Web Coating Blog

Most Prevalent Web Coating Defects

Most Prevalent Web Coating Defects

I am in the process of preparing a series of articles on web coating process. They will appear in the newly launched Converting Quarterly. The series would deal with defects and each article would go into some depth on a specific defect. The first one is streaks and will discuss various causes, show relevant images and discuss cure and prevention.

The initial step in this effort has been to determine the most common defects that are encountered in the web coating production process. The data was obtained by conducting a survey of users and reviewing blog queries and literature articles.

The following tables summarizes the most common coating defects that are encountered:
Most Common defects

rank Percent
total defects

Streaks 1 16.42%
Ribbing 2 7.46%
Bubbles 3 7.46%
Coating uniformity Md, Td, reproducibility 4 7.46%
Chatter 5 7.46%
Wetting failure dewetting 6 5.97%
Scratches 7 5.97%
Craters voids 8 5.97%
Fat edges 9 4.48%
Curl 10 4.48%
Base Contamination 11 2.99%
Edge crawl with emulsions 12 1.49%
Coating skips 13 1.49%
Dirt contamination 14 1.49%
Web break 15 1.49%
Waves 16 1.49%
Cracks 17 1.49%
Mottle 18 1.49%
Pinholes 19 1.49%
Uneven diffusion between layers 20 1.49%
wrinkles 21 1.49%
Scrapes (air bar) 22 1.49%
Misses 23 1.49%
Skip outs (to clean coating head) 24 1.49%
Orange peel 25 1.49%
Haze 26 1.49%
Distortion 27 1.49%

My conclusions from this data are:

• The results are consistent with my defect experience and those of colleagues.
• There are over 27 defect classes that are routinely encountered.
• An ongoing defect reduction and quality control effort is needed to minimize these defects.
• A good technical understanding of defect causes, mechanisms and process behavior is needed for a successful defect reduction program.
• A good economically efficient approach to eliminate defects is to focus on the top 1-3 defects.

The second step in this effort was to determine the effect that the coating method has on defect rate. In addition to defect type, the coating method for each defect was included in the original data set. The methods used for each defect was determined.

The following is the result from the Streak Method analysis:

RANK PERCENT DEFECT
EACH METHOD
SLOT DIE 1 50.00%
KNIFE OVER ROLL 2 16.67%
CASCADE 3 11.11%
ROLL 4 11.11%
CURTAIN 5 5.56%
MAYER ROD 6 5.56%

My conclusion is that streaks can occur in several methods; however, they are one of the major coating defect problems of slot die coating. Another factor is that since slot die is one of the major methods it should have more reported defects.

The results of the ribbing method analysis are:

RANK PERCENT DEFECT
EACH METHOD
SLOT DIE 1 42.86%
ROLL 2 28.57%
ALL 3 28.57%

This data indicates that ribbing is a problem with all coating methods

A second report will discuss additional results.

Coating Methods In Use 2010

Coating Methods in Use 2010

For many years, I have been monitoring the coating methods being industrially used in order to determine the most widely used web coating methods. This information is very useful to me for preparing lectures, consulting and R&D.

The results of the most recent survey are as follows:
Coating Method Rank Percentage in use

Gravure coating, forward or reverse, direct or indirect 1 19.0%
Mayer rod 2 17.2%
Slot die 3 15.5%
Roll coaters (forward, reverse, multiroll) 4 10.3%
Multi-layer cascade(slide) 5 6.9%
Knife-over-roll 6 5.2%
Curtain 7 3.4%
Dip 8 3.4%
Blade 9 3.4%
Spray 10 3.4%
Dahlgren 11 1.7%
Screen printing 12 1.7%
Extrusion 13 1.7%
Comma Coating 14 1.7%
Air Knife 15 1.7%
Transfer 16 1.7%
Micro Gravure(tm) 17 1.7%

Data was obtained by conducting a survey of users, blog queries and literature searches. The number of reported usage for each method was summarized and then used to calculate rating.

My conclusions are:

• Many different methods are still in widespread use
• Older methods, gravure, Mayer Rod, Roll coating are still popular. This ranking has not changed over the years.
• Slot die coating usage is increasing faster then other methods. In the original survey it was at the bottom of the reference rating and may soon be at the top.
• It is surprising that Mayer Rod coating, which is over 100 years old, is still very popular.

Coating methods in use

Coating methods in use

I would appreciate your help in determining the various web coating methods that are routinely used in the coating and drying process, by answering the following questions

Please list the manufacturing coating methods that are routinely used in your facility.

Coating Method

1.

2.

3.

4.

5.

6.

