This article will give you detailed insight into urethane casting and its uses.
You will learn:
Urethane casting is the process of injecting polyurethane and additive resins into a soft mold made of a silicone elastomer. The casting process is similar to injection molding but does not use hard, tooled metal molds. It is used for short-runs and low to medium volume production, which is due to the rate at which the silicone molds wear, Urethane casting is a less expensive process faster, due to the elimination of metal molds, but retains the quality of the molded products.
Castable polyurethane is a part of the polyurethane family, which can be made into high performance, engineering-grade products. Mechanical properties of cast polyurethanes can vary from soft and flexible to hard and rigid materials.
There are four components to creating urethane cast: which are the polyol compound, the diisocyanate compound, the chain extender or curatives, and the additives. Mechanical properties depend on the formulation of the prepolymer resin (the mixture of polyol and diisocyanate compounds) and the curatives. Additives are added to further improve the properties of the polyurethane such as resin curing time, machinability, color, and UV protection. Additives are carefully proportioned relative to the resin mixture since they can weaken the urethane cast.
One of the concerns for urethane molding is the coefficient of friction (COF), which is an indication of when plastic materials stick to each other. To lower the COF, organic compounds called "slip additivies" designed to cover the surfaces of plastic forms and reduce the friction between them, are added to the urethane casting. The most popular types of slip additives are acid amides, erucamide, and oleamide. Erucamide is a slow moving additive while oleamide is fast-moving.
The polyurethane reaction involves the formation of a simple polymer chain from the reaction of a polyol component (a carbon-chained molecule with alcohol on both ends) to a diisocyanate component (a molecule with isocyanate on both ends). This results in a molecule with a reactive alcohol on one end and a reactive isocyanate on the other. The alcohol end further links to another isocyanate end or terminal, while the isocyanate end of the same chain further reacts with chain extender compounds (curatives such as hydroxyl and amines). This process continues on by making long-chained polyurethane.
Preparing the formulation can be done through different processes: single shot, prepolymer, and quasi-prepolymer process. The single shot process involves having all components in separate chambers. These will then be blended by a mixing head and poured or injected into the mold. The prepolymer process, on the other hand, mixes the polyols and diisocyanates prior to pouring them into the mold. This process helps dissipate the heat produced from the exothermic reaction of the compounds. Last will be the quasi-prepolymer. Quasi-prepolymers consist of polyols partially reacted with the diisocyanate compounds. This simplifies the formulation process since the quasi-prepolymers are less viscous and require low processing temperature.
Urethane casting is an easy, straightforward process. It only involves making the pattern, making the mold, and pouring the resin. Below are the steps in making urethane cast products.
The most common process in creating a master pattern is by 3D printing, either by SLA (stereolithography), by PolyJet, or by FDM (fused deposition modelling). Other methods such as CNC machining may be used. Both methods start by making a CAD model of the part to be cast. In making the 3D model, it is important to keep in mind how the pattern will be molded and how the urethane cast will be removed. It is worth considering the following suggestions.
Remove problematic features such as deep and narrow holes, internal cavities, and channels. These features can be added by secondary processes after the urethane part has been cast.
Unlike die casting and injection molding, urethane casting does not require draft angles. Silicone molds are flexible and can be deformed to remove the molded product.
Add gates and vents to the CAD model. The gates are where the resin and curatives will be injected; while the vents allow the escape of air trapped inside the mold. The size and location of the gates depend on the volume and profile of the pattern.
Section the model or separate its parts as needed. This depends on how much is the build volume of the 3D printer.
Add locators on the mold to prevent any shifting of the mold halves. If there is shifting, the parting line of the molds will become significantly visible, and the dimensions of the product will be off tolerances.
Silicone formulations usually come in as a viscous fluid of siloxane polymer which is then combined with a cross-linker and a catalyst. Creating a semi-solid from this siloxane fluid is usually done by two-component room temperature vulcanizing (RTV-2). This method is divided into two types: condensation cure and addition cure.
