1 June 2007

Just one word: Plastics

Infrared temperature measurement improves thermoforming processes

FAST FORWARD

  • Non-contact IR temperature rig works in steel, glass, ceramics, forging, heat-treating, plastics, and semiconductor operations.
  • There are performance and economic benefits to using an IR solution for oven control.
  • An infrared sensor can maintain consistent cooling temperatures across molds.
 
By Bob Elder

In the plastics thermoforming industry, temperature monitoring is critical.

Thermoformers traditionally have relied upon specialized manual techniques for sheet temperature control. However, this approach often yields less-than-desired results in terms of product consistency and quality.

Increasingly, thermoformers are discovering infrared (IR) technology for precision temperature measurement and control. Non-contact IR thermometers measure temperatures of fast moving processes quickly and efficiently, measuring product temperature directly instead of the oven or dryer. Users can then easily adjust process parameters to ensure top product quality.

We encounter thermoformed plastics everyday. Widely used across a diverse range of industries-aerospace, medical, sporting goods, home appliances, and others-thermoformed plastic has replaced many parts previously crafted from wood, paper, glass, and metal.

Thermoforming is a generic term for the process of producing articles from a flat sheet of plastic under temperature and pressure. This technology offers close tolerances, tight specifications, and sharp detail; and when combined with advanced finishing techniques, it results in products comparable to those formed by injection molding.

Plastics lending themselves best to thermoforming include: acrylonitrile-butadiene-styrene copolymer (ABS), high-impact polystyrene (HIPS), high density polyethylene (HDPE), high molecular weight polyethylene (HMWPE), polypropylene (PP), polyvinyl chloride (PVC), polymethyl methacrylate (or "acrylic") (PMMA), and polyethylene terephthalate modified with CHDM (PETG).

Temperature control is a vital aspect of thermoforming processes. The core temperature of the plastic sheet, its thickness, and temperature of the manufacturing environment affect how plastic polymer chains flow into a moldable state and reform into a semi-crystalline polymer structure.
The final frozen molecular structure determines the physical characteristics of the material and performance of the final product.

Process description

Manufacturers use thermoforming to convert various gauges of plastic sheet into finished or semi-finished goods. This conversion process is heating a selected thermoplastic material in a "thermoforming oven" to generate a desired "sag."

Heater zones at the top and bottom of the oven provide heating on both sides of the sheet. Ideally, the sheet should heat up uniformly to its appropriate forming temperature. The sheet then transfers to a molding station where an apparatus presses it against the mold to form the part using either a vacuum or pressured air, sometimes with the assistance of a mechanical plug. Finally, the part ejects from the mold for the cooling stage of the process.

View factors

One of the most important parameters of a thermoforming oven is the geometric configuration of its heater zones with respect to the sheet. The radiant power from each heater to each sheet zone is a function of this geometry through view factors. One must adjust these view factors during the sheet reheat phase as the sheet sags, thus changing the sheet-to-heater distance.

The majority of thermoforming production is by roll-fed machines, while sheet-fed machines are for smaller volume applications. With very large volume operations, a fully integrated, in-line, closed-loop thermoforming system can be justified. The line receives raw material plastic and extruders feed directly into the thermoforming machine.

Certain types of thermoforming tools enable the cropping of the formed article being within the thermoforming tool. Greater accuracy of cut is possible using this method because the product and skeletal (scrap) do not need repositioning. Alternatives are where the formed sheet, including skeletal, index directly to the cropping station.

High production volume typically requires the integration of a parts stacker with the thermoforming machine. Once stacked, the finished articles pack into boxes for transportation to the end customer. The separated skeletal is wound onto a mandrill for subsequent chopping or passes through a chopping machine in line with the thermoforming machine.

Large sheet thermoforming is a complex operation susceptible to perturbations greatly increasing the number of rejected parts. Today's stringent manufacturing requirements of part surface quality, thickness accuracy, cycle time, and yield, compounded with the small processing window of new designer polymers and multi-layer sheets, prompted researchers to look for ways to improve the control of this process.

During thermoforming, sheet heating occurs through radiation, convection, and conduction. These mechanisms introduce a great deal of uncertainty, as well as time-variations and nonlinearities, in the heat transfer dynamics. Furthermore, sheet heating is a spatially distributed process best described by partial differential equations.

Thermoforming requires a precise, multi-zone temperature map prior to the forming of complex parts. This problem is compounded by the fact that temperature is typically controlled at the heating elements, while the temperature distribution across the thickness of the sheet is the main process variable.

In order to optimize productivity and quality, the actual sheet temperature distribution before forming must adhere to the optimized temperature map as predicted by a process simulation, or the recipe as determined by previous runs.

Traditionally, thermoformers relied upon specialized manual techniques for sheet temperature control. Operators tried to minimize the difference between the sheet's core and surface temperatures, while ensuring both areas stayed within the material's minimum and maximum forming temperatures.

It is a difficult balancing act, as low heating temperatures can produce stresses in thermoformed parts and high temperatures cause problems such as blistering and loss of color or gloss.

plastics
This former produces thermoplastic kayak parts. The system has one infrared sensor for controlling heating temperature and a second unit for auto-cycle advance for demolding parts.

Measurement solution

For General Plastics Machines (GPM) a manufacturer of single-ended, double-ended, and rotary thermoforming machines, IR sensor technology provided an automated temperature measurement solution improving sheet temperature monitoring and cycle control. The company's custom thermoforming machines are in vacuum, pressure, stretch, and drape forming applications.

