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With China Plastic Machinery
Multi-layer blow molding is a blow molding technology developed to meet the special requirements of certain industries such as medicine, food and cosmetics for packaging containers, such as air tightness and corrosion resistance. Its basic process principles are the same as those of single-layer product blow molding technology. There are mainly two types: co-extrusion blow molding and injection (injection) blow molding. When using the co-extrusion method, multiple extruders are used to squeeze different resins into the multi-layer die at the same time to form a multi-layer concentric product parison, and then blow molding. When using injection blow molding, a layer is injected on the male mold first, and then the mold cavity is changed to inject the second layer on the first layer. Repeat this operation to form a multi-layer product parison, and then blow molding. At present, the multi-layer systems that have been used in industry include polyamide/polyolefin, polystyrene/polyacrylonitrile/polypropylene, polyolefin/polyvinyl chloride, etc.
Multi-layer blow molding is a technology that uses more than two extruders to melt and mix the same or different plastics in different extruders, compound and extrude them in the same die head, and then blow mold to make multi-layer hollow containers.
Multi-layer blow molding is developed on the basis of injection blow molding and extrusion blow molding. The process is not much different, except that the product wall is not a single layer but a multi-layer. The multi-layer structures currently used include: nylon/polyolefin, polyvinyl alcohol/polyolefin, polyethylene/polyvinyl chloride/polyethylene, polystyrene/polyacrylonitrile/polypropylene, etc. Multi-layer blow molding containers are mainly to meet the requirements of air tightness for the growing cosmetics, medicines, and food packaging. Therefore, the raw materials and number of layers of multi-layer containers should be selected according to needs. Two plastics two-layer structure, two plastics three-layer structure, three plastics four-layer structure or four plastics four-layer structure can be used, as shown in the figure. The more layers, the higher the technical requirements.
The key to multilayer blow molding is to control the melt adhesion between the layers of resin. There are two bonding methods. The first is to mix in resin with bonding properties, which can reduce the number of layers while maintaining a certain strength. The second is to add a layer of bonding material, which requires the purchase of an extruder for extruding the bonding material, making the equipment and operation complicated.
Multilayer blow molding is a process of directly blowing or stretching a multilayer preform to form a multilayer hollow container. The multilayer preform is co-extruded by multiple extruders and then blow-molded, which is called co-extrusion blow molding; if the multilayer preform is made by an injection machine and then blow-molded, it is called co-injection blow molding.
The use of multilayer composite technology is usually based on the following considerations:
(1) Using a small amount of resin with excellent barrier properties but expensive in the middle layer of the multilayer structure can improve the barrier properties of the container and extend the shelf life without too high a cost.
(2) Recycling resources and reducing costs. For example, recycled materials can be placed in the middle layer, or a layer of resin mixed with expensive colorants or UV-resistant resin can be placed in the outer layer to reduce costs.
(3) Other functional needs. Including optical effects, hand feel effects and usage effects produced by the combination of materials with different properties.
In short, the ultimate goal of multi-layer composite is to optimize the combination of different functional materials, complement each other and enhance the value of the product. The materials used can be the same material or materials with large differences in chemical structure. In the latter case, adhesives are required to increase the bonding force between layers. However, the difference in rheological behavior of different materials will increase the difficulty of the process.
The typical application of multi-layer blow molding products is the packaging of milk, natural juice, tea drinks, ketchup, carbonated soft drinks, beer, condiments and cosmetics with high barrier properties.
Multi-layer blow molding includes co-extrusion blow molding and multi-layer injection blow molding. The co-extrusion blow molding process uses several extruders to plasticize resins, which are squeezed into a multi-layer die head at the same time to form a multi-layer and concentric tube blank, and then pass through the core rod to become a multi-layer parison, and then blow molding. There are also special storage cylinder heads to form 3-5 layers of parisons. The various plasticized resins in the storage cylinder are separated from each other, and then the annular piston is used to push various plastics out along the core rod at the same time to form a multi-layer parison.
Multi-layer injection blow molding is to inject the first layer on the male mold, change the mold cavity to form the second layer on the first layer, repeat the operation to form a multi-layer parison, and then blow molding.
The characteristics of the multi-layer injection blow molding process are: no waste edge; no cutting marks on the bottom of the bottle; no need for hot melting or chemical action to make multi-layer containers: However, the equipment cost is relatively high and is limited to the production of large quantities and wide-mouth containers.
(1) Material selection of multi-layer structure
The development of multi-layer co-extrusion blow molding hollow container technology and equipment has made it possible to select the best material (layer) combination scheme and manufacture containers with ideal performance.
According to the product capacity range and performance requirements, 3 to 6 layers of structure can be produced. The principles for selecting layer structure and material are as follows: The barrier layer plastic can be selected from polyamide (PA), polyacrylonitrile (PAN) or ethylene/vinyl alcohol copolymer (EVOH); the inner and outer layer plastics can be selected from polyethylene (PE), polypropylene (PP) or polycarbonate (PC), etc., which should have good heat sealing and printability: the recycled layer can be selected from the flash and residual material of the preform: the thickness of the inner shell layer, recycled layer or outer layer should be greater than that of the adhesive layer and barrier layer. Generally, a combined adjustable co-extrusion die head and program logic control or microcomputer monitoring are selected to evenly distribute the multi-layer plastics according to the selected material quantity, co-extrude into a preform, and the preform is top-blown by a mobile station.
Multi-layer co-extrusion containers have high chemical resistance (anti-oxidation, light aging resistance), resistance to harmful substances, and resistance to odor migration; they have pressure resistance, impact resistance, smooth surface, heat resistance, and protection against surface scratches.
