Reason for use:
To offer a fast and low cost alternative, reducing the potential in some cases of tool distortion due to high residual plating stresses.
The most commonly used backing material is copper, sprayed by either the Flamespray or the Arcspray process. Copper provides an ideal blend of thermal conductivity, strength and low residual stress. Successful tools will be produced if the correct processing techniques are used.
Bond Coat Arcspray
Metallisation 75E Nickel Aluminium
This Arcspray Bonding Wire exotherms during spraying, producing very high bond strength coatings.
Bond Coat Flamespray
The Flamespray Bonding Material will exotherm during spraying to produce very high bond strength coatings.
Generally used to back electroforms because of its high thermal conductivity
The back of the electroform must be free of any grease, oil, water or dirt. It must be roughened by grit blasting, preferably using aluminium oxide grit. The front face of the electroform must be adequately protected during preparation and subsequent processing.
Good bonding of the backing to the electroform is essential. Metallisation 75E is recommended when using the Arcspray Process. Metallisation T405/1 Bonding Material is recommended when Flamespraying. The bonding material should be sprayed with acetylene fuel gas. Bond coats are intended to provide a key between the electroform and the backing coat. They should be sprayed to a thickness of 100 – 150μm (0.004-0.006 in). Excessive bond coat thicknesses offer no technical advantage and may impair the tool performance.
Metallisation 05E is applied with Metallisation Arcspray or Flamespray Equipment, immediately after bonding. It is important that heat input is minimised and rapid movement of the pistol relative to the electroform is recommended. Additional cooling air jets should also be used to maintain the surface temperature below 50°C.
Pistol operating parameters should be set according to the equipment manual. When Arcspraying, the current should not exceed 150A, for Flamespraying 2mm diameter wires should be used. The backing should be built up gradually, until the required thickness has been achieved.
Temperature Control Tubes
If required, temperature control tubes may be built into the mould at any time after the initial deposit has been applied. The tubes should be prepared by degreasing and gritblasting and sprayed with a thin layer of the backing material before placing them in position.
Finishing The Electroforms
When sufficient material has been applied, the reinforced electroform should be machined to fit the moulding machine tool set. Turning, milling or shaping may be employed, but care must be taken to avoid excessive cuts. The use of cutting fluids is not recommended.
Alternative Backing Materials
Where the strength and thermal conductivity of the tool is less important than low residual stress, the copper backing may be replaced by a special brass (Metallisation 13E). This backing is mainly used for larger – less rigid electroforms.
Where strength is of primary importance and thermal conductivity is relatively unimportant, electroforms may be backed with steels. Only low stress steels such as Metallisation 60E or Metallisation 65E should be used. Because of the higher melting points of these materials, extra precautions must be taken to avoid overheating the tool during backing. Steel backings must be carefully ground to size (preferably without coolant) after spraying.
REFERENCE TECHNICAL BULLETIN N°S :-
2.2.3 Metallisation 05E Copper
2.3.2 Metallisation 13E Brass