Traceability and scrap reduction in extruded products.
* Productivity and profit improvement initiatives;
* increased governmental regulations;
* the introduction of increased quality standards and certifications; and
* the need for manufacturers to limit risk and warranty exposure.
The rubber products manufacturing industry is particularly affected by these forces. Tire manufacturing, in particular, has necessarily taken leadership in many advancements in product marking technology.
Although relevant to a wide range of rubber manufacturing industries, this article primarily focuses on issues and solutions developed through and for the tire manufacturing industry and, more specifically, on new developments in the marking and tracking of uncured rubber products.
Evolution of marking in tire manufacturing
From the onset, fire manufacturing components and products have been marked in some way. Calendered stock, tread extrusions, plies, beads and finished fires have been marked with tags, offset printers, indenting wheels, paint, acetate labels, hand stamps, even house paints. These early marks provided basic information used for in-house parts tracking and assembly.
A number of relatively recent events began to affect both the necessity and methods for marking tire products, including:
* A 1978 recall of 14 million tires at a cost of $325 million;
* a 2000 recall of 17 million tires at an ultimate cost of $7.1 billion;
* the desire by auto manufacturers for a well-dressed product;
* the introduction of ISO standardization;
* increased government regulatory requirements;
* amplified price and profit pressures forcing an ever-in-creasing need to cut production and scrap costs; and
* the development of new, more complex tire designs.
The requirement for real-time production information has rapidly escalated in conjunction with advanced manufacturing techniques, and to all stages of manufacturing. Internal mixing, calendering, assembly, final finish and post-cure marking all have begun to move toward integrated marking and product tracking. In essence, modern marking equipment has begun to take on a more meaningful role as a way to increase profit, productivity and quality output. However, despite significant effort, non-contact marking of uncured rubber extrusions has remained elusive.
Marking uncured rubber extrusions--the challenge While advancements in product marking and tracking have been introduced to various areas of the manufacturing process, the marking of uncured stock, extruded tread and calendered products, in particular, has remained unchanged for decades. Most uncured extrusions are still marked with a 24" (circumference) contact printing wheel that applies pigmented ink. The marking typically indicates a lot or batch number, the date and sometimes the shift identifier. The limitations of this marking method have been many and include:
* The extruder operator is required to change the printing type on the contact print wheel with almost every change of the tire extrusion. Extrusion changeovers can occur many times per shift. Multiple changeovers create opportunities to allow sections of product to be mis-marked due to errors in inserting (or not inserting) the correct printing type.
* Contact between the print wheel and the extruded rubber can lead to deformed tread profiles, or a "pick up" of the extruded rubber by the contact wheel as they make contact. This creates out-of-spec sections of material that need to be scrapped or recycled.
* A degradation in mark clarity due to worn or dirty printer type can lead to illegible print on tread extrusion.
* With certain new tire designs, the use of traditional contact printing inks can introduce a contaminant to the tread extrusion. In essence, should traditional contact ink be applied to the wrong location on a tread, it could eventually be cured into the folds of the tread, resulting in potential delamination.
For these reasons, tire manufacturers have continued to experience a significant amount of lost product and production time (man-hours) due to the production, identification and tracking of mis-built or sub-standard tires.
Calendered fabric and calendered wire are marked in similar fashion. Modern final inspection techniques can now identify manufacturing defects that reveal faulty production. It is difficult to identify production problems within a specific time frame due to the insufficient information marked by contact printers on internal components. Consequently, finished products are scrapped due to faulty components, and accompanying production time and profits are lost.
In essence, there has long been a real opportunity to generate manufacturing savings through a more advanced, integrated marking method that would be able to place real-time tracking information on a moving substrate without contacting the surface. The logical choice for this application was an ink jet marking system.
Early attempts to place traceable marks on uncured rubber with ink jet printers In the mid 1990s there were notable attempts by several ink jet printer manufacturers to mark uncured rubber with pigmented ink.
Of the two basic ink jet technologies available, the "drop-on-demand" (a.k.a. large character) technology was the primary choice for this application due to the "DOD's" ability to print large characters, operate in a rugged manufacturing environment, and potentially apply a pigmented ink.
Traditional "DOD" ink jet was (and remains) a simple technology whereby ink, under low pressure, is introduced to a printhead that uses a series of valved orifices. As the substrate travels beneath the printhead, a controller signals solenoids within the printhead to eject drops of ink, forming characters in a dot matrix pattern on the substrate (in this case, extruded rubber).
