This article presents all the information you need to know about plastic coatings and dip molded plastics. Read further and learn more about:
- An overview of plastic coatings and dip molded plastics
- Different polymers used
- Production processes
- Machinery used in dip molding and dip coating processes
- And much more…
Chapter 1: Overview of Plastic Coatings and Dip Molded Plastics
Plastic coating is the application of liquid polymers or plastic on the surface of a workpiece by dipping or immersion. The result is a thick plastic finish for protective and decorative purposes. This gives the material additional resistance against scratches, wear, corrosion, and external elements. This makes the metal piece more durable, and obtain a longer service life. It also offers convenience and protection to the end-user by providing surfaces for gripping and insulation.
Plastic coating is usually seen in hand tools, handles, and grips. A typical example of a hand tool with plastic coating is pliers. With the plastic coating applied on the handle, compression or bending of a rigid material is convenient to the user and more efficient while preserving the hand tool itself. Other applications benefiting from its rigidity are in shopping carts, baskets, forceps, covers, caps, plugs, and much more.
Plastic coatings are also known to be heat and electrical insulators. These can be also seen in many hand tools serving as additional protection when handling hot objects such as tongs and spatulas. Plastic coatings also serve as insulation in electrical and electronic components such as wires, cables, digital meter probes, and so forth.
The process of casting a plastic material into a pre-existing metal piece is called dip molding. The metal that is coated acts as the mold where the plastic or polymer adheres to during the process. The portion (or sometimes the whole piece) to be dipped or immersed is treated and preheated before lowering it directly into the liquid polymer. The liquid polymer adheres to the metal and hardens upon cooling.
The process of dip molding is similar to dip coating; except that dip molding has an additional demolding or unloading step. Dip molding creates single, hollow, and double-walled parts. This eliminates the need for additional downstream processes such as trimming and deflashing. By having less subtractive secondary operations, the process saves raw materials. Some of the dip molded plastic products are latex gloves, fashion and costume accessories, cups, plastic closures, and recreational equipment parts.
Chapter 2: Different Coating and Molding Polymers
The most common types of polymers used as coating materials are plastisol, latex, neoprene, polyurethane, and epoxy. A notable desired characteristic of a polymer is that the polymer mixture must be readily available in liquid form at room temperature without the need for additional processing. Also, before the liquid polymer is considered for dip coating and dip molding processes, it must be viscous enough to be able to resist flowing off the surface of the mold; hence, the polymer settles on the surface until it is cured.
Plastisol is the most commonly used polymer in dip molding and dip coating processes. It is composed of fine polyvinyl chloride (PVC) resins suspended in a liquid plasticizer. When exposed to heat, it solidifies into a soft, flexible, and rubber-like material upon cooling. Plastisol coating is known for its toughness, excellent corrosion, and impact resistance. It has high dielectric strength which makes it suitable for electrical applications. Colorants are added to customize the finish of the end product.
Latex is an emulsion of very small polymer particles (30-40% of which are rubber particles). It is the raw material for rubber production, which may be either natural or synthetic. This polymer is readily available and non-toxic. However, according to some studies, there were allergic reactions developed by some individuals once it degrades into powder, making it less popular than it was before.
Neoprene is produced by polymerization of chloroprene and is a substitute for latex. It is known for its chemical resistance and flexibility.
Polyurethane is composed of urethane (or ethyl carbamate) groups joined by carbamate links. It is known for its flexibility and high resiliency to deformation.
Epoxy is a thermosetting polymer that forms high-strength, chemical and thermal resistant coatings once the polymeric chains have been cross-linked.
Chapter 3: Production Processes
As mentioned in the previous chapters, dip coating and dip molding have the same operating concept. In this chapter, the procedures in both processes will be discussed.
Pre-treatment steps are important to properly apply the plastic coating to the metal substrate. These steps are critical for both processes.
The pre-treatment of a dip coating process is more delicate compared to dip molding processes since the polymer is intended to be fixed in the metal permanently to be covered which acts as the mold. The steps are enumerated as follows:
Removal of surface impurities
Impurities interfere with the adhesion of the polymer to the mold or part. It acts as weak regions where damage can initiate and propagate.
Oils and greases present on the surface of the metal substrate act as impurities that prevent proper adhesion and deposition of the plastic coating and decrease water resistance. Oils and greases are removed by means of alkali washes, acidic washes, or thermal degreasing.
For coating previously coated parts, the old coat is thoroughly removed before the new coat is applied.
Modification of substrate properties
Aside from cleaning the surface, additional properties can be imparted to further produce desirable characteristics. These characteristics may be helpful in the dip coating process itself or in the end-use of the product.
Phosphating (or phosphate conversion) is the process of applying a thin phosphate layer before plastic coating. The phosphate layer gives additional corrosion resistance to the substrate for cases where damage to the plastic coating is inevitable. Common phosphate layers used in dip coating are zinc phosphate, iron phosphate, and tricationic phosphate.