Please return your response to edcohen@aimcalblogs.com I will post a summary on this webcoating blog

Thank you for the help.

DEFECTS SURVEY

I would appreciate your help in determining the various types of defects are encountered in the coating and drying process, by answering the following questions

Please list the 6 highest loss coating defects encountered in your facility.

Defect coater type

1.

2.

3.

4.

5.

6.

When complete, Please return to edcohen@aimcalblogs.com I will post a summary on this webcoating blog

Thank you for the help.

ed cohen

Web Coating Streaks Part 2

Web Coating Streaks
Part 2

Streak Elimination

Once streaks are observed on the coated web, it is obviously essential to start a technical program should be undertaken to determine cause of the streak and to implement corrective action.

As discussed in Part 1, there are many different types of streaks. Figure 1 has some examples of different types of streaks. As a result the first step in the program is to accurately characterize the type of streak so that proper corrective action can be taken. Once the streak type is determined, it dictates the type of technical effort that should be undertaken. For example, if there are multiple streaks across the web, this suggests the defect is ribbing and is a result of coating bead instability. Therefore, coating application conditions should be investigated.

The characterization is done by observing the coating and drying process to determine where the streak starts. Streaks that are caused by contamination in the coating bead should be seen at the coating station and often the contaminants can be seen in the bead. This observation dictates that sources of contamination must be identified. Drying induced streaks are observed at the end of the dryer and not upon entrance. Therefore, drying conditions and the fluidity of the wet coating should be studied. Streaks caused by irregularities in the coating lip or doctor blade can be seen in the bead or after it emerges from the bead. Close examination of coating hardware with edges no coating will determine if there are knicks etc, which are causing streaks. A microscopic analysis is often needed to locate small defects

In addition to visual observation of coating process, microscopic analysis of streak can help determine cause. A bubble streak is very distinctive. If a sample of the streak at its beginning or end is available microscopic analysis will help determine cause. Also, all images of streaks obtained should be stored in a database for easy retrieval in subsequent streak episodes.

Technical actions to eliminate streaks.
This section will discuss some specific actions that can be taken to eliminate streaks.

Contamination Caused Streaks

Streaks can be caused by a variety of contaminants, bubbles, dirt, agglomerates, gels, substrate particles which are held up in the coating deform in the coating and leads to streaks. Therefore these contaminates should be eliminated from the coating solution. This can be done by:

• Uniform mixing to insure all solids dissolved
• Filtration of solution at the end of mixing cycle and in the feed lines to the coating applicator.
• Effective cleaning of all hardware and feed lines to insure no contaminants remain after their use.
• Removing bubbles from feed lines.
• Clean substrate to remove particulates, which are on surface and can transfer to the coating.

Other factors are:

• Widen the gap between applicator and substrate to help purge particles.
• Use higher flow rate to applicator before start to assist in purging
• Determine shear rate sensitivity of coating solution to insure agglomerates are not formed in the feed system.
• If contaminants are seen in gap, remove them with sharp wooden stick.

Flow Instability In The Coating Bead,

Ribbing is the most prevalent streak defect from this instability Figure 3. It occurs because coating the coating process is operating outside of the defect free coating window.

The basic actions to eliminate ribs are to:

• Dilute coating solution to reduce and give a thicker coating weight and lower viscosity solution.
• Smaller diameter coating roll
• Change surfactant to modify surface tension
• Change solvent to modify surface tension

Non-uniformities in the coating applicator,

The edges of coating applicators, such as slot die and doctor blades can wear out or be damaged resulting in Non-uniformities in the coating applicator, which can cause streaks. They should be continually monitored to insure they are straight and defect free. When Non-uniformities are observed they should be replaced or refinished. It is more cost effective to refinish die lip then to replace it. However, it may be cheaper to buy a new blade then to try and to refinish it. A preventative maintenance program to refinish applicators and rolls on a routine basis is a very effective way to insure this type of defect does not occur.

Figure 1
Streak types

Figure 2
Bubble streak

Figure 3 Ribbing

Web Coating Streaks, Part 1

Introduction

Streaks are a general class of defects, which are characterized by a long narrow line in the machine direction of the coated web; that have a different density than the overall coating. They have a significantly difference coating weight in the streak then in the main coating and a sharp edge, which increases their visibility and allows them to be detected visually. They are found that in all coating process and have varying characteristics depending on their cause. They can be a single individual narrow streak with a low coating weight in the center or they can be a wide band with increased coating weight. There can also be a series of streaks streak or a series of streaks. They are one of largest categories of defect product losses; because they can run for a few feet or throughout the entire coated roll. Large losses can result because the location of the streak may result in the entire roll being scrapped because the location prevents a useable width from being obtained. This is very different then a spot defect in which it is possible to cut around the defect in the final finishing process; thus, avoiding scrapping the entire coated roll.

