A polylactic acid model coloring process method for fused deposition molding

Abstract

The invention provides a polylactic acid model coloring process method for fused deposition modeling, comprising the following steps: S1 cleaning the surface of the model; S2 treating the surface of the model; S3 repeatedly polishing the model in sequence by using 100-2000 mesh water-abrasive sandpaper to polish it to be flat and smooth, and cleaning and drying it; S4 forming a primer layer on the surface film of the polylactic acid model by setting three ratios to form a primer and a nano-spraying process; S5 uniformly coloring the model, using a spraying device to apply a color paint; the potholes and depressions on the surface of the model can be filled and trimmed, and while providing the model with color adhesion strength and long-term adhesion, the model color can also be ensured to be uniform, smooth and flat.

Description

Technical Field

[0001] The invention relates to the technical field of model coloring technology, in particular to a polylactic acid model coloring process method for fused deposition molding. Background Art

[0002] Most of the popular 3D printers on the market are primary color fused deposition modeling (FDM) 3D printers. Polylactic acid (PLA) and acrylonitrile butadiene styrene copolymer (ABS) are the two most commonly used engineering plastics in additive manufacturing. The printed three-dimensional models need to be colored on the surface; ABS and PLA both belong to the category of thermoplastics, which are polymers characterized by the ability to change state from molten plastic to solid plastic and back again when reheated to a specific melting point. This is in contrast to thermosetting plastics, which cannot be melted again once cooled and solidified. Part of the reason why polylactic acid (PLA) is becoming more and more popular is that it is made from renewable bio-based resources, which makes it a more environmentally friendly material than non-renewable ABS. Due to its biodegradable and lower cost characteristics, polylactic acid (PLA) is more suitable for making objects that require fineness and appearance quality, such as artworks, models and decorations. However, due to the fused deposition modeling process technology and materials, it is difficult for color paint to adhere to the surface of 3D models, which often easily causes uneven coloring and cannot be maintained for a long time.

[0003] In order to solve the above problems, the Chinese patent application number is 201911196777.9, and the publication date is 2020.03.31, which discloses a 3D printing sand model surface effect process treatment method, including the following steps S1: model making and surface treatment; S2: bonding; S3: covering spraying; S4: spraying weather-resistant material; S5: coloring. The above method is aimed at the defects of the existing sand model or other powder-molded models, such as low strength and hardness performance, poor effect after direct coloring of the sand model, obvious seam marks when assembling and splicing the sand model, moisture return and pigment shedding of the sand model, and short product life. A 3D printing sand model surface effect process treatment method is proposed. The treatment method greatly improves the strength and hardness of the model, gives the model a strong artistic expression, and even in a humid, acidic and alkaline environment, the model can be used or placed for a long time, thereby improving the product life. The method of the present invention is low-cost, easy to operate, and can be industrialized and scaled production and application, which is very practical.

[0004] This method uses sandpaper to polish twice after masking spraying, then uses weather-resistant material to spray the model surface twice, and finally colors it, applying the pigment to the concave potholes. Since this method is used on sand models, and the surface roughness of the sand model is relatively large, the weather-resistant material only needs to be sprayed twice. For the polylactic acid model, its surface is relatively smoother than that of the sand model, so directly using the spraying method will result in insufficient adhesion of the weather-resistant material, resulting in poor coloring effect. In addition, the final coloring of the above method is by brushing, which only evenly brushes the coloring pigment on the protrusions of the model surface structure. This causes the weather-resistant material to be easily discolored when the adhesion to the coloring layer is insufficient, the gloss is different, and it is also easy to cause uneven coloring. At the same time, since the final coloring is by brushing, the thickness of the surface glue penetration layer is relatively large, so after long-term use, when the adhesion of the surface glue layer is insufficient, it is easy to cause the surface glue layer to fall off, resulting in poor long-term adhesion. Summary of the invention

[0005] The purpose of the present invention is to provide a polylactic acid model coloring process method for fused deposition molding, which can fill and repair the potholes and depressions on the surface of the model, and the color has good adhesion and long-term adhesion, and can also ensure that the color of the model is uniform and smooth.

