DESCRIPTION OF LIQUID RESIN CASTING TECHNOLOGY

Liquid Resin Casting (LRC) is a replication technique for the production of plastic parts from thermoset liquid resins, such as polyurethanes, epoxies and silicones.

LRC is unparalleled in its ability to accommodate complex design, and yet it offers unmatched flexibility for potential future design revisions. Complex geometry that includes undercuts, internal and external threads, compound curves, internal passages, and unusual shapes can be created inexpensively. LRC is also not constrained by variations in mass and wall thickness.

The broad variety of LRC resin systems produces a whole spectrum of polymers. Among these materials are soft pliable rubber-like elastomers with durometer hardness of 10-20 Shore A, and much harder materials similar to polypropylene or polystyrene with durometer hardness of 85-90 Shore D. Polymers can be water-clear, tinted in a variety of transparent tones or colored with conventional plastic dyes or pigments.

The advantages of LRC technology include:

• The capital cost of the LRC-associated tooling is orders of magnitude less that that of other techniques, such as CM (Compression Molding), IM (Injection Molding) or even RIM (Reactive Injection Molding).
• LRC-associated tooling is not only much less expensive, but can also be manufactured much faster. The first finished parts can often be produced only 2-4 weeks after the drawings are ready.
• LRC molds are often easily modifiable. The manufacture of any product from polymeric resins always requires tooling that is specific for the produced parts. Practically any modification of products manufactured by CM, IM or RIM requires a completely new set of expensive tooling. On the other hand, LRC tooling often can accommodate "on the fly" engineering revisions rapidly and inexpensively. The lower mold fill pressures with LRC materials allow a designer an unparalleled ability to revise or modify an existing tool. Short of complete design revisions, the original tooling in most cases can be quickly salvaged for the next design revision.
• LRC molds can accommodate multiple and interchangeable inserts, which makes it possible to use one mold to produce several configurations of the same product that differ in the geometry or physical shape of certain elements. Alternate mounting bosses, through-hole panels or alternate shapes are all possible within the same tooling set, if required. Limitations exist, but LRC's design flexibility is unique.
• LRC technology allows completely or partially encapsulate other objects into the body of the product. Foreign objects in a variety of configurations can be inserted into a mold cavity and encapsulated in the final finished form. The strength and isolation ability of cured plastic makes an encapsulated element impervious to external damage or contamination. Instead of relying on secondary bonding or attachment processes, the elements can be overmolded directly into a part, often with very close tolerances due to the low mold fill pressures. The low pressure of an LRC process allows easy control of weep or blow-out that causes the migration of encapsulated parts. This is significantly more user-friendly than encapsulations with higher pressure molding techniques, such as Injection Molding or RIM.

  The most common foreign objects routinely cast into LRC parts are threaded inserts and other fasteners, such as threaded nut plates and anchor plates. They are positioned inside a mold cavity and cast in-place within a finished part. Threaded metal screw inserts can also be added as a secondary operation when they are spun into the molded plastic. Metal threaded inserts are recommended over tapped threads when multiple re-assembly is anticipated.

  Other types of commonly imbedded objects include wiring, delicate electronics, fiber optics, structural reinforcement, tubing and other items that benefit from the protection of the plastic cured around them.

• LRC parts come out of production with a near perfect surface finish. Details on all surfaces of the products replicate the designed textures. Cast part finishes can include a variety of machined and textured surfaces, such as a highly polished glass-like finish, light grain, aggressive grooving, graphics and heavily textured surfaces. Finished parts are created with no voids or porosity. LRC technology, unlike Pressure or Vacuum Forming, controls both sides of any product, as all the surfaces are contained.
• LRC includes techniques that allow the manufacture of products with complex internal passages and cavities, such as those found in manifolds and vascular models. Internal forms that are impossible to obtain with processes like RIM are easily created. Design possibilities with LRC processes include reverse draft openings, undercuts, through-hole geometry with increasing diameters larger than the initial opening hole size, through-part holes, internal and external threads, internal voids, irregular passage ways, and manifold chambers.
• LRC can combine thin and very thick walls without compromising the dimensions and finish of the product, whereas Injection Molding causes shrinkage or chill in similar parts.
• LRC plastic parts can be used in conjunction with other manufacturing techniques. For example, it is possible to use an LRC bezel as a sculpted control panel for an unattractive sheet metal housing. This combination of techniques allows a designer to be simultaneously cost-effective and creative in achieving the best final product.

Depending on technical considerations including the complexity of the product, ongoing design needs and the required volume of production, it is possible to economically justify using LRC to fabricate prototypes, unique parts and short runs of 5-500 units. In the majority of cases, LRC of 1,000 or more units is unfeasible, as other techniques allow the manufacture of large batches of products at much lower per unit costs. However, CM, IM or RIM process tooling and molds are extremely expensive, and any production using these techniques requires a significant capital investment and a long mold production time.

