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In this paper, Nd:YAG nanosecond pulsed lasers were used to remove contaminants from chrome molds and contribute to environmental protection.
summary
The removal of rubber contaminants from slotted chrome plating molds was studied with a acousto-optic tone Q Nd:YAG nanosecond pulsed laser with an average power of 100. Contaminants that are unevenly deposited on the mold can be distinguished as rubber particles with a nominal diameter of 10 μm, mainly located in the flat area (PR), while thick rubber layers up to 50 μm can be found in the slit area (SR). The results show that by positive defocusing, the energy density of the single-pulse laser is 0.97 J/cm2, and argon gas is added, which can realize the efficient cleaning of the single scan of the tank, and can also clean the mesh cracks on the chromium layer. Mold surfaces before and after cleaning were studied using light microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), and atomic force microscopy (AFM).
1. Introduction
Rubber vulcanization molds are contaminated by synthetic rubber deposits, compounding agents and release agents in the production of rubber products. The main pollutants contain sulfides, inorganic oxides, silicone oil dirt deposits, carbon black scale, etc., which will become hard and thick after repeated production, and if produced using contaminated molds, these attached pollutants will deform the product. Therefore, the mold must be cleaned regularly to ensure product quality and extend the service life of the mold. At present, chemical cleaning is widely used in cleaning rubber curing molds, and the method of high temperature alkaline water soaking combined with mechanical bristles is used to decompose and remove pollutants. However, long-term use of chemical cleaning can cause the mold to fail due to corrosion and mechanical damage.
Laser cleaning is a non-contact environmental protection technology that has been widely used to remove microparticles and thin layers of materials. In previous studies, Kong et al. studied CO2 pulsed laser cleaning rubber curing molds. The complete cleaning threshold for rubber is 27.5 J/cm2 and the damage threshold for substrate is 30 J/cm2. Wang et al. studied 1064 nm Nd:YAG pulsed laser cleaning tire molds. The cleaning threshold for rubber is 18.6 J/cm2 and the damage threshold for substrate is 22.0 J/cm2. Lu et al. studied organic dirt on the surface of excimer laser cleaning IC molds. The organic matter cleaning threshold is 100 mJ/cm2 and the matrix damage threshold is 1.05 J/cm2. In these cleaning studies, the mold is mostly flat structure, and the thickness of the deposited rubber is uniform. The complete cleaning threshold of rubber differs greatly from the damage threshold of the substrate. Laser cleaning has a suitable laser flux range, which can obtain a good cleaning effect. However, due to functional requirements, commonly used molds have many grooves, which can lead to thicker deposits of rubber contaminants. The full cleaning threshold for thick groove rubber (SR) is much higher than the full cleaning threshold for flat area thin rubber (PR) and even higher than the damage threshold for mold substrates. Therefore, whether there is a suitable laser injection range for the laser cleaning of the mold slot is a question worth studying.
A cross-section of an old iron product, with a shell of corrosive products and embedded mineral particles.
In archaeology, most artifacts are extracted from the environment in which they were placed for ten to thousands of years. Since all substances decay, it is very rare to find artifacts in their original state. Metal objects react with chemicals in the environment such as moisture, oxygen, carbon dioxide, salts, etc., forming various corrosion products. These occur on the inside and surface of metalwork and become fragile and porous. Contaminants such as mineral particles and organic matter in the soil can also bind to corrosion products to form a hard shell on the surface (above). Corrosion of archaeological metals continues and accelerates even after excavation. Therefore, in order to protect the valuable scientific information carried by materials and artifacts, it is necessary to use appropriate conservation treatments with caution.
In fact, in order to protect the substrate and improve the surface performance, the surface of the rubber curing mold is usually coated with a functional chromium layer. Kumar et al. reported on laser cleaning of powder coatings on the surface of galvanized steel. The results showed that with a 1064 nm laser, the amount of 0.7 J/cm2 was injected, and 0.2 μm thick zinc was evaporated, and a shock wave was generated to remove the paint from the surface. Although the paint surface can be effectively removed, the zinc layer is gone. Due to the difficulty and high cost of maintenance of the chromium layer of the mold, laser cleaning should not damage the chromium layer. In addition, the crisscrossing of the chrome layer of mesh cracks can store oil, which acts as lubrication and reduces friction. However, previous studies have paid little attention to cracks in the functional layer during laser cleaning.
