Whether curing powder coat paint or vacuum casting turbine fan blades from high nickel alloy, the amount of time and temperature in combination is crucial to achieving product perfection. Company and employee reputations rely upon the quality assurance that only direct work piece time-and-temperature recording and analysis can provide.
Taking a cure schedule from a data sheet or translating metallurgical charts and tables into furnace settings is one thing. However, to prove that the staging of theoretical best practices actually imparts the desired material properties to produce long-term product reliability requires direct measurement of the work piece throughout the heating and, in most cases, cooling, cycles in their entirety.
To better accomplish production volume automation and transfers between manufacturing steps, many process heating lines are conveyorized, whereas batch furnaces are often designed with a pass-through port to facilitate sensor wires to an outside chart recorder. So, how does one efficiently accomplish in-transit thermal profiling when products are conveyed through a heated tunnel?
In-Transit Thermal Profiling
A purpose-built data recording system that can repeatedly survive the process heating cycle as it rides along with parts and report back to a computer upon retrieval, or broadcast live via an RF wireless link, can provide the solution. The term for such a device is a “thermal profiler.” Thermal profilers are used on a daily basis to enable quality assurance for a wide array of global manufacturing processes. Such portable, versatile and affordable instrument systems are available with up to 20 input channels with software that supports multiple sensor types, the most common of which are thermocouples. Vertically integrated operations will typically have multiple manufacturing processes to profile and certify. For larger box furnaces, a thermal profiling kit can serve the dual purpose of furnace surveying and load verification.
Whether batch or conveyorized, the zone setpoints of an oven, furnace, kiln or freezer do not necessarily correlate with the temperatures to which products are subjected. A manufacturer’s maintenance philosophy for production equipment (burners tuning and chamber integrity) and controller calibration frequency are variables, as well as additional factors such as:
- Distance of burner control thermocouples compared to product travel locations.
- Natural gas quality fluctuations and interval between analyses.
- Proximity of oven to an outside wall (induces seasonal variations).
- Placement in relation to an outdoor source of draft.
- Refractory condition, door seals and baffling integrity.
Thus, thermal profiling for oven tuning and product optimization helps the process engineer reduce key variables that could otherwise impact product quality.
Specifying a Thermal Profiling System
A complete and ready-to-use thermal profiling system includes four main components: a thermal profiler, software package, thermal barrier and thermocouples.
Thermal Profiler. A compact portable electronic data recorder, it is powered by a rechargeable battery, reliant upon software for Windows, and designed to capture the characteristic process heating cycle for a given application while traveling within a protective thermal barrier for the purpose of quality control, which means to verify that a thermal process meets specification from beginning to end. A precision instrument, the profiler will ship with an N.I.S.T. traceable calibration certificate. The manufacturer will recommend periodic calibration service that suits individual documentation needs.
Control buttons and status LEDs comprise on-board controls while the profiler manufacturer’s software programs handle sample rates, memory allocation and other setup parameters, live RF data transceiving and data downloads upon retrieval from the oven. Often referred to by such names as a ‘“M.O.L.E.”, “Squirrel”, “Paq” or temperture recorder, the thermal profiler should be expected to connect via USB, have multi-run memory, sensor and profiler condition LEDs and be as intuitive to use as a digital camera. And like a system camera, the thermal profiler should also have an available array of optional accessories – specifically, thermal barriers and sensors that enable its use in a wide variety of work situations.
Software. The manufacturer’s software communicates with the profiler for setup and download, organizes profiling work onto shared or local drives for R&D collaboration and should provide customizable report formats that are useful for lot shipment verification, personnel training presentations, marketing the company to new clients and audit protection documents. New software versions should be easily obtainable and not orphan the previous generation of hardware.
Profiling software built to run on Windows brings data to life within an easily interpretable graph of product and internal process time and temperatures superimposed over a virtual model of the oven, showing zone temperatures and lengths in time or distance. Once a profile is made, this model can become interactive, providing the ability to do what-if prediction analysis by altering zone temperatures and/or heat cycle duration to obtain optimum production capacity while minimizing energy use. A second profile at the predicted setpoints verifies the process.
Ensure that the software bundled with the profiling system integrates statistical process control (SPC). The ability of profiling software to accept lower and upper specification limits is key to discerning control limits or Cp and CpK (dependent on samples per subgroup selection). One nimble software program with flexible data extraction templates and application environments can serve a multitude of applications, each with unique parameters and spec limits. Discern assignable cause to process deviations beyond the norm before quality is negatively affected with built-in SPC software.
