Method for manufacturing baskets for dishwashing machines and baskets made by this method

Description

[0001] The present invention relates to baskets to be mounted in the washing chamber of a dischwashing machine, particularly of the domestic type, which are suitably configured (upper baskets for tableware and lower baskets for pots and the like) to act as supports for the different types of kitchen- and tableware to be washed.

[0002] Known baskets for this purpose are made of wires which are cut, bent and welded to the shape of the basket, which is subsequently coated with a layer of a plastic material for protection under the operating conditions within the machine.

[0003] The coating is normally accomplished by the use of poly­amide powders in a fluid bed after a pre-treatment of the metal wire involving sand-blasting and pre-heating.

[0004] The coating with polyamide resins requires the wire to be pre-heated to a high temperature (340 - 370°C) for ensuring adequate fluidization of the powder, as a result of which the process involves a considerable energy consumption. In addition, for ensuring sufficient adhesion of the coating to the metal wire, it is convenient to use an adhesive base (primer), whereby the manufacturing process is rendered more complicated and costly.

[0005] The polyamide resin coating finally presents two rather unsatisfactory characteristics, namely, the variability of its rigidity in response to temperature variations, and a noticeable steam-permeability. By way of example, the elasticity module varies from about 17,000 kg/cm² at room temperature to about 5,000 kg(cm² at the washing temper­ature (70°C), while the steam permeability amounts to about 25g/24h/m² at the washing temperature for a coating having a thickness of 0.5 mm.

[0006] The multiple shortcomings cited above were conducive to the search for and experimentation with other materials for the plastic-coating of dishwasher baskets, keeping in mind the requirements of hygienic and non-toxic character­istics in view of the intended use of the baskets, during which they come into contact with food-handling utensils.

[0007] This search and experimentation did not in the end lead to any positive results; polypropylene has been found to be rather unsuitable for this purpose due to inadequate adhesion to the metal wire (thus still requiring an adhesive base coating), brittleness (due to its crystalline struct­ure), and a tendency to get permanently discoloured by the action of certain alimentary substances (e.g. tomator juice).

[0008] Recently, however, there hyve been developed novel types of modified polypropylenes which, while presenting favour­able mechanical characteristics and sufficient resistance against organic solvents, have a lower melting temperature than stereo-regular polypropylene and a lower degree of crystallinity.

[0009] Such a novel modified polypropylene is marketed for instance by the firm of HIMONT under the commercial name POLISIVE, for use mainly as an outer coating for pipe conduits in view of its outstanding characteristics of stability and resistance against atmospheric influences under different ambient conditions.

[0010] In addition, experiments carried out at the present applicant's laboratories have shown that these modified polypropylenes are suitable for use also under the oper­ating conditions prevailing in a dishwater, being able to withstand relatively high temperatures and the aggressive action of vapours and detergent solutions.

[0011] It is therefor an object of the present invention to sub­stitute the polyamide resins in the coating of diswasher baskets to thereby obtain an improved and more reliable performance, and to provide a simplified manufacturing method involving a reduced energy consumption.

[0012] These objects are attained according to the invention in a method for manufacturing baskets of sand-blasted metal wire, comprising the steps of cutting, bending and welding the wires, pre-heating the thus formed baskets, and coating them with a plastic resin in a fluid bed, characterized in that the pre-heating step comprises heating the baskets to a temperature of no more than 320°C, and the coating step in the fluid bed involves the use of a modified polypropylene which id directly deposited on the sand-blasted wire.

[0013] This process is more simple than the one employing poly­amide resins, because it does not necessitate the inter­mediate step of applying the adhesive coating to the wire prior to its immersion in the fluid bed. This process also involves a reduced energy consumotion, because this type of a modified polypropylene has a considerably lower melting temperature (300 - 320°C) the the polyamide resin (340 - 370°C).

[0014] The present invention is also directed to dishwasher baskets manufactured by the novel method.

[0015] Also of interest in view of this particular employ, that is, as a coating for dishwasher baskets, is the mechanical characteristic represented by the elasticity module, which in the case of propylene modified with ethylene and maleic anhydride varies rather insignificantly in response to temperature variations (from about 6,000 - 7,000 kg/cm² at room temperature to about 4,000 to 5,000 kg/cm² at 80°C), in contrast to what happens in the case of poly­amide resins, the latter having in fact an elasticity module which decreases rapidly as the temperature rises from 20 to 80 °C.

[0016] As the temperature within the washing chamber of a dish­washer rises to a maximum of about 70°C, the polypropylene modified with ethylene and maleic anhydride proves to be better adapted to withstand the temperature variations of repetitive washing cycles.

[0017] The most important advantage resulting from the employ of the modified polypropylene is due, however, to the low steam-permeability of this substance, which amounts to about 15g/24h/m² at 80°C and is thus, all other conditions being equal, much lower than the permeability of polyamide amounting to 25g/24h/m².

[0018] This results in an improved durability under operating conditions, in view of the steam or vapours generated in a dishwasher, particularly during the final rinsing and sterilization stages.

[0019] As already stated, the determination of polypropylene modified with ethylene and maleic anhydride as a substance which, in contrast to polyamide resins, presents advant­ageous characteristics for employ as a coating for dish­washer baskets, has been arrived at by suitable experiment­ation.

[0020] In this context it is to be noted that there is a wide range of modified polypropylenes differring from one another by their molecular structure and idividual performance.

[0021] The extensive experimentation carried out has thus permitted to precisely determine the ranges within which the principal physical parameters of the modified poly­propylene for use in attaining the stated object of the invention are preferably contained.

[0022] These parameters, as determined according to the approp­riate ASTM methods, are the following:
melt flow index      20-50 g/10 min
density      0.945 - 1.000 g/cm³
elastic module      700 - 800 M Pa
tensile strength      17 - 22 M Pa
Izod resiliency (at 23°C)      50 - 100 J/m
Izod resiliency (at 0°C)      30 - 60 J/m
Izod resiliency (at -40°C)      10 - 30 J/m.

[0023] Merely by way of reference it is pointed out that the already named polypropylene POLISIVE of the HIMONT company satisfies the above enumerated requriements.

[0024] The dishwasher baskets made of sand-blasted metal wire and coated with modified polypropylene without the inter­position of an adhesive have been subjected to the teste prescribed by national and international standards, and have been found to be fully satisfactory under every aspect.

Claims

1. A method for manufacturing sand-blasted metal wire baskets for dishwashing machines, particularly of the domestic type, comprising the steps of cutting, bending and welding the metal wire, of pre-heating the thus formed baskets, and of coating them with a plastic resin in a fluid bed process, characterized in that in the pre-heating step the baskets are heated to a temperature of no more then 320°C, and the fluid bed coating step involves the employ of a modified polypropylene which is directly deposited on the sand-blasted wire.

2. A method according to claim 1, characterized in that the temperature attained in the step of pre-heating the baskets is preferably 300 °C.

3. A method according to claim 1, characterized in that the modified polypropylene employed in the fluid bed coating step has the following physical character­istics:
melt flow index      20 - 50 g/10 min
density      0.945 - 1.000 g/cm³
elastic module      700 - 800 M Pa
tensile strength      17 - 22 M Pa
Izod resiliency (at 23°C)      50 - 100 J/m
Izod resiliency (at 0°C)      30 - 60 J/m
Izod resiliency (at -40°C)      10 - 30 J/m.

4. A basket for a dishwashing machine, characterized by being manufactured by the method according to any of the preceding claims.