Auto Body Putty Inspection

Auto body filler (putty) testing mainly conducts scientific testing and analysis on the appearance and colour, mixing ratio and pot life, drying time, sandability, spreading and filling properties, impact resistance and flexibility and other items, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for the production enterprise quality control and downstream user product acceptance.

Testing items

Appearance and colour: Inspect whether the colours of the filler base and hardener are uniform, and free of the impurities, lumps or separation. The base is usually white, grey or of the specific colours, and the hardener colours need to be easy to distinguish. Through the visual comparison with the standard colour plates, record whether their colours are within the acceptable ranges, and the abnormal colours may indicate the problems existing in the raw materials or storage processes.

Mixing ratio and pot life: Verify the recommended mixing ratios of the base and hardener, and determine their pot lives under the standard conditions. Start the timing from the uniform mixing, observe the times required for the significant viscosity rises or the loss of the spreading fluidity at the specific temperatures, and this data is crucial for the on-site construction operations, affecting the coating qualities and construction efficiencies.

Drying time: Systematically test the time parameters of the fillers at the different stages, including the surface drying times, hard drying times and sanding-available times. Under the constant temperature and humidity conditions, judge the time points at which the coating surfaces are not sticky to the touch, completely cured and can be sanded and trimmed according to the standard methods, to evaluate their construction efficiencies and the arrangement of the subsequent processes.

Sandability: Evaluate whether the cured filler coatings are easy to sand smooth without sticking to the sandpapers. Use the specified model sandpapers or sanding machines, carry out the sanding under the standard pressures and speeds, and check whether the phenomena such as the dust flying, sandpaper clogging or coating film overheating and softening occur, and this is the key indicator for the measurement of their construction performance.

Spreading and filling properties: Inspect the construction performance of the mixed fillers, including whether they are easy to spread and whether the dragging or rolling back phenomena occur. Evaluate their filling capabilities for the substrate defects (such as the pits and scratches) at the specified thicknesses, and observe whether the obvious shrinkage, cracking or sinking occurs after the drying, directly affecting the final flatness.

Impact resistance and flexibility: Through the falling weight impact tests and bending tests, determine the impact resistance and flexibility of the cured coatings. This is related to whether the filler coatings will show the defects such as the cracking and peeling when subjected to the external impact forces or slight substrate deformations, and is the important basis for the evaluation of their adhesion to the substrates and system compatibility.

Heat resistance and media resistance: Test the performance changes of the filler coatings after the exposure to the high temperature environments or the contact with the specific chemical media (such as the gasoline, engine oil and antifreeze). Observe whether the phenomena such as the blistering, softening, discolouration or adhesion decline occur, to evaluate their durability in the complex environments such as the automotive maintenance and industrial equipment.

Testing scope

Automotive body repair fillers: Mainly applied in the fields of the automotive post-manufacture repair and after-sales maintenance, applicable to the body panels such as the steel and aluminium alloys. The testing needs to emphatically focus on their compatibility with the various primers, fast drying properties, sandability as well as the tolerance to the common automotive oil products, to meet the high-efficiency and high-quality repair requirements.

Fillers for the industrial equipment and metal components: Applied to the surface levelling of the industrial products such as the machine tools, steel structures, pipelines and containers. The testing scope needs to cover their adaptability to the complex industrial environments (such as the temperature differences, humidities and corrosive atmospheres), as well as the crack resistance under the high film thicknesses and long-term durability.

Special fillers for the glass fibre reinforced plastics (FRP) products: Specially designed for the FRP substrates, and the testing of their adhesion to the unsaturated polyester resin substrates is the key. Need to verify whether their curing shrinkage rates match with the FRP, avoiding the cracking caused by the stresses, and at the same time investigate their fineness after the sanding, to meet the appearance requirements of the high-end FRP products.

Fillers for the plastic part repair: Applicable to the repair and modelling of the common plastic substrates such as the PP, ABS and PVC. The testing emphases lie in their adhesion and flexibility to the plastic substrates, need to ensure that they do not crack during the plastic thermal expansion and contraction processes, and at the same time verify whether their solubility or corrosivity to the plastics is within the safe ranges.

Fillers for the wood and furniture hole filling: Used for the nail hole and crack filling and surface levelling of the solid wood or wood-based panels. The testing needs to focus on the matching of their colours with the woods, the wood grain visibility after the sanding, the sealing performance for the subsequent paint layers as well as whether they contain the components affecting the woods.

Fillers of the different curing systems: The testing scope needs to cover the products of the different curing mechanisms, such as the room temperature curing types, low temperature baking fast drying types and ultraviolet (UV) curing types. For the different systems, set the corresponding testing conditions and methods, for example, the UV curing fillers need to test their photoinitiation efficiencies, penetration depths and shadow area curing degrees.

Testing methods

Visual method and comparison method: For the preliminary inspections of the items such as the appearances, colours and mixing uniformities, mainly rely on the visual observations by the inspection personnel under the standard light sources or natural light. Through the comparison with the standard panels or colour cards, qualitatively or semi-quantitatively judge the product states, and this method is simple and intuitive, and is the first step of the laboratory sample reception.

