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Optimised inspection technology for quality control after cleaning and activation

Many parts cleaning processes require surfaces that are sufficiently clean and wettable. Mobile hand-held measuring devices for fluorescence and contact angle measurement make it possible to inspect the surface condition directly in the production area. The basic characteristics of the processes and practical examples provide orientation in selecting such devices.

The surface condition of metal and plastic parts represent a significant quality characteristic in process chains. Contamination
from preliminary processes impairs the quality of cleanliness-critical processing steps, such as coating, bonding and welding.
The effect of this depends on the type and quantity of contamination, as well as the specific technical process.

Pre-treatment processes such as cleaning or activation are necessary to prepare the surface for the next process step. The quality assurance of manufactured products requires monitoring of the cleaning media and inspection of the surfaces in production.
There is no general measure for the relevance of a parameter to the process.
The interaction of the material, contamination and requirements of the subsequent process step is decisive, and also affects the suitability of the method for quality control. A testing, measuring or analytical method can only represent certain characteristics of a surface. It is necessary to be able to transfer the quantitative result to different qualities of subsequent processing and this transfer must also first be carried out. When selecting the inspection method it is necessary to consider the respective strengths and weaknesses with regard to the specific monitoring task. Detection limits, cross influences due to the surface structure or cross contamination, handling properties, the ability to conduct contact-free or inline measurements are just a few examples. Other requirements include the features of the measuring device
itself, as determined by the goal of the quality control in the production environment including robustness, user-friendly
operation, monitoring of threshold values, and the capability of fast and easy inspection and calibration of the device by inhouse personnel. 

Choice of suitable measurement method
Every production process is different. The diversity of production processes therefore requires the choice of a suitable
measurement method for the particular application and monitoring task. An easyto-use test or measurement procedure is
useless if it cannot describe the surface condition that is decisive for good or bad quality in the subsequent process. On the other hand, it is insufficient for the measuring method to assess the surface condition if the measuring device is not
suitable for use in production because it cannot measure the relevant surfaces due to the geometry of the parts. Whether an inspection method is suitable therefore always depends on the particular process and should be evaluated with respect to the task at hand. As a result of intensive discussions with customers in many sectors, SITA Messtechnik has acquired a
broad understanding of processes, in addition to expertise on the use of measuring technology. Due to many years of experience in the area of monitoring cleaning processes, SITA devices are especially adapted to inspection tasks in production.
Measuring devices SITA feature a robust and user-friendly design, which makes them ideal for use in quality assurance directly in production.

SITA CleanoSpector
SITA CleanoSpector

Calibration standards are provided to enable the fast and easy check of devices by the user for the monitoring of measuring equipment. The devices feature data storage and export functions in compliance with modern quality assurance requirements for documented processes.
The measuring devices SITA CleanoSpector
and SITA SurfaSpector use different measuring methods – fluorescence and contact angle measurement – to provide
optimal solutions to different monitoring tasks. Application experts at SITA use their experience in selecting and assessing the suitability of the device for the particular application.
The common features of the two handheld measuring devices and the extensive accessories that are available for them allows custom quality control in the production environment.

They use established manual and subjective methods – visual inspection under black light and wetting tests with water or
test inks – to implement detectionsensitive, objective and robust inspection technology. The punctual measurement,
which can thus be used specifically on functional surfaces, requires only a few seconds to enable fast and detailed statistical series of measurements. Position the sensor, start the measurement with a single keystroke and the measuring device takes care of everything else. The two-part design with an operating and display unit and specially developed measuring process variants allow compact sensor heads with a small contact surface, without the need for a PC.