The absorption characteristics of different metals act as a function of wavelengths. The wavelengths used in this work are indicated. Since they are not easily absorbed by metal substrates, lasers with longer wavelengths, in this case, CO2 lasers are unlikely to damage metal products. Organic compounds and minerals, on the other hand, are usually translucent or translucent in visible and near-infrared, but absorb strongly in the far infrared.
The TEA CO2 laser used (ALLTEC ALLMARK 870) emits radiation with a wavelength of 10,600 nm in the form of short pulses with an energy of 2-4 J, a typical duration of 100-1000 ns, and a repetition rate of up to 20 Hz. As can be seen from the figure above, most metals are poorly absorbed at λ = 10600 nm; However, organic materials exhibit strong absorption at this wavelength. Therefore, TEA CO2 lasers are best suited for cleaning metal products. The absorption of radiation by organic compounds is due to the interaction of photons with different free radicals. Photons with energy of E = hν = 0.116 eV are strongly excited -OH, -CH, -CN free radicals, causing the organic compounds to be heated and evaporated.
In this study, a sound-optic q Nd:YAG nanosecond pulsed laser with an average power of 100 W was used to perform laser cleaning experiments on rubber contaminants that were not uniformly deposited in the groove of the chrome plating mold. The mesh cracks on the chromium layer after cleaning were further studied. The surface of the mold sample before and after laser cleaning was analyzed using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM).
2. Materials and Methods
2． 1． Sample preparation
Figure 1 shows a schematic diagram of a rubber curing mold with a radius of 72 mm and a height of 35 mm. As can be seen from Figure 1(c), contaminants deposited on the PR are nominally 10 μm in diameter and dispersed in a dispersed granular manner, with part of the chromium layer directly exposed. As shown in Figure 1(d), the thickness of thick layered structural contaminants deposited on the SR is 30 μm to 50 μm. Therefore, the rubber deposited in the groove is different from the common homogeneous contaminants. The mold specimen is P20 steel, the composition of which is shown in Table 1.
Figure 1 Schematic diagram of rubber contaminants deposited on molds and grooves. (a) Schematic of the mold, (b) the slot, (c) rubber particles on the PR, and (d) thick layers of rubber contaminants on the SR.
Table 1 Chemical composition of P20 steel.
This die cuts into 5 mm × 5 mm × 5 mm small samples. Samples are soaked with acetone and then carefully scrubbed with alcohol, with a clean chromium layer surface as shown in Figure 2. As can be seen from Figure 2(a), the reticular cracks are interconnected in a cross shape, and some spherical convex parts appear on the PR surface. As can be seen from Figure 2(b) (c), the chromium layer on the surface of the SR is partially destroyed, and some residue can even be found in the damaged area. The chromium layer cross-section obtained after grinding and polishing is shown in Figure 2(d). The thickness of the chromium layer is approximately 8 μm.
Fig. 2 The surface and section of the mold. (a) The surface of the PR, (b) (c) the surface of the SR, and (d) the cross-section of the PR and the SR.
2.2 Laser cleaning
The schematic of the laser cleaning system is shown in Figure 3. The pulsed laser is a homemade 100 W acousto-optic q-nanosecond pulsed laser with a repetition rate tuned from 5 kHz to 30 kHz. The scanning speed of the 2D galvanometer is continuously adjustable from 500 mm/s to 3000 mm/s, which is faster than the usual laser cleaning speed. The focusing lens used in the experiment is an F-θ lens with a diameter of 35 mm and a focal length of 100 mm. The laser scanning area is 10 mm × 10 mm. By changing the pumping current of the pumping module, the cleaning threshold is found, and after the system study is completed, the sample is cleaned with multiple sets of parameters, as shown in Table 2.
Figure 3 Schematic diagram of laser cleaning system.
Table 2 Laser parameters.
3. Results and discussion
The EDS results for PR and SR surface contaminants before laser cleaning are shown in Table 3(a). Before cleaning, the total content of C and O reached 89.87 wt, and the Cr content was 0 wt%.
Table 3 The average chemical composition of the surface is measured with EDS before and after cleaning (wt.%). (a) EDS results of contaminants, (b) PR surface EDS results after cleaning, (c) SR surface EDS results after cleaning, (c) EDS results of net cracks after cleaning.
3． 1. Laser cleaning of rubber particles on PR
As can be seen from Figure 4, after laser parameter I cleans the rubber particles on the PR, a clear boundary can be seen (Table 2), indicating that the complete cleaning threshold of the rubber particles is 0.59 J/cm2. After cleaning, as shown in Table 3(b), the content of Cr reaches 93.24 wt, and the content of C and O is 6.06 wt%, indicating that the rubber particles on the surface of PR have achieved a good cleaning effect.
