Avoiding Powder Coating Quality Issues - Komaspec

Powder coating is one of the most common for sheet metal fabricated parts, and for good reason. It can be used to improve the aesthetics of a part, it can have excellent corrosion, impact, abrasion and chemical resistance, come in a variety of colors and finishes and is relatively low cost.

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The problem is that it is a demanding and finicky process as well. Poor pretreatment, poor process control, incorrect material selection, and a wide number of other factors can create serious problems from the aesthetic (orange peel, scratching, coloration) to the functional (peeling, cracking or chipping).

This won’t necessarily always even be visible at the production stage, and no one wants ten thousand parts in the market which start peeling after six months. Getting the process and the material right is a better idea than to try to fix the issue later.

In this article, we’ll look at how powder coating finishes work and how problems can be diagnosed and troubleshot.The exact properties a powder coating finish provides and its effectiveness depend on a number of factors related to:

• The choice of powder
• Pre-treatment processes
• The application method
• The curing process

Often, you can avoid problems by choosing the right powder and making the right decisions about pre-treatment processes. However, it’s also important to be aware of what happens in the application and curing stages as well and how this can impact the quality of a finish.See how car wheels are powder coated in this video.

Fig. 1: Powder Coated Part

A clean and properly prepared surface is vital for powder coating to be successful. A poorly treated surface will never result in a good powder coating finish. With a well-prepared surface, great finishes can be achieved.

Preparing a part for powder coating is a multi-step process, but it normally consists of the following key steps:

1. Surface Cleaning: The first and most critical step in preparing a surface for powder coating is cleaning. This step removes any contaminants such as oil, dirt, or other debris that could interfere with the coating's adhesion. Cleaning methods can vary depending on the type of contaminant and may include rinsing, washing, wiping, chemical cleaning, or blasting.
2. Pre-treatment: After cleaning, pre-treatment processes such as phosphating or etching are often employed. These processes create a more chemically reactive surface, which improves the adhesion of the powder coating, resulting in a more durable finish. The specific pre-treatment method used will depend on the material of the part and the requirements of the finished product.
3. Rinsing: Rinsing is typically performed after each cleaning or pre-treatment step to remove any residual chemicals or debris that could affect the coating process.
4. Conditioning: Conditioning the surface helps to prepare it further by neutralizing any residues and promoting uniformity in the coating application. This step is especially important for complex or highly reactive metals.
5. Phosphating: In many cases, phosphating is a crucial step that applies a phosphate coating to the metal surface, enhancing corrosion resistance and serving as a bonding layer for the powder coating.
6. Neutralizing Rinse (Optional): An additional neutralizing rinse may be employed to remove any acidic or alkaline residues from previous steps, ensuring that the surface is completely ready for powder coating.
7. Drying: Proper drying is essential to remove any moisture that could interfere with the powder coating application. The part must be thoroughly dried to prevent any defects in the final finish.
8. Applying Lubricants or Sealers: In some cases, lubricants or sealers are applied to enhance the performance of the powder coating, especially in parts that will be exposed to harsh environments or require a particularly smooth finish.

The exact steps taken should be tailored to match the part in question. Sometimes, problems occur when pre-finishing is not tailored to suit the part being powder coated. It’s a good idea to work with your manufacturer to determine the best set of pre-treatment steps for your production order.

To reiterate, pre-treatment is an important process that needs careful planning. It can also be lengthy, and your manufacturer should be prepared to take all steps necessary. A thorough pre-treatment process can include anywhere from 8 to 12 steps.

The main factors that determine the correct choice of pre-treatment process are:

• The type of metal being finished
• Processes the part will undergo before powder coating, such as welding
• How the part will need to perform in use
• The environment the part will be used in
• Other considerations, such as the presence of sharp edges

Knowing which steps to take requires consideration of the part through its entire lifecycle and the processes it undergoes before powder coating. We’ll look at the different pre-treatment processes below to help you understand what happens in each process and where things might go wrong.

Fig. 2: Sand Blasting Sheet Metal Parts

Pre-treating a sheet metal part before powder coating usually starts with cleaning.

There are several ways in which cleaning can be carried out, and a sheet metal part may need to undergo more than one type of cleaning. Parts may also need to be recleaned after other pre-treatment processes are completed as well.

Shot Blasting

Shot blasting is used to clean the surface. It can also be used to alter the texture of the surface to encourage the powder coat to adhere better.

In powder coating, the surface of the part is blasted with sand, walnut shells, metal soda ash or another substance to clean off scale, corrosion or surface imperfections. The profile left on the surface by the blasting action helps the powder adhere to the surface.

A primer can also be added to a shot blasted metal part to further improve powder adhesion.

