Lubomír Zeman: Plastic injection molding and surface quality, part 2

Surface Roughness Measurement

Surface roughness measurement can be divided into four basic groups:

• qualitative measurement – visual – uses the human senses of touch and sight. The material surface being inspected is compared with reference surfaces of specified roughness; visual inspection can be aided by a magnifying glass or microscope
• comparison method – reference specimens with defined roughness are used to evaluate surfaces – Figure 15; comparisons are made at several locations on the surface being evaluated

Figure 15: Surface roughness comparator

• contact measurements – measurement uses the uniform movement of a measuring stylus across the surface of the sample being inspected; the contact force of the diamond sensing tip is 0,00075 N – ČSN EN 3274, Figure 16

Figure 16: From the top – contact roughness measurement on a CNC machine tool – left; contact measuring instrument – right/diagram of contact roughness measurement/diagram of an inductive sensor/contact sensing tip – its radius is electronically corrected

• non-contact measurements – a description of the three-dimensional surface and its roughness; unlike the contact measurement method, which is profile-based – measurement is performed along a single profile – here the surface is composed of many profiles, and the profile parameters Ra, Rz, Rmax are also derived from the measured data; instruments for non-contact roughness measurement usually use the light-section method – this method uses a bundle of parallel rays directed through a narrow gap (slit) to form a thin plane, with the light rays incident on the measured surface at an angle of 45° through a defined optical lens system – or the laser profiling method – this method uses the triangulation principle – a laser beam is projected onto the measured surface, and the image of the laser beam on the measured surface is captured at a specified angle by a CCD camera; using the sensor, we evaluate the object's profile in the section defined by the laser beam from the captured image. Confocal microscopes are used for the most accurate assessment of surface roughness – a laser scanning microscope, in which the light source is a laser beam, or a white-light spinning-disc confocal microscope – Figure 17

1 – Illumination unit, 2 – Colour filter, 3 – Condenser, 4 – Slit, 5 – Illumination objective, 6 – Observation objective, 7 – Sliding plate with a mark, 8 – Lens, 9 – Measured surface

Figure 17: From the top – optical diagram for the light-section method/measurement of micro-irregularities – roughness using a laser beam – beam diameter 0,04 µm = 0,00004 mm/left – diagram of a laser scanning confocal microscope (it scans the evaluated surface line by line, capturing it point by point), right – diagram of a spinning-disc confocal microscope

• in addition to the non-contact techniques for measuring surface quality mentioned above, instruments with CLA-type sensors – Chromatic Length Aberration – are very frequently used; white light is dispersed and directed onto the surface being inspected by the optics. The optics separate the light according to wavelength, and only a specific wavelength is focused at each point on the surface. Light reflected from the surface passes through an aperture that transmits only light of the focused wavelength. A spectrometer deflects the light onto an array sensor, where each point is assigned a spatial position, which is subsequently processed and evaluated by computer. Laser sensors can also be used instead of CLA sensors. However, they are not as accurate as CLA sensors. The captured spatial positions are then processed using special programs that enable 3D data processing, including 3D visualisation; a diagram of a CLA sensor is shown in Figure 18  

Figure 18: Diagram of a CLA surface roughness sensor

Gloss and Texture Measurement

Gloss – perception of gloss – gloss is a visual perception, an impression created when observing the surface of a given object, and depends significantly on the surface quality of the part being evaluated, the way it is illuminated and the angle from which we view the surface. The more direct light is reflected from the surface, the brighter and glossier the surface will appear. In other words – gloss is a physical quantity that assesses the ability of the surface of an object made from a given material to reflect light under certain geometrical conditions with different directional reflection properties.

Here I will emphasise again – the concept of “smoothness” or “roughness” in gloss assessment and in machining is completely different – roughness is a succession of small surface irregularities in materials resulting from machining. 

High gloss corresponds to a high reflection of light incident on an object, which means that the incident light is reflected in one main direction of reflection, with the angle of reflection equal to the angle of incidence.

The perception of semi-gloss and matt finishes – this perception corresponds to a less intense reflection of light incident on an object and means that the incident light is reflected diffusely. The surface consists of micro-roughness that diffusely scatters the incident light in all directions of reflection; the more the light is scattered, the lower the intensity of reflection in the main direction and the more matt the surface will appear.

The relationships between gloss and matt finishes described above are shown in Figure 19.

Figure 19: Perception of gloss – surface, surface 01 – mirror gloss through high gloss – surface 02, technical gloss – surface 03 to surface 04 – matt Blue arrow – incident ray Red arrow – specular reflection from the surface at the given point

Gloss measurement is covered by the standard STN EN ISO 2813:2016 Paints and varnishes – Determination of gloss value at 20°, 60° and 85°. The intensity of reflected light is measured at a small angle of reflection; the light source, which simulates CIE (International Commission on Illumination – Commision Internationale de l´ Éclairage) illuminant C, is positioned at the focal point of collimating lenses (these lenses are designed to capture divergent or convergent light rays and transform them into parallel rays). The measurement results are compared with a reference standard, with the amount of light reflected by a black body with a defined refractive index np = 1,567 – the measured gloss value for the reference standard is 100 gloss units – 100 GU (Gloss Unit).