[0006] To achieve the above object, the present invention provides a polylactic acid model coloring process method by fused deposition modeling, comprising the following steps:

[0007] S1: cleaning the surface of the model: cleaning the outer surface of the polylactic acid model, wiping it clean and drying it;

[0008] S2: Surface treatment of the model: filing the potholes on the surface of the model and filling the depressions with putty;

[0009] S3 repeatedly polishes the PLA model with water-abrasive sandpaper until it is flat and smooth, cleans it and lets it dry;

[0010] S4 Select to mix the pigment and the first treatment water in volume ratios of X and Y respectively to form primer one and primer two, where 1:2 < X < 1:0.5 and 1:2 ≤ Y < 1:0.6. Then mix the water putty and the thinner in volume ratio Z to form primer three, where 1:5.5 < Z < 1:1.5. First, form a layer of primer one on the polylactic acid model by means of nano-spray plating process. Then, use water sandpaper with the first mesh number to polish the layer of primer one. After polishing, spray a layer of primer two on the polished layer of primer one by means of nano-spray plating process to form a second layer of primer. After complete drying, select water sandpaper with the second mesh number to polish. After polishing, spray a layer of primer three on the polished second layer of primer by means of nano-spray plating process to form a third layer of primer. Use water sandpaper with the third mesh number to polish on the third layer of primer to make the surface film of the polylactic acid model form a weather-resistant layer. The first mesh number is less than the second mesh number, and the first mesh number is less than the third mesh number.

[0011] S5 Uniformly color the model. Use the spraying method to first spray the color paint and then spray the clear paint for color paint coating. The finally formed paint film thickness is less than or equal to 60μm.

[0012] For the above method, first clean the outer surface of the polylactic acid model, and then wipe it clean to ensure that no other objects are attached to the model surface. After cleaning, wait to dry, and ensure complete drying; after the polylactic acid model is dry, level the pits on the model surface, and use putty to fill the larger depressions to strengthen the stability of the polylactic acid model. After scraping the putty, then use water sandpaper to polish the model repeatedly in sequence until it is polished to be flat and smooth, and clean the surface, and then dry it; this ensures the flatness and smoothness of the surface of the polylactic acid model to be painted with pigment, thus making it more convenient to apply the pigment.

[0013] In addition, according to the first spraying of primer, the first layer of primer has the first treatment water, which increases the humidity of the first layer of primer and the polylactic acid model to ensure the adhesion of the first layer of primer, and then uses a smaller first mesh water-grinding paper for grinding, so that the first layer of primer is locally smoothed and the adhesion points are increased at the same time, so as to improve the adhesion of the second layer of primer on the first layer of primer, and also prevent the first layer of primer from being excessively polished to affect the adhesion of the first layer of primer, and the third layer of primer can increase the main color component on the first layer of primer, and then use a larger number of second mesh water-grinding paper to grind the first layer of primer. The paper is used for polishing, so that the first and second layers of primer can be polished in different parts to prevent over-polishing of the first and second layers of primer, while also ensuring a certain degree of smoothness. At the same time, local concave and convex surfaces can be formed through local polishing, so that the third layer of primer can be more reliably attached to the second layer of primer. The water putty of the third layer of primer increases the brightness of the color from the bottom layer of coloring. Finally, a larger third number is used to make the surface of the third layer of primer smoother and flatter, so that it can be more reliably colored by spraying when coloring, so that the color of the model is uniform, smooth and flat.

[0014] Primer 1 and primer 2 are formed by first treated water and pigment in volume ratios of X and Y respectively, so that the first and second primer layers have a certain thickness, neither too thick nor too thin. At the same time, primer 3 is formed by water putty and diluent in a volume ratio of Z to ensure color reliability and facilitate the subsequent formation of the entire finished paint film thickness to be neither too thick nor too thin. Too thick will affect the adhesion of the paint film, and too thin will result in poor color uniformity of the entire product.

[0015] Furthermore, the S3 step includes:

[0016] S31, first use 100-300 mesh water-abrasive sandpaper to polish off the irregular parts of the polylactic acid model;

[0017] S32, then use 400-800 mesh water-abrasive sandpaper to further polish the putty filling part;

[0018] S33. Finally, use 1000-2000 mesh water-abrasive sandpaper to polish the putty-filled part until it is smooth.

[0019] The above settings are to use a small number of water-abrasive sandpapers to polish and remove irregularities, then use a medium number of water-abrasive sandpapers to polish the putty filling part to make the putty part polished flat, and finally use a larger number of water-abrasive sandpapers to polish the putty part smooth, so that the polylactic acid model is polished more delicately and the filling part is guaranteed to be more perfect.

