Drum-sized slurry factory includes computer-controlled dip tank management.
A host of issues
The first coatings were applied by hand. Workers would scatter handfuls of coating powder, typically talc, on the stock before stacking and bundling for shipment or later use. Hand dusting was messy, inconsistent, labor-intensive and very unhealthy.
As rubber production became more automated, a move was made to a box with openings on the sides through which the uncured slab would pass. It was called a dust box and it was a better idea, but the lack of consistency and the attendant housekeeping chores remained.
The next advancement in coating technology was to float the coating material on water in the precursor to today's dip tank. The transition to floating coating materials in a dip tank was a welcome improvement, but controlling the coating's consistency remained difficult.
The most recent change, in the beginning of the last century, was the introduction of dispersions and slurries for the dip tank. Consistency has been improved, but there are still issues, for example, when adding slurry, diluted or full strength, to the tank, the take-up of coating on the stock increases. As the solids level drops, the coating becomes thin and the resulting coating can be insufficient for the purpose intended. Measuring the coating during the course of a production run shows clearly that the coating is not comparable end-to-end. The inconsistency is frequently obvious when looking at a pallet of stock from a given production run (figure 1).
The logical solution is to monitor the dip tank's solids level and use a feedback loop to maintain the desired solids percentage. This approach presents two variables that need to be addressed. The first variable is the obvious solids level management. The second variable to be controlled is fluid level management. Solutions for the second problem exist: Level sensors, that trigger addition of slurry and/or water, can hold the tank's fluid level to a consistent range. These sensors are typically pairs of float switches mounted at different heights. The lower float switch triggers the introduction of fluid, usually with a pump, while the upper float switch is used to indicate the dip tank's upper fill level.
The solution to the solids level problem, however, is not as straightforward as depth-based set-point triggers. The ability to continuously monitor the solids percentage and add slurry, or water, or both, is a complex problem best left to computers and specialized sensors.
Throughout the industry, dip tank solids level "management" often consists of plant personnel making subjective judgements about the coating being applied. When the coating looks light, some form of concentrated dip is added to the dip tank. It is surprisingly common for the dip replenishment "system" to consist of a drum of diluted dip and a pail used to transfer the dip from the drum to the dip tank (figure 2). These "systems" are best described as "inexact."
There have been attempts to solve the solids level problem using, for example, the conductivity of the dip in the tank. These approaches represent a valiant effort, but it is clear that changes in the water's properties, temperature, chlorine level, etc., have a significant effect on the measurements. Further, the conductivity of, for example, calcium carbonate or kaolin clay is essentially non-existent, so doping the slurry with conductive materials is risky at best because their concentration is unlikely to vary in direct proportion to the concentration of the carbonate or clay.
In addition to the dip tank's fluid level and the dip's percent solids level, there are many other problems related to the dip tank (figure 3).
Transferring dip from one vessel to another can be quite messy with the "drum-and-pail" system. Slab dip splashes and dribbles on the equipment's exterior and on the floor as it drips off the outside of the pail. The negative effects range from equipment shutdowns due to spills, to an unsafe work environment and unnecessary injuries.
The dispersions and slurries in use today typically have issues with foam. When stock is coated with a foamy froth, sticking becomes a significant problem which causes rejects. As the foam builds up in the tank, the bursting of bubbles coating the stock presents sticking points which work counter to the coating's intended purpose. When the foam breaches the top of the tank, workplace safety is compromised and the well-being of employees is at risk. Dedicating or re-tasking employees to cleaning the dip tank area is wasteful.
Setting aside the worker safety, housekeeping and reject issues caused by today's dip products, there continue to remain significant problems that need to be solved.
When dispersions and slurries are brought into the plant, the percent solids ranges from typically about 36% to as much as 50%. These solids level concentrations are far too high for direct use in the dip tank. Adding high solids percentage dip product directly to the tank is problematic in that only small amounts can be added at a time if the coating is to be even somewhat consistent. Further, agitation is often required in an attempt to maintain consistent dip concentration throughout the tank. If the dip is prone to foaming, that agitation, no matter how necessary, works against you.
The obvious solution is not optimal: Dilute the delivered product with plant water to a level that is at or near the desired use level in the dip tank. This introduces a handling step to meter dip as delivered into a vessel, add water, and mix for some period of time. Thankfully, some dip suppliers provide so-called dilution systems to take care of that element of the problem. For those not so fortunate as to have a dilution system, the process is labor intensive and error prone. The "drum-and-pail" system mentioned above is a common approach to solving this problem.
The issues relating to consistency are a constant struggle. When the delivered dip is diluted to use level, heat-related water evaporation causes the solids level to climb, necessitating the addition of water to the dip tank. When evaporation is minimal, the solids level drops, and adding dip diluted to use level can never bring the dip tank's solids level back up to the desired solids level. So, the delivered dip is typically diluted to a level above the desired use level. The side effect is the continuous judicious addition of water in an effort to compensate for the higher solids level in the replenishment dip.
