Section 6 Printing of Plastic Products - -1

The printing of plastic products refers to a special printing process using plastic as a substrate. Plastic flexible packaging technology includes plate making, blow molding, corona treatment, printing, compounding, slitting, heat sealing, and bag making.
Definition of plastic film: a general term for a flexible packaging material in which a high-molecular-weight organic compound containing a synthetic resin as a basic component can be rolled into a flat shape. The film made by blow molding method is called "tubular film", and the film made by calendering method is called "flat film".
The thickness of a plastic film is less than 0.25mm, which is more than what is called "sheet", and thicker than the film is called "plate".

I. Overview (I) Printing characteristics of plastic products and application Printing of plastic products is a kind of printing technology that belongs to the edge disciplines. It is related to plastics, electronics, physics, polymer chemistry, color, optics, fluid mechanics and surface chemistry. Other modern sciences have a close relationship. The advanced plastic composite printing production line has adopted microcomputer processing and light control systems. Now plastic printing technology in advanced countries, there are several hundred kinds of plastic composite packaging materials, food preservation can be not bad over the years, can be high temperature 135 °C, low temperature -40 °C, a gas barrier, moisture, shading and other functions, and use Full, vacuum, deoxygenation, bacteria and other modern science and technology have expanded the use of plastic packaging. In the printing industry, color gradation printing has been widely adopted, and images are formed using thick, light, bright, and dark colors, so the prints are rich in color, clear and lively, and vivid in appearance and strong in three-dimensional. In the art design, like the paper printing, the special function of the full-page image processing system has been applied, and the colors are arbitrarily imposed and changed arbitrarily, and the plastic color package printing is also icing on the cake. In recent years, plastic packaging composite materials have been studied to suppress heat, sound, light, radioactivity, static electricity, and electromagnetic wave energy penetration.
(2) Types and properties of printing plastic films The printing of plastic films is generally based on gravure printing and flexographic printing. The printing of plastic sheets and plates is generally based on screen printing.
There are many methods for classifying plastic films or sheets, among which the most frequently used products are classified according to the chemical composition of the films, such as polyethylene film, polyvinyl chloride film, polyester film, etc., and classified according to the method of manufacturing the film. Melt extrusion method, calendar method, solution casting method, and other according to the use of film classification, such as packaging films, agricultural film, electrical insulation film. According to the structure of the film, it can be divided into single-layer film and composite film.
1. Monolayer Films Monolayer films of different resins have different properties. For example, polyethylene films, polyvinyl chloride films, polyester films, and the like, their moisture resistance, airtightness, oil resistance, cold resistance, ultraviolet radiation resistance, aging resistance, printability and the like are not the same.
(1) Polyethylene film (PE) Polyethylene film production is the most commonly used polyethylene resin melt extrusion, blow molding, its simple operation, easy to control. Blown films tend to have the directionality of mechanical strength, ie, the longitudinal strength is greater than the transverse strength.
The properties of the polyethylene film mainly depend on the density of the resin, the melt flow rate and the molding method. The resin is divided into low density, medium density, high density, and ultra high molecular weight polyethylene resin, and correspondingly there are these four types of films. Generally, as the density of polyethylene increases, the tensile strength, rigidity, softening temperature, and impermeability of steam and gas of the film are all improved; transparency, plasticity, thermal adhesiveness, elongation and The impact strength is reduced accordingly.
Linear low density polyethylene can be made thinner and tougher thin and tough film. General low density polyethylenes with similar melt flow rates have higher tensile strength, tear strength, impact strength, and better airtightness. Its durability, puncture resistance, printability, heat resistance and heat sealability have all been improved.
Polyethylene films have poor print fastness and are generally treated with corona discharge to increase their fastness.
(2) Polypropylene film (PP) Polypropylene film production, commonly used blow molding method and T-die extrusion method, a total of medium unstretched, uniaxial stretching and biaxial stretching of several films .
The biaxially stretched film (BOPP) of polypropylene has two kinds of plane stretching method and bubble tube stretching method. The plane stretching method is generally 5-8 times of longitudinal stretching and 6-10 times of horizontal stretching. The plane stretching method is better than the film made by the tube stretching method in uniformity of thickness and thickness. After being biaxially stretched, the gloss, transparency, airtightness, and strength of the film are significantly improved. As a transparent and soft packaging material, the film can be comparable to cellophane. Commonly used in the manufacture of composite films, such as biaxially oriented polypropylene/polyethylene composite films are ideal for moisture resistance, transparency, strength, stiffness and printability. They are suitable for dressing instant noodles, dry cakes, etc., in clothing and knitwear. The packaging is also widely used.
The main disadvantage of BOPP is the poor heat sealing performance. Commonly used chlorinated polypropylene or polyvinylidene chloride copolymers are coated on BOPP, and a small amount of anti-blocking agent such as silica is used to prevent blocking.
(3) Polyvinyl chloride film (PVC) Polyvinyl chloride film is divided into soft film, hard film and heat shrinkable film.
Soft polyvinyl chloride film, its plasticizer content is generally higher than 20%, the type and amount of plasticizer has a great impact on the performance of the film, the higher the plasticizer content, the film elongation and low temperature resistance increased , while the hardness and tensile strength decreased. However, non-toxic plasticizers such as epoxy soybean oil and dioctyl sebacate are used for food packaging.
