Types of Metal Finishing

Chapter 1: Principle of Metal Finishing

This chapter will discuss what metal finishing is and the processes involved.

What is Metal Finishing?

Metal finishing describes various procedures or processes that can be as simple as buffering to adding a special type of coat to the substrate. Included in the description of metal finishing are a number of cleaning methods, polishing methods as well as other procedures which are meant to improve and enhance the surface of the specific metal product.

Electroplating is used for the finishing process. This process is when an electrical current is used to put metal ions right on the substrate. Although it may look different, the process of plating and finishing are often thought of as being similar.

The plating process is an electronic way of placing metal on a surface that is conductive and is one of the various finishing methods being applied today. The term finishing is very broad and points to a range of processes meant to improve and enhance the physical appearance of the metal product. These different kinds of metal products are available in homes and at work.

Taken underground in its original and raw form, metal is very crude, hard, and unattractive. To make the best use of metal, it needs to be polished, finished, and processed. These methods are done through various processes and procedures. The last stage of metal work is the finishing process.

During this process the metal product goes through different techniques to make it look attractive and much appealing. The process of metal finishing reduces wear greatly on metal products.

Metal finishing also maximizes its electrical conductivity, durability, chemical resistance, electrical resistance, and increases its vulcanization. Although the metal finishing process adds to the aesthetic look of the metal, it also helps in industrial scenarios like resistance to torque and soldering.

A major concern for all metals is their vulnerability to corrosion. The process of finishing greatly reduces chances of corrosion and also serves as a primer when paint is added. Additionally, thorough finishing can minimize cleaning with the aim of removing any defects or deformities. Industries that produce metals have many finishing methods they use to maximize the longevity and durability of their metal products. The finishing process is very important and an essential part of the last steps in making the best quality metal products.

Choosing a Metal Finishing Process or Method

Considering all the factors which are necessary for coming out with a manufacturing decision, it is very important to select a metal finishing procedure that meets the required standards of the end product. All of the metal finishing procedures apply a special treatment to the surface of the metal.

They also apply and make certain adjustments to the surface of the metals by means of a mechanical way. The appropriate procedure to use has to be considered very carefully as each has its advantages.

The process of finishing is the final step in producing the quality appearance and texture for a metal product that fulfills its design as well as giving coverage from rust and tarnishing whilst maximizing the metal product’s strength, thickness, durability, and hardness.

Determining the method of finishing to use can be very crucial to the value of the end product. The first consideration to be taken into account is the cost. The typical costs of the metal finishing process are water, type of coatings, energy expended, various consumables, labor, cleanup, and the amount of waste.

Aside from the costs associated with the task at hand, fixed cost of the maintenance and operation of the equipment being used.one other main factor to be taken into consideration is the type of metal that is to be to be fabricated, which can cover a wide range from stainless steel metals to different types of aluminum. In order to make the end result much more profitable, each of these elements has to be taken into consideration very carefully. Another factor or aspect of the cost decision to note is the amount of time that will be required to finish fabrication.

The process of electroplating can be too slow and consume time while the buffing and polishing can be done instantly. This crucial stage of the finishing process determines the ability to keep up with delivery deadlines and it greatly affects the supply chain.

Chapter 2: Types of Metal Finishing Processes

The process of metal finishing is a very crucial part of metal product making. Metal finishing consists of a variety of processes like metal plating, chemical finishing or coating, grinding, buffing, electroplating, sand blasting, etc.

Metal finishing processes can include anything from technical processes to buffers. The procedure or method to use when processing is dependent on the requirements of the end product. The metallic structure of the metal and its final use should also be considered.

Plating or Conversion Coatings

The process of plating involves the application of a metal to a substrate. A specific line is set apart to a specific plating and a specific base metal. A line of plating may be made to perform parts plating of steel by using a layer of copper and then applying a layer of chrome. Because plating is induced to the part, the dimensions of the part will alter to a direct relationship to the quantity of plating that is applied.

The process of plating can either be applied with the use of electrical current or without. Nickel plating needs an electric current to drive the nickel in order for it to plate the parts. Electroless nickel plating plates out in a natural manner though very slowly with no application of an electric current.

