The introduction and widespread use of new plastic materials and the development and production of related ink systems have led to the rapid development of the screen printing industry. Because the end-use products of printing houses include inks and printing materials, the cooperation between ink and printing material manufacturers is naturally close, in order to improve the understanding and understanding of mutual products.
From the perspective of a screen printing plant, it is important to maintain close cooperation with his ink and printing material suppliers. Therefore, this article will discuss the main plastic substrates, their identification, and matching with specialized ink systems.
The two main types of plastic materials
The two main areas of concern are thermoplastics and thermosetting plastics.
Thermoplastics are plastics that can be reshaped or changed in shape by heating. It includes acrylic resins, fiber derivatives, polystyrene (PS), and vinyl materials.
Thermosetting plastics, once shaped and cured, cannot be re-transformed, mainly phenolic resin, polyester, melamine, epoxy resin. It should be known, however, that polyester may also be thermoplastic, such as polyethylene (PE). Thermosetting polyester has a cross-chain connection structure and is used in glass-reinforced plastics. The printing house must know: before picking it up, it should be clear about the end use of the product to be printed, and the corresponding performance requirements, such as heat / grinding / solvent and chemical resistance, as well as weather resistance and sun protection, must also be known and convenient Choose the right ink for each specific print.
In many cases, plastic substrates contain plasticizers, which will sneak into the ink layer on the surface of the substrate. Especially for thermoplastic products, although there are some exceptions here: Mylar, ABS, polystyrene (PS) and acrylic resins (Acrylics) do not contain removable plasticizers. However, when these plasticizers appear, it is necessary to bring the ink to a level that is balanced with plastic products. In most cases, this can only be achieved 24 hours after printing. This is why it is necessary to wait at least so long after the trial printing before the official printing. The tests required for different situations will vary, except for the most obvious adhesion test. For example, for embossed relief, hot die-cutting and other processing, the printing plant must conduct deformation and stability tests on the plastic surface. Similarly, with regard to resistance, is the ink layer resistant to items that the plastic product surface may come into contact with? When printing plastic containers, for example, the printing house must understand the nature of the substance that the container will contain, as this will be directly related to the choice of ink.
Several major inks
Plastic inks can be classified according to different drying methods: volatilization, oxidation or polymerization. Varnish, for example, is completely dried by solvent evaporation to form a physicochemical glue layer, which involves the effect of the solvent on the surface of the substrate, which dissolves the plastic surface to some extent and causes some corrosion. (This physicochemical bonding is sometimes incorrectly called chemical bonding, but this so-called chemical reaction does not happen here).
Oxidized and polymerized inks, on the other hand, are glues formed as a result of specialized high-strength adhesion. This is called mechanical gluing.
Among the oxidation and polymerization curing inks, there are alkyd enamels, prepared alkyds, epoxy esters and epoxy inks. They found their most widespread application in thermosetting plastics. Once thermosetting plastics are shaped and cured, they cannot be changed again. This type of ink can also be used, however, for some thermoplastic substrates resistant to strong solvents, such as polyethylene (PE) and polyester, such as Mylar It can be used for decoration of other thermoplastic materials, assuming that no post-printing process is performed. These inks are highly cross-linked after drying and curing, and do not have the characteristics of reshaping.
When considering the end use of the printed item, it is important to check whether high strength solvents and scratch resistance are required. If you really need it, you need epoxy ink. On the other hand, products such as polyethylene (PE) bottles often require high-strength solvent or product resistance, and scratch resistance is generally sufficient, because they are often thrown away after use. Alkyd enamel, in most cases, can obtain satisfactory results on these products, compared with epoxy ink, it should be preferred. Because the outdoor weather resistance of epoxy ink is not good. At the same time, either double packaging, with ink and catalyst, or single packaging, requires curing at high temperatures.
Lacquer screen inks are basically used for printing thermoplastic materials. There are many different types of resins for these inks: Acrylic resins, Acetate resins, Acetate butyric acid (CAB) resins, Ethyl fiber resins, Nitrocellulose resins, Polyvinyl chloride (PVC) resins Combination of partial resins.
