Environment-friendly lead-free enamel glaze metal surface glazing method

Abstract

The present application relates to the technical field of metal enamel processing, and particularly discloses an environmentally-friendly lead-free enamel glazing method for metal surface, which comprises the following steps: S1, metal substrate pretreatment; S2, environmentally-friendly lead-free enamel material blending; S3, metal substrate preheating treatment; S4, glaze preheating treatment; S5, hierarchical double-layer glazing; S6, gradient section drying; S7, section constant-temperature sintering; and S8, constant-temperature slow cooling post-treatment. In the present application, the metal substrate preheating and the environmentally-friendly lead-free enamel material preheating are simultaneously arranged, so that the temperature difference between the substrate and the glaze is eliminated, the glaze meltability is stable in the spraying moment, the lead-free glaze powder can uniformly infiltrate the passivated surface of the metal substrate, and the adhesion of the glaze layer is improved compared with the traditional process of only preheating the substrate.

Description

Technical Field

[0001] This invention relates to the field of metal enamel processing technology, and in particular to an environmentally friendly lead-free enamel glaze application method for metal surfaces. Background Technology

[0002] The traditional metal enamel processing industry has long used lead-containing enamel glazes. Lead is easily volatilized during high-temperature sintering, forming toxic waste gas. When the finished enamel is in contact with water and food for a long time, lead ions will continue to leach out, posing serious food safety and environmental protection risks. Currently, the industry is gradually promoting lead-free glaze alternatives.

[0003] Existing conventional glazing processes only perform simple preheating of the metal substrate without a simultaneous glaze preheating process. This results in a large temperature difference between the substrate and the glaze, leading to inconsistent cooling rates of the glaze after spraying. Consequently, the glaze layer is prone to problems such as pinholes, bubbles, and peeling. Furthermore, existing processes often employ single-layer, one-time spraying, making it difficult to precisely control the glaze thickness. Thinner layers lack corrosion resistance, while thicker layers are prone to cracking during sintering. Traditional drying processes use a single high-temperature, one-time drying process, causing rapid and intense vaporization of moisture within the glaze layer, resulting in numerous pinholes and pitting on the glaze surface. The sintering process involves directly raising the temperature to a high temperature and holding it there, causing rapid evaporation and expansion of organic matter and moisture within the glaze, damaging the interface between the glaze layer and the metal substrate. Therefore, to address these issues, an environmentally friendly lead-free enamel glaze method for glazing metal surfaces has been proposed. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing an environmentally friendly lead-free enamel glaze application method for metal surfaces.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] An environmentally friendly, lead-free enamel glaze application method for metal surfaces, comprising the following steps:

[0007] S1 Metal substrate pretreatment, S2 Environmentally friendly lead-free enamel glaze preparation, S3 Metal substrate preheating treatment, S4 Glaze preheating treatment, S5 Graded double-layer glazing, S6 Gradient segmented drying, S7 Segmented constant temperature sintering, S8 Constant temperature slow cooling posttreatment.

[0008] The S1 process removes oil, oxide scale, and impurities from the surface of the metal substrate;

[0009] The S2 process prepares lead-free glaze and completes ball milling, viscosity adjustment, and filtration.

[0010] S3 and S4 preheat the metal substrate and glaze respectively to improve the adhesion of the glaze layer;

[0011] S5 uses a double-layer spraying of base glaze and top glaze to control the total thickness of the glaze layer;

[0012] S6 uses a three-stage gradient heating and drying process to remove moisture from the glaze layer;

[0013] S7 stepped heating completes pre-melting and high-temperature sintering;

[0014] After the S8 is slow-cooled in sections, it is polished, dusted, and inspected to obtain the finished enamel product.

[0015] Preferably, the raw materials for the S2 environmentally friendly lead-free enamel glaze preparation process are proportioned by weight as follows: 30-40 parts quartz powder, 20-28 parts borax, 15-22 parts feldspar, 5-10 parts alumina powder, 3-6 parts titanium dioxide, 2-5 parts zinc oxide, 1-3 parts bentonite, and 35-45 parts deionized water; the glaze is wet-milled to a fineness of 300-400 mesh, the viscosity is adjusted to 25-35s, and the glaze is filtered to remove impurities before use.

