A Communication from the PLACE Div. of TAPPI
- Published: September 30, 2005, By David J. Bentley, Jr., Contributing Editor
Providing practical information to the converting and packaging industries…
EXECUTIVE SUMMARY
Control Of Aerosol Contamination
by Wolf Ranzenbacher, Norbert Jedrzejak, and Alwin Braun, United Air Specialists, Inc.
Coating, laminating, extrusion, and the production of plastic granulates produce as a side effect smoke and aerosol contaminations. These consist of polymer products, plasticizers, fine particles, steam, and other materials. They are dispersed into the atmosphere or directly into the surrounding working area. What can we do about this? Leaving it to disperse into the work shop is certainly not a good idea. Consequences of such practice would include contaminated product, high factory maintenance costs, and health risks to workers. As an example, a laminator in one particular plant produced an aerosol volume of 1.72 l per day. Assuming a production year of 320 working days, this gives 520 l of effluent.
Various filtration technologies are available. Cost and performance ratio should influence which to select. Simple mechanical filters can provide some protection, but the filtration components require changing often and have poor collection efficiency. A better solution is the use of electrostatic precipitation systems.
A two-stage electrostatic precipitator has two sections—a charging or ionizing section and a collection section. The charging section contains a series of fine wires suspended between metal plates. The collection section is a series of parallel, flat metal plates spaced approximately 6 mm apart. The field is non-uniform because it is very strong near the wire. It decreases rapidly as distance from the wire increases to a relatively low value at the surface of the grounded plates.
The best aerosol control derives from a good filtration plant designed to fit the specific pollutant problem as closely as possible. This encompasses effective collection of particles and aerosols, excellent collection efficiency, simple operational requirements, a low cost of operation, and safety.
EXECUTIVE SUMMARY
Heat Transfer Rolls: Function, Application and Design
by George R. Cozzarin and Timothy W. Womer, Xaloy, Inc.
The two factors that define the function of a roll are the process and the product. Common processes such as cast film, extrusion coating, extrusion coating laminating, sheet extrusion, metallizing, printing, flame treating, and corona treating typically use cooling rolls. Laminating, embossing, curing, annealing, orienting, and coating are examples of processes that require heating rolls and perhaps cooling rolls too.
After determining basic function, a user needs more detailed information to begin evaluating options for determining the internal design of a heat transfer roll. Is the product LDPE, HDPE, polyester, polypropylene, paper, EPDM, etc. Is it single layer, multi-layer, woven, or nonwoven? What are the properties of the product? What are the process temperatures? What is the desired production rate or line speed? These are but some of the necessary information.
The criticality of the nature of an application will dictate the internal design of the heat transfer roll. The internal design options listed in order of heat transfer efficiency are single shell, double shell, double shell-gap spiral, and double shell-sealed spiral.
Of the variety of heat transfer roll constructions available, the sealed spiral roll design offers a processor the most flexibility. By determining optimization of design through process analysis, the efficiency achieved results in lower capital costs and decreased operating expenses. Increased product throughput and quality improvements are also possible. The process analysis tool aids a designer in determining what the processor can do to maximize the use of heat transfer rolls within a process.
Double shell rolls incorporate the annulus created between the two shells as the fluid flow path. This requires the fluid to stay within this cylindrical area. Improvement in heat transfer efficiency occurs over the single shell design. Nevertheless, fluid velocities sufficiently high to promote turbulent flow and the inconsistent flow throughout the entire heat transfer area of the roll make this design less than optimal for critical applications. The design has had successful use in cooling rubber-covered rolls. Some amount of heat may be absorbed into the rubber, and the roll will facilitate keeping the bond of the rubber to the shell sufficiently cool to prevent delamination. The use of double shell rolls is also a good design choice for nipping against rubber back-up rolls to keep the surface of the rubber cool.
EXECUTIVE SUMMARY
What Does Aluminum Foil Have To Do With Hot Tack In Packaging?
by Gunter Schubert, Hydro Aluminum Deutschland GmbH
The primary reason for using aluminium foil in flexible packaging is its outstanding barrier properties. The absolute barrier performance achievable with aluminium foil and the unrivalled barrier and cost efficiency of foil laminates are not surpassed by any other material.
