Backing Nickel Electroforms
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.
Nickel electroforms are extensively used as tools for injection moulded plastics. Whilst they give excellent service, their production is slow and costly. In some cases, tool distortion may be caused by the high residual plating stresses. For these reasons, the Metallisation spraying processes have been adopted to reinforce thin electroformed shells to produce tools which are strong enough and thick enough to incorporate in injection moulding machine tool sets.
The most commonly used backing material is copper wire, sprayed by either the Flame spray or the Arc spray 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.
Equipment: Metallisation Arc Spray or Flame Spray System (Contact us for further information)
Materials
Bond Coat Arc Spray with MSSA 75 Arc Bond - This Arc Spray Bonding Wire exotherms during spraying, producing very high bond strength coatings.
OR
Bond Coat Flame Spray with Metallisation T 405/1 - The Flame Spray Bonding Material will exotherm during spraying to produce very high bond strength coatings.
Main Deposit
Preparation
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.
Bonding
Good bonding of the backing to the electroform is essential. MSSA 75 Arc Bond is recommended when using the Arc Spray process. Metallisation T405/1 Bonding Material is recommended when Flame Spraying. 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.
Backing
Copper Wire is applied with Arc Spray or Flame Spray 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 Arc Spraying, the current should not exceed 150A, for Flame Spraying 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 grit blasting 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 (MSSA 13). 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 MSSA S2 or MSSA 65 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.
