Research on new all-plastic barrel and steel-plastic composite barrel
Research on new all-plastic barrel and steel-plastic composite barrel
Yang Wenliang
Since entering the new century, especially after China's accession to the WTO, the barrel industry has ushered in new development opportunities. Mainly steel drums are emerging in an endless stream. Many foreign standard steel drums have also been put into production and use in China. In recent years, all-plastic barrels and steel-plastic composite barrels have become more powerful, and the impact on the steel barrel market is not small.
Recently, a large-scale equipment for the production of 200 liter plastic drums imported from Taiwan has been officially put into production. At present, the blue-looking 200-liter steel drum-like plastic barrel has been seen in large and small chemical companies. The traditional steel drum seems to be less and less. This industrial name is double L-ring closed. Plastic barrel products have grown from nothing in the country, from mature to mature, and have experienced eight years of hardships and winds, and now it has become a new trend.
In recent years, steel-plastic composite barrels have developed most rapidly in China, and currently three companies are producing. The inner container is made of plastic, and the outer container is made of steel. It is popular among exporters. They are used for packaging ketchup, jam and chemical raw materials. The composite barrel has excellent chemical resistance and steel barrel for polyethylene containers. The high-strength protection function is a high-grade packaging container widely used in the fields of chemical industry and food.
Whether it is a plastic bucket or a steel-plastic composite drum, the main production link is the manufacture of plastic drums. It is different from ordinary plastic containers. Whether it is process, technology, equipment or raw materials, it is very complicated. All are imported equipment, and some raw materials also need to be imported. In fact, these technologies and equipment are capable of solving our own problems, but our steps are one step late.
The inner container of the plastic bucket and the steel-plastic composite barrel is mainly made of polyethylene as a raw material and blow molded. Compared with other plastics, polyethylene has the following main features: (1) a wide range of volume spans, from small bottles of several cubic centimeters to large storage tanks of 10 cubic meters are industrialized; (2) wide temperature range, It is used in the range of -40 ° C ~ 80 ° C; (3) resistant to a variety of chemicals, comprehensive physical and mechanical properties, and a wide range of applications.
Polyethylene blow molding has two methods of extrusion blow molding and injection blow molding, in which extrusion blow molding is dominant. Direct extrusion blow molding in extrusion blow molding for the production of medium and small polyethylene barrels; storage cylinder extrusion blow molding for the production of large and extra large barrels.
Blow molding matching of polyethylene barrel
High quality parisons are a prerequisite for successful blow molding. It is necessary to obtain a good parison by high-speed extrusion of a ring die fixed in the vertical direction, and it is necessary to grasp the basic characteristics of the polyethylene melt flow. Common problems encountered in the parison extrusion process include fish skin, Shanghai melt fracture and parison squat. The quality of the parison has a great relationship with the molecular weight, molecular weight distribution and molecular structure of the selected polyethylene material, and is also affected by the molding conditions.
Like many thermoplastics, the polyethylene melt is a non-Newtonian liquid. The relationship between the shear rate D and the shear stress T when flowing is: D = KTn
Where n is a non-Newtonian index and is related to the molecular weight of the polyethylene. If the molecular weight distribution is wide, the non-Newtonian property is increased, the value of n is increased, and the fluidity at the time of blow molding is improved. Therefore, the flow properties at the time of blow molding can be estimated by measuring the value of n.
When the shearing force is increased, the flow of polyethylene will become extremely unstable when the shear stress is increased. The surface of the extruded billet is rough and the phenomenon of heavy fish skin occurs until the irregularity of the billet is caused. The entire melt ruptured. The shear rate and shear force at which melt fracture begins to appear are called critical shear velocity and critical shear stress, respectively. The critical shear velocity and critical shear stress decrease with the increase of polyethylene molecular weight.
The shark skin phenomenon is based on the fine concave and convex skin produced by the sliding adhesion between the polyethylene melt and the die. The smaller the molecular weight (the larger the melt index), the wider the molecular weight distribution (the larger the value of n in the formula), the less likely the shark skin phenomenon will occur. The low extrusion speed also helps to overcome the phenomenon of heavy fish skin. However, if the temperature is too low, the squid skin phenomenon may occur even if the extrusion speed is low.
Both the die swell and the sill of the parison are related to the elasticity of the melt. Polyethylene for extrusion blow molding is expected to have higher elasticity at low shear rates and greater flow at high shear rates. The molecular weight of polyethylene increases, the melt elasticity increases, the die swell increases, and the squeezing of the green tube decreases. The wide molecular weight distribution has little effect on improving the elasticity of the melt at low shear rates, while the long-chain branching has a greater effect on increasing the melt viscosity at low shear rates, which helps to prevent squatting, so relatively speaking LDPE is not easy to produce the squat of the blank tube.