GPM decided on a non-contact infrared temperature monitoring system from Raytek Corporation to handle multi-zone sheet temperature control.

The system's two-piece sensing head combines a 1/8 DIN digital monitor and two-wire IR sensor. The monitor offers a host of infrared processing capabilities, including peak and valley hold, averaging, and a user-adjustable offset.

The smart sensor provides digital communications, as well as 4-20 mA output, allowing remote configuration and temperature monitoring. It also features remotely adjustable temperature and output sub ranges, adjustable emissivity, ambient temperature check, and a user-defined alarm output.

The automated infrared temperature monitoring system includes an operator interface and display for process measurements from the thermoforming oven. A digital panel meter with built-in mechanical relays displays temperature data and outputs alarm signals upon reaching set point temperature.

An IR thermometer mounted in the thermoforming machine measures the temperature of hot, moving plastic sheets with 1% accuracy.

Using the infrared system software, operators can set temperature and output ranges, as well as emissivity and alarm points, and then monitor temperature readings on a real-time basis. We can archive process temperature data or export it to other applications for analysis and process documentation.

plastics1
A thermoforming system with an infrared sensor can maintain consistent cooling temperatures across molds, allowing part removal without losses from sticking or deformation.

How the system operates

As part of the thermoforming machine, an infrared sensor is in between the heating and forming sections where it measures the temperature distribution of plastic sheets passing through the system-allowing the heating cycle to be set by the surface temperature of the material. The machine's controllability helps to properly heat sheets of varying thicknesses and material types.

In plastics applications, measuring thin film or sheets with infrared technology requires a specific narrow band spectral response, while thicker materials are generally measurable using a broadband spectral response. An IR thermometer is able to accurately measure a target's surface temperature through a narrow absorption band on the infrared spectrum.

Using the IR sensors, operators can set a high alarm, providing an output signal to the processor and advancing the plastic part out of the oven to continue the thermoforming process. When the process hits the set point temperature, a relay closes and triggers either an indicator light or an audible alarm to control cycle.

Thanks to data from the IR measurements, operators can determine the optimal oven setting to saturate the sheet completely in the shortest period without overheating the middle section. The result of adding the objective data of the IR thermometer to practical experience is far more successful in drape molding with very few rejects. In addition, more difficult projects with thicker or thinner material have a more uniform final wall thickness when using uniform heat on the plastic.

Thermoforming systems with IR sensor technology can also optimize thermoplastic de-molding processes; operators sometimes run their ovens too hot or leave parts in the mold too long. By using a system with an infrared sensor, they can maintain consistent cooling temperatures across molds-increasing production throughput and allowing the removal of parts without significant losses due to sticking or deformation.

Return on investment

For thermoformers to analyze fully the return on the investment of an automated infrared temperature measurement system, we must look at certain factors.

Getting to the bottom line costs means taking into consideration the time, energy, and amount of scrap reduction that may take place as well as the ability to collect and report information on each sheet passing through the thermoforming process.

The benefits of an automated IR sensing system include:

  • Improved part quality
  • Simplified quality control monitoring and setup capability
  • Identification of over-heated or under-heated sheets before making defective (off-spec) parts
  • Ability to thermoform more difficult parts
  • Ability to archive and provide customers with a thermal image of every part manufactured for quality documentation and ISO compliance
  • Increased throughput by reducing part residence time in the heating section
  • Higher yields by significantly cutting down equipment set-up and qualification times
  • Early detection of heater problems affecting process efficiency and energy consumption

Thermoformers utilizing infrared temperature measurement technology see an improvement in production, and they do not have as much scrap. They also make better quality parts because they get uniform thickness coming out of their thermoforming machine.

About the Author

Bob Elder (gpminc@wa-net.com) is founder and president of General Plastics Machines, an original equipment manufacturer (OEM) in Vancouver. He has 20+ years experience in the plastics thermoforming industry.

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Terminology

Thermoforming is the process of forming a thermoplastic sheet into a three-dimensional shape by clamping the sheet in a frame, heating it to render it soft, then applying differential pressure to make the sheet conform to the shape of a mold or die positioned below the frame.

Infrared (IR) is electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves. It is the part of the near-optical spectrum humans perceive as heat rays emitted by stellar objects such as the Sun. Satellites and ground telescopes detect IR, revealing the centers of galaxies and gas clouds where stars are coming into being.

Convection is the transfer of heat by the motion of or within a fluid. It may arise from temperature differences within the fluid or between the fluid and its boundary, or from the application of an external motive force. It is one of the three primary mechanisms of heat transfer, the others being conduction and radiation.

Conduction is the transfer of heat energy through a substance or from one substance to another by direct contact of atoms or molecules.

Radiation is the transfer of heat energy (or sound) from one object to another through an intermediate space-only the object receiving the radiation, not the space, is heated. The heat is in the form of low frequency, infrared, invisible, light energy, transferring from a "warm" object to a "cold" object.

Partial differential equation: In mathematics, a partial differential equation is an equation involving partial derivatives of an unknown function. The idea is to describe a function indirectly by a relation between itself and its partial derivatives, rather than writing down a function explicitly. A solution of the equation is any function satisfying this relation.

Emissivity is the ability of a material to emit heat. Black iron has high emissivity, while aluminum foil has a low emissivity.