Figure 4 SEM image of the sample on the PR surface.
The rubber on the surface of the PR is granular, and the chromium layer is exposed in some areas, so the laser can directly radiate the chromium layer. When a Gaussian laser radiates the surface of the sample, the temperature of the chromium layer at time t is calculated by the following formula: I0 is the peak power density of the laser, T0 is the initial temperature of 300, and K and R are the reflectivity of Cr. The parameters used in the calculation are shown in Table 4. When t is 100 ns as the pulse width, the calculated surface temperature of the chromium layer is 639 K, below the oxidation temperature (900 K) melting point (2094 K) chromium, so there is no oxidation or melting. On this basis, with reference to Lee et al.'s calculation of laser cleaning of metal particles, the thermoelastic expansion force generated in the vertical direction of the rubber bottom is 5.32 × 10−8 N. The binding force between the rubber particles and the substrate is mainly determined by van der Waals Fa, which is about 5 × 10−8 N. The thermoelastic expansion force generated by the vertical direction of the rubber bottom is greater than that of Fa. Therefore, 0.59 J/cm2 as a complete rubber particle cleaning threshold, experiments and theories conform well. Specifically, the removal mechanism of rubber particles is mainly due to the lamination caused by the effect of thermal expansion.
Table 4 Thermophysical properties of chromium and rubber.
３． ２． Laser cleaning of the rubber layer on the SR
After multiple sets of tests, the single scan and triple scan cleaning results of Laser II (Table 2) are shown in Figure 5(a) and (b), respectively. Comparing Figure 1(d) and Figure 5(a), the cleaning results of a single scan with a pulse energy density of 1.2 J/cm2 are acceptable. As can be seen from Figure 5(a), the chromium layer exposed to SR is not significantly damaged, but a small amount of contaminants and oxides remain in a few areas. After three laser II scans, as shown in Figure 5(b), there is no rubber residue on the surface of the SR, and most of the iron oxide is removed, revealing a distinct pit structure. However, a small amount of iron oxide remains in the damaged chromium layer. Chromium layer peeling to form iron oxide is a manifestation of mold failure, the failed mold should be repaired or discarded, so the residual iron oxide under three scans will not be further cleaned, but at the same time our laser cleaning can be used as a new way to judge whether the mold has failed.
Figure 5 Results of cleaning the old rubber layer (a) as a result of a single-scan laser II (Table 2). White protrusions of residual rubber, oxidation and test results shown in Table 5 (a) as oxides of iron, (b) the results of triple scans of laser oxidation in the 20s interval, test results are shown in Table 5 (b).
According to previous experiments, the uneven contamination on the laser cleaning chrome mold is similar to The Ref study. A three-time laser scan with a single pulse energy density of 1.2 J/cm2 can obtain a good cleaning effect, but the multi-scan cleaning method will inevitably reduce the cleaning efficiency. To improve cleaning efficiency, laser parameter III with higher repetition rate and average power in Table 2 is used. In this way, while single-scan cleaning can effectively remove contaminants and oxides, the chromium layer on the PR and SR is also oxidized, as shown in Figure 6. The results of the EDS analysis are shown in Table 5(c), and the XRD analysis found that the oxide was Cr2O3. Oxidation is due to the accumulation of heat, which is due to the increased number of pulses, due to the increased repetition rate.
Fig. 6 Surface damage to the chromium layer.
Table 5 Average chemical composition (wt.%) of the substrate after laser cleaning. (a) (b) EDS results for iron oxides, (c) EDS results for chromium oxide.
We know that argon is the most commonly used inert gas, and the density of argon is 25% higher than the density of air, which is an ideal protective gas. To further improve the cleaning effect, the sample is placed in an argon atmosphere for the same cleaning experiment. The results show that argon not only prevents oxidation of the chromium layer, but also cools the surface of the chromium layer, and there is no oxidation on the PR surface, as shown in Figure 7. After a scan of laser parameter III (Table 2) in an argon atmosphere, the chemical composition of the sample is measured with EDS as shown in Table 3(c). The EDS results are shown in Figure 8. It can be seen that the content of C, O reaches 90 wt, and the content of Cr is 0 wt%. There are many additional elements on the surface before cleaning, such as Si, Ga, S, Zn; After cleaning, all extra elements are removed. The C and O content is less than 5.5 wt, and the Cr content reaches 93.5 wt, indicating that the chromium layer covered by rubber is completely exposed and the cleaning effect is good.
