Included in this article is information regarding plastic water tanks and their use.
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A plastic water tank is a large container designed to store water for various uses, including residential, agricultural, irrigation, and industrial applications. There are different types of water tanks available to suit specific needs, with some designed to meet particular requirements and standards. The term "plastic water tank" broadly refers to various plastic tanks intended for water storage. These tanks are made from plastics that are chemically engineered to ensure safety and comply with standards set by the Food and Drug Administration (FDA). Among the many plastics used for constructing water tanks, polypropylene and polyethylene are the most commonly employed materials.
When selecting a plastic water tank, it is crucial to choose one based on its intended use, as different plastics are suited for various types of water storage. The strictest regulations apply to tanks used for storing drinking or cooking water. For these applications, the tank must meet the standards set by the Food and Drug Administration (FDA).
Plastic water tanks come in a wide range of sizes and types, from portable tanks that can be transported in a truck to large tanks capable of holding thousands of gallons and being buried underground. Essentially, there is a plastic water tank available for virtually every application and requirement. The diversity in types and sizes ensures that plastic water tanks can accommodate any specific need.
There are several methods used to make plastic bottles and containers. The sizes of plastic water tanks limit the number of methods available to produce them, including blow molding or rotational molding. Prior to beginning the molding process, manufacturers choose the appropriate polymer to fit the requirements of the proposed plastic water tank, a choice that is critical to the tank’s performance and use.
When designing a plastic water tank, several critical factors must be taken into account, such as the thickness of the plastic and the desired shape. The manufacturing process is meticulous, and skipping any steps is not feasible, as each stage is essential to ensuring the production of a functional and high-quality product.
Before starting production, manufacturers select the appropriate polymer for making a plastic water tank. The most commonly used polymers are polyethylene and polypropylene.
Polyethylene, also referred to as polythene or polyethene, has a linear molecular structure with ethene as its primary component. Ethene is a stable molecule, and during the polymerization process, a catalyst is introduced to transform ethylene into polyethylene.
Polypropylene is derived from propylene, an organic hydrocarbon, and shares physical properties with polyethylene. However, polypropylene differs in its higher resistance to heat and greater hardness. It is more flexible and offers resistance to certain acids and alkalis.
Rotational molding is a manufacturing technique used to create hollow parts by placing them into a hollow mold. The process involves adding powdered resin to the mold and then rotating it to ensure even coating and distribution of the material.
The first step in the rotational molding process involves loading plastic powder polymer into the mold chamber. The quantity of polymer used determines the size and wall thickness of the plastic water tank. To facilitate easy removal of the molded tank, a release agent is applied to the mold's interior surfaces.
Although rotational molding is often described as producing seamless parts, there is a visible seam where the mold's top and bottom sections are joined. This seam appears because the mold halves are bolted together with a slight offset before coating and polymer powder loading. Despite this seam, all rotationally molded plastic water tanks are designed as a single, leak-proof unit.
The coated mold, containing the powdered polymer, is then placed in a large oven where it is heated to gradually melt the polymer. As the temperature increases, the mold rotates on multiple axes, causing the melted plastic to coat the interior surfaces evenly.
The diagram below illustrates the heating process and the mold's rotation.
While the mold is heated, it rotates to ensure that the melted polymer is evenly distributed across its surface. This process efficiently spreads the material, achieving the desired thickness for the container and shaping it accurately.
Once the heating phase is complete, the mold is transferred from the heating rotation to a cooling rotation. This cooling phase solidifies the plastic against the mold's interior, typically taking less than an hour. The cooling process is carefully controlled to prevent warping and ensure a smooth, even finish. The temperature must be reduced to a level where the material is fully solidified, making the plastic water tank safe to handle.
In the image below, the green circle represents the finished plastic water tank undergoing the cooling stage.