Condensation curing uses organo-tin compounds as the catalyst. Polymerization is done by mixing the reactive component, the silane crosslinker and catalyst, and the unreactive component, the polymer and filler. This is the inferior method between the two since this curing process produces shrinkages of about 0.5% as the curing agent leaches out of the mold over time. Nevertheless, one advantage can be mentioned which is its resistance to inhibition. Inhibition happens when contaminants on the surface of the pattern prevents the silicone from properly curing. A common contaminant is sulfur, which is usually found in modeling clays. If modeling clay is used for the master pattern, then condensation curing may be the better option.
Addition curing is done by mixing the siloxane polymer and the platinum complex catalyst. This type of curing produces no by-products which in turn makes the mold odorless, gives it a longer shelf life, and keeps it shrinkage-free. They have better mechanical properties than condensation cured silicones but are critically sensitive to sulfur, phosphorus, arsenic, organo-tin compounds, PVC stabilizers, epoxy resin stabilizers, and natural rubbers.
Apart from two-component room temperature vulcanizing, there are other methods of polymerizing silicone such as one-component room temperature vulcanizing (RTV-1) and high temperature vulcanizing (HTV). With RTV-1, polymerization happens when the mixture of silicone fluid and cross-linkers is exposed to atmospheric humidity by undergoing hydrolysis. This process releases alcohols, acetic acid, ketones, and so forth. The release of these compounds produces the characteristic smell of the product. With HTV, on the other hand, high-temperatures break down the peroxides into free radicals, which cross-links the polymer. This process is also known as peroxide curing. The downside of HTV is the shrinkage of the silicone mold after cooling due to the high coefficient of thermal expansion of silicone.
The elastomer components are then mixed in a container. This should be larger than the silicone volume to allow for expansion during degassing. Degassing is not required but is recommended to eliminate air bubbles caused by mixing. Bubbles trapped between the silicone and the surface of the pattern will cause bumps in the urethane cast. During degassing, the silicone mixture will rise until the air dispersed into the silicone is released. Afterward, the volume will return to its starting level.
The next step is to prepare the silicone mold. A mold box or a frame is prepared to contain the silicone while being poured. This is similar to a flask used in metal casting.
The master pattern is placed in the mold box, where the silicone and catalyst mixture is then poured. This process can be book mold, two-part mold, or skin mold. The book mold, or single stage mold, involves suspending the master pattern inside the mold box. The silicone is then poured into the mold box until it fully covers the pattern. Once cured, the silicone block is then cut in half to remove the pattern.
The two-part mold, on the other hand, has a predefined parting line on the master pattern. In this process, the mold box is filled with silicone only up to the parting line. After curing, the pattern is removed, and a second mold is prepared from the remainder.
Skin mold involves applying the silicone mixture by pouring it on the pattern layer by layer until a desired thickness is achieved.
After curing the silicone mold, it is optional to apply a release agent before introducing the urethane. The release agent helps in the removal of the cast from the mold. Since silicone is flexible and does not readily bond with urethane, a release agent is not required.
Open silicone molds are the most basic form of molds and are widely used by DIY hobbyists. The creation of an open silicone mold follows many of the steps associated with producing silicone molds, with two halves of an open silicone mold being much easier to create and involving less precision.
The process of making an open silicone mold begins with a pattern that has been created by 3D printing or CNC machining. It is essential that the pattern be inspected for any errors such as cracks, chips, or uneven surfaces since they will appear in the molded shape. Sanding and painting the pattern multiple times can guarantee a proper surface finish. After the pattern is definitively clear of all errors, it is sprayed with a release agent and placed, face up, in the bottom of a tightly sealed wooden box with a sufficient amount of clearance on all sides to ensure proper coverage of the pattern.
The selection of the type of silicone is dependent on the use for which the mold is being made. The choice of silicone is related to its hardness, which can be hard and inflexible or soft and pliable. Additionally, the complexity of the pattern influences the choice of silicone. The mixing of the silicone can be based on weight or volume, another factor related to the use of the final product.
With the open silicone mold process, the silicone is poured over the pattern, a process that requires excellent control. Prior to pouring the silicone, the depth or height at which it will be poured has to be predetermined. The silicone is poured from the lowest point of the mold and allowed to fill up any crevices, intricate patterns, and openings.
A common error associated with open silicone molds is bubbles that are created during the pouring process. To avoid the pimples and deformities of bubbles, the silicone solution is placed in a vacuum degasser under pressure. This will pop any bubbles and create a smooth, even solution.
