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In order to enhance the adhesion of the ink on the surface of ABS products, the surface of the ABS product may be roughened, ie 200 mL of a chromic acid solution having a concentration of 400 g/L, dissolved in 1 L of water at 50° C., and treated for 15 min. The net dry, so that the surface of the substrate formed a lot of micro-holes, thereby improving the adhesion of ink on the surface of the ABS product. The rework of ABS plastic screen printing products is difficult, and even though printing inks can be removed with strong solvents, the surface of plastic products will be damaged.
In recent years, with the localization of computers, more and more work has been required to print characters on ABS plastic keys. Although domestic plastic printing is introduced, most of them are used for decorative purposes, and the printing area is large. The accuracy of the writing and the firm procedure are not very high. The key area is small (minimum area of ​​6.8mm × 3.5mm), high precision requirements, and the word key often contact with the finger, so the character on the plastic key requires a strong attachment requirements, wear resistance is strong, and should have resistance Solvent, sweat resistance and other properties.
Because of the small size and number of keys, it is obviously not appropriate to use the reverse affixing method. Therefore, screen printing should be adopted. In order to solve the phenomenon of bumps and dimples between the keys, in the printing when the word key arrangement, create a special fixture.
In the printing materials, due to the poor brightness and color of plastic ink, and easy to seal the net, ordinary ink adhesion is not ideal, therefore, the choice of light brightness is good, not easy to seal the net, the strong adhesion of ABS plastic printing material is The key to printing.
Silk screen printing ink viscosity is not easy to be too large, otherwise the printed words from the Taiwan, with your fingers can be knocked off, if the viscosity is appropriate, with your fingers can not afford to drop (refer to the normal normal force). In addition, after the ink is dried, a thin layer of varnish (nitrocellulose varnish) is sprayed thinly, which can significantly increase the brightness of the surface of the parts and can maintain the ink layer.
1 screen character silk screen printing process: The keyboard character silk screen printing process is generally:
Screen preparation → plate making → prepress processing of plastic work → screen printing → initial inspection → drying → inspection 2 wire mesh preparation. Use 240 mesh than the net, stretching machine stretch net, cleaning and drying.
3 plate making. The use of commonly used photosensitive plastic plate method.
4 plastic workpiece cleaning. CK-821 low temperature ink remover, temperature about 50 °C, soak 5min to oil, dilute acid, rinse, dry.
5 silk screen. The plastic keys are placed with a special fixture. Screen printing is carried out using ST-71 (black) and ST-81 (white) screen printing ink. If the material is too dry, it can be diluted with ST thinner.
6 first inspection. Failed to clean again with alcohol.
7 Dry. 60 °C oven bake 0.5h, or dry at room temperature for 2h.
8 inspection, inspection items are generally:
·Combination force: After printing is dry, use a fingernail to scratch with a force of about 500g.
·Abrasion resistance: Using a cathode moving device, friction medium sponge, pressurizing 50g, rubbing 100000 times, writing without falling off.
· High and low temperature experiments: High temperature 70°C (30mm) → Normal temperature (10min) → Low temperature -20°C (30min) A total of three cycles are performed in one cycle. The writings are free of discoloration and shedding.
·Sweat resistance: Saturate artificial sweat with filter paper (formula: 7g/L of sodium chloride, 1g/L of urea, 4g/L of lactic acid) and cover it with the key. After 2 hours, wipe it cleanly. There is no shedding of handwriting.
· Water resistance: The word key is soaked in water for 4 hours and wiped without force.
· Solvent resistance: The word keys were soaked in alcohol and gasoline, respectively. After 4 hours, they were wiped cleanly. Some soaked in alcohol fell off, and there was no shedding of keys in gasoline.
(2) Screen-printing of electronic components for electronic products Electronic products, especially radio products such as radios, tape recorders, televisions, etc., have very high requirements for appearance. The designers of the art design attach great importance to the appearance and decoration of the products. However, the current technology used for exterior decoration in our country is limited, which limits the thinking of the artists and enables them to pass only the size of the products and the use of various lines. The plastic itself is limited. The color to change the appearance of the product. After the development of plastic screen printing, it has brought convenience to the decoration of electronic products. Therefore, in recent years, various kinds of electronic products in China have been widely used in plastic screen printing.