Shot peening is done by striking a surface with round particles inducing cold working. This introduces compressive residual stress on the surface of the substrate to strengthen and relieve any residual stress. Residual stress causes microcracks that may be induced in the proceeding processes.
Blasting modifies the surface of the substrate by inducing microscopic holes. This increases the surface area on which the primer, undercoat, and plastic coating can stick to. Common types of blasting processes used are sand, metal grit, glass bead blast, plastic bead blasting, and so on.
De-embrittlement is a heat treatment process that removes the hydrogen diffused into the metal substrate which increases the risk of brittle fracture under stress. The diffused hydrogen is introduced during the preceding pre-treatment steps such as acidic washes and phosphate application.
Improvement of plastic coating quality
The following are applied on the surface of the substrate to improve coating quality:
Primers serve as a preparatory coating layer which increases the adhesion between the substrate and plastic coating. It also gives additional protection to the substrate being coated.
Undercoats are additional coatings that give special characteristics to the finished part, such as UV resistance and scratch resistance. Undercoats are generally not used on their own and typically serve their function when coupled with the main plastic coating.
For the dip molding process, a layer of mold release agent is applied or fixed on the surface of the mold to aid in removing the molded part. Silicone and permanent polytetrafluoroethylene (PTFE) are the most widely used mold release agents.
Once the pre-treatment steps are done, the mold is dried to remove moisture. Retained moisture causes expansion brought by heat introduced in the succeeding steps. This results in void or bubble formation.
The mold is heated in an oven to a predetermined temperature and time. The heating temperature is one of the parameters which determines the coating thickness of the part. The quality of heat distribution depends on the design of the mold and airflow within the oven. It is important to heat the mold uniformly to ensure consistent coating thickness distribution within the material.
The heated mold is partially or fully immersed into the liquid polymer. The polymer attaches to the surface of the mold. The exterior dimensions of the mold assume the internal space of the part. Dwell time or the duration in which the mold is submerged in the liquid polymer, is also one of the parameters which determine the final coating thickness of the part. Longer dwell times result in thicker coatings produced.
The rate of immersing and withdrawing the mold is also a critical parameter to consider, depending on the properties of the liquid polymer to be used. These rates are determined by the manufacturers during the optimization phase. As a rule of thumb, these rates must be slow in order to control the flow of the liquid polymer, hence obtaining a smooth finish. Withdrawing the mold too quickly after the dwell time will result in surface irregularities. However, if the mold is immersed and withdrawn too slowly, the resulting coat will be too thick.
More advanced factories use a fluidized bed of fine polymer powder to replace the conventional liquid polymer solution. The concept is also similar to the conventional one; the fine polymer powder melts and adheres on the heated surface of the mold once they become in contact.
The excess liquid polymer is allowed to drain from the surface of the mold. Multiple dipping steps may be performed to achieve a desired thicker coating, or a special coating may be applied.
The coating is then cured in an oven to set the polymer more thoroughly and to completely evaporate the excess moisture, solvents, and additives. In this step, the final mechanical properties of the polymer such as rigidity and flexibility are acquired. In thermosetting polymers, the curing step allows the polymeric chains to be completely cross-linked.
The cured coating is cooled by means of dipping in a water tank, wherein the water temperature is between 50 - 600C, or by means of forced or natural air convection. The rate of cooling is not considered a critical parameter compared to other types of molding.
Unloading and Demolding:
The finished parts are removed from the frame. In the case of a dip molding operation, the plastic coating is removed from the mold manually or by mechanical means. This separates a single part from its batch. The result is a finished plastic component or part.
The dip molded plastic or the plastic coating may undergo additional finishing steps such as notching, punching, printing, and decorating.
Chapter 4: Equipment
Due to the similarities in the concept of dip coating and dip molding processes, it shares almost the same types of machinery. In a dip coating or dip molding operation, several machines and components are used to produce the dip molded plastic. The following are the equipment and apparatuses involved in an industrial dip coating and dip molding process set-up.
This is where the mandrel or the metal to be coated is heated in preparation for the dipping process. The preheat oven uses blowers or fans to assist in the forced convection (hot air circulation) inside the chamber.
The mold tool used in a dip molding operation is called a mandrel. It is a male mold, custom-made tool containing the three-dimensional form of the part to be molded. The external figure of the mandrel will give the interior space of the finished part. As mentioned, in the case of a dip coating operation, the piece of metal to be coated also acts as the mold or mandrel. The mandrel can be fabricated from a solid metal block that is shaped by machining, cast aluminum from a wood pattern, or formed sheet aluminum.
The mandrels are transported along the process, dipped into a tank of polymer solution. These are rested before curing by means of a carrier frame. The frame contains multiple mandrels and may be attached to a mechanical arm digitally controlled by computer software to improve molding cycle time.
This is where the polymer solution is contained and the mandrels are dipped. The solution bath is often agitated to ensure uniform temperature and concentration. De-aeration equipment is used to remove air and moisture from the mixture. Before the solution is transferred to the dipping tank, an offline tank equipped with a mixer is usually utilized to blend the resin, additives, and colorant.