[0016] Preferably, the S5 graded double-layer glazing process adopts a double-layer spraying process of base glaze and top glaze. The thickness of the base glaze is 0.15-0.25mm. After the base glaze is sprayed, it is leveled for 3-5 minutes before the top glaze is sprayed. The thickness of the top glaze is 0.20-0.30mm. The total thickness of the glaze layer is controlled at 0.35-0.55mm.

[0017] Preferably, the S1 metal substrate pretreatment process sequentially performs degreasing, water washing, pickling and rust removal, neutralization and passivation, and drying to completely remove impurities adhering to the surface of the metal substrate.

[0018] Preferably, the S6 gradient segmented drying process is divided into three constant temperature drying stages: the first stage is drying at 60-80℃ for 15-20 minutes, the second stage is drying at 90-110℃ for 20-25 minutes, and the third stage is drying at 120-140℃ for 10-15 minutes.

[0019] Preferably, the S7 segmented isothermal sintering process adopts a stepped heating mode, first heating to 350-400℃ and holding for 8-12 minutes to complete the pre-melting treatment, and then heating to 580-650℃ and holding for 25-35 minutes to sinter and form the product.

[0020] Preferably, the S8 constant temperature slow cooling post-processing procedure is as follows: after sintering, the temperature is slowly reduced to below 200°C, the workpiece is taken out of the furnace and naturally cooled to room temperature, and then polishing, dust removal and finished product quality inspection are carried out in sequence.

[0021] Preferably, the S3 metal substrate preheating treatment and the S4 glaze preheating treatment are carried out simultaneously. Simultaneous preheating shortens the overall process time and ensures that the temperature of the substrate and the glaze is matched during glazing.

[0022] Preferably, wet ball milling uses an alumina-lined ball mill jar with high-alumina grinding balls, and continuous stirring throughout the milling process to prevent the glaze powder from settling and agglomerating.

[0023] Preferably, the double-layer spraying uses an air electrostatic spraying device, with the spraying air pressure stably controlled at 0.3-0.5MPa to ensure that the glaze layer is uniform and free of drips and pinholes.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] 1. This invention simultaneously incorporates two processes: preheating the metal substrate and preheating the environmentally friendly lead-free enamel glaze. This eliminates the temperature difference between the substrate and the glaze, ensuring stable glaze melt flow during spraying. The lead-free glaze powder can evenly wet the passivated surface of the metal substrate. Compared to the traditional process that only preheats the substrate, the glaze adhesion is improved. Lead-free glazes do not contain lead flux, and their melting and wetting ability is naturally weaker. Temperature differences can cause the glaze to solidify instantly, forming interface gaps. The dual preheating process avoids interface gap formation from the source. After sintering, the glaze layer and the metal substrate are tightly bonded. Even after 200 cycles of alternating hot and cold temperatures, there is no peeling or flaking, solving the industry pain point of glaze flaking after long-term use of existing lead-free enamel products.

[0026] 2. This invention employs a graded, double-layer glazing process, with separate control over the thickness of the base glaze (0.15-0.25mm) and the top glaze (0.20-0.30mm). The base glaze prioritizes filling the microscopic pores of the metal substrate, while the top glaze provides a smooth and even finish. This differs from existing single-layer spraying processes where the thickness is uncontrollable. Single-layer spraying results in insufficient corrosion resistance in thin areas and prone to sagging during drying in thicker areas. In this invention, the base glaze is leveled for 3-5 minutes before the top glaze is applied, allowing for the full release of internal air bubbles. This improves the uniformity of electrostatic spraying, reduces pinholes, pitting, and sagging defects by 90%, and increases the gloss of the finished product by 25%. It eliminates the need for subsequent large-area grinding and repair, significantly reducing grinding material and labor costs. Post-processing time per unit is reduced by one-third, and the yield rate for large-scale factory production is consistently increased to over 95%.

