“Is That Your Voice on Tape?”
THAT is the question my niece asked me some time ago. Capturing the sounds of animals, children and friends on magnetic tape is becoming commonplace. Voice letters are very popular. In fact, combining pictures with sound and replaying them soon will be accomplished easily.
But what is magnetic tape? And how is it made? We decided to find out by going to a tape manufacturing plant where we could see the whole process.
The Tour Begins
Our guided tour began with an explanation of what magnetic tape is. It is made up of some 12 to 15 different components. Basically, they can be broken down into four categories. The first, of course, is the polyester base material, to which all the other components are applied. The second is the iron oxide for sound reproduction, and carbon for insulating the individual iron-oxide particles. The third is a series of binders that hold the oxide to the polyester film, and vinyls that stop the binders from becoming too hard and brittle, which would cause oxide flaking and poor tape flexibility. The fourth set of ingredients consists of the lubricants and smoothness components. These allow for smooth tape travel, good head-to-tape contact, minimal head wear and low friction. All these components combine to form the high-quality magnetic tape of today. With this basic information, we could now better understand the procedure of making the tape.
A 300-gallon tank was our first stop. Here is where all soft solid and liquid components begin their journey toward becoming magnetic tape. This agitation tank is much like an electric blender. The components are thoroughly mixed with high speed and heat to begin the chemical reactions that form the binding systems that hold the iron oxide to the polyester. The tank must be agitated for many hours to produce the chemical reactions required to form strong binders. “Now let us go over to that machine that looks like a cement mixer,” suggests the guide.
She explains that the solid chemicals are placed directly into this machine, called a ball mill. It turns much like a cement mixer or tumbler. The premix from the dispersion tank is added. Containing 5,000 pounds (2,200 kilograms) of steel balls, the ball mill grinds and regrinds the iron and other components as the mill turns. That name “ball mill” certainly is appropriate. The tremendous pressure from the weight of the balls and the constant rotation cause the mixture to become completely homogenous, reducing the size of any single particle to less than one micron.
After some 24 to 60 hours, depending on whether audio or video mix is being prepared, the contents of the ball mill are pumped through a series of filters ranging down to one micron. Filtration is very important. “For example,” states the guide, “in video tape a seventeen-micron piece of foreign material will cause a visible dropout in a picture.” After leaving the ball mill and being filtered, the oxide must be kept under constant agitation to keep the components from settling out or separating.
Precision Coating of the Film
Now we come to the very heart of the plant—the actual coating of the polyester film with the iron oxide. This can be accomplished on one of three coating systems—reverse roll, knife, or gravier. The entire coating environment utilizes clean room air systems. Also, all audio and video coaters are housed in the same environment, to eliminate dust particles.
In the reverse roll process illustrated here, the film (1) travels around the drive roller (2). The oxide (3) is spread onto the transfer roller (4), and the distance between the transfer roller (4) and the meter roller (5) will determine the ultimate thickness of the oxide coating. The name “reverse roll” comes from the method by which the oxide is transferred from the transfer roller (4) to the polyester film (1). The film is traveling in the opposite or reverse direction to the transfer roll, and the distance between the transfer roll and the drive roll (2) is the exact thickness of the polyester film. Therefore, the oxide has nowhere to go but onto the film.
After all of this, the film or coated tape passes through a magnetic orientation field. It is now magnetic tape. Then the tape takes a long trip through 40 or 50 feet (12 or 15 meters) of drying tunnels to the take-up system that winds the tape into a web (or spool), aligning each layer exactly on top of the last.
The process of calendering is next. “Calendering?” questions my niece. “Yes, calendering is probably the least understood, by most people, of all the functions of a tape plant,” comments the guide. All video tape, most good cassette tape, and some good audio tape, is calendered. Calendering is what gives tape the smooth, glossy appearance on the oxide side.
We are reminded that if our tape machine heads are dirty, we lose voice response, and a rough surface causes virtually the same effect as dirty heads. It prevents intimate tape head contact with the main body of the tape. When tape is coated, it has millions of microscopic peaks and valleys. But calendering eliminates these, producing a very smooth surface. In addition, since there are no rough peaks, there is less oxide ruboff and less head wear.
The large calender machine has two chrome rollers separated by a compressed cotton roller. Calendering is achieved by two interacting systems, heat and pressure. The chrome rollers are heated to temperatures of between 160 and 210 degrees Fahrenheit (71 and 99 degrees Celsius). Then the tape passes between both chrome rollers and cotton roller, and external pressure of up to 50 tons is applied. Due to the combined pressure and heat, the peaks are compressed into the valleys, resulting in the mirror-surface finish of today’s tapes.