Despite numerous attempts and trial runs by ink jet printer manufacturers, this particular technology failed to perform with this application for one or more of the following reasons:
* Printhead design. Pigment suspended in the ink carrier (solvent) eventually settles into the crevices of printhead valves, ultimately obstructing the orifice(s) and leading to printhead clogs and eventual failure. Clogging occurred frequently and was amplified by elevated temperature of the extrusion being marked.
* Ink supply design. Some DOD equipment did not effectively address the need to keep pigment constantly suspended in the cartier. Ultimately, pigment would separate and settle within the printheads, ink supply lines and the ink source, again leading to printhead clogs requiring significant maintenance.
* Ink composition. Available pigmented inks (normally MEK-based) were designed to prevent transfer to the walls of the mold or obstruction of the press vents during curing. However, these inks could not be applied to internal tire components such as calendered extrusion, nor could they be allowed to accidentally transfer to the underside of a tread extrusion. The inks had no adhesive properties and presented potential for contamination.
More recent attempts to improve the design of existing ink jet printers have all failed, primarily due to printhead and ink design problems. Nevertheless, the need to place traceable marks on uncured extruded rubber continued to grow.
Development of the rubber printing system (RPS)
In late 2002, Pannier spearheaded the development of a specialized ink jet printing system to place traceable marks with pigmented ink on uncured rubber extrusions. The established objectives of the project were to develop an ink jet printing system that would:
* Print real-time production information on uncured tread and calendered stock;
* utilize a pigmented ink that could be applied to both tread stock and internal tire components;
* not require excessive maintenance and downtime;
* eliminate the need for operator interface by enabling serial integration via a PLC;
* be used in most areas of a rubber manufacturing plant; and
* apply disappearing ink that would vanish at curing. (This secondary objective arose as tire plants requested a method for marking sidewall extrusion.)
By early 2004, the Rubber Printing System (RPS) was developed, tested, and installed on a tread extruding line in a major tire manufacturing plant where it continues to operate per initial project objectives.
The success of the Rubber Printing System is due to the following technological advances.
New ink jet technology
The ink jet component of the RPS has been custom developed by REA Elektronic GmbH of Muhltal, Germany. The RPS ink jet printer has four main components. Referencing figure 1 from the ground up, the components are the ink supply pump, controller, marking head and input device. The optional stand is shown supporting the controller, marking head and input device. All main system components of the RPS are IP65 rated (wash down and dust proof, not submersible). The RPS is classified as a large character industrial ink jet printer. Large character means that it can create characters ranging from 13 mm to 67 mm in height. The marking head utilizes 16 vertically arranged nozzles to produce a 16 x 10 dot matrix character. A 7 x 5 font can also be selected to create two separate lines of text, one above the other.
[FIGURE 1 OMITTED]
The maximum character size, shown in figure 2, is marked with the nozzle plate oriented vertically (straight up and down). The character height is reduced by rotating the head clockwise and adjusting the print height parameter in the software. Marking speed is not reduced as the size of the character is increased.
[FIGURE 2 OMITTED]
* The marking head is constructed of an aluminum cylinder enclosed by aluminum disks at each end. O-rings seal the disks. The nozzle plate is stainless steel. Nozzles made of a stone material provide the precision orifice openings that de liver the ink. It is a patented design that utilizes a new type of DOD ink jet technology. The marking head incorporates plungers (no valves) driven by tiny electric solenoids. To place a dot on the substrate, the plunger is quickly raised and then lowered to release a tiny cylinder of ink. When the cylinder of ink hits the substrate, it forms a dot. The self-cleaning design accounts for high reliability with very little need for maintenance.
* The ink supply pump consists of an electric pump enclosed in a stainless steel IP65-rated enclosure. RPS marking heads have two ink supply connectors. The ink supply pump constantly circulates ink through the marking head during normal operation. The pump also contains a timer for periodically circulating ink to the printheads when the system is not operating.
* The input device provides the human interface to the controller. Marking formats are programmed (created and modified) and stored on the input device. The input device can store 99 different marking texts of 500 characters each. When equipped with additional memory, it can store 1,000 marking texts.
* The controller is the brain of the RPS. It times the firing of the solenoids to produce the dot matrix characters. It accepts the inputs of the shaft encoder and product sensor. The shaft encoder is used for marking applications where the conveyor or line speed is unknown, variable or fluctuates. The controller enables variable adjustment of print height, print width, character gap and print repeating. It contains a real-time clock and calendar for printing the date and time. Character orientation can be normal, upside down, forwards or backwards. The controller can maintain incrementing or decrementing counters. It can also print graphics and simple logos. The controller can be used as a stand-alone unit or be integrated as a computer-controlled component of a larger system. The controller is programmed via the input device. Controllers can be optionally configured to control one or two marking heads. The basic parameters and text for the active marking format are stored in the controller's buffer(s). A two-head controller can independently control heads mounted on the same or different marking lines.
