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Surface beautification method of injection molded parts
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Surface beautification method of injection molded parts

Author: Site Editor     Publish Time: 2021-04-30      Origin: Site

Surface beautification method of injection molded parts

Generally speaking, injection molding can produce completely finished parts. Parts with the correct color can be used immediately or assembled with other parts. However, the quality of injection molded parts can be further improved through surface beautification.


There are several reasons for beautifying the surface of injection molded parts. The most common one is the appearance value of the product, which can make the product more favored by customers. Sometimes it may be necessary to meet functional requirements, such as improving the wear resistance or scratch resistance of the workpiece. So what are the common beautification methods for injection molded parts?


01 Hot stamping


Overview: Hot stamping, commonly known as "hot stamping", is mainly used for silver or gold signs. It is a special printing process that does not use ink. It uses a certain pressure and temperature to use the template installed on the hot stamping machine to make the printed matter and hot stamping The foils are pressed against each other in a short time, and the metal foil or pigment foil is transferred to the surface of the hot-stamped print according to the graphics of the hot stamping template.


Features and applications: This printing method is widely used in almost all plastic processing industries due to its scratch-resistant, abrasion-resistant, and peel-resistant characteristics. In addition to important text prompts and instructions, it is mainly used for decorative purposes. Today's embossed film has excellent abrasion and scratch resistance, as well as extremely high physical and chemical resistance.


Process flow: The hot stamping process can be divided into four steps: 1. The hot stamping foil is in contact with the substrate; 2. With heat and pressure, the transfer layer is transferred to the surface of the substrate; 3. The pressure is released and the polyester is peeled off Film; 4. Feed the hot stamping foil and replace it with the substrate to be hot stamped.



02 Pad printing


Overview: Pad printing is an indirect gravure head printing technology, which has become a main method for printing and decorating the surface of various objects. When using pad printing, a soft (rubber-like) silicone head is used to first press the ink-coated printing block, and then press it on the surface of the product.


Features and applications: suitable for printing on irregular substrates (such as instruments, electrical parts, toys, etc.).


Process flow: 1. Put the ink into the gravure; 2. Scrape off the excess ink; 3. Squeeze the curved surface to obtain the ink; 4. Transfer the curved ink to the surface of the molded product; 5. Clean the curved surface and the layout (necessary Time).


03 Silk screen


Overview: Silk screen is an ancient printing method invented by the Chinese 2000 years ago. The basic principle of printing is: part of the mesh of the screen printing plate can pass through the ink and miss printing on the substrate; the remaining part of the mesh on the printing plate is blocked and cannot penetrate the ink, forming a blank on the substrate.


Features and applications: This method can be used for flat and cylindrical surfaces, pad printing is suitable for small area printing, and silk screen printing can be used for large area printing.


Process flow: The photochemical plate-making method is commonly used in modern times. 1. Tighten the screen on the screen frame, and then apply photosensitive glue on the net to form a photosensitive plate film; 2. Then, the positive image base plate is tightly attached to the plate film, and the printing plate is exposed and developed. The part that needs to be inked is exposed to light to form a solidified plate film, which seals the mesh and does not penetrate the ink during printing; the mesh of the part to be inked on the printing plate is not closed, and the ink penetrates during printing, forming ink marks on the substrate; 3. Pour the ink on one end of the screen printing plate during printing. The ink will not leak on the substrate through the mesh without external force. When the ink is scraped by a squeegee at a certain inclination angle and pressure , The ink is transferred to the substrate under the screen through the screen to realize image reproduction. 


04 Water transfer Printing


Overview: Water transfer printing is a kind of printing in which the transfer paper/plastic film with color patterns is hydrolyzed by water pressure. There are two types of technology, one is water mark transfer technology, and the other is water coating transfer technology. The former mainly completes the transfer of text and pictorial patterns, while the latter tends to perform a complete transfer on the entire product surface. The overlay transfer technology uses a water-soluble film that is easily soluble in water to carry graphics and text. Because the water coating film has excellent tension, it is easy to wrap around the surface of the product to form a graphic layer, and the surface of the product has a completely different appearance like spray paint.


Features and applications: The biggest advantage of water transfer printing is that it can form a seamless layer of imitation natural or other desired texture colors on products with complex contours. The adhesion effect is not affected by the material of the product. , At the same time, it can also avoid the vacant position that often appears in general layout printing; and in the printing process, because the product surface does not need to contact the printing film, it can avoid damaging the product surface and its integrity.