The process of conversion coating is a coating that is made on the surface of the metal. This is due to the reaction of the chemical with the metal surface. Some of the few examples of conversion coatings are, iridite on aluminum, chromate, phosphate, and black oxide.

There is a special category of conversion coating which is known as anodizing and is discussed briefly below. In conversion coating a part that has been coated will have dimension growth which is in proportion to the thickness of the conversion coating. This relationship is not direct like in plating.

The main reason for this is the conversion coating is taking in some of the substrate metal when forming the coating. The conversion coating grows into the part as well as expanding when the coating is forming. This is mainly because a conversion coating is getting into the part during its formation but is also of much greater volume as compared to the original metal.

Anodizing Process

The name or term anodizing is defined or usually described as the conversion coating that is applied to the aluminum metal. Anodizing gives a description of the conversion coating process that is applied to magnesium, titanium, niobium, or the tantalum parts in particular. A major distinguishing factor between anodizing and the general conversion coating processes is that anodizing is generated by both electric current and chemical conversion at the metal surface.

The current applied drives the layer anodizing to be able to form in a much thicker and faster way than it would by means of a chemical reaction only. One other interesting factor of anodizing is that the process leaves tiny microscopic pores in the coating. These pores can be filled with dyes to make a range of part colors.

Painting Process

The process of painting is applying a substrate of a liquid organic coating. These substrates can be wood, plastic, metal, ceramic, paper, foam, or metals. The paint being applied varies from solvent based to solids paints considering of course UV curing liquids.

As a result the paint can be a liquid because of numerous carriers like water or solvents to a two part epoxy paint. This paint is cured by cross linking than drying by evaporation of the carrier. Various spraying methods can be used to apply the paint and a few are described briefly below:

Electrophoretic Painting Process

The E-coat painting process is defined as a cross between plating and painting. This is a method where a metal part is dipped in a water based solution and that contains a paint emulsion. Afterwards there is an electric voltage introduced to the part. This causes condensation of the paint emulsion.

Wherever the liquid can reach the metal surface, a part can be painted inside and out. The voltage applied causes a limitation on the coating thickness. The areas with a high voltage become insulators as a coating is built enabling areas with low voltage to build up. Because the exterior is wholly insulated by the coating, the inside can be coated. A rinse tank then takes away all the residual emulsion from the part. It recycles it by ultrafiltration back to the paint tank.

Powder Coating Process

The process of powder coating is the same as the painting process except that the paint is a dry powder and not a liquid. The powder adheres to the parts because of electrostatic charging happening to powder and the grounding of the parts.

Substrates that can endure the heat of powder curing and those that can be grounded electrically in order to enhance attachment of the charged particle can be used. During the heat application is when the powder flows and performs curing. There are several benefits of powder coating over general paints and these are:

• There is powder recovery for reuse.
• There is no VOC that is generated meaning no VOC destruction is needed.
• This process can be more durable than paints.

However, there are also some drawbacks of powder coating over paints and these are that they can have much less leveling in comparison to paint and also because of the higher temperature needed. Curing is more energy intensive compared to paint drying.

Drying and Curing

Curing and drying operations are very energy intensive operations in the powder coating process. The drying and curing operations are bonded with convection ovens. Using convection heating is costly and very slow if the parts are heavy and large. This is because the evaporation or curing will be greatly dependent on the part temperature. The part must be hot enough for the part surface to dry or for curing of the powder coat. Large amounts of air volumes need to be exhausted and heated from the convection oven to dry or cure the parts effectively.

One other good side of heating using infrared is found in the powder coat curing. The continuous and rapid heating of the part surface will give better powder flow as well as minimum chance of dirt defects or dust. The reason being the little air flow that is available to deposit particles rather than convection heating.

A point to note, using infrared curing for the liquid paints has a much helpful effect because infrared light does not heat the paint but the part surface leading the paint to dry from the inside out. As often in convection curing, here the paint doesn’t trap water or solvent inside the paint or neither does it skin over, progressing to the paint surface.