LRC RESINS

The main materials used for Liquid Resin Casting include polyurethanes, epoxies and silicones.

Resins are mixed with hardeners and transferred into a mold at an ambient temperature where they solidify (cure) due to a chemical reaction between the components. Often, in order to achieve the optimal properties of the solidified materials, it is necessary to post-cure them for several hours at an elevated (180-200?F) temperature. Usually the thermosets produced by Liquid Resin Casting do not contain any air inclusions, but, when necessary, it is possible to produce solid foamed thermosets.

POLYURETHANES

Polyurethanes range in hardness from a Shore A-10 (hardness of soft rubber) incrementally to the hardest, Shore D-85 (hardness of acrylic-like plastic). They are tough and abrasion resistant materials with a glass transition temperature below the 225?F. Polyurethanes can be compounded to achieve Underwriters Lab flame retardant specification of 94 VO.

There is a wide variety of polyurethane formulations, and the properties of the cured polyurethanes depend on their chemistry. The diversity of raw materials allows creative designers to generate a multitude of casted products, including instrument housings, bezels, keyboards, cable strain relief, electronic packaging, encapsulations, fluid manifolds, etc.

Polyurethanes are available in clear and opaque formulations, which can be colored or tinted with pigment dies. Finished parts can be readily coated with conventional paints and shielding.

EPOXIES

There are several types of epoxy resins (standard Bisphenol-A, Novolac, and Cycloaliphatic grades, etc.) that can be cured by different types of hardeners and combined with various accelerators, plasticizers and fillers. This produces materials with a wide range of tensile properties that are usually characterized by excellent adhesion to imbedded components, high chemical resistance to corrosive environments, good arc resistance and the capability to withstand high temperatures in the 350-400°F range.

Applications for epoxy parts include wet and dry food contact items, human body implanting parts, and microwave reflection and high voltage resistance areas. Epoxies also can be produced as transparent tinted and colored versions. Epoxies are very suitable for high temperature applications, will accept all conventional sterilization materials (like alcohol and sterilizing solutions), but will not accept extended full Autoclave Sterilization.

RTV SILICONES

RTV (Room Temperature Vulcanizing) Silicones are flexible rubber-like materials that can withstand very high temperatures (up to 600?F) and are famous for their lack of adhesion to any substrates. Practically no other materials can bond to their surfaces. This family of polymers retains its flexibility and elasticity over a broad range of temperatures (including temperatures well below freezing) without losing their original shape. Silicones are very resistant to water-borne corrosive agents, which makes them suitable to medical applications. RTV Silicones are the ideal material for creating soft molds for casting polyurethane and epoxy parts with complex geometry. This is due to their lack of adhesion and the inability of other substances to bond to their surfaces.

RESIN ADDITIVES AND FILLERS

Adding various components to thermoset liquid resins allows engineers to create blended mixes that improve and purposefully adjust the cured polymer's physical properties. Such additives include fillers like chopped fiberglass, glass flakes and microspheres, silica, calcium carbonate, ultra-high molecular polyethylene (UHMW) powder, substances with antioxidant properties, radical scavengers, air release agents, etc. The inclusion of each of these materials changes certain properties of polymers, including specific gravity, strength, toughness, tear resistance, impact resistance, UV and chemical resistance, fire retardation, thermal conductivity, etc. LRC materials can be brought into compliance with FDA, UL flame retardant, and most other product-specific regulations.

LRC TOOLING

The manufacture of plastic products using LRC techniques usually involves

• The production of Master Model or Master Patterns
• The production of a Mold
• Casting of the product

There are two types of molds used for LRC: hard molds and soft molds.

HARD MOLDS

Creating a hard mold involves machining or casting a negative cavity of a part's final form, with shrinkage allowance, in harder substances like aluminum, harder plastics (polycarbonate, ABS, acrylic, etc.), or other machinable materials. Hard molds are used for producing parts without complex geometrical elements, i.e. without undercuts, thread details, reverse draft features, etc. Such molds consist of at least two (sometimes three to four) separate master patterns. Hard Molds do not require the production of Master Model or Master Patterns.

Master patterns are "imprints" of the actual part, where the impression of the convex surface of the part is concave, and the impression of the concave surface of the part is convex. When assembled together, master patterns form a hard mold with a cavity corresponding to the product that is being manufactured. This cavity has a gate to put resins inside, and vents to displace air. Hard molds require mold release agents to allow the easy removal of finished parts. In addition to the lower tooling costs, the benefits of using hard molds include the ability to hold slightly tighter tolerances in finished products.

