Alexei Volkov
Measuring the surface finish of a plastic part is crucial to ensure it meets performance, aesthetic, and functional specifications. Surface finish is characterised by the micro-geometry of the surface, encompassing roughness, waviness, and lay. Accurate and repeatable measurement techniques are essential to achieving the desired surface finish. Various methods are available, including visual inspection, tactile inspection, and instrumentation-based measurements. The most commonly used instrument is a profilometer, which employs a stylus to scan and measure surface variations. GD&T principles, utilising symbols and rules, enable designers to specify surface finish requirements, including maximum and minimum roughness parameters. Ra, the average surface roughness, is a widely used metric, while other parameters like Rmax and Rz address specific surface anomalies. Finishing processes, such as graining and lapping, are applied to achieve different levels of abrasion and desired finishes. Understanding surface finish is vital for producing high-quality plastic parts that meet customer needs and perform optimally in their intended environments.
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What You'll Learn
Using a profilometer
A profilometer is a device used to measure the profile and surface finish of a surface. It is one of the most accurate ways to measure the surface roughness of an area. It uses a stylus to scan the surface of the plastic and measure the variations in surface height. The data collected by the profilometer is then processed to calculate the roughness parameters of the surface, including roughness average (Ra) and root mean square roughness (Rq).
Profilometers come in many shapes and sizes, but they can be divided into two basic types: contact and optical. Contact profilometers work by measuring the displacement of a diamond stylus as it is moved across the surface of a manufactured component. The stylus tips can create scratches in soft material, especially when measurements are repeated, and they are sensitive to contaminants such as dirt and oil. Handheld contact profilometers are commonly used in machine shops for measuring surface finish on machined parts.
Optical profilometers, on the other hand, use light to measure features on a surface. They are relatively large instruments that consist of a light source, optical lenses, and image sensors. They are faster than contact profilometers but require the surface to reflect the light being used, so they may have trouble with translucent or highly reflective surfaces. These surfaces must be free of debris and contaminants for the reflection to accurately characterise the surface.
When selecting a profilometer, it is important to consider the parameters you want to measure, the range of those parameters, and the required measurement accuracy. The size and shape of the surface to be measured, as well as the number of measurements and cycle time, should also be considered. For example, a machine shop that manufactures large parts with varying Ra or Rz requirements may prefer a handheld contact profilometer, while a semiconductor manufacturing plant may opt for an optical profilometer.
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Ra values and their limitations
Ra, or roughness average, is a measure of the surface finish of a material. It is the most commonly used parameter for surface finish and is calculated as the arithmetic average of surface heights measured across a surface. The Ra value is important because it helps to determine the roughness of a surface, which can impact the performance and durability of a product. For example, rough surfaces tend to wear and tear more rapidly due to higher friction levels.
However, there are some limitations to using Ra values to measure surface finish. One limitation is that Ra values are insensitive to certain extremes. For example, the Rz parameter, which measures the average values of the five largest differences between peaks and valleys, may be more sensitive to anomalies such as burrs and scratches. Additionally, Ra values may not always capture the full picture of a surface's texture. For instance, a D-1 dry blasted finish can have the same Ra flatness as a C-1 stone finish, but they will look and feel very different.
Furthermore, the appropriate Ra value depends on the specific application and requirements of the product. For example, while a cast-iron skillet may have an Ra of roughly 2,000, the Hubble Space Telescope has an Ra of 0. In general, a faster machining process will result in a higher Ra value, indicating a rougher surface. However, it is important to strike a balance between the roughness requirements and the limitations of the manufacturing process, as additional processing for smoother surfaces can increase costs.
To overcome the limitations of Ra values, other parameters such as Rz, Rsk, Rq, and Rku can be used in conjunction with Ra to provide a more comprehensive understanding of the surface finish. Additionally, visual inspection, tactile inspection, and instrumentation-based measurements can be employed to further characterize the surface finish.
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GD&T principles
GD&T, or Geometric Dimensioning and Tolerancing, is a system of symbols and rules used to define the size, shape, orientation, and location of a part. GD&T principles are used to ensure that the surface finish of a plastic part meets specifications by allowing designers to specify the surface finish requirements, including the maximum and minimum allowable roughness parameters.