Metal Grinding

Metal grinding processes finishes off all the rough edges, smooths welds, deburrs, creates very sharp edges, and can generate unique effects. Two common types of grinding are stationary or handheld grinding machines with industrial grinding wheels.

The main principle behind grinding is the use of attrition, friction, or compression in order to smoothen the metal surface. The smoothness of the surface that is finished is usually determined by the type of grinder being used. Numerous grinding methods can be adopted to come up with the shape, size, and features of the end product.

There is a wide range of methods of grinding like electrochemical, centerless, cylindrical, surface to mention only a few. It is crucial to decide the right method for the types of metal and end product when using these grinding methods.

Alkaline Cleaning

Alkaline cleaning is also a finishing process. Initially the alkaline cleaning tanks are put in line and then the bulk of the dirt load is taken. The main purpose of the tanks is to remove wax, grease, oils, particulates, and light oxides on the part surfaces.

With regards to the detergent additives in tanks, they could build up oil emulsions, surface oil, suspended solids and sludge forming at the tank bottom. Considering the acids, the chemicals for cleaning are taken in during the process of removing contaminants as well as preventing their redeposition.

Bath Chemistry Control

Initially, there is a need for a method or procedure in order to monitor the alkaline cleaning strength of the bath. It may be as complicated as moving a sample out for chemical verification or very simple as measuring the pH value. The cleaning supplier of the chemicals can either test certain procedures to monitor or provide the test kits or can correct the cleaning chemistry as it ages.

Surface Oils

To remove surface oils, a combination of surface sparging to a weir and using many oil skimmers available today can be used to remove the building up oil from the weir. Precaution should be taken on dead zones on the surface of the tank.

This is where oils build up and be redeposited as the parts move out of the tank. A well-constructed sparger will drive a layer of surface water across the tank as well as over the weir. Various methods can then be brought in such as disk skimmer, belt simmer, and concentrator vanes.

Removing Particulates

All the heavy particles which are found on the tank bottom can be removed by method of bag filtration or any other simple filtration procedure. The removal of heavy solids is very important especially when the cleaning tank consists of ultrasonic transducers on the tank bottom. A dirt layer on top of the ultrasonic transducers greatly minimizes the efficient action of the ultrasonic cleaning. The parts which are heavily soiled may need a dual filter system and this dual filter system caters for switching as the alternate side gets loaded, this allows the filter change-outs on the fly.

Emulsions and Suspended Solids

Emulsified oils and suspended solids are other oils that need to be considered. They are very hard to remove by normal filtration processes. A method that often breaks the oil emulsions is called ultrafiltration and afterward it removes the solids that are suspended without shaking the cleaning chemistry that is active. The ultrafiltration process type highly depends on the pH value of the cleaning chemistry and the bath’s temperature. Capable filters polymers on removing the emulsion oils and suspending solids but cannot endure high temperatures or extreme pH values. There are some commercially available systems of ultrafiltration which are capable of handling pH ranges from 0 to 14 and high temperatures of up to 158°F.

Sand Blasting

Sand blasting carries out many functions with one process. Blasting removes debris, dirt, and particles that accumulate on a surface during processing. Once the completion of the blasting process is up, no other cleaning is necessary.

This helps in saving time and money. Moreover, blasting has a speed that is more than multiple other methods by 75%. This increases efficiency as well as productivity. There are different types of abrasive materials utilized in the process of sand blasting, including silicon carbide, aluminum oxide, corn-cob grit, plastic abrasives, and glass beads.

Brushed Metal

Brushed metal provides friction to the work material diminishing all imperfections while it creates a textured grain that leaves a smooth exterior finish. Fine bristle brushes or an abrasive belt are utilized in this process leaving a surface that is non-reflective and dull.

The configuration and direction of the grain can be determined by the positioning of the abrasive and its type. Brushing metals removes some of their luster and gives them a distinctive look. Different brushed finishes can be produced by using stainless steel that is power driven, wire wheel brushes brush back sander heads, nylon discs, and different types of abrasive cloth.

Phosphating Process

The phosphating process is typically classified as a conversion coating process. This is because the process includes the removal of metal as part of the reaction. It differentiates from processes such as black oxide or anodizing in that the phosphate coating reaction is a precipitation reaction. The last or final surface is a layer of crystals of phosphate sticking to the metal surface.

