Ultrasonic Thickness Measurement of Paint on Plastic
DeFelsko manufactures hand-held, non-destructive coating thickness gages that are ideal for measuring the dry film thickness of coatings on plastic.
Measurement Challenges
The primary challenge is to non-destructively measure the thickness of coatings on plastic. An additional challenge is to identify the individual layer thickness in multi-layer coating applications.
Dry Film Thickness Measurement Solutions
DeFelsko manufactures two non-destructive ultrasonic coating thickness gages are ideal for measuring the coating thickness of most paint on plastic applications.
The PosiTector 200 gage is ideal for measuring total coating thickness on plastic. The PosiTector 200 has a range of 25 to 1000 microns (1 to 40 mils). Figure 1 demonstrates the PosiTector 200 measuring total thickness on plastic.
Figure 1 - PosiTector 200 Measuring Rough Plastic
When measuring the thickness of rough coatings such as the plastic part shown in Figure 1, the statistics mode of the PosiTector 200 should be used. With memory turned on, the PosiTector 200 calculates and displays the number of readings, average of readings, standard deviation of the readings, highest and lowest readings as shown in Figure 2.
Figure 2 - PosiTector 200 Measuring Rough Plastic
The ultrasonic PosiTector 100B gage shown in Figure 3 is ideal for measuring the thickness of multi-coat paints on plastic. The PosiTector 100B which has a measurement range of 8 to 500 um (.3 to 20 mils), typically provides excellent results when measuring two-coat applications such as base and clear coat in automotive applications.
Figure 3 - PosiTector 100B Measuring on Automotive Plastic
A PosiTector 100B screen capture from a 2-layer automotive sample is shown in Figure 4. The two distinct peaks shown in the graphics section demonstrate the ability of the instrument to identify the 53 microns of clear coat over top of the 27 microns of base coat.
Figure 4 - PosiTector 100B Screen Capture of 2 Coating Layers
The PosiTector 100B is capable of identifying up to 5 individual layers. Figure 5 depicts a 5 layer application for which the PosiTector 100B was able to clearly identify each individual layer thickness.
Figure 5
Depending on the application, two factors may limit the ability of the PosiTector 100B to identify individual layer thickness. Paint application processes such as wet-on-wet may not provide a sufficient boundary interface between layers for the gage to pick up the reflective signal. Also to identify the thickness of an individual layer, coatings applied greater than 25 microns (1 mil) provide best measurement results.
Both ultrasonic probes (200 and 100B) discussed in this article have an 8 mm (0.3") diameter flat measurement surface that should fully contact the coated plastic for best measurement results. Measurements taken on curved surfaces may require averaging of multiple readings for meaningful results.
Free Consultation
For current pricing or to order these instruments, please contact us by telephone (315) 393-4450, fax (315) 393-8471, or e-mail techsale@defelsko.com. If you require additional technical information or have questions relating to your particular application, we encourage you to take advantage of our years of experience in recommending the best gage for your application.
What is the Application?
The coating of plastics, particularly in the automotive industry involves the application of several coating layers to attain full aesthetic appearance and protective properties. Not only does exterior finish tend to be a strong reflection of the quality and durability of costly consumer products, but coatings for plastic components need to address the challenges unique to plastic substrates including adhesion, flexibility, and temperature constraints.
Common plastic substrates (i.e. polyethylene, polypropylene, thermoplastic polyolefin, ABS, nylon, PVC) are nonporous, resistant to most solvents, and have low surface energy compared to other materials. This makes the plastic surface difficult to wet and provides the coatings little opportunity to adhere by penetrating or physically locking into surface irregularities. To counter these difficulties, adhesion promoters are used as paint additives or as primers to promote adhesion of coatings to their substrates. An adhesion promoter usually has an affinity for the substrate and the applied coating, enabling applied coatings to meet the intended performance requirements.
In automotive coatings, the term adhesion promoter refers to the primer (typically chlorinated polyolefin or other modified polyolefin's), which facilitates adhesion of the subsequent paint layer to the plastic. Typically adhesion promoters are applied to achieve a dry film thickness of 0.3 – 0.5 mils (7.5 – 12.5µm). As the adhesion promoter thickness is below the recommended 1 mil (25 µm) individual layer thickness it may be difficult for an ultrasonic gage to distinguish it from subsequent layers.
Primers fill any small imperfections from the molding process while providing a conductive layer that facilitates the electrostatic application of subsequent coating layers. Primer also protects the substrate from potentially damaging UV energy from the sun, as well as providing resistance to chemicals (gasoline) and humidity. Often, primers are formulated to be color keyed to allow minimum basecoat film thickness and to minimize the effect of stone chipping.
Basecoat is the coating layer that provides most of the color, physical performance and aesthetic effects. Fade resistant basecoats often include special appearance pigmentation such as the metallic finish common in automotive coatings. Basecoats can be applied as a single or in multiple layers. Depending on the application method, multiple basecoat layers can be challenging for an ultrasonic gage to distinguish between.
Resistant clearcoats form the protective interface from environmental factors such as etch, bird droppings, car wash scratches and stone chips. Though clearcoats are used in combination with the basecoat to form the final finish, accoustically they provide a significant interface between coating layers and are thus distinguishable from previously applied layers.
Since automotive coating is one of the most expensive processes in automobile assembly, manufacturers and assemblers are constantly looking for technology improvements. Once such application method is referred to as wet-on-wet where waterborne coatings are applied directly over top of each other without allowing previous layers to cure. Such methods minimize the use of energy, paint, and retooling requirements, without sacrificing the quality of the finished appearance. Unfortunately a wet-on-wet coating application tends to cause a "transition layer" effect (blending of individual layers). The lack of clear acoustic boundaries minimizes the capability of an ultrasonic instrument to detect individual layer thickness.
Why measure with Ultrasonics?
Manufacturers and applicators alike have long believed that there is no simple and reliable means for non-destructively measuring coatings on plastic substrates. Their common solution was to place metal (steel or aluminum) coupons next to the part and then measure the thickness applied to the coupon with either a mechanical or electronic (magnetic or eddy current) gage. This labor intensive solution is based on the assumption that a flat coupon placed in the general coating area receives the same paint profile as the plastic part in question. An ultrasonic solution enables the user to measure the total coating thickness of the actual part. Dependent on the ultrasonic gage utilized and the coating application process, an added advantage is the ability to identify multiple distinct layers.
Where is the market?
Over the last several years, the use of plastics has expanded rapidly. While the automotive industry has certainly led the way, other industries also make extensive use of plastics. According to the Society of the Plastics Industry, miscellaneous plastics products (which accounts for most of the plastics processing industry), is the fourth largest manufacturing industry in the United States. It is exceeded only by motor vehicles and equipment, petroleum refining and electronic components and accessories. Though plastics are often colored directly as part of the manufacturing process, many plastic parts must be painted to improve appearance, produce a color match with other parts, improve the stability of the plastic surface, or produce a desired special effect.
The global market for automotive paint was $6.6 billion in 2001, according to consultant PG Phillips & Associates. An increasing portion of this market involves the coating of plastics used for bumpers, exterior panels and decorative trim. Coating applicators and assemblers in the competitive automotive industry need to meet critical aesthetic and life expectancy criteria. Since painting is the most costly process in automotive manufacturing there is a conflicting priority to minimize the amount of time, materials and rework involved while still meeting the requirements of performance-enhancing technology and environmental compliance. Thus an efficient measurement method is needed to accurately and reliably detect and correct application problems as early as possible in the coating process.