Refrigerator appliance having at least one inner plastic liner and method for manufacturing the liner

Abstract

It is provided a refrigerator appliance (1) having at least one internal liner (2), in particular a cabinet liner or a door liner, defining an inner compartment (3) and made of a polypropylene-based material comprising a propylene-ethylene copolymer having a main polypropylene chain with ethylene units arranged along the polypropylene chain; and at least one lamellar or fibrolamellar filler. The material makes it possible to effectively manufacture the liner (2) by thermoforming.

Claims

1. A method for manufacturing an internal liner comprising a cabinet liner or a door liner of a refrigerator appliance, the method comprising the steps of: preparing a polypropylene-based material comprising a propylene-ethylene random copolymer in an amount ranging between 60 and 90% w/w having a main polypropylene chain with ethylene units arranged along the polypropylene chain in an amount ranging between about 1% to about 8% w/w of the propylene-ethylene random copolymer, and at least one lamellar or fibrolamellar filler, and one or more alpha or beta nucleating agents; extruding the polypropylene-based material prepared by compounding the propylene-ethylene copolymer and said at least one lamellar or fibrolamellar filler directly in an extruder into sheets; and thermoforming a sheet of said polypropylene-based material onto a male mold to shape the liner, wherein the thermoforming step comprises the steps of: heating the sheet to reach an operating temperature between 125 C. and 155 C., at which the sheet is thermoformable; and shaping the sheet onto an outer shaping surface of the male mold.

2. The method according to claim 1, wherein the propylene-ethylene copolymer has a content of ethylene units ranging between about 2% and about 5% w/w.

3. The method according to claim 1, wherein the at least one lamellar or fibrolamellar filler is selected from the group consisting of: talc, kaolin, mica, glass flakes, nanoclays, montmorillonite and bentonite, graphite, aluminum nitride, and boron nitride.

4. The method according to claim 3, wherein said at least one lamellar or fibrolamellar filler is talc.

5. The method according to claim 1, wherein the at least one lamellar or fibrolamellar filler comprises particles having a substantially platelet shape.

6. The method according to claim 1, wherein the polypropylene-based material contains the at least one lamellar or fibrolamellar filler in an amount ranging between 10% and 40% w/w.

7. The method according to claim 1, wherein the polypropylene-based material contains the at least one lamellar or fibrolamellar filler in an amount ranging between 20% and 30% w/w.

8. The method according to claim 1, wherein the polypropylene-based material further comprises polypropylene homo-polymer.

9. The method according to claim 1, wherein the polypropylene-based material further comprises polypropylene homo-polymer in an amount ranging between 0% and 25% w/w.

10. The method according to claim 1, wherein each sheet consists of a single, substantially uniform layer of the polypropylene-based material; or consists of a main layer, made of the polypropylene-based material, and of a glossy or semiglossy covering layer, covering a face of the main layer, made of neat polypropylene or a polypropylene compound with glossy appearance, said covering layer being co-extruded or laminated with the main layer.

11. The method according to claim 1, wherein the thermoforming step comprises, after the sheet has been heated, a pre-stretching step, in which the heated sheet is pre-stretched before being shaped onto the male mold.

12. The method according to claim 1, wherein the sheet is vacuum formed onto the male mold.

13. The method according to claim 12, wherein in the thermoforming step, vacuum is applied on the side of a first face of the sheet, facing the shaping surface of the male mold; and a compressed gas stream is injected on the side of a second face of the sheet, opposite to the first face.

14. The method according to claim 1, wherein in the thermoforming step the shaping surface of the male mold is at a temperature lower than a recrystallization temperature of the polypropylene-ethylene copolymer.

15. The method according to claim 14, wherein the temperature of the shaping surface is about 90-110 C.

16. The method according to claim 1, comprising a first cooling step effected on the sheet shaped to define the liner and still contacting the shaping surface of the male mold.

17. The method according to claim 16, comprising a second cooling step effected after the liner has been removed from the male mold.

Description

(1) The invention is further described by way of example in the following non-limiting embodiments, with reference to the accompanying drawings in which:

(2) FIGS. 1 to 5 are diagrams showing different characteristics of materials used according to the invention with respect to prior art materials (as previously discussed);

(3) FIG. 6 is a simplified, schematic perspective view of a refrigerator appliance according to the invention;

(4) FIG. 7 is a schematic perspective view of a liner, in particular a cabinet liner, of the refrigerator appliance of FIG. 6;

(5) FIG. 8 is a schematic representation of the main steps of a method according to the invention for manufacturing the liner of FIG. 7, as well as other plastic liners of refrigerator appliances;

(6) FIGS. 9 and 10 show in greater details some steps of the method according to the invention, in particular of an extrusion process and a thermoforming process which are part of the method of FIG. 8.

(7) In FIG. 6, it is indicated as a whole with reference numeral 1 a refrigerator appliance 1 having at least one inner plastic liner 2 defining at least one inner hollow compartment 3.

