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An Overview
Overview:
Plastic Castings are ideal for low-volume production needs when typical annual volumes are 50 to over 1,000. In design terms, anything goes. Complex shapes, bosses, threads, undercuts, varying wall thicknesses, minimal to no draft, delicate component encapsulations, multiple parts combined into one, overmolding, cast-in color and finish, tight tolerances, tiny parts and very large parts. Tooling costs are a fraction of those for injection molding.
The process is accomplished by mixing and pouring liquid plastic resins into molds. Through a chemical reaction the materials undergo an irreversible reaction to form a part. The molds are of various styles and materials depending on the application. Once poured, heat is added to the mix to speed the process of curing the resin to form a solid.
While the process can be automated to some degree, it has its greatest commercial value in prototyping and low volume production. The trade-offs, compared to high volume methods such as injection molding, are lower tooling costs, high unit costs, and a generally shorter tooling cycle.
The greatest economic benefits occur with unusual geometries that are too costly to machine and annual volumes that are too low to justify the expense related to injection mold tooling. Most material formulations of this type need 10 – 12 hours to complete their initial cure therefore most molds are limited to one cycle per day. Typical production runs require the use of multiple copies of molds to support the anticipated production rate.
Molds:
Molds are generally either cast or machined, or a combination of the two. Tolerance requirements, production quantities, and other factors will determine which configuration is best suited for the job. The size of any tooling element may need to be adjusted by a shrink factor.
Patterns:
Cast molds are made from a pattern (also referred to as a master) that represents the actual part being produced. A pattern may be either a metal pattern or rapid prototype such as a stereolithography model (SLA).
As-cast Tolerances:
Features made with silicone mold elements and those dependent on relationships between metal and silicone elements can typically be held within +.004 units per unit (inches per inch or millimeters per millimeter). Features made entirely with metal tooling elements can usually be held within +.002 units per unit. For silicone molds, tolerances across parting lines are typically within +.01 units per unit. These tolerances are general and vary with material selection. Tighter tolerances can be achieved by secondary operations.
Machined Secondary Operation Tolerances:
Hole diameters will normally be held to +.002 inch. Other machined dimensions can normally be held to a standard +.005 inch tolerance. Non-critical overall dimensions created by removing the chimney (or gate) where the mold is filled, are often created by grinding or sanding to blend with cast radii and are thus subject to as-cast tolerances. For tight tolerances on large dimensions, the temperature at which the parts are machined and measured must be the same. The coefficient of thermal expansion must be considered when mating plastic parts with metal parts.
As-Cast Material Properties
Cast materials are in Hardness Ranges of Shore A-10 through A-90 (Elastomers) and Shore D-50 through D-90 (Plastics)
Other Properties.
Typical Heat Distortion Temperatures: 130 to 225 degrees F
Epoxies are available with an HDT of 300 to 450 degrees F, allowing autoclaving. A number of our customers have successfully qualified some of these materials for medical applications.
Colors:
Black, white, and several shades of gray are cast in large quantities daily, providing economy of scale. Most colors can be matched in most resins using a customer supplied color identification number or color chip sample. A number of resins, particularly elastomers and high temperature epoxies, have various levels of amber tint, making it difficult or impossible to match light colors. Please note that pigment is color matched for a particular resin and will rarely provide the same color when used in another resin.
Transparent water-clear materials are available in a wide range of flexible and rigid materials. These materials can be tinted to any desired color, subject to a color-match charge. Color stability due to ultraviolet exposure and other factors varies for different resin systems. UV stabilizers can be added to some resins.
Encapsulations:
Castings can be used to encapsulate PC boards, wiring, connectors, and/or other components as required for your application. Threaded inserts can be molded in place or self-tapping inserts installed afterwards.
Disclaimer
DPT may not provide some of the services and/or techniques presented in the Knowledge Center. For a full list of services we do offer, please visit our rapid prototyping page.