Ink manufacturers have been working hard to develop ink formulations so that printing plants can significantly reduce the list of ink types. Varnish-based inks, for example, can be widely used in various plastic materials, and also show good adhesion. They are especially useful in the field of vacuum forming of rigid plastics. Even if the adhesion is still at the lowest level after air drying, they form a partial fusion between the ink and the substrate through the forming process during the vacuum forming process. This also shows that the adhesion test should be carried out after the vacuum forming process is completed.
For varnish inks, the weather resistance must be carefully tested, because on some plastic surfaces, adhesion may be lost after passing ultraviolet light or heating.
Ink manufacturers use a solvent system that is carefully balanced to reduce printing problems. For this reason, screen printers can only use solvents provided or recommended by ink suppliers, which is quite important. An excessive amount of inappropriate slow-drying agent, for example, may make people mistakenly think that the ink layer has dried up, so they are piled up, and as a result, the printed matter continues to volatilize. This will cause the prints to pile up or stick together. In this case, a formula must be developed to allow the screen printing factory to print vinyl pressure-sensitive film and allow it to be stacked immediately after air-drying without blocking. These systems have the additional advantage of being able to dry quickly at lower temperatures. This means that pressure-sensitive stickers can be quickly air-dried on the conveyor belt without curling, or do not require heating in very short aisles. An ecological advantage is that these ink formulations comply with the anti-pollution regulations.
I have already mentioned the example of the incorrect use of slow-drying agent. Undoubtedly, many readers have experienced the problem that the curling of the substrate eventually leads to the inaccurate positioning of the multi-color printing set. Such curling or wrinkling is usually caused by excessive thinner.
Not all lacquer inks rely on the adhesion of solvents. In some cases, the presence of active solvents in varnishes can cause problems, especially after printing plasticized thermoplastics, such as styrene or acrylics. These products, especially after molding or injection molding, will become more "crazy": a result of the cracking of the dry solid ink layer. These cracks are actually the cracks on the surface of the plastic material and then transmitted to the surface of the ink layer. That is, when an excessive amount of active solvent is added, the internal pressure and tension of the plastic material are released, which eventually leads to the result of cracking. On the surface of non-plasticized polystyrene (PS) and acrylic resin (Acrylics) substrates, chipping also occurs, often when printing molded article surfaces. However, the same ink is used for the same substrate material, as long as it is printed before molding to avoid this situation. In order to reduce the cracks on the surface of most products, manufacturers have developed special ink formulations, but the printing house must be aware that the cracks may be discovered very late, sometimes after a long time after completion. This is another reason why substrates must be tested in advance or provided to ink manufacturers for testing before formal printing.
The adhesion of the ink on the surface of the plastic substrate may also be seriously affected by the injection release agent. Many lubricants used in production can be removed from the plastic surface by inert solvents without harming the plastic itself. Silicone lubricants are a special case and may require the use of alkaline cleaning fluids. Many plastic materials are treated with active agents to reduce surface static electricity while removing dust and dirt on the surface.
Ink manufacturers use a variety of colors in ink formulations, and the sun protection and durability of each color should be easily available. "Sun protection" and "durability" are often misunderstood: a single color material, by itself, may have good stability, but if this color is mixed with white or other colors, its stability May be sharply reduced. In the same way, if a color is added with a matching colorless agent and an agent to obtain different degrees of transparency or translucency, its stability may be greatly reduced. [next]
Identification of plastic materials
When not sure about the type of plastic material to be printed, it is recommended to carry out some simple tests to identify its characteristics and generic name. There are various ways to test substrate materials. The following tips may be helpful:
1. Vinyl materials
Vinyl materials are thermoplastics, and there are two kinds of hard and soft. One method of testing vinyl materials is through what I call the "hot copper wire test method." When the heated copper wire is placed on the surface of the vinyl film, it will immediately return to the flame state, producing a yellow flame. Once the copper wire is removed, the flame burning stops. If the flame can touch the vinyl film, if it is polyvinyl chloride (PVC), you can always see a green flame at the bottom of the flame. Burning vinyl materials will produce black smoke with a distinct smell of chlorine.
2. Acrylic resin
Like vinyl materials, acrylic resins (Acrylics) are thermoplastics, but they have different characteristics. For screen printers, this material is widely used in outdoor display. Acrylic resin materials can be vacuum formed, but if you really want to do this, you must choose the inks very carefully. Acrylic resin (Acrylics) ink contains some special resin, this resin and acrylic resin (Acrylics) have the same primer system. Acrylics (Acrylics) materials must be carefully tested for solvent resistance, because in many cases will be subject to some damage, including cleaning fluids, acetone and oxidants. It is recommended to use special acrylic resin (Acrylics) ink formulations, although synthetic coatings and some varnish inks are also suitable.
