Micro weighments of compound ingredients.
The first remotely controlled, automatic weighing system installed for rubber compounding took place around 1955, with the weighing equipment consisting of mechanical scale levers connected to dial indicators having potentiometer assemblies mounted on the dial spindles and simple Wheatstone Bridge circuitry activating electro mechanical relays for material feeding control.
That first system, and several which followed, included the automation of the feeding and weighing of carbon black and oils, with the polymer stocks being manually loaded on a belt conveyor mounted on a mechanical/dial scale.
The control for sequencing the weighing and mixer charging cycles was accomplished through a motor driven cylinder having adjustable wiper contacts to control the variable parameters of the system.
Initially, no attempt was made to automate the micro ingredients.
This article will cover the problems which were encountered in the many attempts made in the area of micro ingredient automation (which have to be considered as learning experiences) and conclude with a description of the latest equipment available which provides the optimum solution to these problems.
Definition of Terms
Whenever automatic weighing is discussed, there are terms that are used that can become confusing unless defined.
* Accuracy. For this presentation, accuracy means the actual amount of material introduced into the process. Since the integrity of the entire batch depends on the actual quantity of each ingredient in the batch, all of the other variables in a weighing system must be regulated or controlled to assure that the actual amount is as close to the laboratory formulated amount as possible.
* Resolution. This term is used in reference to the weight sensor/transducer element in the system and defines the smallest usable increment that a weight signal can generate to be used for display and control.
* Repeatability. It is one thing to be able to provide an automated system that gets the accuracy factor under control some of the time, but a successful system is only acceptable when accuracy is present just about all of the time. The ability of a system to hit accuracy targets consistently is called repeatability.
* Net weighing. This is used in batch weighing when one or more than one material is fed into a hopper mounted on a load sensor/transducer assembly with automatic re-zeroing of the system between individual material weighments. This technique is in contrast to loss-of-weight or weight by subtraction which is described in more detail later.
Original attempts at micro ingredient automation
Since several types of carbon black were fed by screw feeders and successfully automatically weighed in a common weigh hopper connected to the back of a mixer through a down spout, the original attempts at micro ingredient weighing used a smaller version of that same concept. The only difference was that the down spout came into the side of the mixer because of physical layout constraints.
Several serious problems became evident rather quickly using this net weighing technique, including the limited number of materials which could be handled, the fact that some of the materials such as sulfur and zinc oxide stuck to the sides of the weigh hopper and the down spout, the resolution limitation of one part: 1,000 resulting in minimal accuracy, the effects of vibration on the mechanical sale equipment, plus the expense of having one of these systems per mixer, led to the evident conclusion that this was not a viable way of automating micro ingredient weighing.
From these early automation attempts, the problems started to come into focus in the categories of accuracy, materials handling and cost.
Introduction of load cell technology
One major improvement in the instrumentation aspect of the system was made available through the introduction of electronic strain gage load cells instead of mechanical levers and dials. This resulted in resolution capabilities of one part: 5,000, with the ability to provide electronic damping and filtering for better vibration control. This new technology provided some "quantum jumps" in the area of accuracy, one of the focus target problem areas to be resolved.
Load cell instrumentation also opened up new possibilities in the area of formula input control through the use of analog to digital and digital to analog devices. Digital thumbwheels and punched cards became the acceptable mode for formula control, and data recording became a reality.
The other two problem areas of micro ingredient automation, however, namely the materials handling aspect and a total system cost, were not helped with this upgrade in instrumentation.
Attempts at solving the materials handling problems
With the development of low melt index sheet material which could be used to make bags, the idea occurred that putting the micro ingredients into this type of bag and dropping the entire bag into the mixer would eliminate a lot of the sticking problems originally experienced in the common weigh hopper concept tried initially.
This led to the realization that filling bags at a central location would enable one system to serve multiple mixer lines which would help the cost justification for an automated system. But this left the challenge as to the best way to fill the bags.
Preparation of master batch pre-blends was tried with the pre-blended batch being transported to a single scale which would be used to fill the bag through a single filling spout.