Streak Types and yield loss

Typically, all defects that are long and in the machine directions are often referred to, as “Streaks”. However, there are a wide variety of streak types depending on the exact causes of the streaks and it is essential to correctly identify the streak type in order to eliminate it. Since there is no standard nomenclature for streaks, the same streak can have many different names and obtaining data and communicating others about the streak causes can be difficult. A good example of this is the “ribbing defect”, which is series of lines running down the web. They are uniformly spaced are across the entire web. While ribbing is the most common name for this defect they are also called Barring. Comb lines, Corduroy, Phonographing, Rake lines, A wide streak. 1/8 “ to 3” is often called a band

Streak causes
The possible causes of streaks are:

- A physical disturbance by particles of the wet coating in the bead, which disturbs coating and lead to the gouging of the wet coating and the streak. Typical particles are bubbles, dirt, agglomerates, gels,

-A hydrodynamic flow instability in bead, which results in a sinusoidal Td variation of the coating weight, leading to streaks.

- Non-uniformities in the coating applicator, doctor blade or slot die lip which can gouge the wet coating. Also drying of coating on applicator surfaces can gouge the coating.

- A non-uniform transverse direction wet coating weight. A higher coating weight at the edge of substrate can give wide streak. Also, a sharp spike in the coating weight profile can also lead to a streak

- Non-uniform airflow in the dryer can lead to streaks. A localized variable airflow velocity from delivery device can physically disturb the coating and cause localized variations in the thickness and drying rate, which appear as the bands. Low viscosity coatings can be susceptible to this because they are fragile when entering dryer. Also if the non-uniformity is in the same location for several nozzles it will reinforce the disturbance and increase the streak severity

Defect Reduction and Prevention

Defect Reduction and Prevention

Defects are an ongoing source of extra costs and an impediment to the successful operation of an operating coating line. I know of no coater that does not have defect problems with both old and new products. Thus programs to eliminate defects when they occur and preventing their reoccurrence is an essential technical effort for a coating line.

Optimizing a current coating line to reduce defects is essential to insure economic success and remain competitive. It is also an essential process when a new coater is being designed and installed. The new coater should have improvements in design and fabrication so that it performs betters then the old designs. It should not be a duplicate of previous coating lines. If it is the new unit will have all of the deficiencies of the old unit.

Defects are defined as the lack of something necessary or desirable, a deficiency, or an imperfection that impairs worth or utility. Physical defects such as chatter, haze, streaks, repeats, ribbing, spots are typically considered as the largest class of defects in a coating line. However, there are other process and product performance defects that are detrimental, such as
• Coating weight non-uniformity and poor reproducibility
• Inadequate drying which lowers line speed and productivity
• Downtime to eliminate defects
• Drying sensitive product performance properties, adhesion, haze, are not being obtained

In developing technical programs there are several Process Success Factors that need to be considered when formulating the technical program:

• There are ~25 major process elements and each of them have several hardware components. All of them are important and need to be evaluated and deficiencies corrected. A deficiency in one element can limit the performance of the entire unit
• Defects created in any step in the coating line will not be corrected in the subsequent steps,
• The major process elements, web drive components, coating station & applicator, dryer, solution preparation, control systems, safety and Environmental must be robust.
• Online instrumentation to measure coating weight, and defects are necessary for optimum performance.
•Because of the complexity of the process detailed operating procedures are needed along with and on going training of operating personnel.

The program to reduce or to scale-up new product with minimum defects should have a defined protocol that covers all of the steps needed for success. This defines the steps needed to implement the program and insure success

These steps in the Defect Reduction Procedure are as follows:

1.Define all the product and process requirements that are needed for success in the marketplace. This is particularly important because the end product must be competitive and the overall needs should be established at the start of the effort rather then in the middle of the effort.

2.Insure availability of valid analytical techniques to measure and characterize properties and process variables. If you cannot measure an important property with the accuracy needed, then a quality product cannot be produced. Similarity, the process instrumentation needs to be evaluated to determine if it can control process variables with the precision and reproducibility needed. If a coating weight measurement precision of 1% is needed and the current method gives 5 %, the program will not be successful. Instrumentation needed should be ordered at this point in program to insure valid experimental data.

3.Insure that the web coating process can meet the required specifications. Variables such as line speed precision, drying load, drying configuration, tension control, cleanliness, web transport, substrate thickness range need to be analyzed to insure that they are adequate. It is possible that coater may not have the capability to make product at the goal specifications. Once identified, corrective action should be started.