The final step in the rotational molding process is to demold the finished product. Smaller compact plastic water tanks can be easily removed by hand, while larger bulkier ones are removed by a forklift or ejection equipment. A completed plastic water tank is one solid hollow plastic tank that is ready for attaching pipes, fittings, and other assembly components.
Blow molding involves two primary processes for manufacturing plastic water tanks: injection molding and extrusion molding. Like rotational molding, blow molding uses molds to shape the plastic water tanks. The key distinction between these methods lies in how the molten plastic polymer is introduced into the mold.
Before starting the blow molding process, there are two preliminary steps for both injection and extrusion blow molding. In the extrusion process, a parison—a tube of molten plastic—is created as the first step. In injection molding, a preform is produced instead.
In extrusion blow molding, the parison, which is extruded from the machine, is inflated within the mold to take on the shape of the plastic water tank.
For injection molding, the preform is created in an injection mold that is shaped by a core rod that is inside the preform and is shaped like the top of the plastic water tank much like a bottle neck. The preform is attached to a core rod with all of the polymer to shape the plastic water tank.
In the next stage of the extrusion blow molding process, the parison is filled with hot air, causing it to expand and conform to the shape of the mold cavity.
In the injection molding process, the next step involves transferring the preform from the injection mold to the mold for the plastic water tank. An air valve then inflates the preform, pushing it against the mold's interior to shape it into the final form of the plastic water tank.
The final step in both blow molding processes involves cooling the mold and the plastic water tank. Once the mold has cooled to room temperature, its halves are separated, and the finished plastic water tank is removed.
Both injection and extrusion blow molding are capable of producing plastic water tanks with detailed and precise features. However, they are more commonly recognized for their role in mass-producing other plastic products, such as soda bottles and various hollow plastic items.
The image below illustrates several steps involved in the blow molding process for creating a plastic water tank.
Plastics are chosen for manufacturing water tanks due to their versatility in being produced in various sizes and configurations, as well as the ability to incorporate different components and materials suited to the type of water stored. Essentially, water tanks are designed to store water for future use, with their specific design tailored to meet the needs of the stored water.
The size and design of a plastic water tank determine its suitability for different applications, including commercial use, household water storage, agricultural needs, irrigation, or manufacturing. Additionally, plastic water tanks can be utilized in reverse osmosis systems.
Double wall plastic water tanks feature a "wall-in-wall" construction, consisting of an inner tank enclosed by a secondary outer tank. This design offers additional containment, making it particularly effective for environmental safety by preventing potential leaks and pollution. Like other plastic water tanks, double wall or wall-in-wall tanks are available in a diverse range of shapes and sizes.
Elliptical cradle plastic water tanks are designed to lower the tank's center of gravity, improving stability and visibility during transport. Their elliptical shape requires that the tank be fully supported by a cradle. These tanks feature translucent walls that provide visibility for level checking and gallon indicators. While primarily intended for transporting water, they are also suitable for holding fertilizers and compatible chemicals.
Horizontal plastic water tanks are designed for above-ground water storage and share similarities with vertical tanks. However, unlike vertical tanks, horizontal tanks come with either molded-in legs or detachable supports for stability. They are typically made from polyethylene, high-density polyethylene (HDPE), or cross-linked polyethylene (XLPE).
Open top plastic water tanks are designed for use in collection systems or as secondary containment for vertical tanks. Typically featuring tight-fitting tops, these tanks are constructed from HDPE or XLPE. In addition to water collection and storage, they serve various industrial purposes, such as dipping, chroming, batching, and mixing. They are commonly utilized in industrial and manufacturing settings.
There is a specialized type of open top plastic water tank with a cone-shaped bottom, which facilitates easy drainage and emptying. These tanks often have translucent walls that allow for easy monitoring of the contents and may include gallon indicators for precise measurement.
Rectangular plastic water tanks are designed for versatile installation and storage in various positions and locations. Constructed from LLDPE or low-density polyethylene with UV inhibitors, these tanks meet standards for use with food and beverages.
