What are sprue cutters coated with?
Sprue cutters are often advertised as having a black, non-glare surface. This black finish is achieved by a layer of black oxide over the surface of the steel.
What is black oxide?
Black oxide has many trade names. Unlike chrome plating, which only bonds to the surface and does not chemically react, black oxide is a coating formed due to a chemical reaction with the iron in the metal to produce a protective surface of magnetite (Fe3O4).
Black oxide can be applied in either a "hot" or "cold" oxidising process. However, the cold process does not provide a true oxide coating and so offers less resistance to corrosion and abrasion.
Why is black oxide used?
Sprue cutters are coated with a layer of black oxide for two reasons. Firstly black oxide is used as a form of corrosion protection for the steel. The second reason is the black colour helps to eliminate glare and reduce eye strain.
How is black oxide applied?
Black oxide is applied by dipping the parts to be coated in a series of bath solutions. This starts with an alkaline cleaner which cleans the surface of any impurities or dirt. The parts are then placed in a water bath to wash off any remaining alkaline solution.
The next bath is in caustic soda (blackening compound) heated to 140.5°C. This bonds chemically to the surface of the part and forms a porous base layer. A second water bath is then used to rinse off the caustic soda before the final sealing bath.
The sealant is usually an oil or wax. It fills pores formed during the caustic soda bath and it is this that provides the corrosion resistance to the black oxide surface finish.
Other sprue cutter surface finishes and coatings
Some sprue cutters have a brushed steel finish. Unlike black oxide this leaves the metal a silvery grey colour. Brushed steel has a semi matt appearance with the surface covered in a series of small parallel lines.
A brushed steel finish is achieved by polishing the surface with a polishing belt or wheel. Unlike black oxide, which improves corrosion resistance, a brushed steel finish offers no improvement in the corrosion resistance of the product.
The handles of sprue cutters that do not have a plastic handle sleeve will often be painted. The most common types of paints used for this are enamel and acrylic.
Enamel paints dry to form a hard shiny finish. They were traditionally oil based paints but there are now water based paints that also describe themselves as enamels.
Acrylic paint is water based but becomes water resistant once dry. Acrylic paint generally dries faster than enamel paint as the water used in it evaporates faster.
Acrylic paint is not as hard as enamel but is more elastic and flexible, which means it is less likely to crack.
What type of surface finish or coating should you choose?
As brushed steel offers no corrosion protection and does not reduce glare like black oxide, brushed steel sprue cutters should be avoided in favour of those with an anti glare black oxide finish.
Painted handles are less likely to be ergonomically shaped and will not provide the same level of grip as ones with plastic sleeve handles. Painted handles may also become slippery if sweat or moisture gets on their surface, so for these reasons you should buy a sprue cutter with some form of plastic handle covering.