(8) The appliance 1 comprises a hollow cabinet 4 internally provided with at least one cell 5, and having a front opening 6 closed by a door 7.

(9) In the exemplary embodiment of FIG. 6, the appliance 1 is a combined fridge/freezer appliance and comprises a single cabinet 4 housing a refrigerator cell 5a and a freezer cell 5b, closed by respective doors 7.

(10) With reference also to FIG. 7, the cells 5 are advantageously defined by respective compartments 3 of the liner 2, which in this case is advantageously a monolithic cabinet liner.

(11) Hereinbelow, reference is made by way of example to a cabinet liner, but it is clear that the following description applies to any other liner, for example a single cell liner or a door liner, of a refrigerator appliance.

(12) The liner 2 comprises a monolithic hollow body 8 shaped to define one or more (two, in the example of FIG. 7) compartments 3; each compartment 3 is advantageously delimited by lateral walls 9 projecting from a back wall 10 and has a front opening 11 opposite to the bottom wall 10.

(13) The liner 2, i.e. the body 8, is made of a plastic (polymeric) material, in particular a polypropylene-based material.

(14) In greater details, the liner 2 is made of a polypropylene-based material comprising a polypropylene copolymer (a copolymer in which propylene is the main component, i.e. having a content of propylene units greater than 50% w/w) containing ethylene units and compounded with at least one lamellar or fibrolamellar filler, for example and preferably talc.

(15) In other words, the copolymer is a propylene-ethylene copolymer having a main polypropylene chain with ethylene units arranged along the polypropylene chain.

(16) Advantageously, the copolymer has a content of ethylene units ranging between about 1% and about 8% w/w.

(17) More advantageously, the copolymer has a content of ethylene units ranging between about 2% and about 5% w/w.

(18) The polypropylene-based material also comprises at least one lamellar or fibrolamellar filler, i.e. one or more fillers having a lamellar or fibrolamellar structure; preferably this lamellar or fibrolamellar filler is talc.

(19) As previously described, a lamellar or fibrolamellar filler is formed by thin particles having a substantially plate shape (i.e. a platelet or leaf shape).

(20) The filler is preferably selected in the group consisting of: talc, kaolin, mica, glass flakes, nanoclays, montmorillonite and bentonite, graphite, aluminum nitride, boron nitride.

(21) Advantageously, the polypropylene-based material contains one or more lamellar or fibrolamellar fillers in an amount ranging between about 5% w/w and about 40% w/w. More advantageously, the content of lamellar/fibrolamellar filler(s) ranges between about 10% and about 30% w/w.

(22) The material optionally comprises the polypropylene-ethylene copolymer as a main component, and also homo-polypropylene (polypropylene homo-polymer), preferably in an amount ranging between about 0% and about 25% w/w. and more preferably in an amount ranging between about 0% and about 15% w/w.

(23) Optionally, the material comprises additives, like antioxidant chemicals, lubricants, processing agents, and small percentages of other fillers; and/or titanium dioxide.

(24) The liner 2 is advantageously manufactured by the method described hereinbelow with reference to FIGS. 8 to 10.

(25) The manufacturing method of the invention advantageously comprises (FIG. 8) the steps of: preparing the polypropylene-based material of the liner 2, advantageously by compounding the propylene-ethylene copolymer and at least one lamellar or fibrolamellar filler; extruding the material into sheets 12, by using an extruder 13; thermoforming a sheet 12 by means of a male mold 14 to shape the liner 2.

(26) The polypropylene-based material and its components have been previously described.

(27) In a preferred embodiment, the material is prepared directly in the extruding step: all the components, in particular the filler(s) and the PP copolymer (polypropylene-ethylene copolymer), are preferably compounded in the extruder 13. A twin-screw co-rotating extruder may be advantageously used.

(28) However, some (or even all) components can also be pre-mixed in a mixing step before the extrusion step.

(29) As shown in FIG. 9, the extruder 13 preferably has a plane dye 15 to produce a thin, flat planar flow.

(30) Since the extruding process can generate, inside the material, internal stresses that can then be released at high temperature, during the following thermoforming process, it is advantageous to control the extruding process in order to obtain a nearly unoriented sheet; controlling of the extruding process is well known in the art, so it to will not be described in more details.

(31) The extruded material which exit from the dye 15 is preferably pulled through a rolling unit 16, comprising a set of cooling rolls and a calender, where the material is cut into sheets 12, the sheets 12 are cooled and the final thickness of each sheet 12 is determined precisely. The sheets 12 are then preferably stacked.

(32) Each sheet 12 comprises, and preferably consists of, a single, substantially uniform layer of the polypropylene-based material. In another advantageous embodiment, in order to improve the final product surface appearance, each sheet 12 comprises, and preferably consists of, a main layer, made of the above described polypropylene-based material, and a glossy or semiglossy covering layer, covering a face of the main layer and made of neat polypropylene or a PP compound with glossy appearance. Advantageously, the covering layer is co-extruded or laminated with the main layer.