[0020] Further, the step S4 comprises the steps of:

[0021] S41 forms a primer layer on the surface of the polylactic acid model through a nano-spraying process, and after drying, polishes it with a first number of water-abrasive sandpapers, and after polishing, sprays a second primer layer to form a second primer layer, and after completely drying, polishes it with a second number of water-abrasive sandpapers;

[0022] S42 Use filling material to fill the surface layer of the PLA model, and after drying, use the fourth number of water-abrasive sandpaper to polish it. After polishing, remove the residual dust;

[0023] S43 repeats step S42 until the surface layer lines are completely filled;

[0024] S44 forms a third layer of primer on the second layer of primer through nano-spraying process, and after it is completely dried, it is polished with the third number of water-abrasive sandpaper to remove the residual dust;

[0025] S45: Repeat step S44 until the surface of the model is flat and the color of the primer is uniform.

[0026] With the above settings, the object of the existing FDM process is generally PLA or ABS material. Because of the molten deposition process, the FMD printer will produce layer patterns on the surface of the polylactic acid model. Therefore, it is necessary to process the surface layer patterns of the polylactic acid model before spraying the primer three to ensure the reliability of subsequent coloring. After processing the potholes on the polylactic acid model, the small layer patterns are polished to make the formed primer layer two more uniform, thereby improving the reliability of coloring.

[0027] Further, the first number is 400-800 meshes, the second number is 1000-2000 meshes, the third number is 1000-2000 meshes, and the fourth number is 600-1000 meshes.

[0028] In the above arrangement, the third number and the second number are the same and larger, so that the uniformity of the second layer of primer and the third layer of primer is closer.

[0029] Furthermore, the nano-spraying process includes:

[0030] S00 PLA ​​model surface treatment: clean the PLA model with white oil, remove dust from the PLA model surface with an electrostatic dust removal gun, and clean the PLA model surface with sandpaper above No. 1000;

[0031] S01 sprays a primer with a thickness of 15-25 μm on the surface of the treated polylactic acid model, then puts it into a drying oven and heats it until it is completely dry, and then puts it into a dust-free device and leaves it to stand until it cools down;

[0032] S02 is sprayed to form a silver mirror film, a surfactant is sprayed onto the surface of the cooled polylactic acid model by a spray gun, and then the excess surfactant is thoroughly cleaned and removed by a spray gun using pure water to form a polylactic acid model after spraying, and a reaction liquid is sprayed onto the polylactic acid model by using a spray gun to form a mirror layer of 0.05-0.3 microns;

[0033] S03 cleaning: Use a spray gun to use pure water to thoroughly clean and remove excess reaction liquid, and then use a spray gun to thoroughly clean the surface of the polylactic acid model with the stabilizer, and then use a blow gun to blow dry the surface of the polylactic acid model to ensure that there is no water on the surface of the silver mirror, and finally heat and dry it in a dehydration drying furnace.

[0034] According to the above settings, the PLA model white oil is first cleaned, dusted and polished, and then a certain thickness of primer is sprayed. After drying and cooling, the surfactant is sprayed to adjust the surface tension. Then, pure water is used to remove excess surfactant, and then the reaction liquid is sprayed to form a mirror surface, and pure water is used for cleaning. Finally, the antioxidant capacity of the metal layer is increased by a stabilizer. Pure water is used for cleaning throughout the process to prevent other water qualities from affecting the cleaning effect.

[0035] Furthermore, the surfactant in S02 is ammonia water, the reaction solution in S03 is silver nitrate; and the stabilizer in S03 is sodium thiosulfate.

[0036] In the above configuration, the surfactant can provide conditions for forming a silver mirror film, the reaction liquid forms a mirror surface, and the stabilizer can increase the anti-oxidation ability of the metal city.

[0037] Furthermore, the filling material in S42 is at least one of eye filler and toothpaste putty.

[0038] The above arrangement ensures that the supplements are more suitable for the polylactic acid model by providing a variety of supplement choices.

[0039] Furthermore, the S5 further comprises the steps of:

[0040] S51 dilutes the paint and varnish respectively by a volume ratio of the paint to the thinner of 1:2.5 and a volume ratio of the varnish to the thinner of 1:3 to form diluted paint and varnish;

[0041] S52 makes the volume ratio of the diluted color paint and the clear varnish be 1:1;

[0042] S53 is painted by spraying the color paint first and then spraying the varnish on the polylactic acid model through the spraying equipment to complete the painting.