1 electronic products plastic parts on silk screen requirements. The requirements for screen printing of plastic parts for electronic products are reflected in both the decorative and functional aspects. The so-called decorative, refers to the appearance of the product can give a person a beautiful enjoyment. Such as: soft and elegant colors, appearance with a sense of the times and so on. Functionality refers to the attachment of imprints on plastics, abrasion resistance, clarity and position accuracy.
2 electronic products plastic parts commonly used plastics. The commonly used plastics for electronic products plastic parts are modified polystyrene, high impact strength polystyrene, ABS plastics, organic glass and so on.
The softening point of the modified polystyrene plastics is about 78°C, and the softening point of the organic glass is about 94°C, so these two types of plastics are not resistant to high temperatures. Plastics used for plastic parts of electronic products are generally soluble in most organic solvents.
3 handwriting, pattern clarity. Sharpness and the accuracy of the print position are the functional requirements of electronic products for silk screen printing. In order to achieve this goal, pay attention to the following points in the screen printing process:
a. Requirements for the bottom plate: In the silk screen plate of the electronic product plastic part, the edges of the writing and graphics must be flat, so that the edge jaggies can be reduced during screen printing. In addition, the distance between the strokes of the handwriting must be increased by about 0.1mm from the original effect, so that the error caused by the inevitable leveling of the ink can be avoided.
b. Screen selection and stretching requirements: When using direct sensitization method to make pure screen, the choice of screen is the key, general application of polyester screen above 280 mesh, due to the high stability of this screen, Affected by the temperature change is not large, the reduction rate is low. Therefore, it can improve the uniformity of the print edge and reduce the distortion of the screen. Tension measurement with a tensiometer should reach about 1.5mm (mm tension meter).
c. Production of screen-printed version: Screen-printing plates should be made of solvent-resistant screen-sensitive adhesive or film, so as to be suitable for printing inks.
d. Screen printing methods, generally divided into mechanical and manual screen printing two kinds, the majority of the domestic still use manual methods for screen printing, especially when printing plastic products thicker, such as: TV light, recording shell and so on. Therefore, the technical level of the operator will directly affect the quality of screen printing products. In addition to mastering the basic skills of ink deployment, positioning, and squeegee selection, operators should also be able to deal with some temporary problems, such as handling stencils. In addition, the problem of how to keep the pressure of the scraper in the silk screen printing process evenly requires more long-term practice and exploration.
e. Defective rework: The rework of silk screens for plastic products is difficult, because its ink adhesion and abrasion resistance are very strong, and the substrates are plastic, so the choice of solvents is very difficult. Although printing inks can be removed with strong solvents, the surface of plastic products will also be destroyed and become waste products. Although the use of weak solvents does not destroy the plastic surface, the ink cannot be removed. It is recommended to use the following formula for removing ink: 2 parts of alcohol and 1 part of tetrahydrofuran (volume ratio). The product can be used after mixing evenly. It is convenient to use without damaging the surface of plastic products.
4 print bit accuracy. The accuracy of the printing position means that silk screen printing should be carried out strictly according to the requirements of the design drawings, but it is not easy to do this in the large production process. Some people have done such tests, in the general injection molding lithographic screen printing, if the tolerance required ± 0.1mm, the defective rate is between 15% -20%, this is mainly due to inappropriate ink viscosity adjustment or squeegee When the pressure is not uniform, etc., the technical level of silk screen printers must be improved. In addition to this, it should be noted from the following two aspects:
a. Make sure to strictly check the size of the photographic film before making the screen plate, whether it meets the requirements of the drawings. In particular, a film made of a film should be inspected at intervals to prevent the size of the film from changing.
b. The design of screen printing tooling should be considered from two aspects in terms of size positioning: one is the adjustment of the lateral positioning, the other is the adjustment of the longitudinal positioning, and the third is the adjustment of the angle. Therefore, a good silk screen tooling must have the performance of these three aspects. When printing thick substrates, such as the case of the recorder, you should also choose the part that is not easily deformed as the positioning rule to prevent injection. Different process conditions cause the shrinkage of the plastic to be inconsistent, resulting in positioning deviations.