Things to be careful of:

• The correct shot medium should be chosen for the metal type being finished. Some heavier shots can cause damage to softer metals.
• Can remove existing metal coating if a part is being refinished
• Does not remove oils or contaminants on surface
• Can produce enough heat to warp aluminum

As well as helping to prepare for powder coating, shot blasting can also help by removing surface defects. Another thing to note is that shot blasting is a good option for parts that are too big for traditional chemical treatments.

Shot blasting is best used on durable metals such as aluminum, steel and titanium. It can also be used on softer metals, such as copper.  

Washing

Sheet metal parts are usually washed to remove soiling on the surface. To do this, the part is washed or dipped in water and the right type of detergent. The detergent should be specific to the fabrication process being used. Hot water or steam can also be used to break down oils on the surface.

What you should know:

• Can remove oils, waxes, polishes and other surface contaminants
• Process can clean difficult to reach spots or gaps in the part

Washing is used for all powder coated metal types.

Acid Treatment

If washing with water and detergent is not effective, acids can be used to dissolve oils and soiling from the surface of a sheet metal part. As well as this, chemicals can also be used to remove rust. Removing rust is vital for good powder adhesion.

It is important to make the right choice of acid for the type of metal being powder coated. The wrong acids can dissolve and weaken the metal, which will reduce the strength of the part and affect the adhesion of the powder coat.

What you should know:

• Very effective for removing rust and contaminants
• The wrong choice of acid can cause problems

Fig. 3: Micro-Polished Part

Pre-treatment is the most important part of the powder coating process, and it’s important that it’s planned and executed carefully.

Firstly, your manufacturer should consider what type of metal your part is made of and what processes it has already undergone. Following on from this, it’s important that they investigate how the part needs to perform in use, what kind of environment it will be used in and what kind of lifespan is expected of it. With this knowledge, they should select pre-treatment processes that match the part.

Expertise is also needed to ensure that the right chemicals and methods are used at the different stages of the cleaning process.

Process Main Benefit Main Considerations Shot Blasting Cleans and improves adhesion Process needs to match with the metal type. Does not remove oil Washing Removes oils, chemicals and soiling May not be effective for all oils, chemicals and soiling. Fast drying may be needed Acid Treatment Removes oils, chemicals and soiling Acid type needs to match metal type. Expertise required Phosphating/Conditioning Improves corrosion resistance and adhesion Phosphate needs to match metal type. Care required Rinsing Removes soiling Important to do at all necessary stages. Fast drying may be needed Drying Prevents rust and other forms of corrosion May need to be carried out quickly

Table 1: Metal Surface Preparation Processes: Benefits and Key Considerations

There is a range of considerations that need to be made when choosing the right primer and powder coating. Your fabricator should discuss the options with you before selecting products for your sheet metal parts. Often, trade-offs need to be made so that you end up with a powder formula that suits your part. You may need to sacrifice chemical resistance to obtain a finish with superior hardness, for example.

Powder coatings are generally classified as class A, B or C:

• Class A – these finishes are the costliest and are used for parts requiring both high performance standards and an aesthetically pleasing finish.
• Class B – these arethe middle option in terms of cost and performance
• Class C - these are the lowest cost option and is appropriate where a certain degree of defects can be allowed for. Also often used where a part doesn’t require high corrosion protection and isn’t visible to the end user.

First, we’ll look at primers and resin types. Then we’ll discuss the factors that influence the choice of primer and resin type.

Primers

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Before a powder coating is applied, the surface of a part may need to be primed, though it is not always necessary.

The type of metal, type of powder coating and the choice of primer all need to be considered together. This can be a complicated choice. It’s a good idea to work with your fabricator to determine the best choice for your part.

What you should know:

• Primers improve adhesion of the powder coating and also add corrosion and chip resistance
• An epoxy or zinc-epoxy primer is recommended when a part is exposed to a corrosive environment (e.g. weather, saltwater or high humidity)

Resin Types

Powders are available in several different resin types. The main types are polyester, epoxy epoxy-polyester.

What you should know:

• Polyester gives excellent durability and UV resistance, making it a good choice for outdoor parts
• Superdurable polyesters give superior durability and hold their color for up to 10 years or even more. They also have better gloss finishes and better corrosion resistance
• Epoxy creates a hard surface with good corrosion and impact resistance. It’s best used for indoor parts as UV exposure creates a chalky appearance.
• Epoxy-polyester blends the properties of both and is a popular, well-rounded finish option
• Fluoropolymers are a common choice for architectural products like windows and doors. They have good weatherability and corrosion resistance along with excellent color and gloss retention
• Urethanes provide a smooth surface as well as good chemical and corrosion resistance

The main factors to consider are resistance to UV light, corrosion, impact, abrasion, chemicals and staining.

