In a very early blog written in 2011 I talked about the “Scotch” tape test for cleanliness. At that time I did not address quantification of the results other than as a visual assessment. In addition to evaluating general cleanliness, this technique is being found particularly applicable when determining the cleanliness of substrates that “shed” including such things as graphite and some forms of cast iron. This blog provides a little more explanation which I hope will be helpful to people applying the tape test method.
The first step is to select the tape to be used. An “invisible” or “clear” adhesive tape is the usual choice. Alternatives may be considered depending on particle color and contrast. Most commercially available tapes are consistent in “stickiness” but it is best to select one type of a readily available brand to minimize variables.
Tape can be applied to surfaces using “finger pressure” in most cases. Although finger pressure can vary from one individual’s finger to another, some degree of consistency can be achieved by observation of another person applying the tape and duplicating the technique as much as possible. Other methods such as pressing the tape with a rubber roller, a straight edge or a rubber eraser have been used. The important thing is that the technique be as consistent as possible.
Establishing a Base Line
Materials like graphite shed particles endlessly until their entire volume is consumed. Think of the “lead” in a pencil. As you write with the pencil, particles of graphite (carbon) are abraded from the “lead” and deposited on the paper as a visible trail. As the lead is consumed, the particle trail it leaves remains relatively stable in appearance which means that the particles deposited have actually been abraded from the surface and were not just particles that happened to be adhered to its surface. Establishing a base line means finding the level of shedding that is present if there are no “free” particles on the surface. This can be done in at least 2 ways.
- Apply tape to the same area of a surface repeatedly until no difference can be detected between the tape most recently applied and the tape applied previously. The base line is the tape showing no difference from the previous one as shown below. This may require taking many samples before the base line is established.
- An alternative to the above is to apply successive tapes in a X pattern repeatedly with the crossing point of the X maintained in successive applications. This results in the crossing point having seen successively more applications of tape than the arms of the X. The base line has been established when there is no longer a distinguishing difference between the crossing point and the arms. Sometimes this is easier to judge than the method described above.
Once the base line has been found, it is necessary to preserve the final tape as a reference for comparison when evaluating the cleanliness of other parts.
Visual evaluation is frequently adequate for the tape test. The human eye is quite sensitive making it relatively easy to distinguish between samples held side by side. However, for purposes of documentation and establishing a precise and quantifiable comparison, one can use a densitometer. This device measures the reflection from an opaque sample or the transmission through a transparent or translucent sample. Again, consistency in the tape, mounting paper (if used), and measurement technique is paramount.
A new base line should be established periodically or when anything changes.
The base line may be different for different materials of composition or when different fabrication techniques are used on the same or different materials.
All of the above is for consideration only. Each application may require different techniques but this should give some general direction.