Process flow:

1. Film printing: printing various patterns on the polymer film;

2. Primer spray: Many materials must be coated with a layer of adhesive, such as metal, ceramics, etc. If you want to transfer different patterns, you must use different background colors, such as brown, brown, ocher, etc. for wood grain. The stone grain is basically white, etc.;

3. Film extension: let the film lay flat on the water surface, and wait for the film to stretch and level;

4. Activation: Use a special solvent (activator) to activate the pattern of the transfer film into an ink state;

5. Transfer printing: use water pressure to print the activated pattern on the printed material;

6. Water washing: Wash the remaining impurities of the printed workpiece with water;

7. Drying: Dry the printed work piece, the temperature depends on the quality and melting point of the material;

8. Spray paint: spray transparent protective paint to protect the surface of the printed object.


05 UV transfer

Overview: UV transfer process is also called UV perfusion process or UV coating process. It uses the non-stick characteristics of UV transfer glue and metal to transfer the ultra-thin button effects of various mobile phones to PET, PMMA or PC through UV transfer process. On the sheet, a variety of effects can be made, including CD pattern, drawing pattern, weaving pattern, sandblasting pattern, leather pattern, 3D surface, illusion effect, cat-eye effect, matte surface, high-gloss surface effect, etc.


Features and applications: It has the advantages of ultra-thin, high hardness, high wear resistance, high solvent resistance and exquisite appearance. It is used in 3C industry, various remote control buttons, home appliance control panel components, ceiling decorations, etc.


Process flow: 1. Mold opening: UV transfer effect and positioning holes for positioning pillars and printing inks should be made on the mold (to make a transfer mold); 2. Transfer: UV glue is printed on by screen printing On the transfer mold; 3. Pressing: Put the PC sheet on the transfer mold, and then press the sheet onto the transfer mold with a glue stick; 4. Light curing: Put the sheet and the transfer mold on Light-curing in the light-curing machine; 5. Demolding: Take out the transferred product from the mold and place it in the designated position (the process is basically completed).


06 IML


Overview: IML refers to in-mold insert injection molding. Its distinguishing features are: a hardened transparent film on the surface, a printed pattern layer in the middle, and a plastic layer on the back. Because the ink is sandwiched in the middle, the product can prevent the surface from being scratched and resistant to friction, and can keep the color bright for a long time, and it is not easy to fade. It can realize matte, leather grain, wood grain, and various 3D patterns, but the outermost layer is made of film material, so the texture is not tactile.


Features and applications: The film production cycle is short and can express multiple colors; the pattern and color can be changed at any time during production; it has excellent scratch resistance; IML production batch quantity is very flexible, suitable for small-scale production of multiple varieties. At present, IML is developing in a variety of innovative directions, such as antibacterial, automatic repair of surface scratches, real fabric in-mold injection, pearlescent effect, 3D texture, and so on.



Process flow: It is mainly divided into three steps: printing, molding and film injection molding. 1. Cutting material: Cut the roll-shaped film into square blocks with a designed size for printing and forming processes. 2. Plane printing: According to the required icons and texts, a film net is made, and the icons and texts are printed on the cut film square blocks. 3. Ink drying and fixing: Place the printed film square in a high-temperature oven to dry, the purpose is to fix the IML ink. 4. Paste protective film: Avoid fouling the printed film surface during the process of punching positioning holes. Sometimes a single-layer or double-layer protective film is required. 5. Punching positioning holes: the positioning holes for thermoforming must be punched accurately. The positioning holes in the shearing process are sometimes punched in advance. 6. Thermoforming (high pressure or copper mold): After heating the printed film, use a high pressure machine or copper mold to form it in a preheated state. 7. Cut the outer shape: cut off the waste of the formed three-dimensional film. 8. Material injection: Put the film exactly the same as the three-dimensional shape of the front mold on the front mold, and then inject the finished IML.  

07 IMR

Overview: IMR is the in-mold decoration technology. The decorative patterns and functional patterns are printed on the film by a high-precision printer, and the film is fed into a special molding mold for precise positioning. The film is injected through the high temperature and high pressure of the plastic raw material The pattern on the top is transferred to the surface of the plastic product, which is a technology that can realize the integral molding of the decorative pattern and the plastic. There is no transparent protective film on the surface of the final product, and the film is only a carrier in the production process.


Features and applications: IMR products are not easy to deform, the edges of the products are completely covered, and the edges have strong adhesion. It is very easy to switch to the IMR transfer foil. The process is simple. Various textures such as wood, bamboo, marble, and leather can be realized. However, the thickness of the printed pattern layer on the surface of the product is only a few microns, and the printed pattern layer will wear off after the product is used for a period of time, and it is easy to fade. In addition, the new product development cycle is long and the development cost is high.


Process flow: Print the pattern on a layer of film, and then attach the film to the plastic mold cavity for injection molding. After injection molding, the patterned ink layer is separated from the foil film, and the ink layer is left on the plastic part to obtain a surface Decorative figure. 1. The film frame is opened, the transfer film is fed in and positioned; 2. The film frame is pressed tightly, and vacuum is sucked to make the transfer foil adhere to the cavity; 3. The mold is closed, injection molded, and thermal transfer and cooling are performed at the same time ; 4. Open the mold, open the film frame, and take out the molded product;


08 Laser carving


Overview: Laser engraving is an important application of the principle of laser processing, that is, the processed laser beam is used to irradiate the surface of the material, and the light energy is instantly converted into heat or photochemical reaction, so that the surface material is instantly decomposed, evaporated, carbonized, discolored, Bubbles, thereby forming a mark.

