Description
Testex is the manufacturer of Press-O-Film replica tape.
Testex tape Press-O-Film provides inspectors and also researchers a simple way to obtain an impression of a surface.
Press-O-Film (POF) consists of a layer of crush-able plastic micro-foam coated onto a polyester film of highly uniform thickness.
When compressed against a roughened material, Press-O-Film accurately replicates details of its surface roughness.
The impression can be studied in several ways:
- The firstly is a simple micrometric dial thickness gage can be applied to the replica for field measurement of the average maximum peak-to-valley roughness. Or also called “profile,” of a replicated surface.
- The second is a digital thickness gage such as the PosiTector RTR also can field-measure profile and bridge the gap between tape grades by incorporating automatic linearization.
- Finally is to produce maps of the replica’s surface topography in laboratory setting can use an optical profiling interferometer or confocal microscope. This is most appropriate to study of subtle features at the microinch (25 nanometer) level.
Testex Press O Film Tape Grades
Testex tape Press-O-Film or also called replica tape is available in a variety of thicknesses to facilitate profile measurement in differing ranges.
The firstly range for measurement with replica tape is 0.8 to 4.5 mils (20 to 115 um). Then use the Coarse Minus grade (<0.8 mil or <20 um) or X-Coarse Plus grade tape (>4.5 mil or >115 um) should be restricted primarily to checking measurements at the lower and also upper ends of the primary range.
Linearization and the PosiTector RTR Replica Tape Reader.
Few methods of measurement are perfectly proportional in their response to changes in the property they measure. This is true of replica tape, an averaging procedure applied to the “overlap region” between Coarse and X-Coarse grades of tape. Is uses to compensate for this phenomenon when inspectors use a spring micrometer.
An alternative to spring micrometer measurement of replica tape thickness is use of an instrument like the PosiTector RTR (Replica Tape Reader). Which combines height measurement with “linearization” to improve the accuracy and also ease of use of the replica method. Linearization is a technique uses to insure uniform accuracy of measurement in many types of advanced sensors. Its use in connection with replica tape.
Linearization of Roughness Measurements.
Many sensors used to measure physical parameters respond in ways that are not directly proportional to the properties. They are design to quantify devices for which this is the case are say to be non-linear. Photographic film, certain types of thermometers and also chemical detection instruments are examples of sensors that are inherently non-linear.
Digital processors can be use to compensate for non-linear sensor response and are commonly use for this purpose in modern measurement devices.
Replica tape, in wide-spread and long-standing use to assess surface profile in the Coatings and Linings industry, displays a somewhat non-linear replication response to roughness. This paper describes procedures that can be use to linearize roughness measurements made with replica tape.
Replica Tape.
Replica Tape consists of a layer of compressible microfoam coated onto a durable incompressible polyester substrate of uniform thickness. When collapsed against a surface that has been clean and roughened by blasting, the foam acquires an impression of the surface “profile”. Such that the highest peaks on the original surface come to rest against the polyester layer and the deepest valleys are replicates as peaks in the foam. Measuring the thickness of the resulting replica, and subtracting out the thickness of the incompressible substrate, gives a good measure of peak-to-valley height of the profile, a parameter important to determining whether paint will adhere.
The standard mechanism for measuring a replica’s thickness is a spring micrometer.
The most commonly used grade, or thickness, of replica tape has the designation “X-Coarse”. This grade covers, approximately, the range of profile heights extending from 38 um to 115 um (1.5 to 4.5 mils). Because some applications require measurements to lower profiles.
A second grade, “Coarse” can be use to extend the range downward to 20 um (0.8 mils). Coarse grade covers the range of 20um to 63um (0.8 to 2.5 mils).
Each grade of tape responds non-linearly at the lower end of its range, then where the foam becomes fully compressed. And at the upper end of its range is where the peak heights are greater than the foam’s thickness.
As the tape’s response becomes increasingly non-linear, measurements become increasingly inaccurate. Testex addresses this by setting conservative limits on the tape’s useful range. For “X-Coarse” this range is 63um to 115um (2.5 to 4.5 mils). For “Coarse” the range is 20 to 38 um (0.8 to 1.5 mils). Small inaccuracies appear at the upper (115 um, or 4.5 mils) of “X-Coarse” and lower (20 um, or 0.8 mils) end of “Coarse”. In the overlap region between the two grades an inconvenient averaging procedure must be applied.
Linearization.
Use of a thickness gage incorporating a digital processor permits the option of displaying linearized peak-to-valley profile. If the replica foam’s response function, relating replica thickness to height of the profile is known.
Suitable “response curves” can be deduced from experiments in which replica tape determinations of profile are plotted against profiles obtained. With electronic stylus roughness instruments for many different abrasively-blasted test surfaces. A separate response curve is deduce for each grade, or thickness, of replica tape. In general, stylus measurements display greater statistical noise than replica tape, but have the advantage of better linearity. Linearizing replica tape’s response combines the low statistical noise of the replica method with the linear behavior of the stylus method.
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