[0043] The above setting makes the coloring effect better by diluting the paint and varnish before spraying.

[0044] Furthermore, step S1 also includes: cleaning the surface of the polylactic acid model with industrial acetone and wiping it clean with a cloth or a sponge.

[0045] With the above settings, industrial acetone can better clean the surface of the polylactic acid model and has a strong cleaning effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a flowchart of the present invention.

[0047] Figure 2 This is a judgment chart of the hundred-grid test results.

[0048] Figure 3 The effect diagram after coloring the present invention.

[0049] Figure 4 This is a rendering of the effect of a hundred-grid test after the primer is applied to the present invention.

[0050] Figure 5 This is a rendering of a hundred-grid test after the present invention is finished with color paint. DETAILED DESCRIPTION

[0051] The utility model is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0052] like Figures 1 to 5 As shown, the present invention provides a polylactic acid model coloring process method for fused deposition modeling, comprising the following steps:

[0053] S1: cleaning the surface of the model: cleaning the outer surface of the polylactic acid model, wiping it clean and drying it; in this embodiment, the surface of the polylactic acid model is cleaned with industrial acetone and wiped clean with a cloth or a sponge.

[0054] S2: Surface treatment of the model: filing the potholes on the surface of the model and filling the depressions with putty;

[0055] S3 repeatedly polishes the polylactic acid model in sequence using 100-2000 mesh water-abrasive sandpaper to make it smooth and flat, and then cleans and dries it; the S3 step specifically includes:

[0056] S31, first use 100-300 mesh water-abrasive sandpaper to polish off the irregular parts of the polylactic acid model;

[0057] S32, then use 400-800 mesh water-abrasive sandpaper to further polish the putty filling part;

[0058] S33. Finally, use 1000-2000 mesh water-abrasive sandpaper to polish the putty-filled part until it is smooth.

[0059] S4 Select to mix the pigment and the first treatment water at volume ratios of X and Y respectively to form primer one and primer two. 1:2 < X < 1:0.5, 1:2 ≤ Y < 1:0.6. Then mix the putty and the thinner at a volume ratio of Z to form primer three. 1:5.5 < Z < 1:1.5. First, form a layer of primer one on the polylactic acid model by means of nano-spray plating process. Then, use water sandpaper with the first mesh number to polish the primer one layer. After polishing, spray a layer of primer two on the polished primer one layer by means of nano-spray plating process to form a primer two layer. After complete drying, select water sandpaper with the second mesh number for polishing. After polishing, spray a layer of primer three on the polished primer two layer by means of nano-spray plating process to form a primer three layer. Use water sandpaper with the third mesh number to polish the surface of the polylactic acid model on the primer three layer to form a weather-resistant layer. The first mesh number is less than the second mesh number, and the first mesh number is less than the third mesh number. In this embodiment, the first number is 400 - 800 meshes, the second number is 1000 - 2000 meshes, the third number is 1000 - 2000 meshes, the fourth number is 600 - 1000 meshes, and the first treatment water is thinners.

[0060] S5 Uniformly color the model. First spray the color paint and then spray the clear paint by spraying method for color paint coating. The finally formed paint film thickness is less than or equal to 60μm. In this embodiment, the paint film thickness is the sum of the thickness of the primer one layer, the primer two layer, and the primer three layer. And the thickness of the primer one layer and the primer two layer is 20 - 25μm. The thickness of each layer of primer can be controlled by the amount sprayed during spraying, or the thickness of each layer of primer can be controlled by the number of polishing repetitions.

[0061] The specific steps of S5 include:

[0062] S51 Dilute the color paint and the clear paint respectively by a volume ratio of 1:2.5 for the color paint and the thinner and a volume ratio of 1:3 for the clear paint and the thinner to form the diluted color paint and the clear paint;

[0063] S52 Make the volume ratio of the diluted color paint and the clear paint be 1:1;

[0064] S53 First spray the color paint and then spray the clear paint on the polylactic acid model through the spraying equipment to complete the coating. In this embodiment, the thickness of the color paint and the clear paint can be 15 - 25μm.

[0065] The nano-spray plating process used in the primer includes:

[0066] S00 Surface treatment of the polylactic acid model. Clean the polylactic acid model with white spirit, remove dust from the surface of the polylactic acid model with an electrostatic dust removal gun, and clean the surface of the polylactic acid model with sandpaper above 1000 mesh.

