Gainsborough Hardware Industries Ltd was founded in 1967, and is a world leader in the manufacture of door furniture such as door handles, key locks and dead bolts. The company distributes door furniture to more than 26 countries, with principal markets being in the US, UK and Canada.
Gainsborough manufacture metal door handles from zinc die cast components. In order to provide an acceptable finish and corrosion resistance, these components are given a range of surface finishing treatments. A number of electroplating processes were used, culminating with a cyanide gold plating operation for some product ranges. Pre-plating with copper and nickel was required to give a suitable substrate for the gold finish.
All the electroplating operations required continuous rinse stages, which generated waste water. Waste water from the gold cyanide plating process contained cyanide, which required specialised treatment with sodium hypochlorite. Metals in the effluent were precipitated as a hydroxide sludge, which was sent off-site for treatment and disposal.
The electroplating operation was generating approximately 1.5 million litres of wastewater and 60,000 litres of sludge per annum.
Cleaner Production Initiative
The gold cyanide plating process was replaced by an electrophoretic deposition process using a 'Clearclad' system. The electrophoretic process still requires copper and nickel pre-plating, but has completely eliminated the use of gold cyanide plating.
The Clearclad system is also available in brass and copper colours. These colours are coated directly onto polished die cast components, thereby eliminating the need for electroplating on these product lines.
The electrophoretic system involves the electrodeposition of a water-based acrylic lacquer onto the work piece. The lacquer solution is slightly acidic (pH 5). When the workpieces are placed in the solution and a current is applied, electrolysis at the cathode (workpiece) creates a layer of alkalinity. This alkalinity causes coagulation of the resinous solids in the solution, followed by deposition onto the surface of the workpiece. After coating, the workpieces are rinsed in dip tanks and then cured in an oven.
The oven curing is required to cross-link the resin to form a durable, high gloss finish.
The rinse tanks flow back into the resin tank, with fresh rinse water being provided by the filtrate from ultrafiltration of the resin tank. This has resulted in a 60 per cent reduction in effluent generation, compared with the previous electroplating system. The filtrate is also passed through a resin ion exchange cartridge to remove any metal ions, as a buildup of metal contaminants is detrimental to the electrodeposition process. The system currently operates with an efficiency of 98 per cent.
Advantages of the Process
The electrophoretic process has achieved cost savings from the elimination of gold cyanide and reduced effluent treatment and disposal costs. The quality of the finish has also been substantially improved, with 100 hour salt spray testing causing no noticeable deterioration of the surface coating. This has allowed Gainsborough to increase the guarantee on its internal door furniture product range, from 1 year to 10 years.The adoption of an electrophoretic process has also allowed Gainsborough to cease using cyanide based decorative plating for its coloured finishes.