Commonly used plastics processing methods, but also the development of a faster plastic molding methods. Blow molding with only negative mold (concave mold), compared with injection molding, equipment cost is lower, adaptability
Stronger, can be molded with good performance (such as low stress), can be molded with complex undulating curves (shape) of the product. Blow molding originated in the 1830s. It was not until after 1979
that blow molding came into widespread use. At this stage, blow molding level of plastics include: polyolefins, engineering plastics and elastomers; blow molded products involved in the application of automotive, office
public equipment, household appliances, medical and other aspects; hourly production of 60,000 bottles can also be manufactured large-scale blow molded parts (pieces weighing up to 180kg), multi-layer blow molding technology has been a greater development;
blow molding equipment has been used in the microcomputer, solid-state electronic closed-loop control systems, computer CAE, and other equipment. Blow molding equipment has used microcomputer, solid-state electronics closed-loop control system, computer CAE/CAM technology is also increasingly mature; and blow molding machinery more specialized, more characteristics.
1 Blow Molding Methods
1.1 Molding Methods
Different blow molding methods, due to the differences in raw materials, processing requirements, production and its cost, have different advantages in processing different products. Detailed blow molding process can be found in the literature.
The characteristics of blow molding are introduced here from a macro point of view. Blow molding of hollow products includes three main methods: extrusion blow molding: mainly used for unsupported parison processing; injection blow molding: mainly used for parison processing supported by
metal core; stretch blow molding: including extrusion a stretch and a blow molding, injection a stretch and a blow molding of two methods, which can be processed biaxially oriented products, greatly reducing the cost of production
costs and improve the performance of products. In addition, there are multi-layer blow molding, compression blow molding, dip coating blow molding, foam blow molding, three-dimensional blow molding. But 75% of the blow molded products with extrusion blow molding, 24%
with injection blow molding, 1% with other blow molding; in all the blow molded products, 75% belong to the two-way stretch products. The advantages of extrusion blow molding is high production efficiency, low equipment costs,
molds and machinery, a wide range of options, the disadvantage is the high rate of scrap, waste recycling, utilization of poor, product thickness control, the dispersion of raw materials is limited, molding must be trimmed
operations. Advantages of injection blow molding is that there is no waste in the process, can well control the wall thickness of the product and the dispersion of the material, fine-necked products with high molding accuracy, the product surface is smooth and clean, and can be economically
Small batch production. The disadvantage is the high cost of molding equipment, and to a certain extent only suitable for small blow molded products.
Hollow blow molding process conditions, the compressed air required to blow mold medium-sized billet must be clean. Injection blow molding air pressure of 0.55 ~ 1MPa; extrusion blow molding pressure of 0.2l ~
0.62MPa, and stretch blow molding pressure is often required up to 4MPa. In the plastic solidification, low pressure so that the products produce low internal stress, stress dispersion is more uniform, and low stress can improve the product
Tensile, impact, bending and other properties. 1.2 Types of products blow molded products containers, industrial parts of two categories. Containers include: packaging containers, large volume storage drums / tanks, and collapsible
containers. But with the maturity of the blow molding process, industrial parts of the blow molded products more and more, the application range is increasingly wide. Currently, containers account for about 80% of the market, growing at about 4% per year
, while industrial and structural products account for 20% of the total, with an annual growth rate of 12%. The growth in container consumption is due to the expanding range of applications for twistable plastic containers, while the growth in industrial
consumption is mainly due to improvements in new processing technologies, such as multilayer billet extrusion, biaxial extrusion, and non-axisymmetric blow molding. Table 2 lists some of the applications for blow molded products
and their performance requirements.