Fig. 7 SEM results of laser cleaning of SR surface under argon atmosphere (Table 2).
Figure 8 EDS results before and after cleaning. (a) pre-cleaning results, (b) post-cleaning results.
Thick rubber contaminants with a layered structure absorb laser radiation and can be gasified and removed. However, due to the obvious splashing phenomenon of rubber particles during the laser cleaning process, the cleaning mechanism of thick rubber is much more complicated than gasification. Researchers have shown that sound waves produced during laser cleaning can produce elastic forces to remove contamination. And due to the previous laser cleaning particle rubber analysis of PR, thermoelastic expansion can be another mechanism for cleaning layered rubber. Therefore, when the laser cleans the thick rubber contaminants on the SR, the thickness of the rubber can be reduced by the vaporization or elastic force generated by the reflected sound waves. After the thickness is reduced, part of the laser will be absorbed by the chromium layer, and the remaining rubber can be removed in time through thermal elastic expansion. As can be seen from the experimental results, in a single scanning laser cleaning, these mechanisms combined can remove rubber contaminants of 50 μm thickness.
Figure 9 AFM test results before and after laser cleaning. (a) before laser cleaning, (b) after laser cleaning.
Figures 9 (a) and (b) show a comparison of the three-dimensional topography of the mold surface before and after cleaning with AFM. Sa is the average roughness of the area. Avoids net cracks in the chromium layer. As can be seen from Figure 9(a), the rubber surface distribution is uneven before cleaning at Sa 100 nm. The maximum height on the Z axis can reach 797 nm. After cleaning, the rubber is removed and the chromium layer is exposed, resulting in a lower surface roughness of 35 nm. The maximum height on the Z axis is reduced to 292 nm.
In addition, for laser cleaning of the slot, Yue et al. proposed a small slot for off-focus axial laser cleaning of 2 mm × 24 mm. This article selects aggressive defocusing and laser concentrations above 2 mm of the bottom slot, with a single pulse energy density of about 0.97 J/cm2, because thick rubber contaminants in the old thick layer of rubber and rubber particles can be cleaned without damage to the chromium layer in an argon atmosphere.
3.3 Laser cleaning of chrome mesh seams
After forward defocus laser cleaning of the PR and SR surfaces of parameter III under an argon atmosphere, the mesh cracks on the chromium layer after cleaning were further studied. The topography of the crack in the chromium layer after cleaning is shown in Figure 10. It can be seen that after cleaning, the crack of the chromium layer with a width of 1 μm is clearly outlined. The EDS results are shown in Table 3(d) with a chromium content of 88.59 wt after cleaning. Therefore, under the action of cleaning parameters and cleaning mechanism, the rubber on the surface groove of the chrome plating mold can be cleaned without damage, and the mesh crack of the chrome layer can also get a good cleaning effect. The function of the mesh crack in the chrome layer can be restored in time.
Fig. 10 Effect of clear cracks in the chrome layer.
4. Conclusion
Single-scan efficient cleaning of uneven rubber contaminants on slotted chrome molds was studied. Under the condition of a single pulse energy density of about 0.59 J/cm2, loose spalling due to thermal expansion effect can remove contaminants deposited on the PR, which has a nominal diameter of 10 μm; Contaminants deposited in the old thick hierarchical structure of 50 μm under the influence of a complex cleaning mechanism of 0.97 J/cm2, where the thickness of the rubber can reduce the elastic force generated by SR gasification or reflected waves, and then the remaining rubber can be removed for thermal elastic expansion. By laser positive defocusing with a single pulse energy density of 0.97 J/cm2, argon gas is added, the chromium layer and the chromium layer can be cleaned well, and the net cracks on the chromium layer are also cleaned.
This study not only provides a damage-free cleaning method for the cleaning of molds with groove structures, but also can be widely used in the cleaning of molds with flat structures, and can even be widely used in other products, such as automobiles with decorative coatings, with universal applicability. Replacing the traditional chemical cleaning technology with laser technology can not only improve the service life of the mold, but also eliminate chemical waste and be friendly to the environment.
来源：Laser cleaning of slots of chrome-plated die，Optics &Laser Technology，doi.org/10.1016/j.optlastec.2019.105659
参考文献：N. Gong，New cleaning technology of the tire pattern，Cleaningword, 20 (2004), pp. 29-31，(in Chinese)，G. Schrems,M.P. Delamare, N. Arnold, P. Leiderer, D. Bauerle，Influence of storagetime on laser cleaning of SiO2 on Si，Appl. Phys. A, 76(2003), pp. 847-849
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