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Measuring product time at temperature has been an accepted industry practice for more than 40 years. The earliest temperature profiling systems plotted data on a chart which process engineers had to laboriously analyze.
Today, profiling systems that are both sophisticated and easy to use can give a clear pass/fail finding, document regulatory compliance, let operators know when and where their ovens are trending out of acceptable limits or show that a process is within suppliers’ or customers’ specifications. In addition, regular and frequent temperature profiling can help increase throughput, reduce rejects and lower energy costs.
Temperature profiling is a means of compiling product and process heating and cooling data over time. A profiling system generally consists of thermocouples, which are either attached to the product or measure ambient air; a data logger, which captures data from the thermocouples; a thermal barrier to protect the data logger from the heat and/or cold; and a software package for data analysis.
Temperature profiling gives engineers power over their process. They can identify and correct hot and cold spots, reduce fuel costs by increasing line speed and/or lowering oven temperatures, optimize their process, prove adherence to customers’ specifications and prove quality without destruction. With the wide variety of temperature profiling system configurations and components currently available, almost every thermal processor can benefit from routine profiling.
The first things to consider before purchasing a system are: What are you producing? How many ovens do you have? How many runs per day are required? What are the times, temperatures and clearances of the process? What information does your quality program call for? Most finishers don’t have clearance issues and can use either one of two types of systems: one with exceptional analysis capabilities and customization or an easy-to-use system that provides basic data acquisition and analysis.
Automotive OEMs or large suppliers with multiple paint lines most often want an easy-to-use yet sophisticated profiling system with all the bells and whistles. Process engineers in such plants need to know that product quality is the same on all of their paint lines. They want data that will help them schedule preventive maintenance, and they require documentation certifying regulatory compliance. Time-saving features are especially critical because temperature profiling is routinely required many times throughout the day on multiple coating processes (E-coat, primer surface, topcoat and clear coat).
When considering the size of many large-scale manufacturing facilities and their increasing erosion of man power, making the profile process quick and efficient is essential and can decrease labor costs significantly. Recent developments allow routine monitoring of different processes with a single system without any need to keep returning to the QA office to download profile data. Combining multiple run capability and process Pass/Fail indication direct from the profiler, the operator can confirm quality at the oven exit. Such protocols can save up to 30% of the conventional profiling labor.
Conventionally, sequential profile testing is performed on production car bodies as they travel through each coating process. For such purpose the barrier must not only provide reliable thermal protection but be light-weight enough to ensure easy, safe installation and retrieval from the car body. Another important requirement for the therrmal barrier is the absence of any silicone contamination. To avoid catastrophic paint contamination issues such as craters, the barrier must be designed to use insulation material that does not contain silicone or any silicone derivatives.
Installing the system into a car body on a moving production line requires skill and efficiency. To help with this, magnetic surface and air thermocouples are designed to be both quick and efficient to install direct onto the body. Being only approximately 1 inch high and an inch in diameter. the micro-magnetic probes can be easily fitted into tight recesses or onto curved surfaces.
A relatively new approach to temperature profiling of multiple automotive coating lines is the single-pass method. Rather than testing each oven separately, the profile system is configured to travel the whole paint line in one operation. The system is installed in a purpose-built test car used only for profile testing. Thermocouples are permanently attached to the test body at strategic points to ensure that readings are consistent from run to run. A high-performance thermal barrier capable of handling temperatures to 400°F for nine hours provides the thermal protection necessary to handle exposure in multiple ovens and can even withstand line stoppages or delays.
In practice, the system can be installed at the start of day one and retrieved at the end of the line at the start of day two. For operators it is simple and safe: While the system travels through the line like any other car body, they can get on with other work.
At the end of a run, the memory portion of the data logger is retrieved from the cool barrier and the process data is downloaded to a PC in a single step. If necessary the single process trace can be separated, allowing analysis and reporting of data from each oven separately.
It is estimated that the single-pass method could provide up to a 60% labor saving compared with established profiling strategies where ovens are tested individually.
Collecting oven temperature data is only part of the story. Data must be reviewed and analyzed in such a way that it can help line managers make sense of what is actually happening to the product being cured. Analysis software is therefore a key part of any profiling system.
Software is often password-protected with two-tiered access. Line operators are guided through profiling setup and start procedures, while process engineers can drill down to more closely examine every analysis function. Manager-created process files function as templates identifying critical process and analysis parameters.
The software should also provide fast analysis of alarm conditions. If the user’s custom-defined parameters fall out of tolerance, an alarm can alert the operator to take corrective action. This approach immediately addresses quality issues, minimizing rejects and ensuring only high-quality products reach the market.