The GD&T symbol for surface finish is a wavy line placed adjacent to a feature of size. The wavy line represents the surface roughness, and the number next to it represents the maximum allowable roughness in microinches (µin). By using GD&T, designers can ensure that the part is manufactured to the correct specifications and that the surface finish is accurately measured.
Surface finish is a critical aspect of plastic parts as it affects both the part's performance and aesthetic appeal. It is composed of three elements: roughness, lay, and waviness. Roughness refers to small irregularities in surface geometry, while waviness refers to undulations in the surface. Lay is the direction of the predominant surface pattern, usually determined by the production method used to process the surface.
To measure surface finish, a profilometer is used. This instrument traces the contour of the surface with a stylus or sensor, generating a graph showing variations in surface height with changes in position. This graph is known as the measured surface profile, which includes roughness, waviness, and flatness defects. To examine only roughness, the wave and flatness defects must be removed from the profile by averaging the surface height over intervals smaller than the waviness spacing but larger than the roughness spacing. This creates a straighter line known as the surface roughness profile.
There are various techniques for measuring surface roughness, including area techniques such as optical scattering, ultrasonic scattering, and capacitance probes, as well as microscopy techniques that rely on measuring the contrast to provide information about peaks and valleys. The profilometer measurement can be supplemented by visual inspection to evaluate the surface finish's appearance, texture, or defects.
By using GD&T principles and profilometers, manufacturers can produce high-quality plastic parts that meet customer needs and performance standards.
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Visual and tactile inspection
Visual inspection is a non-contact test method that is inexpensive and suitable for roughly inspecting objects in the context of single and small series production. It is a good way of identifying defects in the original surface. This method has been used in the food industry to check the quality of packaging and products. Almost all soda, beer, and juice bottles are made of transparent materials such as plastic or glass, which makes it easy to inspect large batches. Modern pipe and sewer cleaning also works with this method. Visual surface inspection is also used in automobile manufacturing, metal processing, and electrical engineering to ensure high-quality standards during production. With the help of efficient microchips and high-resolution cameras, visual surface inspection can be carried out even with fast production processes.
Tactile inspection, on the other hand, involves the use of control devices with diamond tips to measure the roughness of surfaces. This method delivers precise values in the nano range.
Plastiform is a product that can be used for visual and tactile inspection. It accurately reproduces surface details, making it easier to spot defects. It is cost-effective, fast, easy to use, safe, and clean.
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Gloss meters
There are two main types of gloss meters: the SoloGloss® and the PolyGloss®. The SoloGloss® is a single-angle gloss tester, suitable for most applications and is preferred for measurements in the semi-gloss range. The light source and the detector are positioned under an angle of 60° of the surface to be measured. The PolyGloss®, on the other hand, is a triple-angle measurement device that inspects across the entire gloss spectrum from angles of 20°, 60°, and 85°. The 20° angle is ideal for the most reliable gloss measurements on high-gloss surfaces.
The choice of measurement angle depends on the gloss level of the surface being measured. For high-gloss surfaces, a 20° measuring angle is recommended, while an 85° angle is more suitable for low-gloss surfaces. The reported gloss value should always mention the measurement angle used. No special training or skills are required to operate a gloss meter. The user simply needs to position the gloss testing gauge in place, press the scan button, and read the values.
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Frequently asked questions
Surface finish is a critical aspect of plastic parts as it affects both performance and aesthetic appeal. It refers to the roughness, waviness, and lay of a surface.
There are several methods for measuring surface finish, including visual inspection, tactile inspection, and instrumentation-based measurements. Instrumentation-based methods include using a profilometer, which uses a stylus to scan the surface and measure variations in height. Other methods include using a gloss meter, which measures the gloss units (GU) of a surface, and non-contact methods, which use optical sensors, lights, or ultrasonic pulses.
The most commonly used metric for measuring surface finish is Ra, which measures the average surface roughness of a part. Other units include Rmax, which measures the vertical distances of a surface's peaks and valleys, and Rz, which measures the average maximum height of a surface profile.
A rough surface finish can be prone to wear and tear and can create opportunities for breaks and corrosion. On the other hand, a perfectly smooth finish may not be desirable as it can make scratches and imperfections more noticeable.
There are several avenues for finishing plastic parts, including graining and lapping, which involve moving the piece against a series of finer gradient textures to achieve different levels of abrasion. Another technique is flame polishing, which applies heat to level out uneven spots and create a smooth and uniform surface.