In a perfect world, the dip tank headaches that have been reviewed thus far would not exist. Unfortunately, there are common issues with the coating itself, either in use or as delivered. We have already mentioned foam, but the story does not end there.
Dispersions and slurries are necessarily delivered in drams, totes and tankers. While handling drums and totes, and offloading from tankers into intermediate storage tanks, are mature technologies, issues with spills are serious. Workplace safety is a major issue, and cleaning up after a spill is time consuming and expensive. Just one dropped 400-plus pound drum, or an accidentally punctured tote or intermediate tank failure can have a significant negative effect on workplace safety, productivity and profit.
Even in the best of circumstances, today's coatings can be fraught with problems. Depending on the stock, coating pick-up can be hard to control. Many coatings dry very slowly, necessitating festoons and fans. Poorly formulated coatings can flake and dust off of the stock. Other coatings are absorbed by the stock and are rendered ineffective. Coatings that sludge in the dip tank are common. Dips that thicken in storage present a handling nightmare. And build-up on the equipment can cause production of scrap, instead of usable stock.
Clearly, many serious issues surround the dip tank and coatings.
What is wrong with common practice?
The current approach to coatings and the dip tank is accepted as the "best practice" in the industry, despite all of the negatives. There has not been any challenge to the status quo.
In 2010, Nicholas Hill, a seasoned formulating chemist who at that time had already spent more than three decades developing dips and consulting for many dip producers, realized that he was perpetuating mediocrity. Dip system technology had not evolved in decades, despite the many advances in material science and automation. It was time for a change.
If you step back and look at the big picture, it becomes clear that the problems with the current dip technology are many manifestations of a core set of just three issues:
* Dip quality
* Dip delivery and handling
* Dip tank management
Each of these three core issues has been partially addressed, but not with a single, complete, cohesive solution.
Producing a quality dip requires an intimate understanding of the products being coated. Improvement of the delivery and handling requirements necessitates a new, unconventional approach. Managing the dip tank itself is the critical bridge between the dip and its proper application.
Stocks are generally either productive stocks used for end products or non-productive intennediate stocks. Productive stocks are typically coated with a stearate-based product, while non-productive stocks are typically coated with a mineral-based product.
As the industry has evolved, dips have appeared to evolve. These appearances, however, are false. Having a multitude of different products underscores the quality issue. Suppliers have thus far been unable to produce high quality general purpose dips. An old-fashioned dip designed for a highly oiled stock is not optimal for other stocks. The standard approach is "if this dip does not work, try this other dip." That is not evolution.
Delivery and handling have stagnated. When the industry moved to dispersions and slurries, products were packaged in pails, drums, totes and tankers. Dip technology has not evolved, so delivery and handling are unchanged.
Effective dip tank management is non-existent. Plant personnel making judgement calls about the sufficiency of a coating is no more than guesswork. Dilution systems available today do no more than that - dilute the delivered dip. They do not manage the dip tank's solids level.
The "good enough is good enough" mentality does not lend itself to innovation, and these accepted practices are deeply ingrained throughout the industry.
A new approach
The common thread for all but very specialized dips in use today is water. Whether a mineral-based dip is provided in powder form or a slurry, or a stearate-based dip is provided as a dispersion, all dips today are applied in a water-based system. Mineral-based powders are added to water. Slurries and dispersions are diluted with water. Most of what is in the dip tank is water.
Water is heavy. Consider a typical calcium stearate-based dip. The solids level is on the order of 36%. This means that there are just over 275 pounds of water in every 430 pound drum shipped to your plant. And in a 2,000 pound tote, there are 1,280 pounds of water. Buying someone else's water and then shipping that water from their production facility to your plant is an expensive proposition.
The new paradigm for high-quality slab dips that are easily handled is dips delivered in powder form, whether they are mineral-or stearate-based. The addition of water at the last possible moment provides many substantive savings, including a significant reduction in freight costs and storage space requirements. There are no full or empty totes or drums to take up space.
The new breed of slab dips must be held to a very high set of standards. They cannot thicken in storage or build up on equipment. They must be immune to freezing and cannot sludge out in the dip tank. They must dry quickly, be picked up consistently, not flake or dust off of the coated stock, and cannot be absorbed. Most importantly, foam and housekeeping issues must be eliminated.
The delivery and management of the new breed of slab dips should take advantage of the equipment that exists in plants today, and not force onerous procedural changes or major plant alteration to support the new dips.
Furthering the paradigm shift is effective dip tank management. Management of the dip tank needs to be completely automated with a true "set it and forget it" system that works with the plant personnel.
