(57) The baskets for dishwashing machines particularly of the domestic type are made of
metal wires which are cut, bent and welded to provide a basket of suitable shape.
The thus formed baskets are subsequently coated with a plastic resin, usually a polyamide-based
resin, in a fluid bed process.
According to the invention, the coating is made of a modified polypropylene, with
the baskets being pre-heated to a reduced temperature, and without the need of an
adhesive base coating.
[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.
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.