Microcomponent dosing in the film production industry

PC-based precision dosing

The pharmaceutical industry always demands the very highest quality, even in its packaging, and increasing requires an unbroken chain of evidence to substantiate this quality. In the area of microcomponent dosing, the Pharmaceutical Division of a European film manufacturer recently began responding to this demand with a PC-based control system which integrates not only a software PLC but also extensive program and database management elements.

Thorsten Buchta

Pharmaceuticals are sensitive products which must often be protected against environmental influences over long periods of time by means of suitable packaging. The typical demands laid down for mono or coated films for blister packaging, ampoule packaging or suppository packaging include water vapour, gas and aroma barriers as well as fragrance protection and neutral taste or odour. Other requirements include an attractive design, good haptics and of course subsequent processability – all at a competitive price. The exact properties profile is normally defined on an individual basis through the addition of special additives.
One of the biggest European manufacturers of PVC film for the pharmaceutical industry is the ISO 9001-certified EVC Rigid Film GmbH, with a factory in Boet-zingen, Kaiserstuhl, Germany. Around 80% of the company’s production consists of blister film for packaging pills. Special stretchable film rounds off the company’s product line. The Pharmaceutical Division of EVC (European Vinyls Corporation) currently has around 280 employees and manufactures its products in Boetzingen on five modern calander lines and a coating plant. EVC is the first manufacturer in Europe to produce rigid film as well as single-layer and multi-layer film at one and the same location.
Mixing in large and small batches
Five mixing lines are supplied with the main components for the various PVC master batches from large, central storage bins. The results are batches of 200 or 400 kg for use in film production. A mixture of various microcomponents such as colour pigments, titanium dioxide, processing aids and lubricants, which must be precisely weighed and proportioned to within a few grams, is added at the instant specified in the recipe. The exact composition of a batch is crucial for the constant quality of the mixture and thus for colour fidelity, consistency, dimensional stability, surface quality and other characteristics of the final product. These microcomponents are proportioned separately to enable amounts that are small in relation to the overall batch to be weighed to within the neces-sarily tight tolerance limits. In addition to systems for large batches, EVC has operated a self-contained microcomponent dosing plant for several years. It consists of a revolving system of mixing containers equipped with foil pouches that pass through a row of twelve dosing/weighing stations, during which time they are precision-filled from smaller storage tanks.
Since mid-2002, the supply of spare parts for the somewhat outdated weighing system in the microcomponent dosing plant has no longer been guaranteed. In addition, end users from the pharmaceutical industry demanded labelling and tracing of every single mixing bag. EVC wanted to meet this requirement and combine it with additional automation and simplification of the process: the recipes were to be read from a central database and transferred to the microcomponent dosing controller, which in turn was to automatically report the amounts added back to the system.
EVC contracted Contec GmbH Automation-Technologie in Meckenbeuren to handle both planning and implementation. Over the last five years this service enterprise, employing mainly electrical and process engineers, has acquired core competencies particularly in the fields of pneumatic conveyor systems, weighing and proportioning, test stand engineering and CNC applications. This experience has allowed the company to develop several of its own products, including a batch package (proportioning block) for single-component and multi-component proportioning control systems. This proportioning block is suitable for both liquids and solid bulk materials, and in differential dosing systems for precise continuous proportioning. The Meckenbeuren engineers have also developed a software add-on for precise multi-axis positioning using controllers in the Siemens Simatic S7 family.
Automation systems from the world market leader is the common denominator here as well, and was decisive in the awarding of the contract.
System-wide modernisation
The most radical innovation in the interconnected automation systems at EVC’s microcomponent proportioning plant is an on-site control PC with Simatic WinAC Basis software PLC from Siemens. A purely soft PLC, it facilitates economical conversion of non-deterministic processes in conjunction with data-intensive and comprehensive PC tasks. The resulting attainable command execution times are 0.8 µs for binary commands and 1.2 µs for floating-point calculations – a more than adequate result for microcomponent proportioning at EVC.
The soft PLC is connected via Profibus DP with two new compact weighing controllers belonging to the Simatic S7–300 family, each designed for three two-channel Siwarex U weighing modules. The old control system is also still being used. It has now assumed the function of coordinating the distributed motors, sensors and actuators, and is seamlessly integrated in the overall concept. Multi Panel Simatic MP370s, with a Simatic PP17 pushbutton panel for emergency operation, are used to control manual operation and to monitor the revolving containers/bags. The new solution is rounded off by a label printer, which allows EVC to fulfil the pharmaceutical industry’s demand for complete documentation.
„The new control PC with soft PLC,“ according to Ludwig Guetle, who is responsible for engineering tasks at EVC, „now integrates the tasks performed by the old proportioning computer with the functions of the previous PLC software. It is also easily interfaced to the higher-level coordinator PC, which in turn is linked up to our central PPS host for job and materials management as well as to the report PC.“ Higher-level tasks, such as long-term archiving of production data over up to 10 years, are thus separate from local tasks, ensuring the highest possible performance for each.
Optimised processes
In addition to the WinAC Basis soft PLC, a variety of special software modules in the microcomponent proportioning plant’s control PC ensure that all processes run smoothly and that the proportioning results are of consistently high quality. The front end for the operator is the MAWCtrl module, via which as many as 10 mixing instructions can be called up from the coordinator PC’s Oracle database, edited and managed. The module, which is programmed in C++, functions as an OPC client and edits the recipe data so that the soft PLC’s OPC server can process them and precisely coordinate the transport of the mixing containers through the proportioning/weighing stations on indexing, roller and belt conveyors.
Via Profibus-DP, the WinAC soft PLC also forwards the setpoint data for all twelve weighing stations to the two weighing controllers, receives the actual dosage amounts in return and balances these values in the background, after which they are sent back to the PPS host or report PC via the coordinator PC. The processing status is visualised on the control PC’s monitor. The container transport can be monitored on the MP370 Multi Panel and the operator can switch to manual mode using the PP17 pushbutton panel.
Benefits of the software PLC
„The PC-based and WinAC Basis pure software solution was perfect for this situation because on-site PC functionality was needed and the controller overhead was within reasonable limits,“ explains Contec General Manager Guenter Tiefensee. „Integration and code compatibility within the Simatic S7 world were there from the start, making it easy to reuse existing sections of the program originally written for the old basic hardware.“ The company was able to develop and coordinate all the required software blocks itself. WinAC can execute on virtually any popular PC under Windows NT or 2000, and the latest version can also execute under Windows XP, so it does not place any special demands on the computer hardware.
The amount of time available for implementation was extremely limited, as production could hardly be interrupted or switched to manual mode for long. „It was an advantage to everyone that it was pos-sible to develop and thoroughly test the new processes without additional hardware and entirely within an office setting,“ emphasises Joerg Preusch (former Contec employee and now of Ingenieurbuero Preusch engineering consultants, Tettnang), who specialised in programming the application in C++. The engineering took just under six weeks. Integration in the existing plant, commissioning and finally the installation of a new coordinator PC were completed in less than two weeks.
„Plant operators and plant management at EVC have thoroughly embraced our PC-based solution for dosing microcomponents,“ says Guenter Tiefensee, a statement which is confirmed by Ludwig Guetle: „The processes are more user-friendly, more transparent, and therefore also more efficient.“
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