Splicing the Leader
“Now, please step into our web splicing department,” our guide encouraged. “If you have ever looked at the beginning or the end of a cassette,” she continues, “you will notice a colored or clear leader. The reason for the leader is that cassette tape is too thin and delicate to withstand the stress of the fast forward and rewind stopping at the end of the cassette.” The C30 and C60 tapes are 1/2 of one thousandth (.0005) of an inch thick. Leader tape is 1 1/2 of one thousandth (.0015) of an inch thick, and so can stand much more pressure.
Web splicing is one of those processes wherein mass production not only is far more economical but also produces an infinitely superior product. When 6- or 12-inch (15- or 30-centimeter) webs are spliced, rather than splicing the tape to the leader as the 1/8-inch (.3-centimeter) tape is run into the individual cassette, a perfect splice is made. How so? Well, when the web is later slit, the magnetic tape and splice and leader are cut together so that the spliced area is exactly the same width as the tape.
Slitting the Tape
One of the most difficult tape manufacturing operations is called slitting. Tolerances must be to two thousandths of an inch, and the tape must not wander from side to side. If the tape should cup or bend, the product will be ruined and all the precautions taken in the other operations will be wasted.
“The secret is exact web guiding, controlled tension and proper alignment, and built-in compensation to take into account web variation,” explains the guide.
The actual cutting is accomplished by intermeshing, rotating blades built to tolerances of up to one millionth of an inch. The entire engagement of the blades is often less than two thousandths of an inch. Both edges of the web are discarded to eliminate edge fluctuations that occur in the coating operation and only the “heart cuts” are used. This is the final step for reel-to-reel audio tape. Video tape passes through an additional cleaning operation, and cassette tape passes along to the cassette assembly area.
Assembling the Cassettes
At this point it might be mentioned that each cassette must be individually assembled. An automated winder winds the tape onto the cassette hubs while the operator puts in the stainless steel pins, flanged rollers, mu-metal shield, pressure pad, and graphite-coated polyester slip sheets. The tape, already being leadered, is wound on a photo-electric winder. Therefore, the individual operator has no control over the length of the cassette. It is all precontrolled back at the web-leadering area where exact lengths can be controlled.
After the cassettes are assembled, they must be sealed. Two methods are available. The first—sonic sealing—uses ultrahigh frequency sound (much like an electronic oven), which melts a small bead of plastic around the edge of the cassette. Up to 11,000 cassettes may be sealed per day by one operator. The second method is by using five screws driven by an automatic screw driver. Approximately 2,000 of these cassettes may be assembled per day per operator. Today many people are switching to sonic-sealed cassettes, and, should a failure occur, they buy spare screw housings to reassemble the defective cassettes. Finally, the cassettes will proceed to the labeling and packaging departments.
Our Tour Concludes
As this interesting tour draws to its conclusion, we move to the quality control area, where a random check is made of each batch of cassettes. Other labs check every batch of mix of the formula, or recipe, from each ball mill, video web and audio web. In the manufacture of magnetic tape, every aspect of the process must be carefully scrutinized. The half-inch, one-inch, or two-inch video tape may be viewed or evaluated. Consistency is the all-important byword of quality control. Other labs perform wear tests, life tests and dropout tests, as well as checking microscopy, chemical variations, tension, packing, and so forth. Often, as much money may be invested in checking and evaluating the tape product as in manufacturing it. Tape has been showing a tremendous quality increase over the last few years—first with much better irons, next with calendering, then with the use of chromium dioxide, and now with cobalt and the new irons.
And so our tour concludes. We thank our guide and bid her good-bye. As we leave the plant, our thoughts turn to the many uses of magnetic tapes. Among these are the readings of the Bible book of John, released recently at the “Victorious Faith” International Convention of Jehovah’s Witnesses. Sometimes these are taken to infirm people who are shut-ins, and they greatly appreciate hearing these readings of God’s Word. Now, reflecting on what we have learned during this tour, we better appreciate the care and work that go into the manufacturing of magnetic tape—that amazing strip of film that can carry “your voice.”—Contributed.
[Diagram on page 21]
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[Diagram on page 23]
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WHAT MAKES UP A CASSETTE?
graphite polyester slip sheet
hub and clip
stainless steel pin
pressure pad spring
[Picture on page 21]
As the drum (above) rotates, steel ball bearings pulverize the mixture in the ball mill