Spreader method for the determination of the spreading properties: Use the wet film applicators of the specified clearances (such as the spreaders) to uniformly spread the mixed fillers on the standard test panels. Through the observation of the smoothness of the spreading processes, whether the films are continuous and uniform, and whether the particles or streaks exist, quantitatively evaluate their construction viscosities and levelling properties, and the results can be recorded as "good", "acceptable" or "poor".

Drying time determination method: The surface drying times usually adopt the finger touch method, and the hard drying times adopt the filter paper pressing method or cotton ball pressing method. The determination of the sanding-available times needs to periodically use the standard sanding tools to attempt during the drying processes until the uniform and smooth powders can be sanded out without the sandpaper sticking phenomena, recording the times used.

Instrumental sanding test method: Use the standard sandability testers or electric sanding machines, equipped with the specified model sandpapers, and carry out the sanding on the cured filler panels under the fixed loads, strokes and speeds. Through the weighing of the weight differences before and after the sanding, calculate the sanding consumptions, and comprehensively evaluate through the observation of the sandpaper clogging conditions and the states of the sanded surfaces.

Mechanical performance testing methods: Carry out the falling weight impact tests according to the GB/T 1732, "Determination of impact resistance of film". The flexibility testing mostly adopts the cylindrical mandrel bending test method in the GB/T 6742, "Bending test (cylindrical mandrel)", observing whether the coatings crack or peel after the bending on the standard mandrels, to evaluate their elasticity and adhesion.

Chemical media immersion method: Partially immerse the cured and sanded smooth filler panels in or cover with the filter papers soaked with the specific chemical media, and maintain for the specified times at the standard temperatures. After taking out, immediately or after a recovery period, check whether the coating phenomena such as the gloss loss, discolouration, blistering, softening and shedding occur, and rate them.

Instrumental analysis auxiliary methods: For the composition analyses or the diagnosis of the difficult problems, will use the Fourier transform infrared spectrometers (FTIR) to analyse the resin binder types, thermogravimetric analyzers (TGA) to analyse the filler contents and thermal stabilities, scanning electron microscopes (SEM) to observe the cross-section morphologies to analyse the adhesion failure causes, etc.

Testing standards

Domestic standards:

1. GB/T 1732, Determination of impact resistance of film

2. GB/T 6742, Paints and varnishes — Bending test (cylindrical mandrel)

3. GB/T 6682-2008, Water for analytical laboratory use — Specification and test methods

4. GB/T 602, Chemical reagent — Preparations of standard solutions for impurity determination

5. GB/T 603, Chemical reagent — Preparations of reagent solutions for use in test methods

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Electronic balances: Used for the precise weighing of the mixing ratios of the filler bases and hardeners, as well as the sample weight differences before and after the sanding when testing the sandability. Require the accuracies of at least 0.01 g, to ensure the accuracy of the proportioning and the reliability of the test data, and are the basic metrological equipment of the laboratories.

Constant temperature and humidity chambers: Provide the standard environmental conditions (such as the temperatures of 23 plus or minus 2 degrees Celsius and relative humidities of 50 plus or minus 5%) for the filler mixing, spreading, curing and various performance testing. All the time-related items (such as the drying times and pot lives) and physical property tests need to be carried out in this stable environment, to ensure the repeatability and comparability of the results.

Wet film applicators (spreaders): Composed of a set of stainless steel or aluminium double-edged scrapers of the different clearance depths, used for the preparation of the wet films with the uniform thicknesses and consistent widths on the test panels. Through the selection of the different clearances (such as 100 micrometres, 500 micrometres and 1000 micrometres) to simulate the different construction thicknesses, evaluate the filling properties and levelling properties.

Drying time testers: A kind of equipment with the higher automation degrees, can automatically carry out the testing of the multiple drying stages such as the finger touch, paper pressing and scratch methods, and record the times. They reduce the human errors, can more objectively and precisely determine the drying processes, and are especially applicable to the research and development work requiring the monitoring of the entire drying curves.

Film impact testers: Used for the determination of the coating impact resistance. Composed of the guide tubes, weights and punches, change the impact energies by the adjustment of the weight heights, making the weights free-fall to impact the coatings on the back sides of the panels. Observe the cracking or peeling conditions of the coatings in the impacted areas, to judge their capabilities to resist the instantaneous deformations.

Flexibility testers (cylindrical mandrel bending testers): Composed of a set of steel cylindrical mandrels of the different diameters. During the testing, bend the panels with the coatings facing outward around the mandrels for 180 degrees within the specified times, and subsequently check whether the coatings at the bending places crack. Through the use of the mandrels of the different diameters, can quantify the flexibility limits of the coatings.

Sandability testers: A kind of special equipment simulating the manual sanding, usually containing a sanding head that can reciprocate or rotate, and can apply the constant pressures. Through the setting of the fixed sanding strokes, times or counts, and the installation of the standard sandpapers, carry out the standardised sanding on the panels, thereby objectively evaluating the difficulty degrees of the sanding and the sandpaper wear conditions.