[0067] S01 sprays a primer with a thickness of 15-25 μm on the surface of the treated polylactic acid model, then puts it into a drying oven and heats it until it is completely dry, and then puts it into a dust-free device and leaves it to stand until it cools down;

[0068] S02 is sprayed to form a silver mirror film, a surfactant is sprayed onto the surface of the cooled polylactic acid model by a spray gun, and then the excess surfactant is thoroughly cleaned and removed by a spray gun using pure water to form a polylactic acid model after spraying, and a reaction liquid is sprayed onto the polylactic acid model by using a spray gun to form a mirror layer of 0.05-0.3 microns;

[0069] S03 cleaning, using a spray gun to use pure water to thoroughly clean and remove excess reaction liquid, and then using a spray gun to thoroughly clean the surface of the polylactic acid model with a stabilizer, and then using a blow gun to blow dry the surface of the polylactic acid model to ensure that there is no water on the surface of the silver mirror, and finally heating and drying in a dehydration drying furnace. In this embodiment, the surfactant in S02 is ammonia water, the reaction liquid in S03 is silver nitrate; and the stabilizer in S03 is sodium thiosulfate.

[0070] In another embodiment, the step S4 comprises the steps of:

[0071] S41 forms a primer layer on the surface of the polylactic acid model through a nano-spraying process, and after drying, polishes it with a first number of water-abrasive sandpapers, and after polishing, sprays a second primer layer to form a second primer layer, and after completely drying, polishes it with a second number of water-abrasive sandpapers;

[0072] S42 uses a filling material to fill the surface layer lines of the polylactic acid model, and after drying, uses a fourth number of water-abrasive sandpaper to polish, and removes residual dust after polishing. In this embodiment, the filling material in S42 is at least one of eye filler and toothpaste putty.

[0073] S43 repeats step S42 until the surface layer lines are completely filled;

[0074] S44 forms a third layer of primer on the second layer of primer through nano-spraying process, and after it is completely dried, it is polished with the third number of water-abrasive sandpaper to remove the residual dust;

[0075] S45: Repeat step S44 until the surface of the model is flat and the color of the primer is uniform.

[0076] In order to further verify the reliability of coloring, nine different embodiments were selected for comparison. The specific experimental data are shown in the following table, wherein primer one and primer two are formed by pouring the corresponding volumes of pigment and the first treated water into the container according to the volume ratio in the table and stirring them. Primer three is also formed by pouring the corresponding volumes of water putty and diluent into the container according to the volume ratio in the table and stirring and mixing them.

[0077] Table 1 Experimental data table of nine embodiments

[0078]

[0079] Then, after performing adhesion tests, wear tests, impact resistance tests, solvent resistance tests, hardness tests, and anti-aging tests, the test data table is formed as shown in Table 2 below:

[0080]

[0081] As can be seen from the above table, the suitable range of the volume ratio X of primer one is: 1:2 < X < 1:0.5, the suitable range of the volume ratio Y of primer two is: 1:2 ≤ Y < 1:0.6; the suitable range of the volume ratio Z of primer three is: 1:5.5 < Z < 1:1.5. The results tested in this way can not only meet the adhesion test but also meet various other test requirements. However, in Examples 1, 9, and Comparative Example 7, the wear is excessive, which is due to the too thick paint film thickness, and the thickness range of the finished paint film is greater than or equal to 38μm and less than 60μm; the spraying temperature is 25 - 30°C, and the relative humidity of the spraying environment is 80 - 90.

[0082] Among them, the adhesion test is carried out in the way of scribing a hundred grids, also known as the hundred-grid test method. The specific method is as follows: Use a hundred-grid knife to scribe knife marks with a width of about 10mm - 12mm, at intervals of 1mm - 1.2mm each, and there are 10 grids in total. When scribing straight down, 10 straight knife marks with the same interval will appear. Scribe perpendicular to the straight knife marks, and it will become a 100-grid square of 10*10. When using the hundred-grid knife, it should cut through to see the substrate, and it cannot be only cut on the paint, otherwise the test will not be valid. After the hundred-grid knife has finished scribing, it is also necessary to use tape to test whether it will fall off. First, stick the tape on the hundred-grid position, press it down with your finger to closely attach the tape, and then tear up the tape with an instant force. By observing whether there is any peeling phenomenon of the paint layer on the model, such as visual inspection or magnifying glass inspection, verify the adhesion performance of the paint layer on the product surface through the cross-cross method or the tape method, which is used to verify the surface treatment process of the product.