1.3 Advances in Blow Molding
(1) The raw material polymer in the molding process, first of all, through the mouth of the mold by high shear, and then the material shows extrusion expansion and shrinkage phenomenon, in the formation of the sagging parison, the expansion rate
near zero. Then the parison is blown up against the mold, which shows a low expansion rate. Excessive mouth mold expansion produces scrap. Excessive shrinkage leads to uneven wall thickness from the top to the bottom of the part
and in severe cases, the part cannot even be molded. Therefore, in the selection of polymers suitable for blow molding, it is necessary to find out its shear and expansion of the viscoelastic properties. HDPE has become the most widely used plastic in blow molding due to its good thermal stability and
a variety of modified products. Through *** polymerization and *** blending, the study of raw materials for blow molding in continuous extrusion of blow molding grade resins have
made some progress, such as PA6, PP and PET. Intermittent billet blow molding, theoretically suitable for structural panels and large parts of the secondary processing, requires the use of engineering plastics, such as
Flame-retardant ABS, reinforced PVC, modified PPO and PC, etc., and this type of extruded plastics are generally poor high-temperature performance, only a few resins can be in the conventional equipment blow molding large-scale
Parts. In the polyethylene naphthalene dicarboxylate (PEN)/PET *** blend blow molding, the need for oxygen permeation and water vapor permeation of resins such as ( ethylene / vinyl acetate ) *** polymer ( EVOH )
and HDPE and PET to form a composite layer, and anchor layer, in order to improve the permeability of the PEN / PET material and thermal stability. Research is currently underway to use multilayer blow molding of HDPE and PA6 to
produce fuel tanks.
(2) equipment and process technology
Blow molding machinery and equipment has been greatly improved. The newer results are:
① Re-blow molding with improved infrared heating technology;
② Very high speed rotary extrusion pressure, mainly used in the production of milk bottles;
③ Die attached to the shuttle press to compensate for the phenomenon of jetting;
④ Multi-layer continuous extrusion blow molding impermeable containers;
⑤ Through a combination of ⑤ Continuous thermoforming PET bottle production through strict control of orientation and thermal crystallization, preform blank and mold temperature, blowing pressure, and residence time of the parison in the mold cavity
.
Demand for complex, convoluted tubing components has led to the development of off-axis extrusion blow molding technology, which is loosely referred to as 3D or 3-D blow molding.
Theoretically, the process is quite simple: the parison is extruded, partially blown and attached to one side of the mold, and then the extruder head or mold rotates, following a programmed 2- or 3-axis program
. The difficulty is that large blow molding machines with very high inertia are required to close the mold at high speeds with an error of less than 10%. The multilayer blow molding process is often used to process impermeable containers, and has been modified by adding a valve system that allows the plastic feedstock to be changed during continuous extrusion, thus alternating between hard and soft products. When producing large parts such as fuel tanks or automotive exterior
structural panels, it is necessary to reduce the pressure in the mold cavity during the cooling process to adjust the processing cycle. The solution is to store the melt in a trough at the front of the extrusion screw and then extrude the parison at fairly high speeds to minimize variations in parison wall thickness, thus ensuring the elimination of droop and extrusion expansion.
Modifications to the storage cylinder head allow extrusion of heat-sensitive plastics such as ABS-R, modified PPD and PVC. Moreover, the redesigned head can be quickly assembled and disassembled for easy
cleaning of plastics during production, while the rheological characterization of plastics and computerized runner analysis allows for the design of streamlined runners to facilitate the molding of heat-sensitive plastics.
(3) control program and blow molding simulation of the program control of the parison has decades of experience.
The main issue is the parison can be pulled billet thinning the thinnest degree (such as the neck part), thickening the parison pulled billet the maximum degree (such as the container bottle or corner parts), and the design of a
Wall thickness changes in parts, such as concave edges and shoulders, etc.. The focus should be on the viscoelastic properties of the plastic used. The prediction of the wall thickness of the preform in the form of a test tube is also the basis for the selection of the optimum wall thickness for the design of a
permeability-resistant parison. This is determined by the degree of crystallinity of the preform, the temperature-dependent stress-strain elastic properties of the plastic used, and the degree and distribution of freezing stresses created by the injection molding
process. In 1980, GE developed: PITA Programming for Thermoforming and Blow Molding.