(2) Multi-layer co-extrusion equipment
The extruders for each layer of plastic can be general-purpose extruders, and DC motors are often used for stepless speed regulation. The throat of the extruder hopper is designed to be curved. The feed of the barrier layer extruder is preheated by temperature control. Each extruder should be equipped with a torque monitoring device. The co-extruders are operated in parallel and monitored in different levels: the extruders are started together. When the torque of a certain extruder decreases or the feeding is interrupted, the whole machine can be stopped and can be operated together according to the program; the parison length is controlled, relying on the flow distribution, and can be automatically adjusted synchronously; when the melt temperature and torque of each extruder exceed the parallel operation conditions, and when the pressure of the bonding layer and the barrier layer in the machine exceeds the allowable range, they are monitored and adjusted by the fault display.
The die structure of the multi-layer co-extrusion die is shown in the figure.
The die head of the multi-layer co-extrusion die is often designed as a splicing type: the die shell is composed of several flange-type outer molds, and the inner mold is assembled from several mold cores. The outer mold and inner mold core blocks are precisely processed, and the die runner is chrome-plated and polished to reduce the flow resistance of the plastic melt. The entire die head is heated by a four-stage adjustable power ceramic heater, which cooperates with the rapid start of the die head and has good heat insulation measures to ensure that the die head has the best temperature environment: the whole machine selects program logic control or microcomputer control. The speed distribution in the mold opening and closing stages, and the moving speed of the blow molded bubble tube can be controlled by hydraulic proportional valves and numerical position transducers.
Multi-layer co-extrusion blow molding containers fully demonstrate the advantages of various plastics and can complement each other, but the transparency of the container is poor, and the characteristics of the contents cannot be displayed, which reduces the value of the product.
After the development of ethylene-vinyl alcohol copolymer/polypropylene multi-layer biaxial stretch blow molding technology, it was found that the oxygen permeability of the biaxial stretch blow molded container of single-layer polypropylene was reduced to 1/20 compared with the ethylene-vinyl alcohol copolymer/polypropylene biaxial stretch blow molded multi-layer container, while the barrier property to water did not change. Compared with the unstretched multi-layer container, the transparency of the biaxial stretch multi-layer container is significantly improved, and the mechanical strength is improved, the breakage rate of the container is reduced, and the weight of the container is reduced.
Due to the different types of plastics, their physical properties such as glass transition temperature, melting point, elastic modulus, yield stress, breaking tensile force, etc. are different, so the stretching method, process conditions, etc. are also different. For example, ethylene-vinyl alcohol copolymer is difficult to be biaxially stretched because it contains extremely strong hydrogen bonds.
The multilayer container made by biaxial stretch blow molding solves the problem of biaxial stretching of multiple plastics with completely different properties without peeling between layers to produce satisfactory multilayer containers, which is a new achievement of multilayer blow molding technology. The disadvantages of multilayer blow molding are large equipment investment, complex production control, and high product cost.
Multilayer blow molding is a blow molding method for manufacturing hollow products by making multilayer parisons by injection molding or extrusion. The obtained products have the characteristics of isolating oxygen, carbon dioxide and moisture, and also have the advantages of chemical resistance, but the cost is relatively high. It can be used as a container for beverage bottles, fuel tanks and chemicals.
Multilayer blow molding generally uses co-extrusion to make the parison, and then blow molding to obtain the product. For example, in the manufacture of pesticide bottles, PA-6 can be used as the inner layer, high molecular weight polyethylene as the outer layer, and ionic resin as the adhesive for the middle layer; on the equipment, if a storage cylinder die is used, three extruders can be used to plasticize PA-6, ionic resin and HMWPE respectively, and enter their respective storage cylinders; when the pistons of the three storage cylinders are pressed, the materials converge at the die mouth to form a three-layer parison, which is blown, cooled and shaped to obtain a hollow product.
If the melting point of the polyamide resin used is 220¡æ, the selected HMWPE should have sufficient melt strength at 225-230¡æ, and the polyamide adheres to the polyethylene surface through the adhesive. At this time, the draw of the parison mainly depends on the molecular weight of the polyethylene. However, the blow molding of polyamide requires a resin with a high degree of polymerization, or the viscosity is increased by grafting and cross-linking to reduce the draw.
Plastic fuel tanks for automobiles have been produced since 1973. In 2000, this type of fuel tank had accounted for 90% of the market share. In terms of fuel tank materials, it has developed from high molecular weight polyethylene single-layer products to multi-layer extruded hollow products. For example, the 6-layer co-extruded blow-molded fuel tank using ethylene-vinyl alcohol copolymer (EVOH) as a barrier layer has a hydrocarbon leakage of 0.1g in 24 hours. In the case of 6-layer co-extrusion, the material structure is: HDPE/adhesive/EVOH/adhesive/return material/HDPE; the share of each layer (%) is: 38/2/3/2/43/12. Due to the poor heat resistance of EVOH, only continuous extrusion process can be used. In order to make full use of the unit consisting of 6 extruders and reduce waste to a minimum, two sets of blow molding molds can be configured, and two 6-axis manipulators can be used to transport the parison and take away the product respectively.
In terms of co-extrusion blow molding equipment, Germany's Rikufec produced the world's largest three-layer co-extrusion blow molding equipment for a Spanish customer in 2000. The model is GBMS4000/A300/Coex3. Its discontinuous operating system has a 300L annular pusher pressure die; an extrusion molding machine with an extrusion capacity of 1500kg and a clamping force of 5000kN, a template size of 405mm to 3350mm, and can produce products with a volume of up to 25m.
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