Additional software features often used in these kinds of applications include BakeChart, integrated statistical process control (SPC) and Index of Cure functions. BakeChart proves that a coating’s cure cycle conforms precisely to the paint supplier’s bake window specifications. It automatically analyzes the profile data, providing analysis, cure specification and results in a single file. It also provides full traceability of report information back to the test equipment and raw profile data.
Integrated SPC displays variations in key process parameters (peak temperature, time at temperature, Index of Cure) collected from individual profile runs. It will immediately identify when a process goes out of tolerance and can predict future process failures by watching for gradual changes. Maintenance can then be conveniently scheduled, avoiding costly and unexpected shut-downs.
The Index of Cure calculation is an easy way to quantify cure, because the software performs the analysis. Simply put, Index of Cure is a sum of all time and temperature contributions taken from the profile and calculated into a single index value. A value of 100 indicates a cure that exactly meets specifications, higher than 100 indicates over-cure and lower than 100 indicates under-cure.
Sharing information is an important requirement of any QA procedure. Enhancing the latest IT facilities, it is now possible to convert any comprehensive profile report automatically into a standard format that can be e-mailed to anyone for instant review.
Those with fewer ovens or less frequent profiling requirements may prefer a simpler profiling system that provides only critical process information such as a visual graph along with peak temperature and time-at-temperature data. These users want an easy-to-use, portable system that will work in a variety of applications with good accuracy and repeatability.
Many custom coaters favor the ease of use of a basic four- or six-channel system. These economical systems can provide thousands of readings per channel, ±1°F (±0.5°C) accuracy, a programmable sampling interval and a replaceable battery. Basic analysis functions are time-at-temperature, process details, and Index of Cure. With a single mouse click, users can print a report documenting compliance with customer specifications.
The process clearance of the oven is a common factor impacting profiling system selection. The data logger must be protected from heat by a thermal barrier that will easily fit through the process. Some suppliers of temperature profiling equipment offer "one-size-fits-all" systems that will not function in specialty processes. Others can mix and match their data loggers and thermal barriers to create a system appropriate for a wide variety of applications.
Examples of finishing processes that require process-specific solutions for successful profiling include the following:
PTFE and vitreous enamels. Sequential curing of polytetrafluoroethylene (PTFE or Teflon) and vitreous enamel coatings is frequently performed as part of an automated, continuous finishing process for cookware or appliances. Once the enamel is applied to the product and processed, multiple PTFE coats are applied and dried. The critical areas to watch during PTFE curing are time above temperature, which could result in a brittle coating with poor adhesion; variations in color and texture; or degradation of the coating itself. Cure temperatures can reach 750°F, so careful choice of thermal barrier and thermocouples is required.
Beverage cans. Spray lacquer applied on the internal surfaces of beverage cans is cured in an internal bake oven (IBO). The first phase of the oven raises product temperature to 212°F to evaporate water from the water-based lacquer; the second phase cures it.
For temperature profiling, adhesive patch probes are attached to the internal surface of a test can, which is surrounded by other cans and secured by high-temperature tape to create a stable test rig. As curing is achieved by convection it is important that the belt be covered by cans to guarantee that airflow matches actual production. Consistent curing will help eliminate leakage, product contamination and discoloration.
Three-piece cans. Food or aerosol three-piece cans are fabricated from flat aluminum or tinplate sheet. A variety of coating and curing processes are performed before stamping. Lacquer or powder is cured on one or both sides of the sheet, and decorative print inks and varnish may be cured on the reverse side. Curing takes place in a wicket oven, which transports the sheets vertically through the convection oven.
In such an application a thermal barrier fitted with a clip can be mounted directly to a sheet with thermocouples securely attached to a test piece. Profiling to ensure consistent curing has the same benefits as it does for beverage cans.
Coil coating. Coil strip coating is used extensively in many industrial applications including construction, cookware, domestic appliance and packaging. Coil-coated subsrates provide a significant challenge when it comes to oven profiling. With the constantly moving coil strip running at 100 ft/min through a low height clearance oven, the temperature profiling system must be carefully designed. Combining low-height thermal protection with a fixed arm assembly on which thermocouples are located, the profiling system can be easily placed directly onto the moving coil to ensure that it gets through the oven and provides accurate measurement of both surface and air temperatures across the coil.
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Rotomolding is a rapidly growing manufacturing process used to produce plastic products ranging from ride-on children’s toys to large-capacity water tanks. Temperature control of the process, in which a rotating mold containing the plastic material is heated then cooled to create a hollow plastic product, is essential. An ideal temperature profiling solution for rotomolding processes is a system that provides live data via a radio frequency (RF) signal transmitted from the thermally protected data logger directly a monitoring PC. The live temperature data recorded from the oven, mold or center of the mold allows full control or adjustment of the process as it progresses.