The present invention relates to a metal mould for use in the production of a frozen confectionary product, preferably ice lollies.
It is generally known that frozen confectionary products are produced in metal moulds, so-called freezing pockets. Such metal moulds were originally made from copper alloys, but for very many years they have been made from stainless steel.
When making the frozen confectionary product, a large number of moulds are led through a freezing bath for freezing the confectionary product and afterwards through a defrosting bath for freeing the frozen product from the mould walls. A large number of moulds are firmly mounted in plates, which form sections in the installation. An installation in which the moulds are placed may be constructed as a rotating installation with the freezing bath and the defrosting bath arranged in circular segments. The installation may alternatively be constructed as an installation in which the moulds are led through freezing and defrosting baths which are arranged on a line. When passing through the baths, the moulds are exposed to a temperature difference from down to approx. -55.degree. C. to a temperature of up to about 30.degree. C.
The freezing bath normally comprises a calcium chloride solution (CaCl.sub.2 saline solution). In these saline solutions there will be a considerable risk of corrosion on the moulds. If just one mould in a section has a corrosion hole, it will be necessary to replace the whole section. This involves substantial expenses.
Therefore, in order to reduce the corrosion, inhibitors have been used in the freezing bath. A generally used inhibitor has been sodium bichromate (Na.sub.2 Cr). This inhibitor will form a protection layer on all surfaces which contact the saline solution. The inhibitors are difficult to work with as they irritate the skin. Besides there is a health risk if the inhibitor or the saline solution is allowed to penetrate into the confectionary product through corrosion holes.
In certain cases, owing to requirements from the authorities or internal requirements, it is impossible to use inhibitors because of the health risk involved in using these poisonous substances. In such cases premixed saline solutions are used in the freezing bath instead. Due to the absence of the inhibitor, however, it will be necessary to maintain pH within very narrow limits of between approx. 8.5 and 9.0 in order to avoid serious corrosion on the moulds and the other parts of the installation with which the saline solution comes in contact. Even very short periods with pH beyond the above-mentioned limits may give rise to serious corrosion which necessitate replacement of one or several sections. Thus it is necessary to perform a constant supervision and add pH-adjusting agents if no inhibitors are used.
It has been commonly known in the defrosting bath to protect the moulds against corrosion by using a corrosion plug, e.g., of anodized zinc. It has been necessary to inspect this corrosion plug frequently and make replacements in order to avoid corrosion of the moulds.
A further risk of mould corrosion may be caused by current failures in a plant. Thus, it has been essential that the moulds and other parts of an installation were grounded correctly. Such correct grounding may in certain cases be difficult.
Thus, the attempts that have been made until now to avoid corrosion of metal moulds have only been directed towards adaptation/modification of the involved fluids, inhibitors, etc., in the freezing bath and the defrosting bath.
Although metals which are very resistant to corrosion have been known for years, these have not been used so far because of difficulties related to manufacturing, which involves difficult shaping and welding. Thus, until now it has not been considered a realistic and practical option to use other metals which will give the metal moulds a longer life under subjection to the above-mentioned fluids.
It is the object of the present invention to remedy the drawbacks of known metal moulds by disclosing a new metal mould and a method for making such mould.