Omar Khalid
Plastic dip coating is a process that involves immersing a preheated metal part into liquid Plastisol to form a plastic coating. This coating can enhance the appearance of the part, improve its grip, and protect it from corrosion, chemicals, and abrasion. The plastic dip coating process typically involves four main steps: pre-treatment, pre-heating, immersion, and curing. During pre-treatment, the metal part is cleaned and a primer may be applied. The part is then preheated to a specific temperature to achieve the desired coating thickness. It is then immersed in the liquid Plastisol for a set time, allowing the Plastisol to fuse to the item and build the required thickness. Finally, the coated part is baked or cured at a temperature depending on the formula.
| Characteristics | Values |
|---|---|
| Definition | The method of attaching a plastic coating to an existing metal item is referred to as dip molding. |
| Process | Dip coating and dip molding share a similar operational principle. The metal component acts as a mold for the polymer. The component is initially treated and warmed before being submerged in a liquid polymer. |
| Pre-treatment | Pre-treatment is essential for effectively applying plastic coatings to metal substrates. This includes cleaning the surface and removing any existing coating layers. |
| Polymer | The liquid polymer must possess sufficient viscosity and appropriate surface tension to adhere evenly to the mold. Plastisol, a widely used polymer, consists of finely ground polyvinyl chloride (PVC) resins suspended in a liquid plasticizer. |
| Temperature | The preheated part is immersed in liquid Plastisol at a constant speed. The temperature determines the thickness of the coating. |
| Benefits | Plastic dip coating enhances the appearance of the part, provides a soft grip, and protects the part from corrosion, abrasion, chemicals, UV rays, and weather conditions. |
| Enhancements | Phosphating and shot peening can be used to enhance corrosion resistance and strengthen the substrate. |
| Colors and textures | The coating is available in various colors and textures, and additives can provide special characteristics like fungus resistance, dielectric strength, and flame retardancy. |
| Secondary processes | Secondary processes can include trimming, die cutting, printing, and assembly, enhancing durability and performance. |
| Durability | Dip coating improves the durability and longevity of the component coating, reducing the need for additional processes. |
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What You'll Learn
Dip coating vs dip moulding
The plastic dip coating process involves immersing a preheated part into a liquid polymer to form a plastic coating. This process can add protection and enhance the appearance of the part. It also improves the durability and longevity of the component.
Dip coating and dip moulding share a similar operational principle, but the final products are different. In the dip coating process, a part is first prepared and then dipped into a coating material to fuse and set the coating. This process is used to add new layers to parts such as door handles, hand grips on tools, and electrical insulation. It is also used to round off and soften sharp edges and corners, creating a smooth and attractive finish. The dip coating process is used in the medical, automotive, consumer goods, and electrical industries.
On the other hand, dip moulding is the process of dipping a metal form or tool into a liquid polymer to create an entirely new product. The metal form acts as a mould for the polymer, and the polymer adheres to the metal and solidifies as it cools. This process is used to create all-rubber components that do not require a rigid metal inner. Dip moulding is used in the medical field to create products such as nasal cannula tubes, stethoscope tubing, and enema tips.
Both processes are flexible and highly customisable, allowing for the thickness and hardness of the coating to be adjusted. They are also clean, safe, and widely used in industrial, business, and consumer products. However, dip moulding includes an additional phase for extracting the moulded part, which is not required in dip coating.
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Pre-treatment and cleaning
Firstly, impurities and contaminants must be removed from the metal surface. Oils, greases, and other residues can hinder the adhesion of the polymer, leading to weak spots and potential damage. These contaminants are typically eliminated through alkali or acidic washes, thermal degreasing, or high-pressure cleaning techniques. This thorough cleansing process ensures that the polymer can adhere evenly and securely to the metal substrate.
The pre-treatment process may also involve phosphating, also known as phosphate conversion. This step includes applying a thin layer of phosphate, such as zinc phosphate, iron phosphate, or tricationic phosphate, to the metal surface before dip coating. Phosphating enhances the substrate's corrosion resistance, providing additional protection in case the plastic coating sustains damage.
Another pre-treatment technique is shot peening, which involves bombarding the metal surface with spherical particles to induce cold working. This process creates compressive residual stress, strengthening the substrate and alleviating any pre-existing stress that could lead to microcracks. Blasting, a similar process, creates tiny cavities on the surface, increasing the surface area available for adhesion and enhancing the bond between the metal and the coating.
Additionally, pre-treatment may include de-embrittlement, a heat treatment process designed to eliminate hydrogen from the metal substrate. This step reduces the risk of brittle fractures under stress, further improving the durability of the coated component.
After these cleaning and treatment procedures, the metal substrate is carefully dried to eliminate any moisture. This step is crucial, as retained moisture can lead to expansion during subsequent heating steps, causing undesirable voids or bubbles in the finished product.
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Plastisol and other materials
Plastisol is a widely used polymer in both dip molding and dip coating applications. It consists of finely ground polyvinyl chloride (PVC) resins suspended in a liquid plasticizer, forming a stable and versatile compound. The plastic dip coating process involves immersing a preheated part into liquid Plastisol to form a plastic coating that can add protection and aesthetics to most metal parts. The coating is available in a variety of colours and textures, and special-purpose additives can be used to provide specific characteristics like fungus resistance, dielectric strength, and flame retardancy.