Features and applications: The advantages of this technology are: 1. Precision: the thinnest line width of the material surface can reach 0.015mm, and it is non-contact processing, which will not cause product deformation; 2. High efficiency: new products can be obtained in the shortest time In kind, many varieties and small batches only need to change the vector file; 3. Special processing: to meet special processing needs, inner surface or inclined surface can be processed; 4. Environmental protection and energy saving: no pollution, no harmful substances, Higher than export environmental protection requirements.


09 Spraying


Overview: The paint layer structure is generally composed of a primer, a pigment layer and a protective topcoat, where the pigment paint layer is thinner than the topcoat. Generally, a hand-held spray gun is used to spray paint, or a robot automatic spray paint production line is used. The coordination of the influencing factors such as paint viscosity, compressed air pressure, spray paint volume and spray speed is very important to the surface quality of the workpiece. Modern paint equipment uses a rotary sprayer to obtain high-quality, extremely thin paint layers. Environmental protection often requires the reduction of solvent emissions during painting. One way is to use water-based paint or powder paint.


Process flow:

1. Annealing: Eliminate internal stress, heat the plastic part below its thermal deformation temperature, and keep it for 2 hours.

2. Degreasing: The surface of plastic parts is often stained with oil, hand sweat and release agent, which will make the coating adhesion worse, and the coating will crack, foam and fall off. The plastic parts are usually cleaned with gasoline or alcohol. Then, after chemical degreasing, the residual lye on the surface of the workpiece should be thoroughly cleaned, and finally cleaned with pure water, dried or dried. 3. In addition to static electricity and dust removal: plastic products are prone to static electricity, and they are easy to absorb fine dust in the air and adhere to the surface after being charged. It is very difficult to remove the dust adsorbed by static electricity by the general blowing method, and it is better to use high-pressure ionized air flow to remove static electricity and dust.

4. Spraying: The thickness of the plastic coating is 15-20μm, usually 2-3 sprays to complete. After one spray, let it dry for 15 minutes, and then perform a second spray.

5. Drying: The general paint is baked at 150-180for 20-30 minutes, and the plastic paint is baked at 60-80for 20-30 minutes. Some special paints can be dried in the greenhouse.


10 Spray metal


Overview: There are several methods for metal spraying, and both high-vacuum vapor spraying and electroplating can achieve the best results. High vacuum vapor spraying is preferentially used for decorative purposes. A thin aluminum or thin copper layer (layer thickness of up to 50μm) is sprayed on the surface of the plastic workpiece with high vacuum vapor, which has electromagnetic shielding function and can replace the expensive metal plate covering process. The thickness of the electroplated layer is up to about 40μm. In contrast to vacuum vapor spraying, the electroplated layer is generally very fine and has good conductivity.

Features and applications: metal spraying can increase the aesthetics of plastic parts and give a metal texture. It can improve the scratch resistance of products or protect plastics from ultraviolet rays and heat radiation in function. It can also be used in some occasions. Play the role of shielding electromagnetic field.


Technological process: The high-vacuum vapor spraying process is as follows:

1. Degrease the molded parts thoroughly;

2. Paint and polish the surface of the workpiece into a lacquer layer with a thickness of about 0.005mm. After the lacquer layer (paint layer is the adhesion bottom layer of the metal plating layer) hardens, send the workpiece into the vacuum chamber;

3. In the vacuum chamber, the metal is vaporized in the high-temperature baking hood, and then deposited on the surface of the plastic workpiece; the thickness of the spray coating is generally 0.1-1.0μm;

4. In order to protect the sensitive thin coating, a topcoat with a thickness of about 50μm is often applied outside the coating.


The electroplating process is as follows:

1. Use chromium sulfuric acid pickling to make the surface of the workpiece rough and uneven.

2. Use ferric sulfate for disinfection.

3. Use palladium chloride to activate the surface of the workpiece.

4. Use chemical methods to plate nickel or copper.

5. The decoration layer (generally chromium layer) is coated by electrolysis method.

Suitable plastics for electroplating are polypropylene, polysulfone, and particularly suitable ABS.


11 Flocking coating


Overview: The flocking coating method can obtain the surface of the workpiece with velvet or velvet quality and similar to textiles. First coat a layer of adhesive on the plastic surface, and then throw or blow the used textile scraps on the adhesive layer, or use high-voltage equipment to generate static electricity for adsorption. The flocking coating is generally carried out continuously on the track, and special shaped electrodes can also be used for the molded parts.

Features and applications: It has the advantages of simple process, high production efficiency, strong three-dimensional effect, low production cost, no environmental pollution, wide range of product applications, and good economic benefits.

Process flow:

1. Surface cleaning: Use a degreasing agent to remove grease on the surface of the substrate, so that the flocking material in the subsequent process has good adhesion.

2. Primer: a process only required when the substrate is PP, other substrates only need to remove the oil stains.

3. Apply adhesive

4. Suede spraying

5. Drying

6. Blow dust gun to remove excess flocking

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