This is where heat treatment after molding takes place in order to settle the plastic coating. Similar to a pre-heat oven, it also utilizes forced convection to circulate hot air inside the chamber.
Several set-up configurations involving the same set of equipment were made in order to improve production efficiency. Some of them are:
Multiple mechanical arms containing the carrier frames are attached on a rotating wheel, like a carousel. Each arm is indexed as the wheel slowly rotates while the mandrels are submerged into varying depths.
The frames containing the mandrels are transported throughout the process using a precision conveyor. This allows continuous production mode of the dip coating or dip molding process. The pre-heat and cure ovens are placed on opposite sides of the polymer solution tank. Each oven has an entrance and exit doors to allow movement of the frames. Additional dipping tanks for special coatings may be added to the set-up.
Chapter 5: Advantages and Disadvantages
Dip coating and dip molding are advantageous to some manufacturers due to the simplicity of its concept and flexibility. Below are some benefits of utilizing these methods:
Gives accurate internal dimensions
Since no shrinkage is developed during these processes, the desired internal dimension can be obtained accurately. As mentioned earlier, the rate of cooling is not a critical parameter to be controlled compared to other types of molding.
Produces durable parts
Dip coating and dip molding readily produce seamless, double-walled parts. The seams resulting from joining two components produce stress points which significantly decrease the durability of the part.
Limited restrictions on product size and design
Through dip plastic casting, it is possible to manufacture large plastic components through dip molding by enclosing large metal pieces. If a similar large plastic were to be produced in an injection or blow molding process, it would require large tooling; hence, becomes an expensive operation. The only limitation in dip molding is the size of the tool, the capability of the pre-heat and cure ovens, and the capacity of the polymer solution tank.
Dip coating and dip molding also have the ability to form complicated designs with severe undercuts and angles.
Ease of modification and variability of product design
The designer can readily put additional details on the end product since the tooling material is easy to modify. Varying coating materials and thickness can be produced using the same tooling depending on how the polymer solution is formulated.
Low operational and investment costs
Dip coating and dip molding are ideal for short orders and laboratory-scale production because the set of equipment involved in these processes is simple, unlike in injection and blow molding which require sophisticated machinery and a large amount of floor space. Also, the tooling for dip molding is inexpensive since it does not operate on high pressures.
Minimal wastage of polymer material
In dip coating and dip molding, the excess solution drips back to the dipping tank, allowing it to be reused. In other types of molding, excess polymer material comes in the form of cut-outs and runners which requires additional processing in order to be recycled. Dip coating is more advantageous than spray coating. Compared to the spray method, some of the spray coatings will be transferred to the surrounding walls of the coating chamber or equipment, thus being wasted.
However, there are some limitations in using these processes which other molding methods are advantageous. The following are the disadvantages:
Difficulty in obtaining accurate external dimensions
Though the internal dimensions produced are accurate, it is difficult to obtain a precise coating thickness since it is heavily dependent on a number of variables: dwell time, the temperature of the tooling, rate of immersion and withdrawal of the tooling in the polymer solution, and the properties of the polymer solution. It also makes a good coating thickness distribution throughout the part difficult to achieve.
Dip molding is a slow process
Dip molding is a slow process because it requires undergoing long heating, dipping, and cooling cycles.
- Plastic coating is the application of liquid polymers or plastic on the surface of a workpiece by dipping or immersion. The result is a plastic layer for protective, convenience, and decorative purposes. It increases the piece’s durability, strength, corrosion, and chemical resistance. It also gives heat and electrical insulation.
- The main desirable property of polymers to be used in a dip coating and dip molding process is high viscosity which enables the polymer to resist flowing off the tool’s surface. Also, it must be readily available in liquid form. Commonly used polymers are Plastisol, Latex, Neoprene, Polyurethane, and Epoxy.
- Pre-treatment steps in a dip coating process are categorized as follows: (1) removal of surface impurities (degreasing and stripping); (2) modification of substrate properties (phosphating, shot peening, blasting, and de-embrittlement); and (3) improvement of coating quality (addition of primers and undercoats). Pre-treatment in a dip molding process requires the use of mold release agents.
- Production process steps in a dip coating and dip molding are pre-heating of the tool, dipping in a polymer solution, draining excess solution, curing, cooling of the part and tool, and unloading.
- Coating thickness is dependent on dwell time, the temperature of the tool, rate of immersion and withdrawal of the tooling in the polymer solution, and physical properties of the polymer solution.
- Advantages of dip coating and dip molding over other molding methods are: (1) gives accurate internal dimensions; (2) produces seamless and double-walled parts, hence increasing durability; (3) limited restrictions on product size and design, and flexibility in tooling modification; (4) low operational and investment cost; and (5) minimal polymer wastage.
- The disadvantages of dip coating and dip molding are difficulty in obtaining accurate coating thickness and long cycle times.