[0027] 3. In this invention, the gradient segmented drying is carried out in three stages with slow heating to gradually release the free water and bound water in the glaze layer; the segmented constant temperature sintering adds a pre-melting stage of 350-400℃ to slowly decompose the organic additives in the glaze, and then raises the temperature to 580-650℃ to complete the melting and molding of the glaze. Water vapor and organic matter are slowly discharged without internal expansion stress. Attached Figure Description

[0028] Figure 1 This is a flowchart of the environmentally friendly lead-free enamel glaze application method for metal surfaces proposed in this invention;

[0029] Figure 2 This is a schematic diagram of the metal substrate pretreatment, glaze preparation and dual preheating system of the environmentally friendly lead-free enamel glaze metal surface glazing method proposed in this invention.

[0030] Figure 3This is a flowchart of the finished product quality inspection process for the environmentally friendly lead-free enamel glaze application method for metal surfaces proposed in this invention. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0032] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0033] Reference Figure 1-3 An environmentally friendly, lead-free enamel glaze application method for metal surfaces.

[0034] Example 1

[0035] In this embodiment, the workpiece processed is a 304 stainless steel food-grade enamel soup pot. The metal substrate has a diameter of 24cm, a depth of 12cm, and a wall thickness of 0.8mm. The eight-step operation of this invention is fully implemented. The details of each step, including operation specifications, equipment selection, process parameters, and key points, are detailed as follows: S1 Metal substrate pretreatment process: Take the stainless steel soup pot metal substrate and place it on a suspended conveyor line. The first step is degreasing and oil removal using a water-based degreasing agent composed of 5% sodium hydroxide and 2% sodium carbonate. The temperature of the soaking tank is controlled at 55℃, and the soaking time is 8 minutes. An aeration and stirring device is set at the bottom of the tank to continuously impact the surface of the metal substrate, dissolving residual stretching oil and rust-preventive lubricating oil from stamping. After degreasing, it enters a two-stage counter-current water washing tank. The first stage involves water washing to recover degreasing waste liquid, followed by a second stage using room temperature deionized water to rinse away residual alkaline solution from the substrate surface. After water washing, the substrate enters an acid pickling and rust removal tank. The acid pickling agent is a 10% citric acid + 0.5% corrosion inhibitor aqueous solution, and the substrate is immersed at room temperature for 6 minutes to remove oxide scale and welding spots from the stainless steel surface. After acid pickling, the substrate undergoes two more stages of water washing to thoroughly remove any remaining acidic solution. Subsequently, a neutralization and passivation process is performed using a 0.3% zinc dihydrogen phosphate passivation solution, which is immersed at room temperature for 4 minutes to generate a dense passivation film on the metal substrate surface, improving the adhesion of the glaze. After passivation, the substrate is sent to a hot air drying tunnel at a drying temperature of 130℃ for 6 minutes. After drying, the metal substrate surface is free of moisture and white salt residue. The substrate is then cooled to room temperature and transferred to the next process. S2 Environmentally Friendly Lead-Free Enamel Glaze Preparation Process: Accurately weigh the raw materials according to weight: 35 parts quartz powder, 24 parts borax, 18 parts feldspar, 7 parts alumina powder, 4 parts titanium dioxide, 3 parts zinc oxide, 2 parts bentonite, and 40 parts deionized water. Put all raw materials into a 50L alumina-lined ball mill jar, filled with high-alumina grinding balls at a ball-to-material mass ratio of 1.8:1. Perform wet ball milling in the sealed jar for 14 hours. During the milling process, stop the machine every 2 hours to check the fineness of the glaze. Stop milling when all the glaze passes through a 350-mesh standard sieve. Adjust the glaze viscosity to 30s using a Forte 4 cup viscometer. If the viscosity is too high, add a small amount of deionized water; if the viscosity is too low, add a trace amount of bentonite. After preparation, filter twice using a 200-mesh stainless steel filter to remove powder lumps and hard impurities. After filtration, transfer the glaze to a constant-temperature storage tank and let it stand for later use. Continuously stir the storage tank at low speed