Looking at modern packaging, the focus is not simply on laminate properties. The entire system including packaging scrap, machine speed, and packaging closure safety and the related costs require consideration. When sealing robustness and low reject quota are primarily required but higher throughput and productivity are demanded, the hot tack performance of a packaging laminate needs improvement. Hot tack depends on the multilayer structure, the individual layer thicknesses, the placement of the layers, the choice of material, and the packaging environment and conditions. A general simplification is that hot tack is the responsibility of the sealant only. Higher productivity, improved safety, and lower overall cost per packaging unit are possible by employing more expensive and better performing sealants although other factors require consideration too.
Hot tack is usually defined as the strength or resistance of a hot seal measured at a specified time interval after the completion of a sealing cycle but before the temperature of the seal reaching ambient temperature. Hot tack curves typically describe this maximum force in relation to the heat seal jaw temperature. Another way of describing hot tack from a different point of view is to monitor the resistance of a just-sealed seam being peeled at constant peeling speed over time at a high rate of data collection. This type of measurement describes the solidification time indirectly.
Heat transfer properties are often disregarded when choosing or designing packaging materials for heat sealing. The thermal conductivity of each component used in a laminate contributes to the overall sealing and packaging performance. Seal seams take surprisingly long times to solidify or longer than expected. Aluminium foil plays a significant role in efficient heat transfer during sealing and heat dissipation during cooling to reduce the hot tack time and thereby contributes to sealing performance and safety. The use of aluminium foil in packaging has the potential to increase line speed in processes where seal seams are subject to loads in subsequent handling.
EXECUTIVE SUMMARY
What's Inside: Thermal Process Imaging Permits A Real Time View Of Your Extrusion Process
by Andrew W. Christie, Optex Process Solutions, LLC
Product quality and functional performance are critically related to extruder melt consistency. Melt consistency (MI) has been a topic of intense study since the early days of extrusion processing. The challenge for all production operations today is to improve product quality and production efficiency. For many operations, this challenge is complicated by shorter production runs and more varied products with added complexity and coextrusion. How do you assure that you process a high MI acid copolymer and a fractional melt linear LDPE with equal consistency? How do you establish an optimum extruder temperature profile? Do you increase back pressure to increase work in the screw? Do you add a static mixer? These and many other questions confront the extruder operations manager daily. He often must make a decision with limited or misleading data.
Because of the varied resins run on a single extruder in many extrusion coating operations, the screw is frequently run under conditions far from its original optimum design point. To compensate for these conditions, temperature settings and valve adjustments are often required to achieve the desired melt temperatures. Most operations use a fixed position shielded junction melt thermocouple for melt temperature feedback.
Understanding flow distribution problems in coextrusion applications is often difficult. Consider a converter who was experiencing flow problems in an AB coextrusion coating application. The application was a film to foil lamination using a LDPE/Acid Copolymer (AC) coextrusion. The AC required a processing temperature of less than 310°C. The LDPE was run at approximately 325°C. Although the converter did not have detailed rheology curves on the polymers, an initial thermal image showed that the lower temperature AC polymer was encapsulating the LDPE. The concentration of the low temperature polymers at the edges indicated that at 310°C the AC is still lower viscosity than the LDPE and therefore is encapsulating as it distributes through the die.
Although the analyses in this paper were not conducted as controlled experiments, an IR process imager allows one to begin to see into an extruder. The IR process imager is a useful tool for analyzing and understanding the temperature field variation in extrusion processing. This tool allows simultaneous monitoring of both cross web (position dependent) temperature and down web (time dependent) temperature variation. Coupling this process data with simulation analysis greatly enhances the capability of understanding and interacting with extrusion processes for process improvement and optimization. Software utilities will further allow the use of this system for process control.
For information about the PLACE Division of TAPPI, access the TAPPI web page at tappi.org. To obtain the complete papers whose expanded summaries appear in this section, go to the TAPPI web site at tappi.org., then click on "the PLACE" in the section designated Journals.
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