Process of manufacturing multi-layered hybrid liquid storage tank

Abstract

Disclosed is a process for manufacturing a hybrid multi-layered liquid storage tank comprising pre-fitting a pre-fabricated steel tank (1) inside the roto mould / die (101) before starting the roto moulding process by fitting the legs (4) of steel tank (1), tightening the bottom cover flat mould (103) and fixing the tapered neck (5) of the steel tank inside the cut out portion (107) of the top side of the plastic mould so that it protrudes out from the cut out (107) portion resulting in gripping the steel tank (1) from both sides inside the plastic tank mould thus providing gap / space (106) between plastic mould body and steel tank. Heating and cooling the mould (101) resulting in melting of the plastic material granules which take the shape of the mould. One or multiple layers of insulating material are filled in the gap / space (106) to provide insulation from top to bottom.

Description

[0001] PROCESS OF MANUFACTURING MULTI- LAYERED HYBRID LIQUID STORAGE TANK

[0002] FIELD OF INVENTION

[0003] The present invention relates to field of manufacturing process of storage tank. More particularly it pertains to a process of manufacturing of multi- layered hybrid liquid storage tank for all kinds of liquids primarily water, made up of plastic and coated stainless steel together.

[0004] BACKGROUND OF THE INVENTION

[0005] Definitions:

[0006] Plastic tank- It is a large capacity liquid or granular storage unit that can be vertical, horizontal, below or above ground, and permanent in place or movable. They are designed to hold several gallons of a variety of substances for long periods without experiencing wear, weathering, or deterioration. Manufacturers, farmers, and fuel companies depend on plastic tanks as a means of transferring and saving materials for later use.

[0007] Water storage tank is a good example of a plastic tank as it is most commonly used in households and industry to store water for later use. These are usually installed at the roof top permanently and need regular cleaning and maintenance.

[0008] There are many types of plastic tanks available in market. Three of the common polymers are polyethylene, polypropylene, and fiberglass used in making plastic tanks. These tanks can be multi layered.

[0009] Since multilayering of plastic tanks is a common practice these days, the method of manufacturing these kinds of tanks is also evolving. Tradeoff between cost and quality is always a challenge for the manufacturer. Many companies are manufacturing and selling the single or multi layered plastic water tanks such as Sintex, Plasto Supreme, Kaveri, Aquatech, Penguin etc.

[0010] There are a wide variety of methods used to produce plastic tanks with polyethylene and polypropylene being the most common raw materials. As with most plastic components and parts, plastic tanks are produced using injection, blow, or rotational molding with other methods being variations of these common ones.

[0011] We restrict our discussion to rotational moulding process (also called roto moulding) of manufacturing storage tank.

[0012] Rotational Moulding

[0013] The roto molding technique is exceptionally helpful to make multi-layered tanks. After the outermost layer is molded and solidifies, each of the immediate inner layers is added by putting powdered material from outside which sticks to the inner surface. A roto molded water tank is manufactured in one piece, with smooth edges and curves.

[0014] The principle of rotational molding of plastics is simple. Basically the process consists of introducing a known amount of plastic in powder, granular, or viscous liquid form into a hollow, shell-like mold. The mold is rotated and / or rocked about two principal axes at relatively low speeds as it is heated so that the plastic enclosed in the mold adheres to, and forms a monolithic layer against, the mold surface. The mold rotation continues during the cooling phase so that the plastic retains its desired shape as it solidifies. When the plastic is sufficiently rigid, the cooling and mold rotation is stopped to allow the removal of the plastic product from the mold. At this stage, the cyclic process may be repeated. The basic steps of (a) mold charging, (b) mold heating, (c) mold cooling, and (d) part ejection are shown in Prior art Figure 1.

[0015] Roto moulding can be single axial or bi axial. There can be two arm or four arm roto moulding machines where two pieces or four pieces of the plastic product can be manufactured simultaneously.

[0016] Steel Tank- These tanks are widely used in food, beverage, dairy, medicine, cosmetics, and other manufacturing processes where cleanliness and purity are important. These are also used in industrial plants for storing chemicals and gases where strong resistance from chemical degradation is required. Stainless steel is a type of iron alloy containing a certain percentage of chromium, which imparts corrosion resistance to the metal. Corrosion resistance is achieved by creating a thin film of metal oxides that acts as protection against corrosive materials. Still the outer layer still gets corroded with atmospheric changes after sometime.