SITA SurfaSpector
SITA SurfaSpector

Focus on fluorescence measurement
• Optical method for detecting organic film contamination that is self-illuminating under UV light
• Fluorescence signal increases with higher film thickness/contamination quantity; intensity, however, depends on the existing
  organic conditions
• Layer thickness is typically in range of 10 mg/m² to 10 g/m² (about 10 nm to 10 μm)
• Fast, contact-free operation, non-destructive, inline-capable (for measuring in motion)
• Directly on part surfaces, spatial resolving, partially for functional surfaces
• Confocal principle enables small (e.g. 1 mm wide) or complex (e.g. strong curvature) inspection surfaces
• Requires accessibility to the measuring point and correct measuring distance
• Suitable for non-fluorescent materials such as metals and some ceramics; suitability for glass and plastic is restricted
• Low influence of surface roughness and structure

Quality control implemented prior to bonding and painting.
In the bonding of aluminium die-cast parts for gearboxes, problems repeatedly occurred with the adhesive bond. The customer inspected the wettability with test inks for 42 mN/m in accordance with the specifications of the adhesive manufacturer, but was unable to assure the quality of the bond. A process evaluation and the use of analytical methods revealed that cooling lubricants from processing were the cause of the poor bonding. Residues of water- mixed cooling lubricants contain surface-active substances and therefore, result in excellent wettability with test liquids.
Despite good spreading, however, the adhesive cannot achieve a sufficient bond, since the contamination prevents the necessary contact to the base material.
The use of fluorescence measurement for quality control allowed for reliable detection of the residues and ensured the
necessary cleanliness and thus, good adhesion of the surface. Other convincing arguments in favour of fluorescence measurement included the robustness of the method towards changes in the surface structure, as well as the potential for using it as a 100% inline inspection of the entire adhesive groove.

Solvent-based and alkaline cleaning compared
In another case, stainless steel housings had to be cleaned before painting. Contamination by punching oil was removed by means of solvent degreasing or highly alkaline aqueous cleaning.
Fluorescence measurement detected virtually identical cleaning performance of both methods with suitable process management.
The switch to a water-based paint system due to regulations showed differences, however; all samples degreased with solvents and a small quantity of the parts cleaned with aqueous solution showed defects in the paint. While the solvent
removed only the oil, the alkaline cleaning resulted in surface activation due to removal of the reaction layers. This was
likewise evident in the wetting inspection conducted with the contact angle measurement, which made it possible to
optimise and monitor the aqueous cleaning process.

Monitoring of plasma activation
Due to their poor wettability, plastics are activated specifically by generating polar molecular groups. An ABS-PA6 surface had to be pre-treated prior to printing by means of atmospheric pressure plasma within tight process limits. This makes it
necessary to achieve sufficient wettability and adhesion while preventing damage to the material from over-treatment. The
measurement of the water contact angle can ascertain exactly this requirement by detecting and responding to process
changes through regular inspection.
The plasma pre-treatment is also used for precision cleaning and activation of precleaned metal surfaces. A sufficiently clean pre-existing condition is essential; otherwise critical cracking of the residue as well as merely wettable separating layers will develop. In this case the combined use of fluorescence measurement as cleanliness inspection prior to activation and contact angle measurement after activation has proven to be very effective.

Contact angle measurement compact
• Optical detection of the contact or edge angle of test liquid droplets placed on the surface as a measure of wettability
• Wettability increases (spreading droplets) as contact angle decreases
• Contamination-free, easily wettable surfaces or more reactive
• Wettability reacts sensitively to changes in the surface condition:
• Wetting-inhibiting and promoting substances (oils/separating agents, surfactants)
• Oxidation and absorption layers due to reaction with the atmosphere, for example
• Chemical or physical activation/passivation
• High-purity water as test liquid assures this, with no residue or toxins
• Water wettability is material dependent: clean plastics are poorly wettable without pretreatment, clean metals without
  reaction layers are easily wettable
• Layer thickness differentiable only to a few monolayers (a few nm)
• Directly on part surfaces, spatial resolving, partially for functional surfaces
• Surface roughness affects the contact angle, oriented structures (channel, edges) have an effect
• Typical droplet sizes in the microlitre range require test surfaces with a width of several millimetres
• Surface energy can be calculated correctly only for non-contaminated surfaces and using at least two liquids.

Authors
André Lohse, Stefan Büttner
SITA Messtechnik GmbH
Dresden, phone 0351 871 8041
www.sita-messtechnik.de

SITA SurfaSpector metal part
SITA SurfaSpector metal part

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