< CONTINUOUS, CENTFRIGUAL & CUTTLEFISH CASTING
ELECTROFORMING PROCESS
Electroforming or ‘EMF’ process is a method of creating objects in metal using a technology broadly similar in character to electroplating. Creating decorative works by this method was popular throughout the 19th century, during which time design metalworks were mass produced by electroforming processes known variously as GALVANO-PLASTY, ELECTROTYPING and so on.
To create an electroformed design, a rubber mould of the master pattern is first made from, which a wax pattern is produced by injection. This is a procedure very similar to that used for creating jewellery investments. In this case the wax pattern, usually termed a MANDREL, is metallised – a procedure that enables a gold/silver alloy or other appropriate metal to be deposited over the wax image (see also ELECTROPLATING).
The electroform is carried in an electrolytic bath which is composed of gold and silver potassium cyanides. The alloy is deposited on the wax mandrel, settling to a thickness of about 700µm (0.70mm). After the depositing process is completed, the underlying wax can be removed by chemical or thermal methods leaving behind a fine, hollow reproduction with adequate tensile strength to be self supporting. Quite intricate forms can be created using this technique, and the thin deposit (1/32 inch or >1mm) makes the electroforming process a cost effective means of producing designs in precious metals (as well as copper, nickel and other metals) [ref 1].
RESIN AND SINTERED CASTS
The process of cold resin casting is outlined in the PATTERNS section. To briefly recap here, resin ‘bronze’ casts have a fine free-running bronze powder added into the resin’s body, or GEL COAT layer, prior to backing up with impregnated fibre matting. The added powder content imparts a finish to the resin that mimics a hot cast bronze finish. The resulting reproduction can in some cases be patinated after release from the mould. Stone and other fillers can also be applied to emulate the finish of marble, sandstones, concrete and other materials.
The effectiveness of a resin reproduction depends greatly upon the skill of the maker and results can vary from a very poor and obviously cheap substitute for a full metal cast, through to a very fine standard of reproduction, easily capable of passing casual scrutiny. New moulding products are emerging as potential replacements for resin body, some of which are favoured for art and design applications due to their acceptance of a wide range of fillers, ease of use and low toxicity (see also RESIN & CEMENT GYPSUMS).
An alternative metal forming technique which makes use of a process similar to metal sintering can also be used to create 3D designs. Sintering is a method of impregnating a free running metal compound with a hydrocarbon oil, which is then formed into design, before kiln baking in a reducing (oxygen deficient) atmosphere. Patented processes broadly combine a copper alloy powder with a blended wax body. This creates a plastic medium that can either be modelled directly by the artist, or else cast into moulds. Once the model is finished, the medium is fired to 800°C in a kiln to consolidate the compound [ref 2].
CHEMICAL ETCHING
Etching copper or copper alloy materials can be a useful method of producing textured relief surfaces, two dimensional graphic images and text on sheet metal. The most common use of copper alloy etching today is in the production of printed circuit boards for the electronics industry, however founders and fabricators will sometimes use chemical etching as a means of producing signage and other architectural or design detail.
An image can be etched onto metal plate by one of two basic methods. The most straightforward etching process involves directly applying a liquid resist to the plate. The resist is usually a lacquer like substance, SHELLAC and ASPHALT are both suitable. The fluid is applied freehand to the plate’s surfaces (including the back and edges, to prevent these surfaces from being attacked).
The applied resist prevents the etching solution from attacking the plate in the protected areas, leaving a raised surface after the etching process has been completed. Obviously the etching process works in reverse to the resist, so if a letter for example is to be cut into the plate, then the resist must be applied in such a way as to block out the surrounding plate to leave the letter as exposed copper. If the letter is to be raised in relation to the plate’s surface, the resist is applied in ‘the positive’.
The alternative method of producing an etched image is by photo-transfer. The image can be scanned into a computer graphics programme and manipulated as desired. Once the image is processed it can be out-put to a laser printer or photocopier, after which the image is developed onto an adhesive photo-sensitive resist (dry film). The dry film is then heat transferred onto the plate to be etched. Obviously this method of image transfer allows quite complex (photographic) images to be etched into the plate. Whilst the etching solution does not recognise tonal values, various shading effects can be achieved by manipulating the screen frequency of the image’s halftone.
For the etching process to work properly, the exposed plate must be free of grease and dirt, this includes careful handling of the degreased plate to avoid the transfer of sebaceous oils. The etching solution for copper plate is composed of FERRIC CHLORIDE or dissolved SODIUM PERSULPHATE crystals (note that both chemicals are SALTS), heated to no more than 122°F (50°C). The plate is carefully placed in the etching bath and left for an appropriate period for the etch to bite. Once etched, the plate is removed and cleaned to stop the etching process (some methods require the plate to be stop washed in an AMMONIA SOLUTION). The resist is removed using a solvent or proprietory chemical stripper, after which the finished plate can be patinated or finished as required.
SAND CASTING SUMMARY >
|
