Magnetic tape is a material that is commonly used for the storage and playback of analog audio and video recordings. It consists of a thin magnetizable coating applied to a long and narrow strip of plastic film. The first magnetic tape recorder was invented in Germany in the 1930s for recording and broadcasting spoken words. Since then, magnetic tape has been used extensively for audio and video recording due to its reusability, ease of editing, and relative low cost compared to other mediums.
The development of magnetic tape enabled the recording and playback of audio and video content in a practical and cost-effective way. It allowed for repeated recording and erasing on the same tape, making editing possible. Magnetic tape could also be easily mass produced. These advantages led to widespread adoption of magnetic tape for purposes ranging from data storage to home audio and video recording. Though challenged by digital storage in recent decades, magnetic tape remains valued today for its reliability, long shelf life, and high data density.
This article will cover the materials and process needed to make magnetic tape at home. Key steps include preparing a base film, applying a magnetizable coating, orienting the magnetic particles, and adding a back coating.
Sources:
[Paragraph 1: https://en.wikipedia.org/wiki/Tape_recorder]
[Paragraph 2: https://historictech.com/magnetofon-the-birth-of-magnetic-tape/]
Materials Needed
The key materials needed to make magnetic tape are:
- Magnetizable coating – This provides the magnetic properties that allow the tape to store data. The coating is made up of fine magnetic particles, usually ferric oxide or chromium dioxide, suspended in a polymer binder.
- Polyester base film – This provides the base structural support for the tape. Polyester films like polyethylene terephthalate (PET) are commonly used due to their strength, flexibility, and chemical stability. The base film is extremely thin, typically 4-25 μm thick.
- Binding agent – The magnetizable coating needs an adhesive binder to bond it to the base film. Common binding agents include polyurethane resins, vinyl chloride copolymers, and cellulose nitrate.
In addition, lubricants are added to reduce friction and allow smooth tape travel. Conductive particles may also be added to control static charge buildup. But the magnetizable coating, base film, and binder are the three core components.
Sources:
https://www.iasa-web.org/tc05/22111-components-magnetic-tapes-and-their-stability
Prepare Base Film
The first step in creating magnetic tape is to prepare the base film that the magnetic coating will be applied to. Polyester film such as polyethylene terephthalate (PET) is commonly used as the base film. According to the How Magnetic Tape is Made video [1], PET is selected for its strength, flexibility, and ability to withstand high temperatures during the coating process. The PET film should be cut to the desired width for the final tape product, which is commonly 0.5 inches for audio tape and 1-2 inches for video tape [1]. Once cut, the film must be thoroughly cleaned to remove any dust, oils or other surface contaminants that could interfere with proper coating adhesion. This is often done using specialized cleaning solutions or solvent baths.
[1] https://www.youtube.com/watch?v=FeKxC8pLTbI
Make Magnetizable Coating
The magnetizable coating is a critical component of magnetic tape. This coating contains magnetic particles like iron oxide or chromium dioxide dispersed in a binder polymer. The coating allows the tape to store magnetic information.
The type of magnetic particle affects the tape’s storage capacity and sound quality. Iron oxide particles are inexpensive and common, providing decent performance. Chromium dioxide particles have higher magnetism and enable better high-frequency response, but are more expensive. Metal particle coatings like barium ferrite offer the highest performance but cost the most.
The binder polymer holds the magnetic particles in place and bonds them to the tape backing. Common binders include polyurethane, polyester, vinyl chloride copolymers, and cellulose derivatives. The binder must be durable, flexible, and able to withstand temperature changes.
To make the coating, the chosen magnetic particles are milled into a fine powder to maximize surface area. The particles are then mixed and dispersed into a solvent-based solution along with the binder polymer and any additives. Ball mills, attritors, and bead mills are often used for thorough mixing and dispersion of the ingredients.
The concentrations and proportions of magnetic particles and binder must be carefully controlled. Higher particle content increases signal strength, while more binder improves durability. An optimal balance must be struck.
Once dispersed, the coating solution is ready for application onto the tape backing. Coating methods include dip coating, spray coating, roll coating, and knife edge coating. The coating must be applied evenly across the entire width and length of tape (Source: https://patents.google.com/patent/USRE28866E/en).
Apply Coating
The coating containing the magnetic particles must be applied evenly to the base film to create high quality magnetic tape. There are a few common methods for applying the coating, such as using a reverse roll coater, knife coater, extrusion coater, gravure coater, or slot coater.