[0017] One more disadvantage of a steel tank is that it gets heated up / cooled down very quickly when put on roof top. Some of this heat gets transferred to the inner surface despite providing insulation. Tanks can be single-walled or double-walled, horizontal or vertical, insulated or heated, and so on. Manufacturing is usually through cold rolling. Secondary milling operations such as pickling are done to enhance the stainless steel corrosion resistance from inside.

[0018] Steel Tanks need manual work at the very beginning as the steps of manufacturing involve Sheet Cutting, spot or tig welding, Seam welding manually or automatically, Multi Rib rolling on middle surface and single rib rolling on both the ends followed by spot and seam welding to fix the cover parts at the top and bottom edges.

[0019] There are many manufacturers of steel tanks in India such as H2O, Sheetal, pure ever etc.

[0020] Plastic tanks have a life span of approximately 7 years without UV stabilizers and need more maintenance. Stainless steel tanks on the other hand are non-corrosive from inside but corrosive on the outer surface and have a life span of nearly 10 years but these tanks are high in cost, heavy in weight as compared to plastic tanks and are difficult to provide insulation. A few stainless tanks with insulation are available with insulation only around the side body, however from the bottom & to the top, there is no insulation, which leads to a temperature increase quite significantly and provides only partial insulation.

[0021] Present market scenario w.r.t Innovation in water tanks

[0022] No high impact innovation has happened in the last decade in plastic storage tanks except for introduction of additional layers or some sort of thermal insulation in the plastic water tanks. No one has invested in the R & D to bring innovation to the technology. All major brands spend very low on marketing & branding, because it is more of a commodity business and talks about only Life & Durability and to some extent about hygiene - which is almost common for all players, because technology, materials, process all are same.

[0023] Stainless steel tanks do offer better life, higher level of hygiene and health benefits, but these are too expensive for any meaningful adoption to happen.

[0024] No product offers 360-degree insulation.

[0025] On the pricing aspect, the prices of standard plastic tanks vary from Rs 7 to 10 per liter of storage and prices of Stainless-Steel tanks vary from Rs 55 - 70 per liter. There is a huge gap in the market and there is a huge market in this gap. A need is felt to have a plastic cum steel tank (may be called hybrid tank) which should have advantages of both the materials but at the same time, overcome the disadvantage of plastic such as corrosiveness, shorter life span and non-suitability for storing strong chemicals and at the same time overcome the disadvantages of steel tank also such as high cost and less thermal insulation.

[0026] INNOVATIVE SOLUTION PROPOSED BY THE INVENTORS

[0027] One of the inventors of the present invention has duly addressed this gap and came up with a more accessible, affordable Hybrid type water storage tank which gives the benefits of the stainless-steel tank but comes at a significantly reduced price. The same is already granted an Indian Patent No. 438761 titled “An insulated water storage tub”. The patented invention provides a water storage tub / tank which does not allow Leaching of harmful chemicals in the stored water because of exposure to UV rays and high temperature. The water storage tank keeps the water temperature constant due to outer layer made up plastic, being non-resistant. The same is rust and frost resistant and achieves durability at a reasonable price.

[0028] The main body of hybrid tank is built of three layers:

[0029] Outer layer is made of high quality virgin LLDPE, UVSP or any other suitable material to protect it from the outer heat and cold and also from UV rays. Outer layer may have the wall ribs to improve the strength of jacket. Middle layer is a foam layer which does not allow the water to get heated or cool with temperature change. Inner layer is made of stainless steel (or copper) and comes in contact with the water. It may also be MS coated or any other corrosion free type of steel.

[0030] The thickness of all three layers in the present invention are carefully calculated and defined after due experimentation with actual field level trials. One of the inventors and applicant is himself a manufacturer of water storage tanks and has a small scale industrial unit. So he is well conversant with types of tanks available in commercial production and the problems in such storage tubs and tanks. Therefore the present invention is developed after careful thought and inventive thinking.