to prevent powder sedimentation. S3 metal substrate preheating and S4 glaze preheating are carried out simultaneously: the pretreated and dried metal substrate is suspended and sent into an infrared preheating oven. The oven temperature is stabilized at 110℃ and kept at this temperature for 10 minutes to complete the substrate preheating. At the same time, the environmentally friendly lead-free enamel glaze in the storage tank is transported to the jacketed insulated material tank. The jacket is circulated with warm water for constant temperature preheating. The glaze discharge temperature is stabilized at 45℃. The two preheating processes start and end simultaneously to ensure that the temperature difference between the metal substrate and the glaze is controlled within 60℃ during glazing, so as to avoid the glaze from solidifying rapidly due to excessive temperature difference.S5 graded double-layer glazing process: Use air electrostatic spraying equipment with a stable spraying air pressure of 0.4MPa and an electrostatic voltage of 60kV; first, spray the base glaze, with the spray gun 20cm away from the metal substrate surface, spraying in a uniform circular motion, controlling the base glaze thickness to 0.20mm, ensuring the base glaze fully covers the inner and outer surfaces of the substrate. After spraying, let the workpiece stand still for 4 minutes to level, allowing the tiny air bubbles inside the base glaze to fully rise and burst; after leveling, use the same equipment and change the spraying pipeline to spray the top glaze, with the spraying speed slightly slower than the base glaze spraying, controlling the top glaze thickness to 0.25mm, and the total glaze layer thickness of the base glaze + top glaze to 0.45mm. Use a film thickness gauge for fixed-point sampling throughout the process, and immediately adjust the spray gun speed if the thickness exceeds the range to prevent glaze layer thickness deviation. S6 Gradient Segmented Drying Process: The coated soup pot workpiece is suspended and pushed into a three-section hot air drying tunnel, with independent temperature control for each section. The first drying zone is at 70℃ and is held for 18 minutes to slowly evaporate the free moisture on the glaze surface. The moisture vaporization rate is slow and will not impact the glaze surface to create pinholes. The second drying zone is at 100℃ and is held for 22 minutes to precipitate the capillary-bound water inside the glaze layer, and the bentonite binder slowly solidifies and forms the shape. The third drying zone is at 130℃ and is held for 12 minutes to completely remove any residual trace moisture inside the glaze layer. After drying, the glaze layer of the workpiece is completely cured, with no sticky feel to the touch, and no whitening or cracking defects. S7 Segmented Constant Temperature Sintering Process: After drying, the workpiece is sent into a gas-fired roller sintering kiln, and a stepped heating program is adopted, with segmented temperature control in the kiln. In the first heating stage, the temperature is raised to 380℃ and held at a constant temperature for 10 minutes to complete the pre-melting. Bentonite and trace amounts of organic dispersants inside the glaze slowly decompose and volatilize without rapid expansion. In the second heating stage, the temperature is raised to 620℃ at a uniform rate and held at a constant temperature for 30 minutes to complete the sintering and shaping. The quartz, borax, and feldspar components in the lead-free glaze are fully melted and react with the passivation film of the metal substrate to form a stable enamel bonding layer. Throughout the sintering process, the oxygen content in the kiln is maintained at 8%-10% to avoid the reduction and discoloration of titanium dioxide in the glaze. S8 Constant Temperature Slow Cooling Post-Processing: After sintering, the workpiece first enters the slow cooling sealed kiln section and is cooled to 180℃ at a uniform rate of 3℃ / min. Rapid air cooling is prohibited. After the temperature drops to 180℃, the kiln door is opened, the workpiece is taken out and placed in a room temperature ventilated station to cool naturally to room temperature of 25℃. After cooling, the glaze surface is manually polished with non-woven fabric and ultra-fine polishing powder to remove slight sintering dust and minor protrusions. A high-pressure dust removal fan blows away dust and impurities from the inner and outer surfaces of the workpiece. Finally, the workpiece enters the quality inspection station, where the glaze thickness, adhesion, glaze appearance, and acid and alkali resistance are tested in sequence. After passing the tests, the workpiece is packaged and stored. Unqualified workpieces are marked with defects and reworked.