An operator standing at the filling spout placing empty bags and sealing the filled bags was used originally. The next step was the attempt to replace the operator with an automatic bag handling system. One type using bags connected through a tab and pulled from a box in a fan-fold pattern was used, and the next attempt was the use of a form/fill/seal system where tube stock was taken from a roll, a bag formed and filled with the pre-blend and sealed forming the bottom of the next bag.
From a bag handling point of view, these machines worked very well. However, the old problem of materials handling reared its ugly head again.
The pre-blended batches were not absolutely homogeneous as they came out of the preblender, and transporting the batches from the blender to the filling station caused segregation, and as a result, there was no way to assure the correctness of the proportions of the materials that were fed into the bags.
Using a net weigh scale above the filling spout of these automatic bag handling machines did not eliminate the materials handling problem of the blended materials sticking in the weigh hopper and bag spout.
About the same time as the introduction of low melt bags, and automatic bag handling, a new weighing concept was developed called loss-of-weight.
Although you are probably familiar with this term, briefly it means that a weigh hopper and feeder assembly are mounted on a scale platform, the weigh hopper is filled from an overhead supply bin, and after taring-out (re-zeroing of the total load), the feeder runs to weigh-out (or weigh by subtraction) the required batch amount.
The primary advantage of this type of weight control is that there is no weigh hopper or contacting surface to which any materials can stick after they are weighed, resulting in much better weighing accuracy of the material being handled.
The problem of having one filling spout with an automatic bag handling system, however, did not eliminate the segregation and non-homogeneous blend problems already mentioned. All this solution did was put a more accurate amount of unknown quantities of the blend into the bag.
At this point in the development of loss-of-weight feeding, there were still some disadvantages, one being the very large dead load to live load ratio of the hopper and feeder equipment related to the actual ingredient quantity to be weighed. This required that some method of off-setting this dead load through a mechanical device be developed, so that a load cell of a capacity close to the maximum weight required for any given batch could be used. By this time though, load cell technology and instrumentation had increased to the point where resolution capabilities of 1:10,000 were available so the technology trend was in the direction of providing better accuracy.
Another disadvantages of using loss-of-weight control was the fact that each ingredient needed its own supply bin, re-fill feeder and scale system which drove the pricing of a multiple ingredient system to the point where it was hard to cost justify.
Although the accuracy factor in an automated system was heading in the right direction, the demands of some of the chemicals to be used in the feeder/scale combinations available were such that these systems were still not up to a level where all of the requirements could be satisfied.
Because of the material handling characteristics of most of the ingredients to be used, auger type feeders are required. Conventional type feeders using two speed drive control provides a turn-down ratio (fast feed rate to slow feed rate) of only approximately 50:1; and although the feeder ran slower at the dribble rate, the volumetric through-put does not change, resulting in a large amount of material to be concerned with at the conclusion of the weighing. Granted, this factor is not as critical in loss-of-weight control as it is in net weighing, but it is still a significant factor considering the inertia of a weighing system during feeding and material flow cut-off on completion of the weighment.
The "now" generation of transducers and feeders
Everything discussed so far was background information leading up to the equipment available today, which has the capability of meeting all of the challenges including accuracy, materials handling and cost, which have been eluding complete solutions until now.
First let's consider the transducers which are being used. Vibrating wire or strain gage types still have limitations in the 1:10,000 part range of resolution, and although there is a gyro based transducer available, which has much higher resolution capability, it is very difficult to implement in a dynamic weighing application.
This leads us to a transducer now available called magnetic force restoration type. This device starts at a null balance point and as a load is applied, the current required to drive the transducer coil back to that null point can be measured very precisely resulting in a weight sensor having usable resolution and display capability of 1:120,000.
This type of transducer is available now, built into a scale platform sized 600 x 750 mm (23.6" x 29.5") for example, which is large enough for the mounting of a hopper/feeder assembly resulting in a loss-of-weight scale with a capacity of 240 kilograms (529.2 pounds) which can be read to the nearest 2 grams (.004 pounds or .064 ounces).
In addition, due to the flexure design included in this scale platform, it is possible to pre-load this scale with up to 65 kilograms (143 pounds) to offset the "dead load" of the hopper and feeder assembly without eroding the live load capacity and readability as previously mentioned.