With a reverse roll coater, the coating is applied between a rubber applicator roll and a metal reverse roll. The applicator roll picks up coating from a reservoir and transfers it to the reverse roll, where it is evenly applied onto the base film. This allows for precise control of coating thickness and distribution. A knife or doctor blade is used to meter the coating and remove any excess. Similar principles apply for the extrusion, gravure, and slot coaters.
Coating thickness is a key parameter that must be closely monitored and controlled during the coating process. Typical magnetic coatings for audio and video tape are 3-6 microns thick. Insufficient coating thickness will result in poor signal properties. However, if the coating is too thick it can lead to poor flexibility, adhesion issues, and increased manufacturing costs. The thickness and roughness must also be consistent across the full width and length of the tape to prevent recording and playback issues.
Orient Particles
Properly orienting the magnetic particles in the coating is crucial for creating a high-quality magnetic tape. This is done by passing the coated film through powerful magnetic fields to align the particles so they are parallel to the tape’s lengthwise direction. As G. Bate explained, “The paper addresses the design of an orienting magnet for tapes. The undesirable reversed field is reduced in two steps. First the source of mmf is moved to a position where the field is parallel over a large region.” (Bate, 1980).
Aligning the needle-shaped magnetic particles gives the tape more uniform magnetic properties and improved signal strength. As the particles become oriented parallel to each other, the tape’s residual flux density and squareness ratio increase. This results in a stronger magnetic field and output signal when the tape passes by the playback head. Proper particle alignment also reduces noise and electromagnetic interference. Overall, orienting the magnetic particles is essential for optimizing the tape’s magnetic recording properties.
Dry Coating
After the coating has been applied to the base film, the next step is to carefully dry the coating. According to Magnetic Tape Production and Coating Techniques (http://bitsavers.informatik.uni-stuttgart.de/pdf/memorex/tape/Magnetic_Tape_Production_and_Coating_Techniques_Sep65.pdf), this process must be carefully controlled to avoid defects:
Proper drying conditions must be maintained to avoid blistering, pinholes, and other coating imperfections. The coating is passed through drying ovens with precisely controlled temperatures and air circulation. The temperature and dwell time in the ovens must be optimized to thoroughly dry the coating while avoiding damage to the base film.
Any pinholes, blistering, or other defects can degrade performance and quality. Maintaining clean, dust-free conditions and carefully controlling the drying process helps produce a uniform, consistent magnetic coating.
Slit to Width
Once the magnetic coating has dried, the coated film needs to be slit to the desired tape width. This is done using specialized slitting machines with precision ground circular or shear blades. As noted in research from Aggarwal (2005) on finite element analysis of the slitting process, the blades must converge at a precise angle to avoid tearing the film. The slit edges must also be smooth and uniform to ensure proper tracking and head contact during use.
Precision slitting minimizes edge roughness and deviations in tape width. This is critical for preventing tracking errors, tape jamming issues, and reduced signal quality. As described in the Japanese patent JP2632229B2 on magnetic tape slitting methods, maintaining tight tolerances on slit width contributes to reliable performance. Overall, the slitting step transforms the coated film into the desired tape width needed for the final product.
Add Back Coating
After the magnetic coating has been applied and dried, magnetic tape usually receives a back coating on the opposite side. This coating is applied through a similar wet coating process.
The back coating layer serves a few key purposes:
- It provides a smooth surface so the tape can wind evenly on the reel or cassette without sticking.
- It reduces static electricity and friction.
- It helps strengthen the tape and prevent stretching.
- It protects the magnetic layer from physical damage.
Overall, adding a back coating improves the durability, winding, and performance of magnetic recording tape. With a smooth, frictionless back side, the tape will run through equipment with less issues or degradation over time and repeated playing.
Conclusion
The process of making magnetic tape involves preparing a base film, making and applying a magnetizable coating, orienting the particles, and adding a back coating.[1] It is an intricate multi-step process that produces a versatile recording medium with a variety of uses. Magnetic tape enabled the recording and playback of audio and video and served as a reliable means of data storage and transfer for decades.[2] Though largely superseded by digital storage, magnetic tape remains valued for its longevity, capacity, and dependability.[3] Ongoing research aims to boost tape’s capacity and capabilities even further through new materials and production methods. After more than sixty years of innovation, this venerable medium still has potential for continued evolution and application in the future.
[1] https://horizontechnology.com/news/get-that-on-tape-the-past-and-future-of-magnetic-tape-storage/
[2] https://johnrbessant.medium.com/innovation-weve-got-it-taped-ffccdbfe1607
[3] https://webuyusedtape.net/2020/10/21/the-technology-and-innovation-that-keeps-magnetic-tape-alive/