[0036] Example 2

[0037] In this embodiment, the metal workpiece processed is a small test plate of a carbon steel industrial anti-corrosion storage tank. The metal substrate is Q235 carbon steel, with dimensions of 150mm×100mm×2mm. The raw material ratio and upper and lower limits of the process range are adjusted to verify the adaptability of the parameters of this invention. S1 Metal substrate pretreatment process: Degreasing and oil removal are carried out using 6% sodium hydroxide degreasing agent, soaked at 60℃ for 10min; pickling and rust removal are carried out using 12% hydrochloric acid corrosion inhibitor pickling solution, soaked for 7min to thoroughly remove the thick iron oxide rust scale on the carbon steel surface; neutralization and passivation are carried out using 0.5% zinc phosphate passivation solution, soaked for 5min, and dried at 140℃ for 7min. S2 Environmentally friendly lead-free enamel glaze preparation: 30 parts quartz powder, 20 parts borax, 15 parts feldspar, 5 parts alumina powder, 3 parts titanium dioxide, 2 parts zinc oxide, 1 part bentonite, and 35 parts deionized water; wet ball milling to 300 mesh, viscosity adjusted to 25s using a Cotton Cup IV, and impurities removed by filtration through a 200-mesh filter. S3 and S4 Simultaneous Preheating: Preheat the metal substrate oven to 90℃ and hold for 12 minutes; preheat the glaze jacket to 40℃. S5 Graded Double-Layer Glazing: Base glaze thickness 0.15mm, leveling for 3 minutes; top glaze thickness 0.20mm, total glaze thickness 0.35mm, spraying air pressure 0.3MPa. S6 Gradient Segmented Drying: First segment drying at 60℃ for 20 minutes, second segment drying at 90℃ for 25 minutes, third segment drying at 120℃ for 15 minutes. S7 Segmented Constant Temperature Sintering: Pre-melt at 350℃ and hold for 12 minutes, then heat to 580℃ and hold for 35 minutes for sintering. S8 Constant Temperature Slow Cooling Post-Treatment: Slowly cool to 150℃ and remove from the oven, allow to cool naturally to room temperature, polish, remove dust, and inspect for quality. In this embodiment, the carbon steel test plate showed glaze adhesion of 2.8MPa, and no corrosion after 180 hours of acid and alkali immersion, meeting the standards for industrial anti-corrosion equipment use, verifying that the lower limit parameters of this invention are suitable for easily corroded metal substrates such as carbon steel.

[0038] Example 3

[0039] In this embodiment, the workpiece is an aluminum alloy decorative enamel panel, measuring 300mm × 200mm × 1mm, and is processed using the upper limit range of the formula and process parameters. The S2 environmentally friendly lead-free enamel glaze is prepared as follows: 40 parts quartz powder, 28 parts borax, 22 parts feldspar, 10 parts alumina powder, 6 parts titanium dioxide, 5 parts zinc oxide, 3 parts bentonite, and 45 parts deionized water; wet ball milling to 400 mesh, viscosity adjusted to 35s. S3 Preheat the metal substrate to 130℃ and hold for 8 minutes; S4 Preheat the glaze to 50℃; S5 Base glaze thickness 0.25mm, leveling for 5 minutes, top glaze 0.30mm, total glaze layer 0.55mm, spraying air pressure 0.5MPa; S6 Gradient drying: first stage 80℃ for 15 minutes, second stage 110℃ for 20 minutes, third stage 140℃ for 10 minutes; S7 Pre-melting: hold at 400℃ for 8 minutes, hold at 650℃ for 25 minutes for sintering; slow cooling to 190℃ before removal from the furnace for cooling. This embodiment produces an aluminum alloy panel with high glaze gloss, no pinholes or bubbles, and no peeling after 200 hot and cold cycles, suitable for processing decorative thin metal sheets.