With this new technology, the weight signal from this scale platform can be readily converted into an RS232 format for interfacing with just about any microprocessor or micro computer type device.
However, in order to take advantage of this new level of weight sensor/transducer capability, it is necessary to use a new feeder design which has a much higher turn-down ratio than those previously available.
Since auger type feeding is still required (because of the handling characteristics of the ingredients being handled), the use of two speed drive control with two different diameter augers is the answer. Running both augers at the high feed rate for the major portion of the feed cycle, and then running only the small diameter auger at a much lower rate for the dribble feed cycle results in turn-down ratios approaching 500:1, eliminating the previously described feed rate problems as the point of cut-off.
Empirically it has been found that the accuracy of a weighing, when the proper size and type feeder is mounted on a properly sized scale platform, will be not more than plus or minus 3 divisions of the resolution.
From this input, you can see that it is not possible to expect accuracies in the range of up to 10 kg. (22.05 lbs.) to plus or minus 6 gms. (.01 lb.).
With the materials handling problems being resolved, the accuracy capabilities being available to meet today's compounding requirements and on the basis of using low melt bags as the best method of assuring that the right quantity of the right material gets into the compound, the only thing left is the best way to fill the bags. Figure 1 shows the optimum system which incorporates all of the current technology.
Although only three loss-of weight stations are shown, an unlimited number of stations is possible, since they are located in a row along both sides of a container conveyor which can be as long as required.
Each scale platform is mounted on a portable stand, so that they can be positioned as required, and if necessary different capacity units can be moved into position as formulas vary.
In addition, on each scale platform there are inverted V tracks, and under each hopper/feeder assembly there are V-grooved wheels, enabling any hopper/feeder to be placed at any scale station.
With this arrangement, the minimum number of scales can be used (this is where the cost savings occur) and a large number of hopper/feeder assemblies can be kept stored in a convenient location and put into place as required.
Keeping a hopper/feeder dedicated to a particular material also saves time in cleaning and eliminates the possibility of contamination if one hopper/feeder unit is used for more than one material.
Under the discharge nozzles of the feeders which have been wheeled into place, there is a conveyor assembly consisting of equally spaced containers into which the empty bags are placed. The spacing of the loss-of-weight feeder assemblies will coincide with the center lines of these containers; the conveyor will be controlled in such a manner that they will be automatically indexed one increment at a time.
When the indexing cycle is complete, each scale weighs-out its required quantity directly into the bag, with the formulas being retained in memory in the micro computer which runs the entire system.
With the number of scale platforms equal to the maximum number of ingredients ever put into any compound, and portable hopper/feeder units equal to the total number of ingredients ever used, moving these assemblies into the proper position gives unlimited flexibility and ingredient combinations.
With this type of micro ingredient weighing system, one operator stands at the end of this container conveying assembly to place empty bags into the containers as they are positioned in front of him, and he also takes out the filled bags as the arrive, heat seals them and places them on a pallet or into a tote type container for delivery to the mixer stations.
Although not shown on the reference drawing, dust control is built into the system by placing dust "pick-up" points at the feeder discharge location at each scale station.
Of course with today's digital technology, anything that happens on the bag filling line can be recorded, and as a further safe guard, rugged industrial proximity switch identification can be provided so that the system will not run unless the materials that are called for in the formula are in fact in position on the scale platforms.
As a final check that the proper amount of material has been introduced into each mixer batch, the filled bags can be placed on to an automatic bag indexing conveyor at the side of the mixer inlet hopper, and a scale platform can be located at the discharge end of that conveyor to catch and check-weigh the bag weight just prior to its being discharged into the mixer hopper. This type of indexing conveyor arrangement eliminates the need for an operator to be at the mixer at the exact time in the mixer cycle, and the mixer controller can index the bag conveyor at several points in the cycle if required for sophisticated compounds.
Today, the proper application of the current materials handling, weight sensing and bag handling techniques makes it possible to satisfy the demanding requirements of micro ingredient weighing for rubber compounding.
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|Date:||Feb 1, 1990|
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