Once the silicone has filled the box and covered the pattern, it must be allowed to cure. Curing agents can be tin or platinum, with platinum being the more expensive. The amount of time for curing varies depending on the complexity of the pattern and the density of the silicon. Under normal conditions, curing takes six hours or more.
At the end of the curing process, the mold can be removed from the box by ripping the box apart, which has to be done gently to avoid damaging the mold. As with any molding process, there may be bits of the silicone material, called flashing, that have seeped into the mold that has to be removed. Unlike injection molding, the fragments can easily be cut away.
There are a variety of polyurethane resins available for casting, each with its own set of properties. Choosing the right urethane resins and curatives will be discussed in detail later. Once the silicone mold is prepared and the urethane resin has been chosen, casting is ready to begin.
The casting process involves two liquids, the resin, and the curative, chemically reacting to form the shape of the pattern. Like the silicone, the resin and curative are mixed in a container and degassed to remove any trapped bubbles dispersed in the mixture. The mixture is then introduced into the mold by pouring or by pressure fill. Pouring uses gravity only to fill the void inside the mold, while pressure fill requires the use of equipment such as an injection ram or screw-type plunger to push the mixture.
Once the urethane is cured, the silicone mold is split into its two halves and the cast is removed.
Depending on the quality of the silicone mold, the urethane cast can have a smooth surface with minimal rough areas. Gates, vents, and flashings are cut and smoothened. Holes and channels not included in the master pattern are drilled or milled to complete the features of the product. One thing to remember while urethane cast machining is its susceptibility to melting. Machining must not be too aggressive, and the use of coolants is recommended.
Compared to metals and other plastics, urethane castings have a set of properties that makes them stand out. Below are some noted properties of polyurethane resins.
Abrasion can be classified into two types: sliding and impingement abrasion. Sliding occurs when a soft and a hard material slide or rub into each other, with or without contaminants between the surfaces. Impingement abrasion, in contrast, happens when particles impact the surface, causing erosion. Because of these properties, cast urethanes are excellent materials for wheels and rollers.
Cast urethanes with low coefficient of friction and high tear strength offer good sliding abrasion resistance. For impingement abrasion, cast urethanes with good resilience are used. Resilient polyurethanes can easily yield elastically. Forces from the impacting particles are easily distributed on the surface.
Abrasion resistance is achieved by the right composition of the urethane resin. Among the polyol compounds for making polyurethane, polyesters exhibit better tear and abrasion resistance.
Like abrasion resistance, urethanes possess good impact resistance because of their good resilience. Urethane castings can elastically deform to absorb the impact and return to their shape while dissipating the energy throughout the body.
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 then graded according to their relative hardness from one another. For elastomers, hardness is characterized by a Shore hardness number as measured by a durometer. The Shore hardness scale is categorized into 12 scales, each scale has its own indenter configuration, profile and force applied. The most common scales for urethane casts are Shore A and D. Shore A scale measures hardness of soft, semi-rigid polyurethanes. Shore D, on the other hand, measures hard rubbers and rigid polyurethanes. However, keep in mind that high hardness does not correspond to high rigidity or strength.
Elastomer tensile strength, in metals, is the amount of force or stress the material can take without failing. Urethane casts can be formulated to meet strength requirements for up to 14,000 psi by using composite materials such as fiberglass and carbon fibers.
For flexible urethane casts, flexural strength is a key property that tells how much bending force is required to break the material. Urethanes have excellent flexural properties, especially in thin cross-sections. They usually have 17,000 psi flexural strength, up to 39,000psi for composite casts.
As mentioned before, urethane resins are made of four components. Out of these four components, three influence the final mechanical properties of the product. There are a number of formulations possible that serve a specific application. Below are the urethane resin components, their types, and the properties they impart to the product.
A polyol is an organic molecule containing one or more hydroxyl (OH) groups. Polyols used in urethane casting are either polyether or polyester types.
These are made by the reaction of organic oxides and glycol. Polyethers are characterized by having good resilience, high impact resistance, low heat build-up for dynamic applications, hydrolysis resistance, and good low-temperature performance. Common types of polyether used in the polyurethane industry are PTMEG and PPG. Between the two, PTMEG offers superior quality but is more expensive.