Control software for parison blow molding must take into account factors such as: uneven parison wall thickness; parison cutoffs and wrap-around blow molded pipe cutoffs; pre-blowing of the parison prior to closing the mold;
control of the blowing process and where cutoffs are opened; and the positioning of the parison edges in blow molding of structural parts. At present, the commercial blow molding simulation software is mainly the original U.S.
ACTech company's C - PITA, Belgium's POLYFLOw and so on. The difficulty of numerical simulation lies in: large strains, nonlinear material behavior, contact problems, and expansion process of some physical
non-stability, and these complexities will lead to a series of nonlinear equations that need to be solved iteratively. Among them, the study of materials, blow molding mechanism has been a difficult, hot spot in the research,
such as stretch blow molding is widely used, but the mathematical description of the stress-induced crystallization required for the simulation of the process, so far there is no suitable method. As for extrusion blow molding, the parison, which is formed when the polymer
melt flows through an annular die head, the geometry of the annular pipe and the viscoelastic nature of the material will directly affect the expansion of the parison, and the existing knowledge of viscoelasticity has not been able to describe this process.
Compared with the relatively mature injection molding CAE technology, blow molding software is currently in the early stages of development.
1.4 The development trend of blow molding
Blow molding will be with the market demand for its products, in terms of materials, machinery, auxiliary equipment, control systems, software and other aspects of the following development trends.
(1) raw materials to meet the function of blow molded products, performance (medicine, food packaging) requirements, blow molding grade raw materials will be more abundant, better processing performance. Such as PEN materials, not only strong
degree of high, heat-resistant, gas barrier, transparent, ultraviolet radiation, can be applied to blowing a variety of plastic bottles, and high filling temperature, carbon dioxide gas, oxygen barrier
excellent, and chemical resistance.
(2) Product packaging containers, industrial products will have greater growth, and injection blow molding, multi-layer blow molding will have rapid development.
(3) blow molding machinery and equipment blow molding machinery precision and efficiency; auxiliary production ( operation ) equipment automation. "Precision and efficiency" not only refers to machinery and equipment in the production molding process with
higher speed and higher pressure, but also requires the production of products in the appearance of dimensional fluctuations and fluctuations in the weight of the pieces can achieve a high degree of stability, that is to say, the production of products in all parts of the size
Inch and the shape of the geometric shape
High precision, deformation and shrinkage is small. That is to say, all parts of the product size and shape geometry, high precision, deformation and shrinkage is small, the appearance of the product and the inner quality and production efficiency and other indicators should reach a high level. Auxiliary operations include de-flashing, cutting,
weighing, drilling, leakage detection, etc., and the automation of the process is one of the development trends.
(4) blow molding simulation blow molding mechanism of more in-depth research, blow molding simulation of the mathematical model of the reasonable construction of numerical algorithms fast, accurate is the key to simulation, blow molding simulation will be
in the quality of the product prediction, control play an increasingly important role.
2 Factors affecting the quality of blow molded products and the elimination of common defects
2.1 Influence of blow molding
The following analysis of molding parameters from the blow molding process at various stages. Blow molding process can be divided into four stages:
(1) parison formation stage polymer in the extruder conveying, melting, mixing, pumping molding for the formation of the parison stage; in this stage, the main process parameters affecting the wall thickness distribution are:
1) the molecular weight distribution of the material, the average molecular weight;
2) temperature control system for blow molding machine and screw speed, which temperature control system including hopper temperature, screw speed, and the temperature control system of the blow molding machine. The temperature control system includes hopper temperature, barrel 1 zone, 2 zone, 3 zone, 4 zone temperature, flange temperature, and storage
material mold head 1 zone, 2 zone, 3 zone, 4 zone temperature.
(2) billet downstream stage billet from the gap between the mold lip and mold core extrusion for the downstream stage. At this time, the parison from the mold expansion and parison hanging the two phenomena affect the parison molding. Affect the wall thickness distribution
The main process parameters are blow molding machine die head diameter and wall thickness control system, which control system includes axial wall thickness control system and circumferential wall thickness control system, in order to adjust the die lip and the core of the gap
.