4.Use the appropriate coating method. There are a wide range of available coating methods each of which has an operating window, which gives the best defect free product. For example, The Mayer rod gives a coating weight uniformity of 6-10 % and has a maximum line speed of 300 fpm. If you need a uniformity of 1% and need to coat at 600 fpm, this method will not be adequate. The curtain coater functions at high speed and will not coat at speeds below 400 fpm. A low volume product will not give good results. A previous Blog article, Selecting a Coating Method describes this procedure in more detail

5. Develop and implement operational procedures. The coating line is complex and has many variables that need to be controlled. There should be operating procedures for the coater and all products so that all variables are controlled and reproducible. Operating personnel should be trained to use these procedures and they should be kept current.

6. Quantify and understand defect losses. When defects occur, the type of defect and amount of product lost must be recorded. This should include product defects as well as coater defects such as coater downtime and mechanical problems. This will give an accurate depiction of losses and a method to calculate Economic impact due losses to help justify future efforts. It can help determine if any product has a superior or inferior performance

7. Use a structured troubleshooting protocol when defects occur. There will be defects with products as they are manufactured on an ongoing basis. A structured protocol will help determine the reason for the defects and to rapidly correct them and recommend improvements. Next article will discuss this.

8. Set up a computer database to store and share information. The converting process generated a lot of data, which should be analyzed to help understand the process and identify areas for improvements,

9. Implement process improvement technology. Many new technologies for the converting process are being developed each year. These should be evaluated and implemented so that coater performance is always improving.

10. Use statistical process control. The best method to control the process and insure reproducibility and good performance is to utilize one of the many quality control methods. The process is to complex to control without using one of these methods.

Measurement of Paint Coating Thickness on Different Materials

Measurement of Paint Coating Thickness on Different Materials
Javier Marín Zafrilla
PCE Group Ibérica S.L.

Ultrasonic Principle
An ultrasonic technique is used when measuring the thickness of paint. The probe of the instrument contains an ultrasonic transducer that sends a pulse through the coating. The pulse reflects back from the substrate to the transducer and is converted into a high frequency electrical signal that is analyzed to determine coating thickness.
A example is the PCE-CT 28 coating thickness meter / gauge for cars is for measuring the thickness of paint and plastic on ferrous or non-ferrous materials without damaging the coating's surface. The coating thickness meter / gauge stands out due to its high range of measurement. This coating thickness meter / gauge should be one of the tools used by technicians in areas of paint or lacquering such as in production, automotive body repair or quality control.
The coating thickness meter / gauge is particularly useful in detecting damage to vehicles produced by an accident, for preventing possible damage or as a toll in auto body repair. Also, in the industrial sector, this device is essential for quality control in shipping and receiving and during production. It is ergonomincal with an integrated sensor and it shows results of measurements quickly and accurately. It was designed for non-magnetic coatings such as plastic, chrome, zinc, copper, etc. and insulation such as rubber, plastic, glass and paper over steel, iron, aluminium, brass and stainless steel. Thanks to its wide measurement range, the coating thickness detector is ideal for the naval and metal industries.

Paint thickness over exterior automotive materials is best measured with hand-held, electronic instruments.

Could you kindly suggest appropriate steps to prevent edge buildup in Mayer Rod Coating?

Mayer Rod Coating Edge Build-up

There are several reasons for the coating to build-up at the edges of the web:

• The gap between mayer rod(s) and web may not be uniform across the web.

• The web is not laying flat because of tension variations and web planarity. This could create wider gap at the edges leading to higher coating thickness.
• The coating solution surface tension may be too high and it is pulling the coating away from the free surface at the end of the web leading to build-up. surface tension. This should approve edge coating.

• If the web ispaper we would talk about a baggy edge. Is it possible the fabric has less strength ( for lack of a better term ) such that it is a little stretched at the edges thereby reducing web tension at the edges. This could allow a thicker coating at the edges.

I am also forwarding your memo to a Mayer Rod vendor who might be able to help.

How can we remove curling in coating PP/LDPE film on PP substrate?

Response to Blog Query on Curl

There are several actions that can be taken to reduce curl:

• Increase the relative humidity in the drying zone when the coating is in the falling rate zone. This will reduce shrinkage stress from the drying coating, which is causing the curl.

• Increase the relative humidity in space between the end of the dryer and the rewind.

• Plasticize the coating to reduce drying stresses.

• Use a coating on the backside of the substrate. This will counter act the stresses casing curl from the active side of the substrate.

• Consider thicker and higher modulus substrate.

• An increase in web tension can “pull out” some of the curl. However, depending on the physical properties of the substrate, the tension might distort the substrate,