Acrylics (Acrylics) substrates are the easiest to recognize by eyes, but flame tests can also be used. A yellow flame will be produced, and unlike the vinyl, it will continue to burn when the flame is removed. Acrylics also have an unpleasant smell and are unique to Acrylics. In this case, a further identification test can be used: during the combustion process, acrylics (Acrylics) will produce a noticeable crackling sound. Another way to identify Acrylics is to tear the edges of Acrylics or cut a section with a hacksaw: you will notice the unique smell of the monomer.
3. Cellulose derivatives and fiber ester plastics
These are the oldest known thermoplastic materials. Among these materials, the commercial name of one of the earliest materials is "celluloid". Their use is continually decreasing, because some materials have strong resistance to extrusion, but are highly flammable and have poor sun protection. We will mainly look at two of the materials that fall into this category:
Acetate fiber-printing of this material can choose bright vinyl ink, varnish ink and some so-called multi-purpose plastic ink. There are two main types of acetate fiber: binary acetate fiber, which is generally not a big problem for screen printing inks. Triacetate fibers will have more serious adhesion problems and may require special formulations. The flame burns yellow in the center and blue in the surroundings. When the flame is removed, it will continue to burn. It smells of sour vinegar when it starts to burn.
Acetate butyric acid fiber-This material can be used in outdoor purchase point display cards, which has better water resistance and weather resistance than acetate fiber material. The most obvious difference is that butyrate has an unpleasant smell in a closed container. Its printing can choose acrylic resin (Acrylics) ink and some varnish ink. Once burned, it will produce the same flame as acetate fiber, but at the same time there will be molten plastic dripping, the smell is very similar to the rancid smell of grease.
4. Olefin material
The two most common types are polyethylene (PE) and polypropylene (PP). Polyethylene (PE) is widely used in the bottle industry because of its high strength, light weight, and good shatter resistance. It can be injection molded, stamped or formed, while being easy to color. Before printing, the polyethylene (PE) material must be surface-treated, either flame-treated or corona-treated. Epoxy inks and synthetic lacquers are generally recommended for printing on the surface of such materials.
Polypropylene (PP) is one of the lightest thermoplastic materials in common. In addition to good surface hardness and strong scratch resistance, it is more rigid than polyethylene (PE) materials. Widely used in the manufacture of bottle caps, medicine cups and some household products. Similarly, pre-press processing is usually required, and epoxy inks are suitable in most cases.
Olefin materials can be identified by flame characteristics, that is, the center is blue and the surrounding is yellow. In other words, the flame produced is exactly the opposite of the flame of the cellulose derivative. Olefin material can melt and drop into small droplets, and its smell is similar to paraffin wax.
Some substrates contain two or more plastic materials, one of which is a stamped, grooved polyethylene (PE) / polypropylene (PP) copolymer. This material has a wide range of applications, such as point-of-purchase advertising and packaging. They have different business names. Generally speaking, the supply of this product is either corona treated or coated on the surface. It must be remembered, however, that the corona treatment effect will not last indefinitely. Special ink formula can ensure good adhesion, even for products with uneven or insufficient treatment. In addition, this special ink formulation also has good water resistance. Even if some inks show good adhesion on this material, the continuous rain effect may be greatly weakened, which is why it is recommended to use this special ink formulation.
5. Polystyrene
This belongs to the category of thermoplastics. Low-strength polystyrene (PS) differs from other materials in terms of its hardness, rigidity, thermal and dimensional stability, and flexible manufacturing. Although it is relatively hard, however, polystyrene (PS) products are also often easily broken. Low-strength polystyrene (PS) materials are also very sensitive to solvent attack. We, however, also noticed other copolymers, such as high-strength butadiene-styrene, and acrylonitrile-butadiene-styrene terpolymer, often called ABS ). These types are much stronger and less sensitive to solvents than low-strength polystyrene (PS).
Oriented polystyrene (PS) is produced in a biaxial orientation during the stamping process. It overcomes the fragility of polystyrene (PS) material itself and improves its strength, while retaining the original advantages of ordinary polystyrene (PS) material, such as clarity and stability of external dimensions.