(3) Screen Printing of Plastic Signage In recent years, plastic signage and plastic molding components for silk screen graphic characters have developed rapidly. For example, the screen printing on polyvinyl chloride rigid board (PVC board), polycarbonate, ABS, plexiglass, modified polystyrene plastic parts, has been widely used, and achieved a good decorative effect and economy benefit.
1PVC hard board sign silk screen. Due to the advantages of transparency, strength and performance, and the advantages of cutting, forming and not cracking, PVC panels are widely used in televisions, tape recorders and various household appliances, instrumentation, automobile signage and other industries. The thickness of commonly used PVC plates for making signs is 1mm, 0.5mm, 0.3mm, etc. The colors are transparent, matt, and black. Transparent plate screen printing patterns are often printed on the back of the board, and matt and black boards are printed on the front of the board to make various signs. The process of screen printing PVC signs is:
Cutting → Uncovering paper → Screen printing (color printing) → Inspection → Plastering film → Adhesive double-sided tape or sealant → Cutting or stamping screen printing Plastic inks are mostly domestic ink, Japanese ink, thinners Cyclohexanone.
Due to the large odor of plastic ink, quick drying, and easy web blocking, oily inks can also be used. Such as printing black can use 2711-instrument ink, and add 20% alkyd varnish and 3% dry oil, in order to improve the printing effect.
2ABS, modified polystyrene plastic pressure screen printing. After plastic parts are molded, they can be printed directly with plastic ink. Can print monochrome and various colors of the appearance of decoration, the process is simple, low cost, good economic returns, so some computers, household appliances often use this technology directly on the screen printing. If the printing fails, you can erase the reprint with 200# gasoline or turpentine in time, and the parts will not be scrapped.
3 plastic spray screen printing. After the plastic casing and parts are painted, the appearance of the whole machine can be improved, so that the plastic parts can have a metallic effect, and the color difference of the plastic can also be eliminated, thereby increasing the added value of the products.
ABS, PS plastic spray silver paint formula:
Formula 1: Plastic-1 Acrylic Paint (Tianjin Paint Factory) 1000g
Silver paste (above 300 mesh) 80g Plastic-1 Diluent Formula 2: PS plastic paint (Beijing Red Lion) 1000g
Silver paste (above 300 mesh) 80g PS Diluent The above formula is diluted with dilute material to Tu-4 viscometer for 12-13 seconds. After blending well, it is well-mixed and filtered with 150-mesh wire mesh for use. Other colors use plastic color enamel.
Spray painting method: With general painting equipment, the surface of plastic pressure parts should be scrubbed with alcohol in advance, the air pressure should be controlled at 3-5kg/cm2, and the distance between spray gun and plastic parts should be 25-50cm. After painting, dry it at 50-55°C for 20 minutes or at room temperature. After drying, you can use plastic ink screen printing
Section 6 Printing of Plastic Products - 7
2. Example of plastic silk screen printing (1) Screen printing of the keyboard character The keyboard is ABS plastic, and ABS resin plastic is an engineering plastic. In recent years, it has been widely used in televisions, computers and other products as well as in many areas of the national economy. ABS plastics are terpolymers of propionitrile, butadiene, and styrene. Because ABS plastics are terpolymers, they share the common characteristics of three components. Acrylonitrile can make the copolymer chemically resistant and have a certain surface hardness; butadiene can make polymers have rubbery toughness; styrene can be The polymer has easy processing and shaping properties. ABS plastics have a variety of colors, products have a glossy appearance, low water absorption, good impact strength and surface hardness within a certain temperature range, and good dimensional stability. Most ABS resins still have considerable impact strength at -40°C and exhibit toughness. The service temperature of the products ranges from -40 to 60°C. ABS plastics have high relative molecular mass, good physical properties, easy processing, and good printability.