To differentiate (distinguish) gloss over the entire measurement range – from high gloss to a matt surface, three measurement geometries are standardised – 20°, 60°, 85°. There are three because, when comparing different gloss levels, a single measurement geometry, for example 60°, may not provide a result that correlates (correlation = a mutual relationship between quantities, processes, characteristics) with visual observation. Each of the three geometries uses the same aperture for the light source, but a different aperture for the detector.

The choice of geometry depends on whether we want to carry out a general gloss assessment, compare high-gloss surfaces or assess low-gloss parts. The 60° geometry is used for most gloss measurements and to determine whether it is better to use the 20° or 85° geometry. The 20° geometry is advantageous when the 60° geometry indicates values above 70 GU – high gloss. Conversely, the 85° geometry is used to compare samples with gloss close to greyness – when the 60° geometry measures gloss values below 10 GU.  

Reflectance spectrophotometers are used to measure the reflectance spectrum; they use diffraction gratings to determine the intensity of reflected light at individual wavelengths. The light dispersed by the diffraction grating falls on an array of photodetectors, which convert the incident light into an electrical signal.

The spectrophotometer first calibrates itself to absolute white, which absorbs no light, and then calculates the ratio of the intensity of light reflected from the measured object to the intensity of incident light for all wavelengths, thus obtaining the reflectance spectrum of the object.   

In handheld spectrophotometers, the diffraction grating is fixed (in one position) and the instruments have an array of photodetectors for measurements at 10 nm intervals. Laboratory instruments have a single detector and a rotatable diffraction grating, allowing the measurement interval resolution to be set to individual nm. Halogen discharge lamps or LED sources are used as the light source in handheld spectrophotometers. Benchtop laboratory spectrophotometers are equipped with xenon discharge lamps. Both untextured, polished surfaces and structured, textured surfaces are measured

Possible Causes of Discrepancies When Measuring the Gloss of the Same Moulding

In general, where process parameters have been optimised in relation to the quality requirements of a specific moulding, these parameters are usually also valid for the production of mouldings with the required gloss and surface quality – texture. Temperature parameters have the greatest indirect influence on these quality parameters:

• mould temperature, increasing it – is more favourable for better reproduction of the mould cavity surface – gloss, matt finish, texture – higher temperature = higher gloss. In semi-crystalline materials, it increases crystallinity and creates a more uniform structure, reduces filler orientation and, especially in PA, reduces water absorption
• melt temperature, increasing it – reduces orientation effects, increases the crystalline fraction and reduces post-crystallisation
• cycle time – moderate influence on gloss – extending the cycle time has a similar effect to increasing the melt temperature 
• screw speed – low influence on gloss
• injection speed – increasing the injection speed slightly increases surface gloss, even on textured, structured surfaces
• the influence of the holding-pressure phase on the surface quality of the moulding – texture replication – depending on the gate is shown in Figure 20 

Figure 20:  Relationship between gloss x matt finish of the moulding surface depending on the distance of the surface from the gate during the holding-pressure phase of the injection moulding process

K – surface of the cavity-forming part of the injection mould
L – surface of the moulding
A – surface locations close to the gate – high pressure response in the polymer melt during the filling phase and subsequently in the cooling moulding during the holding-pressure phase – the surface of the cavity-forming parts of the mould is well replicated on the moulding surface – the textured moulding surface is matt
B – surface locations further from the gate – lower pressure response in the melt during the filling phase, or in the moulding during the holding-pressure phase – poorer surface replication – the texture has lower gloss
C – locations furthest from the gate – low pressure response in the polymer melt during the filling phase, in the moulding during the holding-pressure phase – the least replicated surface – the moulding surface will have the highest gloss value

Possible causes of discrepancies when measuring the surface quality – gloss – of the same moulding can be found in the following factors:

• spectrophotometer – when the gloss of the same moulding is measured by several assessors, these personnel must always use the same instruments – the same manufacturer, the same instrument type, the same measurement geometry
• measurement location on the moulding – it is always absolutely essential to define the exact surface measurement location on the moulding – the reason for this requirement is shown in Figure 20
• customer measurement standard – for gloss or texture assessment, the customer must provide their reference standard, i.e. a plaque made of the same material as the moulding being assessed, with the required gloss or texture; both parties must have an identical reference standard with which their instruments will be calibrated
• measurement methods – measurements must be carried out strictly in accordance with the manufacturer's recommendations
• environmental influence – the result of measuring the moulding surface may be affected by the environment in which the moulding is assessed – temperature, relative humidity, UV radiation, type of lighting, etc., as well as
• moisture absorption of moulding materials, ageing of mouldings, changes in the crystalline fraction in semi-crystalline polymers, filler distribution in mouldings made from composites with a thermoplastic matrix. Environmental influence can be particularly evident in mouldings produced from semi-crystalline materials, most notably polyamides, which are the most susceptible to the effects of water and moisture absorption. Particularly in PA, depending on the temperature, time and relative humidity of the moulding's operating environment, moisture is absorbed primarily in the surface layer of the moulding, which is the surface being assessed, while, again depending on temperature and time, changes in the crystalline fraction may occur there. Both phenomena – moisture absorption and changes in the crystalline fraction – can affect both gloss and texture. In composite materials with a thermoplastic matrix, filler distribution in the surface layers and filler orientation may also affect the measurement. 

End of the second part.

You can find the first part here.
You can find the third part here.

Cover photo: R & D MOLD MACHINING s. r. o.