These are made by the polycondensation reaction of diacids and glycol. Compared to polyethers, polyesters have good abrasion resistance, heat aging resistance, oil resistance, solvent resistance, good shock absorption properties, and better tear resistance.
The most common are polycarbonate and polycaprolactone polyols. These two polyols are also sometimes classified as polyesters. Polycarbonates are used as engineering materials due to their strength and toughness. Polycaprolactone, on the other hand, gives the cast urethane good water, oil, solvent, and chlorine resistance.
Like the polyols, diisocyanate compounds form the resin side of the polyurethane system. There are two main types of diisocyanate: aliphatic and aromatic.
This type‘s most popular characteristic is its non-yellowing appearance. Also, they have lower reactivity that makes them useful for chemically-resistant coatings. Aliphatic diisocyanates are mostly used in polyurethane coatings, films, and castings where color stability is required. The most common ADIs are hexamethylene (HDI), hexamethylene (HMDI), and isophorone (IPDI).
This type is further divided into NDI, TDI and MDI.
This type is extensively used in Europe as compared to 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 making them difficult to process. Moreover, it is highly reactive resulting in lower storage stability. Thus, it is usually manufactured with special equipment at the custom molder.
This type is popularly used for high hardness applications such as guide rollers, in contrast with MDIs. Typical forms of TDIs used in an industrial scale are the 2,4 and 2,6 isomers at an 80/20 blend. Producing different proportions other than the 80/20 require 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.
Curatives are mixed with the polyol and diisocyanate prepolymer to form a solid or semi-solid elastomer. There are two basic types of curatives: 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 is 4,4-methylenebis (2-chloroaniline) or MOCA as the base curative for TDI prepolymer systems. However, this type was then identified as a carcinogen by OSHA. Other amine chain extenders are now being used such as 4,4-methylenebis (3-chelloro-2,6-diethylaniline) (MCDEA).
Mentioned below are the main advantages of using urethane casting in contrast to die casting and injection molding.
As mentioned earlier, urethane resins can be cast by soft molds such as silicone. Silicone molds are made easier and less expensive than metal tools or molds. This creates an option for producing cast prototypes making any modification cheaper. It also does not need expensive equipment such as injection pumps, heaters, kneading machines, and so forth.
Urethane molds can be created within three days as compared to hard molds that may require a week or two. Master patterns and silicone molds are easy to fabricate, making them suitable for pre-production runs that are usually on trial and error. Product design becomes much faster before transferring to hard tooling.
Urethane casting is preferred for backyard, do-it-yourself projects because of the simple equipment required. At most, one will need a few containers, a weighing scale, an air compressor, a mixer, and a spatula. Silicone molds can be prepared at atmospheric pressure and temperature conditions. The urethane resin, on the other hand, usually requires some heating to lower its viscosity, hasten the curing process, or melt the solid or semi-solid components.
Polyurethane systems, with their different types and proportions of polyols, diisocyanates, and curatives, can have almost any property to suit a particular use. It can be formulated for hard, tough, high-performance parts such as wheels and rollers; or, for soft, shock-absorbing applications such as impact absorbing pads and cushions. There are different formulations available in the market each with its own intended usage.
Because of its wide array of properties, cast urethanes are found in every industry. Urethanes are versatile materials that can be manufactured easily with low initial costs. Below are popular applications of urethane casting.
As mentioned earlier, prototyping or product design is one of the main applications of urethane casting. Urethane castings can conform to any design due to its easy to customize properties.
Due to their toughness, high impingement resistance, shock absorption, and fatigue resistance, urethane resins are a popular choice. Urethane castings can be seen on caster wheels, pulleys, guide rollers, and so forth.
Urethane castings can be formulated as shock and vibration resistant which makes them suitable for automotive applications. Moreover, they can withstand high temperatures, replacing steel.
High vibration from rotating equipment causes rigid materials to crack. Urethane castings can be made to absorb vibrations, as seen from shock absorbers and dampers.
There are urethane formulations available that are FDA compliant. Medical devices serving a niche purpose or having a unique design usually requires low volume production. This makes urethane casting a suitable method of production.
Because of its wide range of properties, urethane castings can be used in many consumer products. Examples of these are shoe soles, sports equipment, electronics casings, and so forth.
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