Another type is polystyrene (PS) foam. This material, however, is particularly sensitive to solvents and usually requires specialized ink formulations. For most polystyrene (PS) materials, acrylic resin (Acrylics) ink formulations are the safest. For the identification of polystyrene (PS) materials, the flame is bright yellow. The material will continue to burn when the flame is removed.
6. ABS
ABS stands for acrylonitrile-butadiene-styrene terpolymer. As the name implies, the main elements are butadiene and styrene. Used for printing, so ABS has many similar properties, and can use many types of acrylic resin (Acrylics) type ink formulations. It can be vacuum forming, blow molding, stamping or calendering. Has particularly good chemical resistance, physical strength and brightness. Unfortunately, the ABS material cannot be identified by flame testing. Obviously, it contains a combination of styrene and butadiene.
7. Phenols
Phenols are thermosetting materials and have a wide range of applications in the electronics and household appliance industries. The material has good heat resistance and insulation properties. It can be molded into a hard object with a good finish. Although it has good solvent resistance and low water absorption, it will yellow when exposed to strong light. For this material, epoxy ink is recommended.
When the flame is close to this material, it will not burn but carbon black-like substances will appear. At the same time, it will also emit a unique smell of phenols.
8. Amine plastics
This thermoset includes melamine and urea. From translucent to opaque colors, there are various amine plastics. This material has high hardness, scratch resistance, and is not corroded by alcohol / oil / fat. The material is also used for filling in the packaging industry and the electronic component industry. Epoxy ink should be used. When approaching the flame, the phenolic material will burn very bright, but the flame will soon be extinguished, followed by formation of carbon black. In other words, it is difficult to identify melamine and urea from other phenols.
9. Polycarbonate material
These are thermoset plastics, which have recently been of interest to printing houses. This material can be injection molded, cast or stamped. Has good moisture resistance, temperature resistance and extrusion resistance. In fact, it cannot be broken, and it is mostly used for warning signs in public places. It is also used in the production of bottles that must be sterilized. It must be pointed out, however, that polycarbonate (PC) is more sensitive to solvents than acrylics (Acrylics) materials.
Some vinyl inks and acrylic resin (Acrylics) formula inks are suitable for this material. Polycarbonate (PC) materials cannot be identified by flame testing, but to distinguish them from acrylics (Acrylics) materials, the crush test method can be used. Acrylics (Acrylics) material will be crushed by the hammer and polycarbonate (PC) material will crush the hammer!
10. Polyester material
This material has a wide range of applications in the field of pressure sensitivity. The most common product is DuPont's Mylar film. This material is thermoplastic, and special ink formulations must be used unless the material surface is pre-coated. After the primer is applied, synthetic paint or coating can be printed on the surface of the primer. [next]
Some suggestions and considerations
Of course, there are many other plastic materials that are not mentioned. The plastic materials discussed above are the most used by screen printing factories. Since they have different adaptability to inks, it becomes difficult for screen printers to choose inks. When printing these materials, the time schedule must require sufficient time for testing. In order to avoid printing problems, each ink manufacturer will have relevant recommendations and instructions. Ink manufacturers will test the printing materials provided by customers under their own laboratory conditions, mainly to test the adhesion of ink. Assuming enough time, ink manufacturers can make recommendations on solvent resistance, weather resistance, and other properties. Even so, it must be remembered that the operating conditions of the ink laboratory will be different from the production workshops of most printing manufacturers. Therefore, there must be a certain error space. Only tests conducted under actual printing conditions are safer.
The printing house must always remember that it cannot completely rely on and trust the information conveyed by the substrate supplier and the ink supplier through oral or even written forms. For example, it must be remembered that if a silk screen factory prints a product provided by a customer, it is recognized as a polyester material based on understanding, and the usual way is to consult the ink supplier to select which ink to use for the polyester material, and the ink supplier Without seeing or conducting physical tests in person, only some relevant suggestions can be given. They cannot make any specific guarantees: First of all, the accuracy of the information obtained by the screen printer from the customer is not guaranteed. Second, if it is a polyester material, it can be coated, so it does not even require special inks for polyester materials. The correct method should be to send samples of the substrate to the ink supplier for printing test.
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