[0031] However the method of manufacturing of the hybrid storage tank needed innovative thinking as it is very difficult to combine three layers one of which is steel and one being plastic. It was difficult to make the innermost layer of steel first and then outermost layer of plastic. This is because the outer layer of plastic needs to be made by roto moulding method and the same method cannot be applied for making inner layer of steel. In other words, it is like putting complete steel tank into the small opening of plastic tank and still maintaining a space / gap for third or subsequent multiple layers to be prepared. But the inventors did this very intelligently and by applying their expertise and intellect.

[0032] They have successfully modified the Roto Moulding process and developed a method of manufacturing a hybrid liquid storage tank having three or multiple layers of different materials.

[0033] PRIOR ART PATENTS / PUBLICATIONS

[0034] Indian Patent No. 438761 discloses “An Insulated water storage tub”

[0035] The above cited patent discloses an insulated water storage tub comprising three layers, wherein Outer layer is made up of high quality virgin LLDPE material to protect it from the UV rays which is UV stabilized and is of 5 to 10 mm thickness; Middle layer is of 45 to 60 mm thick foam layer, which doesn’t allow the water to get heated or cool with outer temperature change and Inner layer is made of stainless steel or copper with thickness in the range of 0.5 to 1 mm.

[0036] The above cited granted patent is of same applicants as that of present invention and the layers of the tub as disclosed in cited patent are similar as the layers disclosed in above cited patent. However the method of production of water tub using roto-moulding and injection moulding as disclosed in present invention is not disclosed in cited patent, hence present invention is basically an extension of above cited patent.

[0037] Patent Application No. CN113002030 discloses “Plastic-lined stainless steel water tank and manufacturing method thereof’

[0038] The above cited patent discloses a method for manufacturing a plastic-lined stainless steel water tank. Firstly, a stainless steel plate into a plate with an outer drum and an inner recess is manufactured through a cold pressing technology, wherein four edges of the plate are two continuous 45 -degree outward-bent flanges, and connecting holes are punched in the outward flanges. The stainless steel plate is then fitted into the mould and further, plastic matrix is injected into the mould to ensure that the surface of the stainless steel is uniformly wrapped by the plastic, cooling and demoulding together with the stainless steel plate to form a composite water tank plate with an inner layer made of the plastic and an outer layer made of the stainless steel. In above cited patent, a double layer water tank is manufactured in which, plastic layer is added inside the stainless steel layer, however there is no provision or method of incorporating additional foam layer in between plastic and steel layer, hence both inventions are different from each other.

[0039] The above patent discloses the steps like first manufacturing a stainless steel plate into a plate with an outer drum concave inwards through a cold pressing technology, wherein four edges of the plate are two continuous 45-degree outward-bent turned edges, and connecting holes are punched in the outward turned edges. In second step, putting the stainless steel plate into a mould, injecting a plastic matrix into the mould by using an injection molding machine to ensure that the surface of the stainless steel is uniformly wrapped by the plastic, cooling and demoulding together with the stainless steel plate to form a composite water tank plate with the inner layer made of the plastic and the outer layer made of the stainless steel.

[0040] On the contrary, the present invention has an outer layer made up of plastic and innermost layer is made up of steel.

[0041] The middle layer is a foam layer which cannot be provided in the above mentioned patent as the steel and plastic layers do not have any gap in between for the third material.

[0042] Thus the advantages provided by the method on manufacturing of Hybrid tank of present invention are missing in cited patent.

[0043] Patent Application No. US 2014 / 0008372 Al discloses “Storage tank containing a black polyethylene inner layer”

[0044] The above cited patent discloses a method for manufacturing water tank with a black polyethylene inner layer. The preparation method comprises steps of adding first pigmented resin to the mould, adding second pigmented resin to the dropbox, attaching dropbox to mold, curing phase 1 , opening dropbox compartment, curing phase II, cooling the mold and removing the tank.

[0045] The method as disclosed in present invention is not disclosed in above cited patent, making present invention different from above cited patent. Patent Application No. CN111980111 discloses “Manufacturing process of PE-lined water tank”

[0046] The above cited patent discloses a manufacturing process of a PE-lined water tank. The method of manufacturing comprises steps of heating an iron plate of a water tank to 240-300 °C, taking out water tank iron plate from the furnace, uniformly spraying HDPE powder on the front surface and the back surface of the heated water tank iron plate, so that the HDPE powder is naturally melted on the surface of the water tank iron plate to form an HDPE protective layer and arranging a silica gel sealing strip between the two water tank iron plates and fastening the two water tank iron plates through screws.

