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Standards in the Coating Industry

Coating Curing

When coatings are applied, for aesthetic or functional reasons, to a manufactured product a critical step is that of curing the coating. In either a batch or continuous oven the requirement is to raise the product mass and its coating material to a specified temperature and hold this temperature for a set time. Typical process times are designed to achieve 25 to 35 minutes at the threshold temperature to ensure a minimum time at the curing temperature of 20 minutes.

Typically the coating manufacturer supplies the time versus temperature specification for the coating materials which must be met to achieve its full performance. Under or over curing can lead to numerous quality problems, such as lack of adhesion, low impact resistance, poor surface finish etc. Reworking of a coated product is time consuming and troublesome; so getting it right every time is critical.

Thermal Profiling

Within the coating industry the need for temperature monitoring is well understood; however simply checking the oven controller setting is not sufficient. The control or monitoring thermocouples installed, within the oven measure the ambient temperature but they provide only a poor indication of the product temperatures. Temperatures across a simple homogeneous product, such as a flat sheet will vary from top to bottom and left to right of the oven and this is further compounded when the product has a varied cross section, or is made up from a number of different materials. 

The decisive factor in achieving an optimum cure is to know the product temperature at a number of key locations and how they interact with the oven controller setting. Measuring the product temperature as it passes through the process is the best way to ensure that the temperature at these key locations is known for every stage of the process.

In the many paint shops, use of a profiling system such as the Datapaq XL2 or EasyTrack 3 for routine oven profiling has become a way of life. Every day in most automotive plants, systems like the Datapaq XL2 will be used to confirm that the cure schedule is being achieved therby guaranteeing paint performance and quality of the final product.

The frequency of profiling for each paint line varies from user to user depending on the type of product and tolerance range of the coating cure window; typically ranging from once per shift per oven to once per month. However, there are events that would drive the need to run a thermal profile where there is risk that the oven operation may have changed. Examples of such events are listed below:

  • New product introduction
  • Use of new coating or change of coating supplier
  • Following routine oven cleaning
  • Following major line breakdowns
  • After oven modifications
  • Changes in production schedules
  • Any extended “down” periods exceeding four days

Thermal Profiling Methods

Thermal profiling can either be accomplished by a direct or indirect method.

Direct Profiling

The direct method involves the testing of a product that is part of the production schedule and as such, will be painted and eventually supplied to a customer. As the product is painted and part of the production run, the thermocouples will be placed on the product just prior to the cure oven and are located so as avoid damage to the surfaces that will be visible on the final product; typically surfaces that will be covered by fixtures and fittings, so paint damage will not be critical.

Indirect Profiling                                 

The indirect method involves the testing of a product that has been instrumented for testing purposes and will have thermocouples permanently connected to it. This product is installed on the paint line when testing is required and run directly through the oven.

The strengths and weaknesses of the two approaches are summarized in the charts below:

Direct Profiling

Strengths

Weaknesses

No need to store instrumented tests products.

Variation in thermocouple placement will result in inconsistent results

Thermocouples easily repositioned to focus on area of interest when trouble shooting

Thermocouple attachment increases handling of uncured product – surface damage

No risk of process contamination resulting from dirty test piece

 

Measure temperatures under true curing conditions.

 

Indirect Profiling

Strengths

Weaknesses

Thermocouples always in the same location ensuring consistency

A test piece will be required for each product size.

Thermocouple lifetime improved as result of reduced handling.

Storage space required for the test pieces

Unskilled operator can conduct the thermal profiling operation.

Cost of thermocouples and test pieces needs to be considered

Thermocouple Placement

A key component of the oven profiling system is the thermocouple. The correct choice and placement of the thermocouple sensor is critical to the accuracy and consistency of the data collected. Placement of the thermocouple will be determined by the type and size of product being monitored. In general, placement is selected to measure areas on the product that are at risk of over curing (such as thin metal sections) or areas that will potentially suffer from under curing (thick structural members or areas where many parts join together).

The choice of thermocouple sensor will be dependent on the product material, as well as the space available. Attachment of the thermocouple could be magnetic or clamp, for direct profiling while screwed or welded to the product for indirect profiling is more appropriate. 

An example of a magnetic attachment is the MicroMag, a thermocouple designed specifically by Datapaq to meet the demands of general automotive profiling. The compact sensor can be placed in the tightest of recesses, allowing those difficult areas to be easily measured.

For regular temperature profiling 6 to 8 points of measurement are commonly used. During new product launch programs, oven set-up and optimization requires more detailed profile information. In this situation thermal profilers with up to 20 thermocouple inputs such as the Datapaq TP3 can be used; when needed profiles from a number of loggers can be merged to create thermal profiles containing in excess of 60 thermocouples.

Accurate documentation of thermocouple placement is critical to the interpretation of product temperature profiles. To help with this task Datapaq Oven Tracker Insight software is offered with the ability to show not only their placement on a schematic image of the car, but a complementary thermocouple library containing detailed photos can be incorporated to show exact thermocouple locations.

Analyzing the Data

To determine whether the process matches the paint supplier’s cure schedule, a variety of analysis tools can be considered. At the simplest level, the 'time at temperature' calculations can give an approximation to whether the coating has experienced enough heat to allow the cure reaction to reach completion.

On a more sophisticated level tools such as the Datapaq Value index and bake chart of cure calculation can be employed. This Datapaq value calculation is based on the Arrhenius equation that describes the first order rate of reaction kinetics followed by many cure type reactions. The calculation uses the sum of all time and temperature contributions to generate a cure index value. A result of 100 indicates that the profile matches the desired cure schedule perfectly. Higher than 100 indicates an over cure condition and under 100 indicates an under cure condition.

Thermal profiles taken at regular intervals with the same oven settings and product are a valuable resource that can be mined to provide the input data needed for Statistical Process Control analysis to be undertaken. This can be used to predict requirements for preventative maintenance, avoiding expensive unplanned interventions.

Conclusion

Although oven profiling has been used for many years in the coatings industry, it has come a long way from its humble beginnings. The latest technologies from Datapaq now provide a range of features that enables the paint shop or production manager to understand, control and optimize oven cure processes with greater accuracy and efficiency.

 

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