[0083] As Figure 2 shown, it is a judgment diagram of the existing hundred-grid test results. Generally speaking, when the grade is between 4B and 5B, the hundred-grid test is qualified.

[0084] As Figure 3 shown, it is the effect diagram after coloring of the present invention. It can be seen from the figure that the coloring uniformity is better, and the model is relatively smooth after coloring. Then, perform the hundred-grid test on the model after coloring. Figure 4This is the effect of the 100-grid test after the primer was applied. From the picture, it can be seen that after being magnified by a 2x magnifying glass, "the edges of the cut are completely smooth and the edges have not fallen off." Figure 5 It can be seen that under an eight-fold magnifying glass, it was observed that "small flakes were peeling off at the intersection of the cuts, and the actual damage in the grid area did not exceed 5%", so the 100-grid test after priming and coloring met the requirements.

[0085] The working principle of the present invention is as follows: firstly, the outer surface of the polylactic acid model is cleaned, and then it is wiped clean, so as to ensure that no other objects are attached to the surface of the model, and after cleaning, it is waited to be dried, and it is ensured to be completely dried; after the polylactic acid model is dried, the potholes on the surface of the model are filed flat, and atomic putty is used to fill larger depressions, so that the stability of the polylactic acid model is enhanced, and after the atomic putty is applied, the model is repeatedly polished in sequence by using water-abrasive sandpaper, so that it is polished to be flat and smooth, and the surface is cleaned and then dried; in this way, the flatness and smoothness of the surface of the polylactic acid model to be painted are ensured, so that it is more convenient to paint.

[0086] In addition, according to the first spraying of primer, the first layer of primer has the first treatment water, which increases the humidity of the first layer of primer and the polylactic acid model to ensure the adhesion of the first layer of primer, and then uses a smaller first mesh water-grinding paper for grinding, so that the first layer of primer is locally smoothed and the adhesion points are increased at the same time, so as to improve the adhesion of the second layer of primer on the first layer of primer, and also prevent the first layer of primer from being excessively polished to affect the adhesion of the first layer of primer, and the third layer of primer can increase the main color component on the first layer of primer, and then use a larger number of second mesh water-grinding paper to grind the first layer of primer. The paper is used for polishing, so that the first and second layers of primer can be polished in different parts to prevent over-polishing of the first and second layers of primer, while also ensuring a certain degree of smoothness. At the same time, local concave and convex surfaces can be formed through local polishing, so that the third layer of primer can be more reliably attached to the second layer of primer. The water putty of the third layer of primer increases the brightness of the color from the bottom layer of coloring. Finally, a larger third number is used to make the surface of the third layer of primer smoother and flatter, so that it can be more reliably colored by spraying when coloring, so that the color of the model is uniform, smooth and flat.

[0087] Primer 1 and primer 2 are formed by first treated water and pigment in volume ratios of X and Y respectively, so that the first and second primer layers have a certain thickness, neither too thick nor too thin. At the same time, primer 3 is formed by water putty and diluent in a volume ratio of Z to ensure color reliability and facilitate the subsequent formation of the entire finished paint film thickness to be neither too thick nor too thin. Too thick will affect the adhesion of the paint film, and too thin will result in poor color uniformity of the entire product.

Claims

1. A polylactic acid model coloring process for fused deposition modeling, characterized in that: It includes the following steps: S1 Clean the model surface: Wash the outer surface of the polylactic acid model, wipe it clean and dry it. S2 Treat the model surface: File the pits and depressions on the model surface flat, and fill the depressions with putty. S3 Repeatedly polish the polylactic acid model with water sandpaper until it is flat and smooth, clean it and dry it. S4 Select pigments and the first treatment water and mix them into primer one and primer two respectively according to volume ratios of X and Y, where 1:2 < X < 1:0.5 and 1:2 ≤ Y < 1:0.