[0047] In present invention along with plastic, stainless steel and foam layers are used for tank manufacturing, whereas above cited patent uses silica gel sealing strip and iron plates. Further, in cited patent, HDPE powder is sprayed on both sides of the iron plate, whereas in present invention, the first layer is plastic, last stainless steel layer and in between plastic and steel, layer of foam is being incorporated. The method as disclosed in present invention is also not disclosed in cited patent. Hence, present invention differs from the cited patent.

[0048] Patent Application no. CN111237854 discloses “Polyethylene PE inner container pressurebearing buffer water tank and manufacturing process thereof’

[0049] Above cited patent discloses a Polyethylene (PE) liner pressure -bearing buffer water tank and a manufacturing process thereof. The polyethylene PE liner pressure -bearing buffer water tank comprises a water tank shell, a liner and a plurality of mounting supports, wherein the liner is arranged in the water tank shell, the mounting supports are uniformly arranged below the water tank shell respectively, a polyurethane foam heat-insulation material layer is arranged between the water tank shell and the liner, the liner is a polyethylene PE liner.

[0050] The above cited patent discloses three layers, outermost layer being steel and inner lining of PE material and foam layer is arranged between the two. In contrast, the present invention is a reverse. The outermost layer is made up of LDDPE, innermost layer being of stainless steel and in between, foam layer is sandwitched.

[0051] The advantage of this reverse arrangement is that utilizing the non-corrosive property of steel in the area where water is stored and getting less heat absorption due to outermost layer being of plastic and constant temperature of water due to Middle layer being of foam, gives better and hygienic water quality and long life in single hybrid tank.

[0052] Patent Application no. CN217755100U discloses “Heat-insulation heat-preservation type stainless steel water tank”

[0053] The above cited patent discloses a thermal-insulated heat preservation type stainless steel water tank, including stainless steel water tank body, thermal-insulated heat preservation layer and resin protection layer.

[0054] Three layer water tank is disclosed in above cited patent, however method of manufacturing of the same is not disclosed. Moreover it seems that the inner layers are provided to the outer layer of stainless steel which is not the case in the present invention.

[0055] Patent application no. CN208715913U discloses “A kind of stainless steel water case apparatus”

[0056] The above cited patent discloses stainless steel water case apparatus, including water tank, polyurethane insulation coating, in which outermost layer of water tank is provided with one layer of stainless steel plate, water tank is internally provided with one layer of PP foamed board, polyurethane insulation coating is provided between the stainless steel plate and PP foamed board. The stainless steel plate has a thickness of 5mm or 10mm. The polyurethane insulation coating has a thickness of 50mm and the PP foamed board has a thickness of 15mm. In the above cited patent, stainless steel plate, polyurethane insulation coating, glued using polyurethane binder between PP foamed board It connects, coating is " rice " font. The method of manufacturing water tank as disclosed in present invention is not disclosed in above cited patent, making present invention different from above cited patent. Moreover it seems that the inner layers are provided to the outer layer of stainless steel which is not the case in the present invention.

[0057] Journals and Publications

[0058] R. Bhaskar Reddya, V. Jyotsna Kalpanab, V. Srinivasc, V.V.S. Prasad; Design formulation and analysis of overhead water tank: Material used for automatic cleaning mechanism; Materials today proceedings; 13thMay, 2023

[0059] The above cited article discloses an automatic cleaning mechanism for domestic overhead water tank. The tank is designed in such a way that the cleaning can be done both in manual and automatic ways. The disclosed tank comprises of three layers including Outer Premium Layer, Insulated Middle Layer and Inner Food grade Polymer Layer.

[0060] The method of manufacturing water tank as disclosed in present invention is not disclosed in above cited article, hence present invention differs from above cited article. Moreover the article seems confined to plastic tank only.