They often feature translucent walls for easy monitoring of the tank’s contents. Like all plastic water tanks, rectangular tanks are made as single, seamless units without joints or seams.
Stackable plastic water tanks are ideal for situations with limited space due to their ability to be stacked vertically. Their freestanding design allows for placement on top of one another. Similar to horizontal tanks, stackable tanks feature robust, built-in legs for stability. Available in sizes ranging from 30 gallons to over 300 gallons, these tanks often come with tote leg stands to ensure safety and support.
Underground plastic water tanks are specifically designed for storing large volumes of water or potable water and are intended for underground installation. Constructed from UV-stabilized polyethylene, a food-grade polymer, these tanks feature ribbing to increase their durability. They come in capacities ranging from 260 gallons to over 2,000 gallons.
Vertical plastic water tanks are intended for above-ground water storage and are typically available in green or black to reduce sunlight exposure. These tanks are used for a variety of purposes, including rainwater collection, emergency water supply, farm irrigation, and fire protection.
Plastic water tanks offer a multitude of applications beyond their primary function of water storage. Their versatility stems from their adaptability and the ability to be customized in terms of shape, design, and engineering to suit various needs.
This flexibility is largely due to the materials used in their production, which can be mixed, blended, and chemically modified to improve their durability and performance.
Reverse osmosis is a water purification method that necessitates a storage tank for holding the processed water. Plastic water tanks are well-suited for this purpose due to their durability, flexibility, and ease of installation. They can be positioned in various locations to accommodate the requirements of the purification process.
A cistern is an underground plastic water storage tank designed for safely storing potable water. Typically, they have capacities ranging from several thousand gallons for agricultural purposes to hundreds of gallons for residential use. Cisterns feature ribbed sides for added structural support and are never fully drained to avoid warping from surrounding soil pressure.
Doorway plastic water tanks are compact, freestanding tanks designed for water storage in areas with limited space. They can be positioned in standard doorways without requiring additional supports, legs, or bands. Constructed from polyethylene and compliant with FDA standards, these tanks feature a seamless design that resists cracking, punctures, and weather-related damage.
Truck bed plastic water tanks are engineered to fit snugly in the bed of a truck, with specific allowances for wheel wells. These tanks are designed to meet FDA standards for transporting potable water. They may also feature translucent walls and gallon indicators for easy monitoring.
Plastic rainwater tanks are commonly used in urban settings as a primary water source. Manufactured through rotational or injection blow molding, these tanks come in sizes ranging from a few hundred gallons to over 10,000 gallons. Over the years, plastic rainwater tanks have increasingly replaced traditional galvanized steel tanks due to their lower cost and greater durability.
The seamless design of plastic water tanks makes them resistant to cracks, punctures, and leaks. They offer exceptional impact resistance and are immune to corrosion and rust. Additionally, plastic rainwater tanks have a long lifespan and are both easy to install and handle.
Plastic water tanks that are designed to be lifted by a forklift are made for light duty water transport. Their base is designed to accommodate forklift forks and have a rectangular metal frame for extra support. Vertical forklift plastic water tanks have a poly base with space for forklift forks. In some designs, their walls are translucent and have gallon markers.
Marine plastic water tanks are designed to store fresh drinking water on boats and withstand the challenging conditions of sea travel.
These tanks can be installed on any boat, regardless of its size, offering flexibility and adaptability for various vessels.
One of the key advantages of marine plastic water tanks is their durability and long lifespan, which is highly valued by boat owners. They are available in a broad range of sizes, from a few gallons to several hundred gallons, catering to different needs and capacities.
Wastewater septic plastic tanks serve various applications, from recreational vehicles and boats to underground tanks for homes and commercial buildings. They come in a range of capacities, from a few gallons for smaller installations to over 1,000 gallons for larger systems.