RPS communications technology
The input device is normally provided with an RS232 or RS422 serial communication port. The serial port is used to receive data from an external host computer such as a PLC or PC. Variable text from an external host can be combined with constant text in the same printed message. The printed message automatically changes when the compound changes, eliminating the need for manual changeover.
The input device can be disconnected from the controller when programming is complete. It is not necessary for marking, unless it is being used to receive data from an external host computer. The controller powers the input device.
APV Engineered Coatings of Akron, OH, a leading producer of chemicals for the tire manufacturing industry, has formulated co-curable and disappearing inks for use in the RPS system.
The co-curable ink has been designed and proven to not only provide a high-quality pigmented contrast, which will neither transfer nor clog vents during the process, but, more importantly, contains adhesive properties that allow the ink to be applied to any internal component of a tire or rubber compound. In fact, the ink possesses more adhesion than the surrounding rubber layers.
Formulating the marking ink
The co-curable marking ink has been developed to obtain good adhesion test results on a wide variety of rubber compounds used in the manufacture of tires. Tire components usually consist of various elastomers or elastomer blends of the following: Styrene butadiene rubber; polybutadiene; polyisoprene; natural rubber and NR blends, various grades; blends of chlorobutyl rubber; and other synthetic rubbers.
In general, inks consisting of an unsaturated base elastomer that is similar in composition to that of the substrate may offer a good possibility for adhesion development. In this regard, formulations can be developed for application to a wide variety of non-tire substrates. A curative package consisting of accelerators and crosslinking agents may be used, although this may not be required to develop adequate adhesion.
Adhesion test description
The objective of an adhesion test is to determine the amount of force required to separate laminate layers of a substrate coated with a marking ink and cured together in a manner similar to the process used to produce a rubber article.
This interfacial adhesion peel force is obtained by mechanically pulling apart layers of compound at a 180[degrees] angle on the tensile tester. One of these compound layers is coated with ink formulated to co-cure with the compound substrate. The test specimen is a sandwich consisting of a square woven backing, milled compound and a Mylar sheet with cutouts of the desired dimension. One of the milled compound layers is coated with a thin layer of ink and dried prior to assembly of the test specimen. Coating of this compound may be achieved by brushing or by using the actual print process for application.
The test specimen assembly is cured in a positive pressure mold using an established time-temperature curing cycle. After cooling, this test specimen is tensile tested. Results are given as the average peel force in Newtons, or the average peel force per specimen cutout width.
Testing conditions can be established to best simulate a given process. Variables consist of:
* Compound composition and thickness;
* cure time, temperature and applied pressure;
* cross sectional width dimension of the Mylar cutout;
* method of ink application; and
* tensile tester jaw speed.
A marking ink has been developed to disappear upon curing. This formulation uses the high temperatures encountered in the curing process to melt the ink solids' composition. The melted ink components are then absorbed into the affected rubber compounds and are no longer visible because the ink is no longer present. Laminate layers of compound are free to co-cure and develop strong adhesive bonds.
Applications and implied savings
In early 2004, the first RPS was installed on the tread extrusion line of a major tire manufacturer. The system has been running since that time and has been printing nearly all product produced on a 24/7 basis with little maintenance and operator interface. Following this initial installation, a number of RPS systems have been installed in tire and non-tire rubber manufacturing facilities. As the capability of this new technology is realized by rubber manufacturing companies, there has been an increasing number of requests to mark uncured and post-cured rubber products that were previously not able to be marked, and can potentially yield even more quality and productivity improvement results.
At this writing, specific savings are not available for publication and the aggregation of industry data of this early-stage product would be considered premature. Some examples of current RPS applications and implied savings are listed in table 1.
The recent development and introduction of an integrated ink jet system provides the rubber manufacturing industry with an entirely new avenue for generating savings in the form of reduced scrap, improved quality and reduced manufacturing man-hours. Early stage results are yielding real returns on both capital investment and operation. New applications for this marking system are being discovered on a daily basis. The introduction of a marking system that contributes to a company's "bottom-line" is an indication of new technologies being developed by the marking equipment manufacturing industry.
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|Date:||Jul 1, 2006|
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