6. Then mix water putty and thinner into primer three according to volume ratio Z, where 1:5.5 < Z < 1:1.

5. First, form a layer of primer one on the polylactic acid model by means of nano spraying process. Then, use water sandpaper of the first mesh number to polish the primer one layer. After polishing, spray a layer of primer two on the polished primer one layer by means of nano spraying process to form a primer two layer. After it is completely dried, select water sandpaper of the second mesh number to polish. After polishing, spray a layer of primer three on the polished primer two layer by means of nano spraying process to form a primer three layer. Use water sandpaper of the third mesh number to polish the surface layer of the polylactic acid model to form a weather-resistant layer. The first mesh number is less than the second mesh number, and the first mesh number is less than the third mesh number. S5 Uniformly color the model. First spray color paint and then spray clear paint by spraying method for color paint coating. The finally formed paint film thickness is less than or equal to 60μm. The nano spraying process includes: S00 Treat the surface of the polylactic acid model. Clean the polylactic acid model with white gasoline, remove dust from the surface of the polylactic acid model with an electrostatic dust removal gun, and clean the surface of the polylactic acid model with sandpaper above 1000 mesh. S01 Spray a primer with a thickness of 15 - 25μm on the treated surface of the polylactic acid model, then put it into a drying furnace and heat until it is completely dry. Then place it in a dust-free device and let it stand until it cools down. S02 Spray and form a silver mirror film. Spray a surfactant onto the cooled polylactic acid model surface with a spray gun, then use pure water to thoroughly clean and remove the excess surfactant with a spray gun to form the polylactic acid model after spraying. Spray a reaction solution onto the polylactic acid model with a spraying gun to form a 0.05 - 0.3 micron mirror surface. S03 Clean. Thoroughly clean and remove the excess reaction solution with pure water using a spray gun. Then use a spray gun to thoroughly clean the surface of the polylactic acid model with a stabilizer. Then blow dry the surface of the polylactic acid model with a hair dryer gun to ensure that there is no water accumulation on the silver mirror surface. Finally, heat and dry it in a dehydration drying furnace.

2. The polylactic acid model coloring process of fused deposition modeling according to claim 1, characterized in that: The S3 step includes: S31 First, polish with 100 - 300 mesh water sandpaper to grind off the irregular parts of the polylactic acid model. S32 Then, use 400 - 800 mesh water sandpaper to further polish and flatten the putty-filled part. S33 Finally, use 1000 - 2000 mesh water sandpaper to polish and buff the putty-filled part until it is smooth.

3. The polylactic acid model coloring process of fused deposition modeling according to claim 1, characterized in that: The step S4 includes the steps: S41 forms a primer layer on the surface of the polylactic acid model through a nano-spraying process, and after drying, polishes it with a first number of water-abrasive sandpapers, and after polishing, sprays a second primer layer to form a second primer layer, and after completely drying, polishes it with a second number of water-abrasive sandpapers; S42 Use filling material to fill the surface layer of the PLA model, and after drying, use the fourth number of water-abrasive sandpaper to polish it. After polishing, remove the residual dust; S43 repeats step S42 until the surface layer lines are completely filled; S44 forms a third layer of primer on the second layer of primer through nano-spraying process, and after it is completely dried, it is polished with the third number of water-abrasive sandpaper to remove the residual dust; S45: Repeat step S44 until the surface of the model is flat and the color of the primer is uniform.

4. The polylactic acid model coloring process of fused deposition modeling according to claim 3, characterized in that: The first number is 400-800 meshes, the second number is 1000-2000 meshes, the third number is 1000-2000 meshes, and the fourth number is 600-1000 meshes.

5. The polylactic acid model coloring process of fused deposition modeling according to claim 1, characterized in that: The surfactant in S02 is ammonia water, the reaction solution in S03 is silver nitrate; and the stabilizer in S03 is sodium thiosulfate.

6. The polylactic acid model coloring process of fused deposition modeling according to claim 3, characterized in that: The filling material in S42 is at least one of eye filler and toothpaste putty.

7. The polylactic acid model coloring process of fused deposition modeling according to claim 1, characterized in that: The S5 further comprises the steps of: S51 dilutes the paint and varnish respectively by a volume ratio of the paint to the thinner of 1:2.5 and a volume ratio of the varnish to the thinner of 1:3 to form diluted paint and varnish; S52 makes the volume ratio of the diluted color paint and the clear varnish be 1:1; S53 is painted by spraying the color paint first and then spraying the varnish on the polylactic acid model through the spraying equipment to complete the painting.

8. The polylactic acid model coloring process of fused deposition modeling according to claim 1, characterized in that: Step S1 also includes: cleaning the surface of the polylactic acid model with industrial acetone and wiping it clean with a cloth or a sponge.

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