The plastic dip coating process involves four main steps: the part is pretreated (cleaned and primed if desired), preheated to a temperature that determines the thickness of the coating, immersed in liquid Plastisol for a set time to build the required thickness, and then baked/cured at the required temperature depending on the formula. The preheating and curing temperatures are determined based on the part's raw material, size, and thickness. The part is then cooled prior to any secondary processes, which can include trimming, die cutting, printing, and assembly.
Other materials used in dip coating include thermoplastic coatings, which are ground into a fine powder and kept in a steel box (fluid bed) open at the top and with a fine gauze across the bottom. Air is then introduced up through the gauze, making the powder particles rise and act like a box of fluid, hence the name "fluidised bed." Metal components are pretreated (usually degreased and shot-blasted), heated to between 250°C and 400°C, and then dipped into the fluid bed of plastic. The powder particles melt onto the metalwork, forming a smooth, continuous, and durable coating.
Phosphating, or phosphate conversion, is another technique used in dip coating, which involves applying a thin layer of phosphate before the plastic coating. This enhances the substrate's corrosion resistance. Common types of phosphate layers include zinc phosphate, iron phosphate, and tricationic phosphate. Shot peening is another process that involves bombarding a surface with spherical particles to induce cold working, creating compressive residual stress on the substrate's surface and alleviating any pre-existing residual stress.
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Benefits of dip coating
The benefits of dip coating are extensive, and this process has emerged as an innovative solution in the manufacturing world. Here are some key advantages:
Enhanced Protection and Durability
Dip coating provides a protective layer to tools, equipment, and other objects. For metal items, it creates a plastic or vinyl exterior, giving desirable properties like corrosion, abrasion, and wear resistance. It also enhances durability, making it resistant to sloughing off or dripping from the substrate. This is achieved through the use of high or low viscosity coatings.
Cost-Effectiveness and Efficiency
Dip coating is a cost-effective method, offering a simple, efficient, and economical process. It is cheap to set up and maintain, and any excess paint simply drips back into the dip tank to be reused, reducing waste. It is also versatile, suitable for a range of industries and applications, from medical devices to electronics, optics, and automotive.
Uniformity and Precision
This process achieves a uniform coating thickness, which is challenging with other methods. It fills in imperfections, creating a smooth, even finish. It can also be controlled precisely, with the ability to adjust variables like speed, curing times, and temperature, to achieve the desired finish.
Versatility and Adaptability
Dip coating is adaptable to complex shapes and intricate designs, making it ideal for coating geometrically challenging parts with nooks and crannies. It can also be formulated for different cure types, including thermal, UV, and infrared techniques, and can incorporate various surface effects for unique finishes.
Safety and Functionality
Liquid plastic dip coating enhances the safety and functionality of tools and equipment. It provides grip, insulation, and protection against corrosion, impact, and other environmental factors. It is also removable, allowing for flexibility in applications.
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Secondary processes
The dip coating process can be separated into five stages: immersion, start-up, deposition, drainage, and evaporation. During the immersion stage, the substrate is immersed in the solution of the coating material at a constant speed. This is followed by the start-up stage, where the substrate remains inside the solution for a period of time before being slowly pulled up during the deposition stage. The speed of withdrawal determines the thickness of the coating, with faster withdrawal resulting in a thicker coating. Excess liquid drains from the surface during the drainage stage, and finally, evaporation occurs, where the solvent evaporates and forms a thin layer.
There are several secondary processes involved in plastic dip coating that are crucial for achieving optimal product quality and performance. One important step is pre-treatment, which ensures proper adhesion of the polymer to the metal substrate. This includes cleaning the surface of any impurities, oils, or greases that may hinder the bonding process and create weak spots.
Another secondary process is priming, which is a pre-processing technique that enhances the adhesion of the plastic coating to the metal. Priming creates an incredibly tight bond between the vinyl and the metal substrate, ensuring that the PVC adheres more strongly to the metal than the cohesion of the vinyl itself.
Additionally, there are various surface treatments that can be applied to achieve specific desirable traits. Phosphating, or phosphate conversion, involves applying a thin layer of phosphate to enhance the substrate's corrosion resistance. Shot peening is another process where a surface is bombarded with spherical particles to induce cold working, creating compressive residual stress that strengthens the substrate and alleviates pre-existing stress.
The selection of polymers is also a critical secondary consideration in the plastic dip coating process. The choice of polymer depends on the desired durability, finish, and performance of the final product. Plastisol, a widely used polymer in dip coating, consists of finely ground polyvinyl chloride (PVC) resins suspended in a liquid plasticizer, resulting in a stable and versatile compound.
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Frequently asked questions
Plastic dip coating is a process that involves immersing a preheated part into liquid Plastisol to form a plastic coating. The Plastisol is made up of finely ground polyvinyl chloride (PVC) resins suspended in a liquid plasticizer. This process can add protection and aesthetics to most metal parts.
The process involves four main steps: pre-treatment and cleaning, preheating, immersion in liquid Plastisol, and baking/curing. The preheating temperature and immersion time determine the thickness of the coating.
Plastic dip coating can enhance the appearance of the part, provide a soft grip, and protect the part from corrosion, abrasion, chemicals, UV rays, and weather conditions. It can also add specific characteristics like fungus resistance, dielectric strength, and flame retardancy.
Plastisol is commonly used for dip coating and dip molding applications. Other materials include thermoplastic powders, fluidized bed PVC powder, nylon powder, and liquid vinyl. The selection of materials depends on the specific requirements and unique specifications of the component.