After years of development, a slab dip system that satisfies all of these requirements has finally been created.
Quality slab dips
The Superior Formulations LLC OverCoat line of slab dips addresses all of the historical issues with slab dips and solves all of the problems. The OverCoat line of slab dips provides an antistick coating that does not interfere with the rubber's chemistry. OverCoat dips are available as calcium carbonate or kaolin clay mineral-based products, or as calcium stearate- or zinc stearate-based products. OverCoat dips are suitable for both non-productive and productive stocks.
OverCoat dips do not foam. Since OverCoat dips are hydrated as needed on-demand, the notion of thickening in storage is non-existent and they are immune to freezing during shipping. OverCoat dips are state-of-the-art formulations that do not sludge out, do not float and will not build up on equipment. Because of the careful formulation, OverCoat dips coat consistently, will not dust or flake off, and are not absorbed into the coated stock.
Handling the dry OverCoat powders takes advantage of existing plant equipment. Nothing more than a forklift to move pallets or bulk sacks is required.
Dip delivery and dip tank management
The Superior Formulations LLC Constituter is a self-contained computer-controlled robotic slurry factory and dip tank manager. Its only requirements are power, a water line and an Internet connection (which is typically WiFi). Internally, there are five computers working together to provide dip delivery and management. It is important to note that Constituters are provided at no charge for use with OverCoat dips (figure 4).
The smallest Constituter has a footprint of just 18" x 27", roughly the size of a 55-gallon drum. Operating a Constituter is easy and no special training is needed. The desired dip solids level is "dialed in" on the control panel. That is it.
Once the desired dip tank solids level has been set, day-today operation consists simply of plant personnel emptying bags of OverCoat powder into the Constituter's hopper, or switching out bulk sacks, as needed. The Constituter does the rest. It meters water and OverCoat powder into its internal mix tank, maintains an appropriate temperature for the water, and mixes the OverCoat to produce OverCoat concentrate. The concentrate is then moved to an internal storage tank and the Constituter starts making the next batch of concentrate. When the storage tank has been depleted, the ready-made batch in the mix tank is moved to the storage tank and the Constituter starts on the next batch. In this way, the Constituter always has ready-to-use OverCoat on board.
Built in to each Constituter is a proprietary device developed by Superior Formulations LLC that monitors the dip. The method used to test the dip in the dip tank is proprietary and is unaffected by the dip's temperature. The device is also immune to treatments such as chlorine in the water supply.
A probe assembly that allows the Constituter to continuously monitor the dip tank's fluid level and the dip's solids percentage is mounted to the side of the dip tank. The Constituter's fluid level monitor does not use switches, acoustics or optics, and is accurate to within a fraction of an inch, so the dip tank's volume is always known with very high accuracy. The solids percentage device tests the dip in the tank every several seconds and is able to discriminate to within a fraction of a percent. The Constituter is then able to accurately compute the amount of concentrate and/or water that needs to be added to the dip tank to maintain the tank's physical level as well as the solids percentage.
During normal operation, the Constituter continuously monitors its performance. Statistics are communicated over the Internet to the Superior Formulations LLC servers for purposes of self-diagnostic reporting and inventory awareness.
One size fits most
Designing and sizing a dip tank management system to meet the needs of a given production line is not a simple or trivial task. The requirements, whether a fully manual "drum-and-pail" system or a semi-automated mixing station supporting a partly manual system, can vary widely. A small line running a high durometer stock has needs that are vastly different from those for a large line running a highly oiled, low durometer stock.
There are a number of factors involved. Flow big is the dip tank? What is the pick-up rate of the coating? How fast is the stock moving through the tank? What is the desired solids percentage? Is water being lost through evaporation? If so, at what rate?
The answers to these and other questions can have a significant impact on the nature of the systems and procedures employed for "managing" the dip tank and producing a coating that is consistent, not excessive, and repeatable. The only component that appears to be static is the dip tank's size. In practice, though, the dip tank's fluid level varies during a production run. The other components are obviously dynamic. Different stocks have different pick-up rates. Speed can vary as a function of thickness or width. Stocks that exhibit cold flow characteristics need a more robust coating than harder, high durometer stocks. Loss of water varies as a function of ambient humidity and the dip's temperature, which will vary based on the stock's temperature and the plant's ambient temperature.
The goal, of course, is to maintain consistent solids and fluid levels in the dip tank. When you factor in all of the variables, it may be possible to make a reasonable guess as to how much dip, and at what solids level, is needed to be on hand for dip tank replenishment during a production ran. Maintaining a consistent solids level, however, remains a daunting task. As the production environment changes, so too must manual dip tank management systems and procedures.
