. Oxidation can lead to the deterioration of physical properties and the formation of surface defects. Common antioxidants include phenolic antioxidants and phosphite antioxidants.
UV stabilizers are used to protect polyurethane products from the harmful effects of ultraviolet (UV) radiation, which can cause degradation, discoloration, and brittleness. Hindered amine light stabilizers (HALS) and benzotriazoles are examples of UV stabilizers commonly employed in polyurethane molding.
Heat stabilizers are additives that help protect the polyurethane material from thermal degradation during the molding process. They prevent the material from breaking down or losing its physical properties when exposed to elevated temperatures. Common heat stabilizers include organotin compounds and hindered amine light stabilizers (HALS).
Biocide stabilizers are additives used in polyurethane molding processes to inhibit the growth of microorganisms such as bacteria, fungi, and mold. These microorganisms can thrive in the presence of moisture, temperature variations, and organic materials commonly found in polyurethane formulations. Biocide stabilizers help to prevent microbial contamination during the production and storage of polyurethane products.
Since this compound is added during the compounding of the polymer, these are specifically termed internal antistatic agents. These additives prevent the build-up of static electricity. Static electricity affects the material by altering its purity, moisture affinity, and surface properties.
Degassing is a process done in molding solid polyurethanes or for making polyurethane coatings and elastomers. Mixing the components of the polymer system tends to dissolve and trap gases; this may lead to a lower quality product. Degassing aids help remove these undesirable trapped gases.
Flame retardants are essential additives in polyurethane molding to improve the material's fire resistance. They work by slowing down or inhibiting the spread of flames when exposed to heat or fire, making polyurethane products safer for various applications, such as in the construction and automotive industries.
Colorants are used to add aesthetic appeal to polyurethane products. They come in various forms, including pigments and dyes, allowing manufacturers to create a wide range of colors and finishes. Colorants are crucial for industries where the appearance of the product is important, such as in consumer goods and furniture manufacturing
COF additives are designed to raise or lower the coefficient of friction of a polyurethane product where a high COF is ideal for applications where there is sliding between surfaces. A lower COF increases the friction between materials and is found in harder polyurethane products. The COF of polyurethane is easily adjusted by changing the additive formulation.
Polyurethane molding is often used in product development and prototyping. It allows designers and engineers to quickly create prototype parts and test their designs before committing to expensive production tooling.
Polyurethane molding is ideal for producing low to medium volumes of custom parts and components. It can replicate complex shapes and intricate details with precision.
Many industries use polyurethane molding to manufacture replacement parts for equipment and machinery. It can be a cost-effective way to produce parts that are no longer available from the original manufacturer.
Polyurethane molding is used to make various automotive components such as bumpers, spoilers, gaskets, seals, and interior trim parts. It offers excellent durability and resistance to chemicals and UV radiation.
Polyurethane is a biocompatible material, making it suitable for producing medical devices such as seals, gaskets, housings, and cushioning components.
Polyurethane molding is used to produce protective covers, cable assemblies, enclosures, and insulating components for electronic and electrical applications.
Polyurethane is commonly used in the production of consumer products like sporting goods (e.g., skateboard wheels), toys, cases, and decorative items.
It is used to create wear-resistant parts for heavy machinery, such as conveyor rollers, wheels, and bushings.
Polyurethane molding is used to manufacture seals, gaskets, and other components for oil and gas exploration and extraction equipment, which need to withstand harsh environmental conditions.
Polyurethane molding can be used to create decorative architectural elements like cornices, columns, and balusters, as it can replicate the look of traditional materials like wood and stone.
Due to its resistance to water and salt, polyurethane is used to create marine parts like boat bumpers, winch drums, and marine hardware.
Polyurethane molding is used in aerospace for producing seals, gaskets, vibration damping pads, and other parts that require lightweight, high-performance materials.
Items like skateboard wheels, inline skate wheels, and shock-absorbing pads in sports equipment can be made using polyurethane molding.
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