For coatings that use paint and powder, there are two primary functions of a phosphate. Initially, the applied coating offers better paint and powder coating adhesion. This is due to the organic behavior of phosphate crystals that anchor sites and secondly, the phosphate layer behaves like a barrier for corrosion in case there is scratching of the organic coating. When testing for rust creep, the rust creep is much minimized when there is phosphate under the powder coat layer or paint layer in comparison to no conversion layer under on organic coating.

The Phosphate can be used as a standalone coating for such purposes like lubricity in parts making. Common phosphating chemistries are the zinc phosphate, iron phosphate, and the manganese phosphate. The other phosphating chemistries like Plaforizing are not traditional in their chemistry and in their application because they are single step .they are organo-phosphate meaning they can react with both the metal surface and the organic contaminants

Part Drying

The various processes under part drying include:

Water Spotting

An easy way to avoid water spotting is using deionized water in the final rinse process. Water spots are usually caused by minerals in normal water. They can be removed by removing the minerals. Another method that will reduce water spots is the air blow off procedure aimed at removing the droplets of water before they have time to dry. This procedure is dependent on part orientation and geometry on the racks. If the air fails to reach specific areas on the parts because of the geometry or if parts block one another then spotting can still happen in such areas.

Drying Process

A drying parts process should conserve energy as much as possible. High pressure blowers should be used in place of compressed air because the cost of electricity is low for blowers compared to compressors. By adding a chunk, the risk of having air carried contaminants on the parts is much higher for compressed air as compared to blowers.

Hot Final Rinse

Hot final rinse is one of the simplest processes of drying parts using deionized water. If the part geometry doesn’t create water puddles the parts automatically will have to flash dry. The hot final rinse process needs energy to heat the water but fully removes the need for any hot air drying.

Air Drying

There are two air drying categories that can be used independently or in conjunction with each other.

High Velocity Air – its main purpose is to remove the water from the parts rather than dry it.

Heated Air–also termed convention drying and its main aim is to evaporate the water. completely from the parts. This means to effectively evaporate the water that the parts have to warm up in the heated air.

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Chapter 3: Applications and Advantages of Metal Finishing

This chapter will discuss the applications and benefits of metal finishing.

Applications of Metal Finishing

There are numerous applications of metal finished products in many industries. In manufacturing, finished metals are used for materials that have a high sensitivity to oxidation. The coatings applied help prevent any chemical reaction from happening.

In the automotive industry, metal finished products are used to protect the internal part of the combustion engine from high temperatures. In the aerospace industry metal finished products are also used to prevent corrosion and wear inside the engine blocks.

For industrial uses finished metal is also of great significance since machines are much prone to corrosion, wear and tear as well as other factors that may lead them to deformation quickly.

There is a vast range of household applications for metal furnished products as they play an important role as decorative material. Metal finishing has such a big impact that this process is often used in preserving the life of metals.

Advantages of Metal Finishing

There are numerous advantages of using finished metals such as resistance to wear and tear. Another factor is that finishing can aid in the adjustment of mechanical properties. The metal finished products are much less likely to corrode. There is also preservation of the aesthetic properties.

Various types of materials come with many different microstructures. Metal finished products can work with low and high atmospheric pressure. Some have different scales of porosity through the thickness. The metal finished products also can be decorative and endure high temperatures. The processes of making them are cost effective. They are also used for biomedical applications. Another benefit is they have a high adhesion quality and are available in complex geometries.

Disadvantages of Metal Finishing

Metal finished products also come with some drawbacks. Each of the metal finished products has its own operation range, because of the mechanical properties of the different coating or finishing material that is used. As a result, the efficiency differs from one metal product to the other in areas with different pressure and temperature conditions. The industrial sector requires very high vacuum or corrosion resistance metals with a high melting point and great tensile strength.

Conclusion

The metal finishing products come in different shapes and sizes. They have impacted the industrial sector where much corrosion, wear and tear of metals do happen. Metal finishing applications are so broad with the available technology. Machines can operate smoothly for quite a long period of time because of these finishing techniques.

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