SOFT MOLDS

Soft molds are made from pliable and flexible materials. The most common material for the production of soft molds is RTV silicone, but sometimes molds are made from other elastomers such as flexible polyurethanes or PVC plastisols. Soft molds are usually used to manufacture parts with a convoluted geometry, including undercuts, severe reverse draft angle configurations, thread details, and other complex details that work counter to the 180? parting plane of the mold. Silicone molds do not usually require release agents to remove the product. LRC engineers /mold makers can assist customers in the best tooling approach for a given project.

In order to make a soft LRC mold, it is necessary to use a Master Model or Master Patterns. LRC Master Models are usually machined from plastic, metal, or some other durable rigid material in a form that is an exact copy of the finished part. The dimensions of Master Models are 3 - 3.5% larger than the dimensions of the finished product in order to accommodate the shrinkage that occurs in the process of curing of silicones. Master Models can be produced not only by machining, but also by such techniques as Stereo Lithography, Fused Deposition Modeling, Laser Prototyping, etc. Sometimes when the shrinkage is within product tolerances, even existing fabricated or machined parts can be used as Master Models for the soft LRC molds.

It is important to understand that the production of Master Models brings into focus issues of tolerance control, flatness, brittleness, material stability, and surface quality that must be considered before Master Model production is undertaken. It is advisable to consult a qualified production casting engineer if you are not sure of the ramifications of using prototype processes or existing models for LRC replication.

After the Master Model is complete, a soft LRC mold is produced by either

• Imbedding the Master Model in liquid silicone inside a rigid box, curing the silicone around it, and splitting the silicone cast into two parts. These parts are converted into the lower and upper shell of the mold, or
• Imbedding one half of the Master Model in liquid silicone and then curing the silicone around it, thus creating the upper shell of the mold. This process is repeated with the second half of the model, creating the lower shell of the mold. Then both shells are fitted together and placed in a rigid box.

  It must be noted that sometimes it is technologically advantageous not to create a single Master Model, but to manufacture two separate Master Patterns, one of which represents one half of the targeted product, and the other the second half. Then each shell of the soft mold is cast around its own Master Pattern.

Following this procedure, a box that contains the soft mold is fitted with a gate (tube that leads from the outside of the box to the bottom of the mold cavity and is used to fill the mold with liquid resin), and vents (narrow channels that connect the mold cavity to the atmosphere to allow the displaced air to leave the mold).

Notes for Cast LRC Mold Tolerances:

Silicone Molds: ±0.004 inch/inch
Epoxy, ABS or Urethane Molds: ±0.002 to ±0.004 inch/inch
Aluminum & Metal Molds: ±0.001 to ±0.002 inch/inch

SURFACE FINISHES

The finish of the cast surface usually depends on the material of the mold. Silicone molds are excellent for detailed reproduction of textures copied from hard patterns. Epoxy and Urethane molds are more limited in surface finish options, and typically can provide only high gloss, smooth, bead blasted and sanded finishes. Metal molds or inserts can replicate an unlimited range of finishes, but are usually not appropriate for excessively aggressive textures, unless draft allowances are incorporated. It is necessary to properly choose the material and finish of the Master Model in order to achieve the targeted finish of the final product.

SHRINKAGE

When liquid resins cure, they usually shrink in size. Therefore, Models or Patterns are typically scaled up in size to accommodate the shrinkage factor of the material to be cast. The degree of shrinkage depends on the properties of the resin used and on the linear dimensions of the product. The larger the product, the more overall shrinkage affects the dimensions of the final cast form. With smaller parts, the shrinkage factor is often negligible and can be disregarded. Shrinkage is overcome by providing a dimensional compensation for the cast parts' final fully cured size, which is usually smaller than the dimensions of the Master Model or Pattern. Incorporating a shrinkage factor allows the final cast part to be replicated at nominal final size and engineering dimensions.

Note: You are not required to include dimensional shrinkage in files supplied to your casting vendor for tooling. Your casting vendor will create your LRC tooling with a material shrinkage factor incorporated in your patterns/tools. It is, however, important to note that when you provide a rapid prototype or machined model for reproduction to your casting vendor, a significant problem may develop if material shrinkage is not first addressed.

COLOR

Clear formulations are available in all classes of thermoset casting resins and can be tinted with pigment colors to achieve tones similar to, for example, those in acrylic and polycarbonate sunglass lenses. The technique involves tinting the base resin with organic dyes to achieve transparent coloration with various levels of intensity. It should be noted that in any given product, the thicker the section, the darker the tinting will appear. Also, certain material formulations will darken over time, dependant on Ultra Violet exposure. Color shift typically has no effect on materials' physical properties.

Custom Opaque Colors are achieved by blending various pigments with any resin formulation. Certain color limitations exist for some epoxy and polyurethane materials due to the base color - usually in the brown and bronze color range. Also, not all resin systems can be used for manufacturing products colored in white, off-white and other very light colors. Please contact a LRC engineer to discuss your specific needs.

