Nickel electroformed tools spread products to market.
Supporting the manufacture of high quality nickel electro-form molds is a new CNC-machinable polyurethane board stock for cutting mandrels called Ren Shape 540 board, supplied by Ciba-Geigy Corp. The material, which produces mandrels with outstanding dimensional accuracy and an excellent, nonporous surface finish, can help cut tooling costs by as much as 60% and production leadtime by 50% compared with conventional materials such as aluminum.
Background -- A time-consuming and often problematic aspect of making nickel electroform tooling is building the mandrels on which the nickel is deposited. Mandrels are typically constructed from a fiberglass/ epoxy or polyester laminate or from machined foam, all of which have shortcomings. In the worst scenario, incompletely cured resin in laminated mandrels can contaminate expensive electroforming solutions and produce unusable tools. In less dramatic failures, the mandrels contain blemishes that produce nickel molds with pitted surfaces requiring extensive secondary finishing.
A further drawback of conventional laminated mandrels is the number of tooling sequences involved. These mandrels are constructed from masters and splashes to generate the reverse shape on which the nickel shell will be formed. The process is time consuming and expensive; it and accounts for some loss of tolerance at each tooling step. In addition, the multiple intermediate tools produced must then be stored.
Although machined foams are cut directly from CAD data to generate the needed reverse shape, many of the materials are too light to remain submerged in the electroforming solution. To counteract the buoyancy of the foam, heavy frames made from steel are used. However, the steel can dissolve in the low pH nickel bath and contaminate the solution. In addition, the difference in the coefficients of thermal expansion (CTE) between the metal frame and the foam frequently results in distortion, cracking, or both in the mandrel surface when it is immersed in the electroforming solution. The CTE difference between the foam and the adhesive used to bond foam blocks can also produce undesirable gaps that may result in raised areas in the finished mandrel. And, conventional foams frequently create porosity in the electroform that reduces the surface quality of the nickel tool.
The Ren Shape 540 board features a combination of physical properties that meet the special requirements of nickel electroformers including: low porosity, high density (1.65 g/cc), submersibility without a back support structure, and resistance to impact and abrasion.
Nickel electroforming -- Once the mandrel is built and sent to the electroformer, it is inspected to detect any problems that might arise during the moldmaking process. The mandrel is then made conductive and immersed in a bath where nickel is electrode-posited on the surface to form a metal shell. The physical characteristics of the nickel shell vary according to the chemical composition and properties of the electroforming solution such as its pH and temperature. Shell thickness is controlled by several factors: the electrical current applied to the mandrel; the length of time the mandrel remains in the electroforming tank and the contours of the mold being formed. (See chart above for examples of contours that are easy and difficult to electro-form.) Depending on the application, shell thickness ranges from a few thousandths of an inch to more than an inch, with a typical thickness of 1/4" for most molds. After deposition is complete and the tool has been removed from the bath, the nickel shell is mounted on a support structure, removed from the mandrel cleaned, and polished.
Here's how electroformed nickel shells perform in various molding applications. Electroformed nickel shells provide good performance in the manufacture of unsupported molds for rotational molding, slush molding, blow molding, and vacuum forming. The nickel molds are also well suited for constructing supported molds used to auto-clave-cure prepreg parts and for RIM and RTM processes.
Rotational molding -- Shells are thin, usually 0.100" to 0.150" thick. Although nickel exhibits slightly lower thermal conductivity than copper, it offers higher strength and hardness for increased mold durability.
Slush molding -- Electro-formed nickel shell molds are widely used as slush molds for fast, easy manufacturing of parts such as gearshift boots.
Blow molding -- Strong heatable nickel blow molds for forming parts with a Class A surface can be produced by attaching copper heating/cooling lines on the back of the nickel shell. Particularly when a multicontoured tool is required, electroformed shells are less expensive to build than machined steel or aluminum. And, because heating and cooling lines are mounted close to the back of the shell, they alter the nickel tool temperature significantly faster than possible with thicker steel molds to reduce cycle time and cost.
Vacuum form molds -- Laser-drilled electroformed nickel has been successfully used for this type of mold on several automotive interior door panel programs. The process produces a pressure difference of about 14 psi across the thickness of the 0.125" thick shell. Because nickel has a tensile modulus of approximately 25 Mpsi, it can easily handle this pressure.
Autoclave molds -- Nickel electroform molds are well suited for producing composite parts fabricated from thermoset epoxy and a cloth reinforcement such as fiberglass, aramid fiber, or graphite. The shells are typically 0.190" thick and supported by an open, welded steel structure to permit contour adjustments. Autoclave curing temperatures greater than 800F (412C) are easily handled by the nickel tooling.
Reaction injection molding - RIM molding involves some pressure that can place on the molds during part injection and press opening. A properly backed and supported nickel shell, 0.200" to 0.300" thick, will produce the same number of parts per year as a steel mold at a price about 40% less.
To support a nickel shell for the RIM process, resin or modified concrete can be poured behind the face. If this backing contains neither aluminum nor another metal, the backing material will act as a thermal insulator behind the shell. Alternatively, heating/cooling lines can be installed directly against the back of the nickel or mounted against a copper mesh to help diffuse heat and produce a more uniform mold temperature.
Resin transfer molding -- Nickel electroform molds are being used successfully in this process because they provide a hard, high gloss surface, can be heated easily, and offer a long life at a price significantly less than steel tooling. One EMF customer has used a nickel shell for RTM, producing approximately 20,000 parts without significant wear on the nickel surface from the glass preform.
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|Publication:||Tooling & Production|
|Date:||Oct 1, 1996|
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