[0061] Commercial products

[0062] Youtube video by Unbox Engineering on How Plastic Water Tank is Made in the Factory

[0063] Ref- https: / / www.youtube.com / watch‘?v=RVrQ2MUQFV60

[0064] The above cited youtube video discloses a general method for manufacturing plastic water tank. The discloses process comprises steps of extrusion process, pulverisation of plastic granules, rotomoudling, by adding plastic granules in mould and further heating and rotating the mould comprising plastic granules, so that plastic melts, spread over the mould to form plastic tank. The next step is cooling in order to solidify the tank and finally the tank is removed from the mould. The method referred above is directed towards manufacturing of plastic tanks by roto moulding which is not the object of the present invention.

[0065] Therefore the need for a hybrid tank was fulfilled by the inventors in their granted patent but the method of manufacturing the same and the technological knowhow is not disclosed prior to filing of this patent application.

[0066] OBJECT OF THE PRESENT INVENTION

[0067] The primary object of the present invention is to disclose a process of manufacturing a hybrid multi-layered liquid storage tank made up of three or multiple layers, each layer of different material.

[0068] One more object of the present invention is to disclose a process of manufacturing a hybrid liquid storage tank using modified roto moulding process.

[0069] Another object of the present invention is to disclose a process of manufacturing a hybrid storage tank with insulation all around 360 Degree from top to bottom.

[0070] Yet another object of the present invention is to disclose a process of manufacturing a hybrid storage tank which is simple and cost effective so that the same is easily adapted by manufacturers.

[0071] SUMMARY OF THE INVENTION

[0072] The present invention is a modified roto moulding process wherein a single or multi-layer layer steel tank is pre fitted into the mould (101) of roto moulding used for making plastic tank only. This pre fitting of steel tank ( 1 ) is carried out in various steps which must provide a gap or space between the plastic layer (6) and the steel tank body so that a third layer of foam (7) or multiple layers of different materials are inserted in between the two making it a hybrid three or multi layered layered tank of different materials. The provision of gap or space is achieved by providing multiple legs (4) at the circumference of opening of the single layered steel tank (1). Fixing of legs (4) in the mould / die (101) of the bottom mould cover (103) of the hybrid tank is done with the help of nut / bolt or screws (105). The top end of the steel tank (1) has a shape suitable for holding a lid. This is due to a narrow neck (5) at the top end. In the present invention, the plastic tank mould is provided with a cut out portion as indicated in fig 3b as 107. The neck (5) of the steel tank (1) is slightly tapered so that it protrudes out from the cut out portion of the plastic tank mould and gets a good grip at that end also. Due to fitting of steel tank (1) inside the cut out, the plastic granules needed for moulding also get stuck in the mould itself and do not fall off out of the mould.

[0073] DETAILED DESCRIPTION OF THE DRAWINGS

[0074] Prior art figure 1

[0075] Figure 1 -Bottom mould cover indicating holes for fitting of legs of steel tank

[0076] Figure 2- Steel tank with legs

[0077] Figure 3a and 3b- Upside down and normal side up views of complete mould with bottom mould cover open and legs protruding out and indicating gap

[0078] Figure 4- Complete mould with bottom mould cover closed with nut / bolt or screws and legs protruding out

[0079] Figure 5- Drawing of plastic tank with steel tank inside

[0080] Figure 6- Drawing of plastic tank with steel tank inside and third layer in between

[0081] Figure 7- Drawing of plastic tank with steel tank inside and multiple layers in between

[0082] Figure 8- Drawing of rock and roll type of roto moulding machine

[0083] Numbering details-

[0084] 1- Steel tank

[0085] 2- Steel tank legs fitting holes

[0086] 3- Steel tank legs guiding boss

[0087] 4- Steel tank Legs

[0088] 5- Steel tank neck

[0089] 6- Plastic layer

[0090] 7- Foam layer

[0091] 8- Multiple layers

[0092] 101- Mould die

[0093] 102- Top mould cover 103- Bottom mould cover

[0094] 104- 60 mm gap between steel tank and die

[0095] 105- Nuts / bolts / screws

[0096] 106- indicating gap between plastic tank and steel tank

[0097] 107- cut out portion in top mould cover 102

[0098] DETAILED DESCRIPTION OF THE INVENTION

[0099] The following description is the best-contemplated mode of carrying out the invention. The description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense.