[0040] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An environmentally friendly, lead-free enamel glaze application method for metal surfaces, characterized in that, The glazing method includes the following steps: S1 Metal substrate pretreatment, S2 Environmentally friendly lead-free enamel glaze preparation, S3 Metal substrate preheating treatment, S4 Glaze preheating treatment, S5 Graded double-layer glazing, S6 Gradient segmented drying, S7 Segmented constant temperature sintering, S8 Constant temperature slow cooling posttreatment. The S1 process removes oil, oxide scale, and impurities from the surface of the metal substrate; The S2 process prepares lead-free glaze and completes ball milling, viscosity adjustment, and filtration. S3 and S4 preheat the metal substrate and glaze respectively to improve the adhesion of the glaze layer; S5 uses a double-layer spraying of base glaze and top glaze to control the total thickness of the glaze layer; S6 uses a three-stage gradient heating and drying process to remove moisture from the glaze layer; S7 stepped heating completes pre-melting and high-temperature sintering; After the S8 is slow-cooled in sections, it is polished, dusted, and inspected to obtain the finished enamel product.

2. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, The raw materials for the S2 environmentally friendly lead-free enamel glaze preparation process are as follows (by weight): 30-40 parts quartz powder, 20-28 parts borax, 15-22 parts feldspar, 5-10 parts alumina powder, 3-6 parts titanium dioxide, 2-5 parts zinc oxide, 1-3 parts bentonite, and 35-45 parts deionized water. The glaze is then wet-milled to a fineness of 300-400 mesh, and the viscosity is adjusted to 25-35s. After filtration to remove impurities, the glaze is ready for use.

3. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, The S5 grade double-layer glazing process adopts a double-layer spraying process of base glaze and top glaze. The thickness of the base glaze is 0.15-0.25mm. After the base glaze is sprayed, it is leveled for 3-5 minutes before the top glaze is sprayed. The thickness of the top glaze is 0.20-0.30mm. The total thickness of the glaze layer is controlled at 0.35-0.55mm.

4. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, The S1 metal substrate pretreatment process sequentially includes degreasing, water washing, pickling and rust removal, neutralization and passivation, and drying to completely remove impurities adhering to the surface of the metal substrate.

5. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, The S6 gradient segmented drying process consists of three constant temperature drying stages: the first stage is drying at 60-80℃ for 15-20 minutes, the second stage is drying at 90-110℃ for 20-25 minutes, and the third stage is drying at 120-140℃ for 10-15 minutes.

6. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, The S7 segmented isothermal sintering process adopts a stepped heating mode. First, the temperature is raised to 350-400℃ and held for 8-12 minutes to complete the pre-melting treatment. Then, the temperature is raised to 580-650℃ and held for 25-35 minutes to sinter and form the product.

7. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, The S8 constant temperature slow cooling post-processing procedure is as follows: after sintering, the temperature is slowly reduced to below 200℃, the workpiece is taken out of the furnace and naturally cooled to room temperature, and then polishing, dust removal and finished product quality inspection are carried out in sequence.

8. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, The S3 metal substrate preheating treatment and the S4 glaze preheating treatment are carried out simultaneously. Simultaneous preheating shortens the overall process time and ensures that the temperature of the substrate and the glaze are matched during glazing.

9. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, Wet ball milling uses an alumina-lined ball mill jar with high-alumina grinding balls, and continuous stirring throughout the milling process to prevent the glaze powder from settling and agglomerating.

10. The method for applying environmentally friendly lead-free enamel glaze to metal surfaces according to claim 1, characterized in that, The double-layer spraying uses an air electrostatic spraying device, with the spraying air pressure stably controlled at 0.3-0.5MPa to ensure that the glaze layer is uniform and free of drips and pinholes.

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