Septic tanks are specifically designed for their intended purpose and should not be repurposed. Made from polyethylene resins, these tanks are resistant to cracks and punctures, offering a cost-effective solution for both purchase and installation. For added convenience, dual and double compartment versions are available, reducing the frequency of required maintenance by minimizing the number of times the tank needs to be emptied.
The eight uses for plastic water tanks listed above represent just a fraction of their many applications. These tanks are exceptionally versatile and are employed in a wide range of scenarios that demand durable, long-lasting, and resilient performance.
Two polymers are predominantly used in the production of plastic water tanks: polypropylene and polyethylene. The exceptional properties and characteristics of these plastics make them ideal for withstanding the pressures, stresses, and potential damage that water tanks may encounter during use.
While polypropylene and polyethylene share similar properties, polyethylene itself comes in various forms. These different types of polyethylene allow for adaptation of the plastic material to meet specific needs, types, and properties of plastic water tanks.
Polypropylene is a thermoplastic addition polymer made from combining propylene monomers. It was introduced in 1951 and developed quickly into a material for the production of commercial products. The major use for polypropylene is for the production of packaging materials.
As a low-density plastic, polypropylene is lightweight, making it an attractive option for manufacturers and producers. Although it has limited structural use, polypropylene is well-suited for the production of plastic water tanks. Additionally, polypropylene can be copolymerized to produce other plastics, such as polyethylene.
Key characteristics of polypropylene include its resistance to chemicals, elasticity within a certain range, resistance to fatigue, and the ability to be made transparent.
Polyethylene comes in five distinct types: Polyethylene (PE), High-Density Linear Polyethylene (HDLPE), Cross-Linked High-Density Polyethylene (XLPE), Ultra High Molecular Weight Polyethylene (UHMW), and Linear Low-Density Polyethylene (LLDPE).
Low-Density Polyethylene (LDPE) – LDPE is known for its flexible properties, making it suitable for products like shopping bags and plastic films. It exhibits high ductility but has relatively low tensile strength, which causes it to stretch under stress. This characteristic limits its application in manufacturing plastic water tanks.
Linear Low-Density Polyethylene (LLDPE) – While similar to LDPE in terms of usage, LLDPE offers enhanced strength and durability. Its superior toughness makes it a good choice for product storage and protection. However, it is still not commonly used for plastic water tanks due to its limitations.
High-Density Polyethylene (HDPE) – HDPE stands out as the strongest among the polyethylene types. It features a high melting point, which maintains its rigidity under elevated temperatures. HDPE's ability to be easily molded when melted makes it suitable for plastic water tanks. Additionally, it resists mold, mildew, decay, and harsh environmental conditions.
Cross-Linked Polyethylene (XLPE) – XLPE is a variant of HDPE with polymer chains linked through heat, chemicals, or radiation. This process enhances its physical attributes, including stress crack resistance, toughness, and chemical resilience. These improved properties contribute to the structural integrity of plastic water tanks made from XLPE.
Ultra High Molecular Weight Polyethylene (UHMW) – UHMW is a highly dense form of polyethylene known for its exceptional strength, light weight, and durability. It excels in resisting abrasion, impact, and wear, and has a very high melting point, which makes it suitable for high-temperature applications. UHMW’s properties are comparable to HDPE but with added resistance to concentrated acids, alkalis, corrosive chemicals, and organic solvents.
UHMW's strength is evident in its use for lining bulletproof vests. Its characteristics, while similar to HDPE, provide enhanced protection against a wider range of harsh substances.
Polyvinylidene fluoride (PVDF) is created through the polymerization of difluoroethylene. It exhibits good tensile strength but has relatively low impact resistance. PVDF is both non-flammable and self-extinguishing. It offers partial resistance to UV light and has a low melting point, making it resistant to corrosion.
Due to its durability against corrosion, absorption, weathering, and degradation, PVDF is widely used in various industrial applications. It can be easily molded and processed, showcasing properties that surpass those of PVC, polyethylene, and steel.
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