(33) Each individual sheet 12 is then supplied to a thermoforming section 17 (FIG. 8), comprising a heating unit 18 and a forming unit 19.

(34) In the thermoforming section 17, the sheet 12 is first heated, for example by passing through the heating unit 18; and then shaped by thermoforming in the forming unit 19 to form the liner 2.

(35) With reference also to FIG. 10, in the heating unit 18, the sheet 12 is heated to a predetermined operating temperature (forming temperature) at which the material is softened to a substantially rubbery state.

(36) Advantageously, the operating temperature is between about 125 C. and about 155 C.

(37) The sheet 12 is preferably advanced through the heating unit 18, having upper and lower heaters 20. The heaters 20 preferably (ma not necessarily) comprise infrared (IR) heating sources.

(38) In order to reach a high efficiency, the heaters 20 have preferably an emission wavelength ranging between 2.9 and 4.2 microns (34502380 cm-1), with the highest emission in the range 3.23.8 microns (31252630 cm-1), corresponding to the absorbing range for PP polymers.

(39) Once the sheet material has reached the predetermined operating (forming) temperature, the material is in a substantially rubbery state and the heated, softened sheet 12 is moved to the forming unit 19, in particular inside a pressure box 21 or bell which houses the male mold 14, having the shape of the liner 2 to form.

(40) For example, in the preferred embodiments of FIG. 10, the sheet 12 is positioned inside a vacuum chamber 22 of the pressure box 21 and the sheet 12 is advantageously clamped along its peripheral edge 23 in order to hold tightly the sheet 12. The clamped sheet 12 divides the vacuum chamber 22 of the pressure box 21 into two zones delimited by opposite faces of sheet 12: a first (inner) face 24, facing the male mold 14 and intended to define, after thermoforming, an inner surface of the liner 2; and a second (outer) face 25, opposite to the first face 24 and intended to define, after thermoforming, an outer surface of the liner 2.

(41) Advantageously, in the pressure box 21 the sheet 12 is first subjected to a pre-stretching step, i.e. the sheet 12 is pre-stretched; and then the pre-stretched sheet 12 is vacuum formed onto the male mold 14.

(42) For pre-stretching the sheet 12, vacuum is applied in the vacuum chamber 22 on the side of the second (outer) face 25 of the sheet 12.

(43) For multi-cavity liners, such as combined cabinet liners for refrigerator and freezer, a double suction is advantageously performed and the sheet is blown into a so called double bubble (having two adjacent cavities) pre-stretching.

(44) Once the sheet 12 is suitably pre-stretched, the male mold 14 is moved in the vacuum chamber 22 against the pre-stretched sheet 12, which is draped around an outer shaping surface 26 of the mold 14.

(45) Advantageously, the mold 14 (in particular the shaping surface 26 thereof) is at a temperature lower than the recrystallization temperature of the PP copolymer. Preferably, the temperature of the mold 14 is about 90-110 C., for example around 100 C.

(46) Vacuum is then applied on the side of the first face 24 of the sheet 12, for example through suitable inner channels in the mold 14, so as to draw the sheet 12 against the shaping surface 26 of the mold 14.

(47) The differential pressure against the sheet 12 is amplified if the pressure inside the pressure box 21 is increased: thus, a compressed gas stream (air) can be advantageously injected in the pressure box 21, on the side of the second face 25 of the sheet 12, simultaneously with the vacuum (acting on the first face 24 of the sheet 12); in this way, it is possible to better replicate even small details on the shaping surface 26 of the mold 14. The sheet pre-stretching in combination with the forming pressure give a more uniform material distribution which is favorable to form complex geometries and undercuts.

(48) Additional draft angles can be provided for very deep parts.

(49) The sheet 12 adheres to the shaping surface 26 of the male mold 14 and assume the shape thereof.

(50) The formed sheet 12, still contacting the shaping surface 26 of the mold 14, is then cooled to harden and form the liner 2, for example by blowing air onto the face 25 and/or by circulating a coolant in cooling conduits 27 inside the male mold 14.

(51) Once sufficiently cooled, the sheet 12, having the shape of the liner 2, is separated from the male mold 14 (for example by blowing air through the inner channels of the male mold 14), and extracted from the pressure box 21.

(52) The additional energy used in the heating step for softening the material should be removed efficiently in order to keep a pace corresponding to short production cycles. An additional cooling step can be added after extraction of the liner 2 from the thermoforming section 17 and it is very useful for thick thermoformed parts. Preferably, the warm rigid liner 2 is hence supplied to an additional cooling station (not shown) and cooled in order to reach a temperature that doesn't cause any further deformation; then the liner 2 is advantageously moved to a trimming and cutting unit (not shown) for removing edges and other scraps.

(53) Clearly, further changes may be made to the refrigerator appliance and to the method for manufacturing the liner of the refrigerator appliance described herein without, however, departing from the scope of the present invention as defined by the enclosed Claims.