Blow molding conditions of polyethylene barrels and their influence on product performance
1, molding temperature
Practical polyethylene blow molding temperatures vary with polyethylene, typically from 0.1 to 210 ° C for HDPE and from 150 to 190 ° C for LDPE.
Blowing large barrels generally use lower temperatures, while blow molding small barrels generally use higher temperatures, but it should be noted that if the molding temperature is too low, it is easy to produce a blank of fish skin or melt fracture, and the temperature will be too high. The squat of the blank tube appears, resulting in a significant unevenness in the barrel wall.
2, extrusion speed
Excessively increasing the extrusion speed and lowering the molding temperature, it is easy to cause the squid skin and melt rupture in the green tube, and at the same time, the large die swell is generated, and the wall of the parison is thickened, resulting in an increase in the weight of the product, so the extrusion speed should be increased. On the premise of not producing stolen fish skin and melt fracture, it is often necessary to adjust the distance between the die core rod and the adjustment blank to maintain the weight of the product within the standard range.
3, wall thickness adjustment
Wall thickness adjustment includes circumferential wall thickness adjustment and longitudinal wall thickness adjustment. Generally, the position of the adjusting ring on the horizontal surface is moved by adjusting the screw, so that the wall thickness of the blank tube tends to be uniform, and the blank tube does not bend and moves downward in parallel. Longitudinal adjustment is accomplished by the parison wall thickness program control system. During the parison extrusion process, the hydraulic cylinder is driven by a servo valve according to a preset procedure, and the mandrel of the die is moved up and down to adjust the die gap, thereby adjusting the longitudinal distribution of the wall thickness of the blank tube. The wall thickness control of the polyethylene blow molded container can be achieved by adjusting the circumferential and longitudinal wall thickness of the extruded blank tube.
4, mold temperature
The mold temperature has an effect on the appearance, molding shrinkage and strength of the polyethylene blow molding drum. In addition, the mold temperature also affects the cycle of blow molding.
The mold temperature is high, the appearance of the polyethylene blow molding barrel is improved, but the dimensional stability is lowered, the mechanical strength (especially the impact strength) is lowered, the production cycle is prolonged, and the production efficiency is lowered. If the mold temperature is too high, it may also cause the product to be too thin in the cut-off portion, so it is advantageous to appropriately lower the temperature of the mold. However, if the mold temperature is too low, some problems may occur. For example, when the mold is in contact with the mold, the contact portion is rapidly cooled, and the parison is difficult to extend before reaching the product design shape, resulting in uneven thickness of the product.
5, blown air pressure
The pressure of the blown pressure of the LDPE is generally from 0.2 to 0.4 MPa, and the blow pressure of the HDPE is generally from 0.4 to 0.7 MPa.
Appropriately increasing the blowing pressure helps the contact between the product and the cavity wall, improves the cooling efficiency, improves the cooling setting effect and helps shorten the molding cycle, but excessive blowing pressure increases the burden on the mold closing device. Higher requirements are placed on molding equipment.
Common problems and solutions for blow molding of polyethylene barrels
(1) Forming of parison (green tube)
1, the surface is rough
(1) Shark skin and melt fluctuations. The reason for this is that the resin melt at the end of the die exceeds a certain shear rate. Common in low temperature and high speed molding. The way to overcome the fluctuation of shark skin and melt is to raise the temperature, reduce the melt viscosity or reduce the extrusion amount, enlarge the gap of the die flow path, reduce the shear rate, and improve it by selecting the resin.
(2) Surface stripes. The surface stripe, also known as the die print, is caused by scratches on the resin and the flow path, or foreign matter and burnt matter adhering to the die. The mold should be cleaned first, and when the resin temperature is too high, the die impression in the product is more obvious, so it is necessary to adjust the molding temperature.
(3) Weld wire. It is caused by poor resin joining condition, such as poor structure of the die flow path, insufficient homogenization of the resin at the junction, and aging of the resin at the junction, which may cause the weld line. In order to eliminate the weld line, an effective solution is to correct the flow path of the cross die, eliminate the stagnant flow, and homogenize the merged portion. In addition, if the resin temperature is too high, it is easy to produce a weld line, so adjusting the set temperature is often considered as a countermeasure.
2, uneven wall thickness
The wall thickness of the parison is not uniform, which causes the parison to bend, which causes perforation when blowing, and also causes a decrease in product strength. The wall thickness of the parison is not uniform. In addition to the reason that the adjusting ring is not adjusted, it may be caused by poor heating of the electric heating element or uneven temperature caused by wire breakage, which needs to be adjusted accordingly. Unusual wall thickness unevenness will increase due to excessive temperature, so care must be taken not to overheat.