Color Note: Designers should be aware that an interrupted batch processing will lead to a wider part-to-part color variation. If you have to color-match with other components, it is better to paint the product rather than tint it. Having all parts painted eliminates any chance of a color mismatch.

PAINTING, PRINTING, GRAPHICS

Painting and printing on polyurethane and epoxy LRC products is easy. One can use standard silk screen and pad printing inks used for graphic applications, as well as multiple types of conventional paints, enamels, lacquers and topcoats. Many of these paints require no coating primers. Cast-in, recessed graphics can be paint-filled with standard printing inks or contrast colored resins. When metal patterns are used in the molding process, graphic replication on parts can be as small as a 6 point font.

Note: Thermoset LRC materials are not compatible with conventional Hot Stamping techniques.

TOLERANCES, DRAFT AND OTHER QC CHARACTERISTICS

LRC finished part tolerances are directly dependent on the chosen tooling approach, the materials being used, and the configuration of the part. Typically, LRC parts produced from harder polyurethanes and finished "as cast" have an accumulated tolerance

• from ±0.003 to ±0.004 inch/inch when created with RTV Silicone molds, and
• from ±0.001 to ±0.003 inch/inch when created with aluminum or steel molds.

These values do not apply across the mold's parting lines, which generally add another 0.005 inch/inch to the tolerances of each half of the mold.

Diameters are usually held to ±0.005 nominally, with tighter tolerances possible on smaller diameters. Machined overall lengths are usually ±0.010 nominally, with 0.001 inch/inch under controlled areas. Hole-to-hole and point/edge-to-hole tolerances are usually ±0.010, with 0.001 inch/inch under controlled areas.

For parts above five inches in length, the measurement temperature must be the same as the machining temperature. The softer the material is, the more critical the temperature differential is.

Secondary operations, such as placement of the machined holes and threads (including tread inserts) can be used to hold tolerances of ±0.001 per inch.

Secondary Machined Operation Tolerances:

• Diameters: ±0.001 to ±0.005 inch/inch. (Temperatures for all measurements must be consistent)
• Machined Overall Lengths: ±0.002 to ±0.004 inch/inch.
• Hole to Hole & Point-Edge to Hole: ±0.001 to ±0.010 inch/inch.

Draft consideration is usually not required on exterior surfaces of LRC produced parts except when using hard molds. Internal draft is not required except for extremely deep draw configurations or for parts with rigid cores. It is possible to produce parts with reverse draft openings and holes using typical RTV silicones. Removable core pins can be used to collapse a core form for removal of the part from the mold's cavity.

Rounded corners are advisable on both exterior and interior edges, as sharp and angular corners can be difficult to control and lead to premature mold deterioration and failure. It is recommended to round all edges and corners where possible with interior radius dimensions at half the part's wall thickness.

WALL THICKNESS

It is possible to create small sections of wall as thin as 0.025", but it may be problematic to produce such thin walls with a large area. To aid in filling the molds, walls should be at least 0.100" thick on small parts (less than 4 - 6" in size). Larger parts should have increasingly thicker walls - from 0.125"up to at least 0.250" for parts 18 - 20" and larger. There is no maximum thickness requirement. Wall sections that are several inches thick can be easily replicated without shrinkage or surface dimpling. Variations in wall thickness are also easily accommodated - is not difficult, for example, to cast 0.125" sections next to 4.00" walls. Parts can also be cast solid, if this is a design requirement.

BOSSES, THREAD MOUNTS, COUNTER SINKS AND COUNTER BORES

Bosses and thread mounts should have a diameter at least two times larger than the major diameter of the threaded insert. If possible, bosses should be attached to side walls or other cast features for additional strength. The following dimensions are a guide to designing boss features for threaded inserts. Self tapping threaded inserts contain a threaded base that is pressed into a pre-machined access hole. For example, a #10-24 threaded stud insert has a major diameter of 0.296". It would require a major boss diameter of 0.592", and a depth of 0.450" for anchoring. Threaded inserts can also be cast in place, providing greater anchoring strength.

Counter Sinks and Counter Bores, which are typically used in aligning mating parts, are best created by machining after casting is complete. Casting these features into the finished part may create misalignment and attachment problems with multi-component assemblies.

IMI/RFI SHIELDING

IMI/RFI Shielding requirements are best met by coating the interior surfaces of LRC parts with a Copper or Nickel finish. This can be accomplished through electroplating or paint spray, and will require parts to be masked to protect finished surfaces. In certain applications, an electronic vacuum deposition process can also be used to copper plate LRC parts. Low levels of conductivity sufficient to provide electrostatic discharge can be achieved by adding special compounds to the resin formulations. Check with your LRC vendor for specifics.

We hope this general guide to Liquid Resin Casting (LRC) technology has been helpful in designing the next generation of your prototype plastic parts.