The Constituter eliminates all of the guesswork relating to managing the dip tank. Once the desired solids level is set for the stock being run, the Constituter ensures that the dip tank environment remains consistent. During installation, the tank is sized and the optimal fluid level is identified. The Constituter's probes provide input to the state-of-the-art software running on the Constituter's main computer. Four computers operate in various support roles to the main computer, handling tasks like taking sensor readings and monitoring the Constituter's wellbeing. The communications processor handles the Internet connection and communicates in spoken English to the personnel on the plant floor.
The Constituter does not require any modifications to the dip tank. There is no need for the dip tank to be mounted on load cells, and no plumbing is needed for external fluid circulation. All that is needed is the compact probe assembly mounted to the side of the dip tank.
The basic Constituter's general-purpose design has been optimized to support almost every installation. One basic Constituter is able to produce, in just one hour, more than 60 gallons of dip at use levels needed to maintain typical production line use levels. Since there is a Constituter dedicated to each line, that is more than sufficient for the majority of plants. In those cases where a basic Constituter is unable to produce enough dip, Constitutes can be linked in cooperative groups. Alternatively, larger Constituters can be pressed into service.
In this way, Superior Formulations LLC has created an easy-to-use system that can be used in virtually every plant on virtually any production line.
What are you going to do with all the money you save?
Freight is a significant cost component of conventional slab dips. We present freight costs for various key locations throughout the U.S. showing the cost as a function of weight in table 1. These rates are non-discounted rates computed from our manufacturing facility in Georgia, and will clearly vary depending on a given supplier's location, but they serve to illustrate our point.
Let us first consider a stearate-based dip and its attendant freight costs. A single skid of OverCoat H, for example, shipped to an Akron, OH, plant, carries a freight cost of $602.49. (We show the 2,000 pounds plus 100 pounds to cover the weight of the skid and packaging). When a comparable 2,000 pounds of 36% solids stearate-based slurry is shipped, the cost jumps by a factor of 2.7 because of the water. Straight math indicates that the cost is $1,626.72, $1,024.23 of which is paid to ship water. Looking at 16,000 pounds of OverCoat powder shipped to Akron, the cost is $2,508.80. A slurry would cost $6,773.76, with water accounting for $4,264.96. Over the course of a year, those savings are substantial. Presuming just 16,000 pounds per month, the saving is more than $51,000 per year.
Think about this. Using 16,000 of slurry pounds per month, the freight paid to ship a conventional slurry for the first four months of the year is slightly more than what would be paid for shipping an entire year's worth of OverCoat powder.
The savings realized when shipping a mineral-based dip, even at 50% solids, is still a substantial saving of 50% of the freight bill.
Additional savings are seen when heated trailers (or tankers) and storage space are no longer needed during the winter months. It is hard to calculate the saving from needing half or less floor space to store your dip, but those savings are real, too.
A complete turnkey solution
With the Superior Formulations LLC OverCoat/Constituter system, slab dips and dip tank management have evolved to the 21st century. This system is a technological game-changer for the industry.
The benefits of computerized dip tank management coupled with delivered-dry slab dips eliminate the problems and fill the voids present in conventional slab dip systems. Using the plant's water results in significant savings in freight and a significant reduction in storage space requirements. The OverCoat line of dips eliminates the problems with conventional dips, including foaming, inconsistent coating, sludging, thickening, and dusting or flaking off of coated stock.
by Kristopher K. Hill, Superior Formulations, LLC
Table 1--freight costs from Pendergrass, GA, to various cities Weight (lbs.) 2,100 4,200 8,400 Greenville, NC $264.72 $498.52 $837.52 Charlotte, NC $313.44 $590.39 $1,003.67 Knoxville, TN $321.70 $607.10 $1,019.92 Memphis, TN $470.12 $875.10 $1,470.17 Akron, OFI $602.49 $1,012.08 $1,493.34 Chicago, IL $621.12 $1,043.44 $1,539.37 St. Louis, MO $635.48 $1,068.68 $1,799.63 Dallas, TX $718.64 $1,330.57 $2,235.35 Des Moines, IA $771.03 $1,428.01 $2,399.05 Detroit, Ml $771.03 $1,428.01 $2,399.05 Weight (lbs.) 16,800 23,100 40,000 Greenville, NC $1,439.93 $1,979.90 $3,428.40 Charlotte, NC $1,700.61 $2,338.33 $4,049.05 Knoxville, TN $1,746.12 $2,400.92 $4,157.44 Memphis, TN $2,480.74 $3,411.03 $5,906.53 Akron, OFI $2,508.80 $3,449.61 $5,973.35 Chicago, IL $2,586.14 $3,555.93 $6,157.47 St. Louis, MO $2,870.38 $3,862.40 $6,523.52 Dallas, TX $4,258.31 $5,232.30 $9,060.25 Des Moines, IA $4,084.06 $5,615.58 $9,723.96 Detroit, Ml $4,084.06 $5,615.58 $9,723.96