[0100] The present invention is a modified roto moulding process wherein a single or multi-layer layer steel tank is pre fitted into the mould (101) of roto moulding used for making plastic tank only. The steel tank (1) may be covered with multiple layers (8) such as Aluminum foil or as required. The steel tank (1) may be pre- fabricated with any type of insulations as per the requirement. This pre fitting of steel tank (1) is carried out in various steps which must provide a gap or space between the plastic layer (6) and the steel tank body so that a third layer of foam (7) or multiple layers of different materials are inserted in between the two making it a hybrid three or multi layered layered tank of different materials. The provision of gap or space is achieved by providing multiple legs (4) at the circumference of opening of the single layered steel tank (1). The bottom cover flat mould (103) is also specially designed with multiple holes at the circumference so that these legs can protrude out from these holes and give a grip to the steel tank inside the mould. Fixing of legs (4) in the mould / die (101) of the bottom mould cover (103) of the hybrid tank is done with the help of nut / bolt or screws (105).

[0101] The top end of the steel tank ( 1 ) has a shape suitable for holding a lid. This is due to a narrow neck (5) at the top end. Similarly normally the plastic tank mould (101) also has a neck portion for fitting of a lid. In the present invention, the plastic tank mould is not provided a neck but instead it is provided with a cut out portion (107) as indicated in fig 3b in Top mould cover (102). The neck (5) of the steel tank (1) is slightly tapered so that it protrudes out from the cut out portion of the Top mould cover (102) and gets a good grip at that end also. Due to fitting of steel tank inside the cut out (107), the plastic powdered material needed for moulding also get stuck in the mould itself and do not fall off out of the mould.

[0102] Since legs of the steel tank get fixed with the flat bottom cover and the neck of the steel tank get fitted in top cut out of plastic tank mould, the steel tank gets tightly fitted from top and bottom and cannot move freely while rotation of the roto mould in moulding process, thereby not touching the inner surface of the mould from anywhere.

[0103] Following this pre fitting, the next steps of loading the mould with LLDPE or any other suitable plastic material such as PP, HDPE, composite material etc., heating the mould in heater chamber with the help of gas burner or oil burner of suitable kind and then cooling the mould with blower fans from outside or any other suitable means remain the same as in normal roto moulding process. After the roto moulding operation, a two layered Hybrid tank with outermost plastic layer (6), innermost steel layer and both layers having a suitable gap / space (106) for injecting at least one insulation layer in between, is thus created. Insulation layer (7) of any suitable material such as Liquid foam is injected manually / automatically in the gap / space resulting in the third layer of the hybrid tank.

[0104] Preferred embodiment of the present invention

[0105] The present invention is a modified Roto Moulding process used for plastic moulding wherein three or more layers of different materials are created to result in a hybrid storage tank. The beauty lies in the fact that the innermost layer is of steel and other layers above it are of insulations and plastic. The tank looks like a plastic tank in appearance but gives the advantages of steel tank as well. The person skilled in art till date could make a steel tank with various layers or plastic tank with multiple layers but no one could envisage a plastic tank containing a steel tank inside as it is resulting in thermal insulation as well as corrosion resistance properties in a single product.

[0106] The inventors being tank manufacturers themselves put in their intellect to put the steel tank with legs inside the roto mould used for plastic tank manufacturing by making few changes in the bottom cover flat mould body so that the legs protrude out of the bottom cover flat mould and are tightly fitted with nut / bolt or screws to prevent the movement inside the mould body and giving gap / space for forming outer tank body of plastic. The top end of the steel tank has a shape suitable for holding a lid. This is due to a narrow neck at the top end. Similarly normally the plastic tank mould also has a neck portion for fitting of a lid. In the present invention, the plastic tank mould is not provided a neck but instead it is provided with a cut out portion (107) as indicated in figure 5. The neck of the steel tank is slightly tapered so that it protrudes out from the cut out portion of the plastic tank mould and gets a good grip at that end also. Due to fitting of steel tank inside the cut out, the plastic granules needed for moulding also get stuck in the mould itself and do not fall off out of the mould

[0107] The complete process steps are as below:

[0108] Procurement of Steel Tank (1) - Steel tank of requisite size and capacity can be manufactured in house by any available means or can be procured from commercial market. The quality of steel in the steel tank is as per the requirement and is not a limiting factor in the method of the present invention.