Thermal Barrier. This component enables in-transit data acquisition and ensures profiler longevity. Heat cycle time and zone temperatures, cooling method, clearance dimensions and atmosphere determine the appropriate selection of a thermal barrier for the profiling system to be viable. Such an enclosure enables in-transit profiling and provides return on investment many times over with the assurance of providing a long, useful life to the thermal profiler.
Profiling system suppliers offer a range of standard sizes with the ability to manufacture custom dimensions and features as required. The exterior is often built from various grades of stainless steel or titanium to survive the process temperatures and withstand corrosive atmospheres. The interior can be a combination of microporous insulation and passive heat sink plates, with the addition of active phase-change heat sink canisters to accomplish longer, hotter applications in a smaller overall space.
Thermal barrier data sheets can depict time/temperature tables and performance graphs along with maximum temperature rating, dimensions and weight for selection. Clearance height above a conveyorized process heating line is the starting point for analyzing applicability of an in-transit profiling solution. Also, on the first inquiry to the profiler system manufacturer, the user should be prepared with zone times and temperature data, known as the process profile.
Whether rapid or gradual, monitoring cooling curves of the actual work pieces is extremely important to verify controlled phase transformations and impart desired metallic structure for hardening and tempering applications. Liquid immersion quench can be challenging for in-transit profiling. However, cylindrical O-ring sealed thermal barriers are available for such applications. Sometimes, marginal above-belt oven clearances are impossibly thin for a profiler inside a thermal barrier to pass through, which results in a “mission impossible.” For such applications, a modern thermal profiler’s capability and software prowess may still be employed, using the feed-and-retrieve approach.
Thermocouples. Accurate and relatively inexpensive, these “self-powered” temperature sensors are constructed of a unique pair of dissimilar insulated metal conductors that, when joined, create a predictable millivoltage per degree for that calibration type (the J,K,T,E,N,R,S,B referred to in charts with their unique color code). They connect to the measurement instrument/software to render these values into temperatures, without requiring power to supply the circuit. The point of measurement is called the “junction,” and this welded tip is the sensing point, cold-junction compensated within the profiler to account for its own rate of temperature change while remaining in spec.
These mV/°C or °F thermoelectric voltages are widely published and thermal profiler software contains these linearization tables for the thermocouple types that the profiler is designed to accept. Channels are either scanned in succession or are “multi occurrent” such that all active channels scan simultaneously at the programmed log interval.
In all, five variables define a thermocouple: calibration type; wire gauge; insulation type; temperature rating and connector style. For the best accuracy, look for “special limits of error” thermocouples and extension wire. And, if a thermocouple welder is not available, replace exposed-junction thermocouples that have severed. It is not worth the loss in accuracy to solder the two wire tips together, or worse, use a wire nut to twist the pair.
Product Profiling and Profiling QA Tips
Once the oven is tuned and balanced, the thermal profiling focus can shift to the products themselves. Maximize the number of available thermocouples to measure surface and/or insertion into the products. Once again, spreading out sensors high and low, left to right as well as thin to thick areas of a work piece will reveal that process specifications are met for the complete product, irrespective of location inside the oven.
Installing a chromatograph on-site is recommended if natural gas quality fluctuations are suspected. Otherwise, monitor the chromatograph reports published for the local compressor station of the gas line company supplying the local utility. Profile the furnace on a regular basis. This is more readily accomplished when the manufacturer owns their thermal profiling system.
Stage the profile rig at least one work-piece length behind that which is being measured so that the mass of the thermal barrier will not affect load data. Near high EMF sources should use ungrounded sheath thermocouples no longer than necessary to avoid noisy data (fuzzy profile lines). And, remember to cool before the next profile run. Immediately retrieve the profiling rig, open the thermal barrier and disconnect the thermocouples while removing the profiler. Relocate the thermal barrier to a cool location (a box fan can halve the time to the next profile run). To ensure that phase change heat sinks have resolidified, shake them.
A situational awareness of all the factors influencing a process heating application is vital to maintaining its consistency. The ability to cite compliance to a given manufacturing standard, with the data to prove it, is an invaluable sales tool. Thermal profiling can help the manufacturing enterprise secure a competitive edge in an ever-changing global market.