For flat and other irregular shapes, it is often impossible to adjust to a uniform wall thickness by a common wall thickness adjusting ring, and it is often effective to adjust the wall thickness uniformity by using a flat die or a trimming method of cutting a part of the die.
3, squat
If the temperature of the resin is too high, it is the main reason for the melt enthalpy. However, there is also a case where the polyethylene melt index is too high and the melt tension is insufficient. In this case, a suitable resin should be used instead. In addition to adjusting the temperature and using appropriate raw materials, measures to eliminate squatting are also to increase the extrusion speed and shorten the mold closing time. The use of the wall thickness controller is very effective in controlling the uniform distribution of the longitudinal wall thickness in order to prevent uneven wall thickness caused by the squat.
(2) Closed mold blank
If the temperature is too high or the temperature is too low, sufficient joint fusion strength cannot be obtained, so it is necessary to set an appropriate resin temperature.
The fusion strength of the blank is also affected by the shape of the pinch-off line and the pinch-off speed. The angle of the cutting edge and the thickness of the blade are appropriately increased, and the molten resin at the cutting edge is raised on the inner side of the product, which helps to eliminate the shrinkage hole at the cutting edge and increase the strength at the cutting edge. The structure of the cut blank portion is somewhat different depending on the thickness of the parison, and it is also required to be compatible with the resin and molding conditions. However, when the large bucket is broken, the cut portion is easily broken, and there is sufficient fusion time at the cut blank, and it is effective to slow down when the mold is closed. By changing the deceleration position (deceleration time), the strength at the cut end can be improved.
(3) Blow molding and cooling
1, the surface is rough
The parison is not in close contact with the mold, and the bean spot appears on the surface of the product. At the same time, the mold line is concave and thin, and the appearance of the product is deteriorated because the air between the parison and the mold is not discharged. Due to the presence of air, the local air enclosed between the parison and the mold causes the portion of the parison to be slowly cooled to be thinned by its stretching. The thinning of the mold line is also a cause of the drop in strength of the product. The countermeasures for solving the above defects are: first, the surface of the cavity is sandblasted to facilitate air discharge, and the second is to design a sufficient exhaust structure for the mold surface.
2, the chamfered part of the wall thickness is evenly poor
For a blown bucket with chamfers, the wall thickness at the chamfer is always the thinnest, because the parison begins to expand after blowing in the air, but after the contact with the mold surface, the parison essentially stops expanding. At the chamfer, the parison is in contact with the cavity at the latest, the expansion time is the longest, and the expansion is the largest, so the wall thickness is the thinnest. In order to improve the uniformity of the wall thickness of the container, the most effective measure is to increase the appearance and practical use, and should use a large chamfer radius as much as possible.
3, the mold line is obvious
The poor quality of the mold processing causes the misalignment of the parting surface, the misalignment of the guide rod, and the insufficient mold closing force are all reasons for the mold line. Corresponding trimming can often eliminate the clamping line. In addition, the aforementioned poor exhaust gas may also present a mold line, which should be solved by the way of exhaust.
4, the bottom is expanded, the container is not self-standing
When the bottom of the container is not sufficiently cooled, the bottom portion expands after the former is taken out, resulting in a defect in the self-standingness of the container. The way to eliminate this defect is to use a chiller to enhance mold cooling.
5, large shrinkage rate
When the cooling is insufficient, post-crystallization occurs after the container is released from the mold, so that the shrinkage is increased. When the blow molding pressure is insufficient, the shrinkage may also increase when the parison is deformed. Therefore, when it is found that the shrinkage is too large, it should be solved from the aspects of sufficient cooling, appropriate increase of the blowing pressure, and elimination of the parison variant.
(4) Other issues
1, parison rewind
The solution to the rewinding of the parison is to reduce the extrusion speed, increase the die die gap, and increase the temperature of the punch side die or the mandrel.
2, the die expansion is too small
To solve the problem of too small expansion of the die is to reduce the temperature, prevent the parison from snagging, and increase the size of the die.
3, there are bubbles in the parison
The solution to the bubble in the parison is to dry the resin, add a splitter plate, a filter net, use a screw with a large compression ratio, and delay the melting point.
4, the emergence of "fisheye"
The solution to the "fisheye" has improved refining effects, using fine filters, cleaning extruders and dies, and replacing polyethylene raw materials.
5, the product is not shiny enough
Solutions to the lack of gloss in the product include raising the resin temperature, increasing the mold temperature, and the hot ring at the lip.
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