[0109] Fixing of legs (4) at the bottom part of the steel tank (1)- Some steel tanks come with a set of legs to rest them on the roof top but most of the time the steel tanks do not have such legs as these are simply kept on a platform and connected to the water supply. So we need to weld the legs (4) with the steel tank (1) body if not already there as this is one of the pre requisites of the present invention.

[0110] Holding the steel tank from Bottom- The mould (101) used for roto moulding of a plastic tank comprises of three parts. Main body mould (101), top lid mould (102) and bottom cover flat mould (103). Usually the bottom cover flat mould (103) has provision for nut / bolt or screws (105) but not for fitting the legs of steel tank so we need to provide holes for the same. Size of these holes must be in coherence with the size of legs of the steel tank so that these protrude easily out of the mould. This helps in providing gap / space between steel tank and plastic layer to be formed during roto moulding as well as it provides ease of handling the hybrid tank while unloading from the mould after cooling. Holding the steel tank from Top end- The neck of the steel tank is slightly tapered so that it protrudes out from the cut out portion (107) of the top end of plastic tank mould and gets a good grip at that end also. Due to fitting of steel tank inside the cut out, the plastic granules needed for moulding also get stuck in the mould itself and do not fall off out of the mould.

[0111] Loading the mould (101) with material- The normal process of roto moulding for manufacturing any plastic storage tank is followed and thus suitable material for making outer tank body of plastic is filled in the mould (101). One such material is LLDPE which is a blended form of LDPE where the film has much more flexibility, tensile strength, and more conformability. It is more pliable and softer. LLDPE gives more flexibility and extra strength, so can be preferred material for these tanks.

[0112] Heating the chamber- The mould (101) is already fitted inside the roto moulding chamber. The roto moulding movement may be single axial or biaxial, the machine can be single arm or upto four arm machine, shuttle machine, automatic using smart electronics or manual using skilled manpower to monitor and control the heating and cooling processes.

[0113] The heating is switched on in the chamber and burners are fired. These burners can be gas burners or coal fired ones or petrol / diesel / kerosene burners.

[0114] The temperature and time may be set manually or using automation during the above process and is not a limiting factor in this method.

[0115] The heating results in melting of the plastic material which takes the shape of the mould by sticking on the inner surface of the mould body. The material does not stick to the steel tank body due to gap in between the inner surface of the mould body and steel tank. The material gets sticked only to the mould body which is being heated from outside but this heat is not transferred to the steel tank body due to gap / space.

[0116] A blower is also installed inside the chamber for uniform distribution of heat during the process.

[0117] Sometimes during the moulding process some kind of gas formation also occurs. The same is given a vent through the steel tank and it gets out into the atmosphere. Cooling the mould (101) - After heating for a preset time, the chamber door is opened and blower fans or any other suitable means of cooling the mould (101) are employed.

[0118] Unloading the mould (101) - Once the mould (101) body gets cooled, the nut / bolt or screws are opened manually and the plastic tank fitted with steel tank (1) inside is taken out. The handling is very easy as the legs protrude out of plastic tank body and can be held easily with hands.

[0119] Forming third or multiple insulation layers- Since the hybrid tank of present invention offers insulation and constant water temperature, it must have an insulating layer to maintain the temperature of water inside the tank during all seasons. Therefore the middle layer of foam (7) as insulating material is filled using injection method only but can be carried out in any manner by a person skilled in art.

[0120] The size, shape and capacity of the steel tank, plastic tank and in totality the new hybrid tank is selected by the manufacturer and is not a limitation in the method.

[0121] Capacity of the hybrid tank may range between 100L to 5000L. The thickness of all layers may also increase or decrease as per the capacity. All these are well within the scope of the present invention. Finally a three layered hybrid storage tank with innermost layer being of steel is successfully formed.

[0122] In another embodiment of the present invention, the gap / space between the plastic layer (6) and steel layer can be suitably increased or decreased so that we can form multiple layers (8) of different materials inside the gap / space (106) which may provide better thermal insulation.

[0123] Such layers can also be provided over the steel tank before fitting it into the mould.

[0124] In yet another embodiment of the said method, the steel tank is fitted in rock and roll type of roto moulding machine.