(3) The pre-blowing stage of the parison in order to avoid the contact and adhesion of the inner surface of the parison, and to improve the uniformity of the wall thickness of the product, the parison should be pre-blown. In the pre-blowing stage of the parison, from the parison below the parison to the parison
Inside the jet, in order to protect the parison, reduce its pendant stretch. In this stage, the main process parameters affecting wall thickness distribution are: pre-blowing pressure, pre-blowing time.
(4) billet high-pressure blowing stage high-pressure blowing billet, so that it is close to the mold cavity, to achieve the product plastic forming stage. At this stage, the impact of product molding is the parison by high-pressure blowing deformation, parison
contact deformation with the mold cavity. The main process parameters affecting wall thickness distribution are: material shrinkage; blowing pressure, time; mold materials, structure, mold exhaust system and mold cooling system
structure, such as cooling water distribution, cooling water inlet temperature, etc.. Although the quality of blow molded products affect more factors, but when the production conditions, product requirements are determined, adjust the blow molding process parameters can be
effective in improving product quality. Optimized process parameters can improve production efficiency, reduce raw material consumption, optimize the overall performance of the product.
2.2 Setting of blow molding process conditions
The purpose of the adjustment of process conditions is to meet the minimum wall thickness requirements of the product on the basis of the wall thickness as uniform as possible, the product weight as small as possible (reduce material consumption).
The rational method for setting process parameters is to combine experience with numerical analysis techniques. The basic process is,
①Use the established computer model, simulation of blow molds, downstream billet, clamping plate and other states;
②Input the parameters of the impact of each stage of the distribution of the billet wall thickness;
③The simulation results obtained by analyzing the computer simulation to show which parts of the wall thickness is not up to the requirements of the part wall thickness is not thick, and which parts of the wall thickness of the thicker;
④ Using manual experience, adjust the input parameters, repeat the process of ① to ③, to ensure that all parts of the product to achieve the minimum wall thickness under the premise of minimizing the wall thickness of all parts of the product as much as possible.
⑤ Analyze and compare the results of multiple process solutions and finally determine the optimized process parameters. Stretch blow molding, also known as biaxially oriented blow molding, is in the polymer in a highly elastic state through mechanical methods
Method of axial stretching of the parison, with compressed air radial blowing (stretching) parison in order to form the method of packaging containers. Stretch blow molding has a one-step method, two-step method.
2.3 blow molding common product defects and their improvement here given extrusion blow molding, injection blow molding, stretch blow molding common problems, causes and solutions.
(1) Extrusion blow molding Extrusion blow molding is the most important molding method of extrusion molding. There are two methods of continuous extrusion and discontinuous extrusion. Table 5 gives the common product defects and improvement methods of extrusion blow molding.
(2) injection blow molding injection blow molding is the first injection method to make the bottom of the billet, and then blow it into the blow mold molding hollow products. Injection blow molding can be precise control of products, can produce
No scratches, high precision, smooth surface products, without secondary processing; of which the weight of the products can be controlled at ± 0.1g, the accuracy of the thread can be ± 0.1g, the weight of the products can be controlled at ± 0.1g. The weight of the product can be controlled at ± 0.1g, and the accuracy of the thread can be ± 100 μm. Injection blow molding common product
Product defects and improvement methods are shown in Table 6.
(3) stretch blow molding
3 Conclusion
Blow molding technology is with the plastics industry, machinery manufacturing and other technological advances and continuous development, in the design of blow molded products, production process, and continue to incorporate modern design thinking
thinking, design tools, engineers and technicians should make full use of the advanced design concepts, combined with manual experience, so that the design and manufacture of products in all aspects of the design, manufacturing, and the production process. Product design, manufacturing efficiency in all aspects, thereby improving the
quality of blow molded products and market competitiveness.