Rigid polyvinyl chloride film is an airtight film material that contains no plasticizer or a small amount of plasticizer and can be cast by using dichloromethane as a solvent. The thickness of the sheet is generally between 0.1-0.5mm, and can be made into a variety of containers by vacuum forming.
The polyvinyl chloride shrink film is a material that is excellent in gloss, transparency, heat sealability, printability, and prevention of odor permeability. However, it is poor in heat resistance and softens when it approaches 70°C.
(4) Polystyrene film (PS) The polystyrene film includes a biaxially stretched film, a film, and a foamed polystyrene film.
Polystyrene film and film have the characteristics of acid and alkali resistance, low temperature resistance, hygroscopicity paper, good gloss, non-toxicity, easy processing and molding. The biaxially stretched polystyrene film has good air permeability and heat shrinkability and can be used for shrink packaging of fruits and vegetables. Its film transparency and gloss are better. Often processed into various cups and containers.
(5) Polyester film (PET) Polyester film is usually produced by the T-die method. The film coming out of the T-die is an amorphous film, and has tough properties after being uniaxially or biaxially stretched. Can be stretched under different conditions to give different shrinkage films. Films heat-set or crystal-stabilized have much lower residual shrinkage when heated (average at 1% at 105°C).
After being stretched by orientation, the polyester film is the toughest thermoplastic film. Its tensile strength can be 9 times that of the polyethylene film compared to the aluminum film. Heat resistance, cold resistance, oil resistance, chemical resistance, air tightness, moisture resistance, water resistance, and transparency are all excellent. However, it is not easy to heat-seal bags, so it is often compounded with polyethylene, polypropylene, and other heat-sealable materials.
The polyester film is coated with a very thin layer of aluminum under vacuum conditions, which has an excellent silver appearance and UV resistance, improves the hermeticity and moisture resistance of the polyester film to oxygen, and reduces the cost. Aluminum-plated polyester film is an ideal composite packaging material that is compounded with heat-sealing properties such as polydecene.
(6) Polyamide film (PA) Polyamide is generally referred to as nylon, and the film has two kinds of unstretched and biaxially stretched films. Polyamide contains strong amide groups (-CONH-) and has high water absorption. It has the advantages of non-toxic, transparent, oil-resistant, wear-resistant, drug-resistant, high strength, no static electricity, fragrantness, and good printability, and its high and low temperature performance is particularly outstanding. Can be used at -60-+200°C. The disadvantages are large moisture permeability, poor strength, and difficulty in heat sealing, so they are often compounded with PE films. This is because the polyethylene film is water-resistant, corrosion-resistant, and heat sealable, but has high permeability to oxygen; the polyamide film is resistant to oxygen and has high strength but poor heat sealability. Both materials Compounding solves the airtightness and heat sealability, so it is an ideal composite film. Polyamide film is a packaging material for oils and fats, vacuum packaged foods, retort foods, and dairy products.
(7) Polyvinylidene chloride film (PVDC) Polyvinylidene chloride film is a vinylidene chloride-vinyl chloride copolymer. The film has excellent air tightness, fragrance protection, transparency, oil color grease resistance and chemical resistance. It is non-toxic and can be used for the packaging of foods and medicines; and it is one of the excellent substrates for improving the airtightness of composite films. Generally, the film is used as the middle layer, and the polyester, nylon, paper or polypropylene are the outer layers, and the poly. Ethylene is a hot melt seal.
Since the film has outstanding heat shrinkability and strong self-adhesion, it is used as shrink film and stretch film for shrink packaging and stretch packaging.
The thermal sealing temperature of the film is relatively low, generally 120-150°C. Can use far infrared seal, pulse seal, ultrasonic seal. Among them, the sealability and strength of the far-infrared seal are the best, and the pulse seal can avoid the wrinkles caused by the common electrothermal seal and reduce the chlorine and hydrogen produced by the thermal degradation. The solvent seal is only used for packing non-food products. Common solvents are tetrahydrofuran or cyclohexanone.
(8) Polyvinyl alcohol film (PVA) The plasticizers used in the production of polyvinyl alcohol include water-soluble compounds such as glycerol, butanediol, and sorbitol. The type and amount of the plasticizer have a great influence on the properties of the film.
General polyvinyl alcohol film is divided into two kinds: water-resistant film and water-soluble film. When the polyvinyl alcohol film with saponification value of over 99% is heat-treated at a certain temperature, the film is crystallized and thus has better water resistance. The resin used for the water-soluble polyvinyl alcohol film is a partially saponified product. Such a film is difficult to crystallize and can be dissolved in cold water.
The transparency, airtightness, oil resistance, printability, and surface gloss of polyvinyl alcohol films are all good, and they have no static electricity, soft touch, and strong toughness. The disadvantage is that the thermal adhesiveness is slightly poor, but The use of high frequency or pulsed welding is an important material for clothing and textile packaging. The film can be dissolved in water and discarded as a package to reduce environmental pollution.
Table 3-29 shows the main physical properties and combustion characteristics of several plastic films.
Table 3-29 Main Properties and Combustion Characteristics of Common Films