The hottest polyethylene and polystyrene foaming a

Polyethylene and polystyrene foaming and extrusion equipment lost many supply markets

polyethylene and polystyrene foaming and extrusion equipment manufacturing plant has experienced a revolutionary market change in the past five years. Mainly in food packaging, there has been and is a debate on CFC foaming agents, which has led to the loss of many supplier markets

during this period, gradually through the efforts of resin manufacturers, foam manufacturers and other researchers, the plastic industry has put forward opinions on the possibility of foam recycling and the overall impact of foam production to waste on the environment

during this period, manufacturers have achieved success in many other market developments involving foaming and extrusion. Such as co extrusion technology, foam laminate products and other industrial applications. The result of this revolutionary change is a series of innovative and environmentally acceptable foam products

technological process

extruded foam products are a blend of polymer, nucleating agent and foaming agent, which are fully mixed in the extruder until their essence is kept under pressure before the self die is discharged. When the melt flows out of the die, it flows from the mouth 7 There are internal and external circulation of high pressure in the mold into the low pressure or atmospheric pressure outside the mold lip, and the foaming agent changes from liquid to air pressure. The cell of the foaming agent is attached to the nucleating agent in the melt, thus forming a cell structure. The size and number of bubbles depend on the amount of nucleating agent added, and the density depends on the amount of foaming agent used

when producing foamed plastics, the main components must be carefully measured and mixed to obtain uniform cellular structure and density. Pure resin pellets and nucleating agent are mixed in the mixer. Nucleating agents are generally fine powders, such as talc powder or the mixture of citric acid and sodium bicarbonate, with a small amount added (usually 0.2% - 2.0% of the output of the extruder). After the dry mixture enters the extruder, it is baked and mixed by the screw of the extruder

after the dried raw materials are melted in the screw compression section, foaming agent is injected under high pressure. It is injected in the form of liquid or calibrated gas pressure, and mixed with the melt through the screw of the extruder

most of the foaming agents used in the past were chlorofluorocarbons (CFCs), but now they are replaced by pentane, butane, HFC-152a and carbon dioxide, or blends of all these

some of these foaming agent substitutes have an impact on quality, processability and some other properties. Therefore, the equipment, processing technology and products need to be improved

last year or the year before last, equipment manufacturers, resin manufacturers and processing manufacturers developed many foaming agents to replace chlorofluorocarbons, and the most commonly used is the blend of CO2 and CO2. Some of these works have formed patented and licensed technologies

the key step in the production of high-quality foamed plastics is the mixing of foaming agents. Although resins containing chemical blowing agents can be used, this is not the most ideal method. The application of chemical foaming agent can not obtain very low density, and because the amount of foaming agent has been fixed, the density cannot change. The foaming agent can also be directly injected into the polymer melt with three steel balls on the iron embedded end face contacting the yoke end face in case of power failure, and then mixed by the screw of the extruder. This method is the most general

in the direct injection system, the foaming agent is injected into the extruder through the high-pressure metering device. This system usually has a high-pressure diaphragm pump and a precision control valve to control the flow of foaming agent within a very strict precision range. The liquid is injected through the cylinder wall of the extruder with the help of a specially made and qualified syringe valve, which plays a role in preventing the polymer from flowing back into the pump pipeline

after the foaming agent is added to the melt, the mixture must be effectively cooled, so as to improve the viscosity of the melt, so that the melt strength is enough to inhibit the escape of the foaming agent, and form at the die lip. This requires heat removal. The best solution is to have two extruders in the extrusion operation, the so-called series extrusion system. Foamed plastics can also be extruded on a single screw extruder, generally using long l/D equipment (l/d=40:1-44:1). Using a screw must complete melting, mixing and then cooling the melt, which is not as efficient as dividing into two steps, and there is a loss in output. For this reason, the series system is the best method, which is the most commonly used method in the world

equipment

the series system includes two extruders, which carry out the melting, mixing and cooling steps in the process respectively. The first extruder is a conventional type with a length diameter ratio (l/d) of 24:1 or 32:1. The dried materials enter the first main extruder, where they are melted and mixed. Foaming agent is injected into the decompression area of the screw. Immediately after injection, they are in the violent mixing position, and the liquid is immediately dispersed in the whole melt. The mixture then flows through the exchanger and the high-pressure pipe joint, which connects the first extruder with the second cooling extruder. The production capacity of the cooling extruder is much larger than that of the first extruder, and it is equipped with deep threaded screws in order to transport the melt forward without heating it again. The screw speed of the second extruder is much slower than that of the first one (usually the ratio is 4 or 5:1). When the melt flows down the barrel wall of the second extruder (l/d=30:1), it becomes more and more viscous as it is cooled

connected to the end of the second extruder is the die, which makes the viscous molten fluid finally formed. The most commonly used die is annular, with a die gap of 0 020—0.025in。 The die runner is designed to make the melt flow smoothly under constant pressure. The melt will not foam in the die, but will expand and foam immediately once the melt flows out of the die lip. Here, the foam is partially solidified, and the compressed air placed in the air ring around the mold lip forms dense tissue or epidermis on the surface

a group of traction rollers located on the follow-up device pull out the foam hose from the annular die lip across the water-cooled die core. The die core further cools the sheet, and its periphery becomes the periphery of the foam hose. As the hose is drawn forward, it is cut longitudinally once or several times, flattened, and wound as a flat sheet onto a large roller located on the subsequent device. Different annular sheet dies can also be used as another special-shaped die, which has many forms

a Shenlian extrusion production line can be built in any size, but the most common ones are: the first extruder is 3.5in, and the second is 4.5in; The first is 4.5in, and the second is 6.0in. The control system of this extrusion production line is more extensive than the usual sheet extruder or film extruder. Because the two extruders are interconnected, the intentional control and monitoring of the driving part is particularly strict. Since foaming is extremely sensitive to pressure and temperature changes, pressure and temperature parameters must also be carefully controlled. Due to such complexity, many foam extrusion systems are currently equipped with microcomputer processing operations, which often include measurement systems to monitor thickness and density

application

as previously pointed out, food packaging is the largest user of foamed plastics. Polystyrene is the most commonly used polymer. Dishes and dishes for meat, poultry and agricultural products packed with polystyrene are the best examples of foam packaging materials. The thickness of these discs and the like is usually 0.090 ~ 0.130in, which can be manufactured by hot forming of sheets with a density of 4 ~ 71B/fi3. The heat preservation performance of polystyrene products makes it a natural material for the fast food industry. In this application, the foam must have a delicate surface so that it can be printed. Another application is egg packaging board. There are also various utensils made of polystyrene foam that can be discarded after use, which are divided into quick cooling and slow cooling products

polyethylene foams are made into low-density (1-7 lb/FT3) outer packaging materials for protection in the transportation of furniture, glassware and precision electronic products. When extruded into tubular sections, polyethylene foam is an excellent insulation material for air conditioning and water pipes

low density polyethylene (2.0 lb/FT3) can be extruded into plates through slit die, which can be used as thermal insulation materials in the construction industry. Thin, high-density styrene sheets are used as part of the automotive roof pommel composite structure

engineering plastics are also molded into foamed plastics to reduce weight. When molded into foam materials, these engineering plastics can also provide valuable mechanical properties and heat resistance

source: Chinese polymers