[0125] Advantages of Hybrid Tank of the present invention

[0126] The said tank is lighter in weight than Stainless steel tank, but somewhat heavier than PE tank. It is much lower in cost than a stainless-Steel tank but slightly higher than PE tank. But this hybrid tank has higher insulation than Stainless Steel and PE tanks and is inert to the water in contact due to inner layer being of Stainless Steel. Thus the Novelty and Inventive step of the method of the present invention lies in disclosing a unique process for manufacturing a hybrid multi-layered liquid storage tank for all kinds of liquids. The process requirement is such that after fitting the steel tank inside the roto mould body, it should not move freely inside the mould during moulding process and also the mould material does not stick to the steel body so an adequate gap / space must be maintained between the inner surface of mould body and the steel tank inside. This is achieved due to modification in the bottom lid cover body of the mould and providing holes in it to get the legs of steel tank fitted inside these holes tightly with nut / bolt and screws. No one could get the innermost layer made up of steel and getting plastic layer over it in the same roto moulding process cycle. Thus the inventive step is clear and without doubt.

[0127] Industrial application is well defined as these tanks are primarily used in water storage and this industry is worth thousands of crores. These tanks are widely used in food, beverage, dairy, medicine, cosmetics, and other manufacturing processes where cleanliness and purity are important. These are also used in industrial plants for storing chemicals and gases where strong resistance from chemical degradation is required. Thus the industrial application is very clear.

Claims

WE CLAIM:

1. A process for manufacturing a hybrid multi-layered liquid storage tank wherein the same comprises the steps of: pre-fitting a pre-fabricated steel tank (1) inside the roto mould / die (101) before starting the roto moulding process, with the following method: a) welding the legs (4) with the steel tank (1) body; b) providing holes in the bottom cover flat mould (103) and fitting the legs (4) of steel tank (1) ; c) fixing the tapered neck (5) of the steel tank inside the cut out portion (107) of the top side of the plastic mould so that it protrudes out from the cut out (107) portion resulting in gripping the steel tank (1) from both sides inside the plastic tank mould thus providing gap / space (106) between plastic mould body and steel tank; filling of plastic material in powdered form in the gap / space (106) for making outer tank body of plastic; heating the mould (101) which is already fitted inside the roto moulding chamber resulting in melting of the plastic material in powdered form which takes the shape of the mould by sticking on the inner surface of the mould body; cooling of the mould (101) after heating for a preset time, resulting in shrinkage of plastic tank; opening the nut / bolt or screws (105) manually to take out the plastic tank fitted with steel tank (1) inside, by holding the legs (4); one or multiple layers of insulating material are filled in the gap / space(106) to provide insulation all around 360 Degree from top to bottom for constant water temperature resulting in hybrid multi-layered liquid storage tank.

2. The process for manufacturing a hybrid multi-layered liquid storage tank as claimed in claim 1 wherein the gap / space (106) between the plastic layer (6) and steel layer is up to 60 mm.

3. The process for manufacturing a hybrid multi-layered liquid storage tank as claimed in claim 1 wherein the gap / space between the plastic layer (6) and steel layer is up to 100 mm so as to form multiple layers (8) of different materials inside the gap / space (106) which provide better thermal insulation.

4. The process for manufacturing a hybrid multi-layered liquid storage tank as claimed in claim 1 wherein the pre-fabricated steel tank (1) is single layered.

5. The process for manufacturing a hybrid multi-layered liquid storage tank as claimed in claim 1 wherein the pre-fabricated steel tank (1) is multi-layered.

6. The process for manufacturing a hybrid multi-layered liquid storage tank as claimed in claim 1 wherein the roto moulding machine is rock and roll type.

7. The process for manufacturing a hybrid multi-layered liquid storage tank as claimed in claim 1 wherein the machine for roto moulding process is single axial or bi axial.

8. The process for manufacturing a hybrid multi-layered liquid storage tank as claimed in claim 1 wherein the machine for roto moulding process is two arm or four arm.

9. The process for manufacturing a hybrid multi-layered liquid storage tank as claimed in claim 1 wherein the legs (4) of the steel tank (1) body are connected together after manufacturing the hybrid multi-layered liquid storage tank to give strength to each other and form a base support.

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