UV Resistance (Weatherability)

Parts that are used outdoors are exposed to UV light. Yellowing can occur if the wrong powder coating is chosen, and the longevity of the finish may be reduced. UV light also affects the powder coating binder, which can impact the gloss and color of the finish

What you should know:

• Chalk, epoxies, and epoxy hybrids are not recommended for parts that will be outdoors
  • Epoxy has 1 to 3 months durability when outdoors (based on southern Florida conditions)
  • Epoxy - polyester hybrids have a 2 to 6 months durability when outdoors (based on southern Florida conditions)
• Polyester and acrylic powder coatings are a good choice for parts exposed to outdoor UV light
  • Standard polyester has 1 to 2 years durability when outdoors (based on Southern Florida conditions)
  • Superdurable polyester has 3 to 5 years durability when outdoors (based on Southern Florida conditions)

Corrosion Resistance

Parts can be exposed to corrosive conditions whether inside or outside. Your manufacturer needs to choose the powder coating that provides the correct corrosion resistance for your sheet metal part. The decision should be based on the corrosiveness of the environment it will be exposed to.

What you should know:

• Epoxies offer the highest corrosion resistance but degrade under UV exposure. They are often used for indoor parts
• Polyurethanes give good corrosion resistance and good UV resistance. They are suitable for saltwater or marine environments
• Polyester offers good corrosion resistance and is often used for gardening equipment
• Parts exposed to extremely corrosive conditions should receive two coats of powder coating. A combination of an epoxy primer and a UV resistant topcoat is a good option
• A single coat of powder coating will generally offer better corrosion resistance than a traditional liquid paint

Hardness, and Abrasion and Impact Resistance

Characteristics like hardness and abrasion resistance need to be considered in light of how the part will be used.

Your parts may require both good abrasion resistance and a surface with low friction if it is repeatedly in contact with other parts or surfaces, for example. Otherwise, flexibility may be a factor to consider if parts will be powder coated before additional forming or other parts are fastened.
What you should know:

• Epoxy and epoxy-polyester hybrids have excellent flexibility and impact resistance
• Polyester has excellent flexibility and impact resistance
• Acrylic and superdurable polyester can be brittle with little mechanical flexibility
• Abrasion issues can result from under curing (lack of curing time and low temperature)
• Impact resistance issues are generally a result of incorrect curing (incorrect curing time or temperature)
• Thicker applications often result in impact resistance issues

If your part requires exceptional durability, it may be a good idea to apply dual layers of powder coating.

Chemical and Stain Resistance

Some powder coatings resist chemical exposure better than others. Your manufacturer should consider whether or not your part will be exposed to oils, solvents, fuels, food or other compounds when in use and make the appropriate choice.

What you should know:

• Epoxies and polyurethanes offer the best chemical and stain resistance
• Fluoropolymers are an option for good chemical resistance but can be expensive
• Polyester offers good resistance to chemicals, some food products and some cleaning agents
• Acrylic is very resistant to chemicals especially gasoline, motor oils and hydraulic fluids

Resin Type Durability Corrosion Resistance UV Resistance Chemical/Stain Resistance Polyester Superdurable Polyester Epoxy Epoxy-Polyester Blends Good all round choice, although may not excel in some conditions Flouropolymers Urethane

Table 2: Comparison of Resin Types and Their Properties

Here are some of the common defects that crop up with powder coated sheet metal parts.

Poor Adhesion

Poor adhesion can occur as a result of poor pre-treatment, poor primer or powder choice or incorrect application or curing. You may need to re-evaluate the entire powder coating process in order to remedy this problem.

Fig. 7: Poor Adhesion

Orange Peel

This defect can be caused by the incorrect choice of powders. However, it is more often caused by poor powder application or the wrong choice of curing schedule. The powder type should be checked, and your manufacturer should also check their application method (including application gun settings) and make sure that the curing schedule is correct.

Fig. 8: Orange Peel Effect

Overly Thick

Overly thick powder coatings can crack or peel, and this is a problem. The usual causes of overly thick coatings are incorrect application procedures or overcharging of powder particles. As well as this, overheating during the curing process can cause excessive thickness. This should be remedied by checking the application procedure and curing temperature.

Overly Thin

Again, application procedures are likely to be at fault here. Another possible cause, however, could be damp powder. Application procedures and humidity levels in the application area should be checked.

For a more complete list of powder coating defects, and an explanation of the roots causes and troubleshooting behind these issues, feel free to review our more in-depth article here.

What Causes Peel-Off Problems With Powder Coating?

Peel off is usually caused by poor adhesion between the powder coating and the sheet metal part. This could be a result of a fault at any stage of the process, from pre-treatment right through to curing.  

How Does Incorrect Curing Cause Powder Coating Issues?

Incorrect curing causes powder coating issues because the reaction that occurs when the powder coating sets is not properly controlled. Common problems that result from this are an altered appearance or a brittle finish.

What Causes Pinholes in a Powder Coating?

The most common cause of pinholes in a powder coating is surface contamination. Typically, solvents and grease present on the surface cause pinholes to form by dissolving the powder coat. Better cleaning with detergent or cleaning with acid should be introduced in the pre-treatment stage to remedy this problem.

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