What Is Blow Molding ?

Blow molding is one of the processes of plastic container molding

Blow molding is to put a tube or parison in a semi-molten state obtained by extrusion or injection molding into a mold, and then close the mold, and introduce a compressed gas into the parison to extend and expand it to adhere to the mold. On the wall of the cavity, after cooling and demolding, a production and processing method of a hollow product of a certain shape is obtained.

It is a fast-growing plastic processing method.

 

According to the production method of the parison, the blow molding can be divided into extrusion blow molding and injection blow molding, and newly developed ones have multi-layer blow molding and stretch blow molding.

 

1. EXTRUSION BLOW MOLDING

Extrusion blow molding is one of the most used methods for forming packaging containers.

Extrusion blow molding is a process in which a thermoplastic raw material is plasticized by an extruder and then passed through a specific die to obtain a tubular parison in a hot melt state. When the tubular thermoplastic parison still in a softened state reaches a predetermined length After entering the mold, the mold is closed, and then the compressed gas is introduced, and the blank is deformed along the cavity by the pressure of the gas, thereby blowing into a hollow product having a short neck opening.

ADVANTAGES AND DISADVANTAGES OF THE EXTRUSION BLOW MOLDING METHOD:

  • High production efficiency.
  • The parison temperature is uniform.
  • The allowable range of the shape, size and wall thickness of the hollow container is large.
  • Strong adaptability.
  • Blow molded products have higher hardness.
  • Simple equipment and low investment.
  • The precision of the product is not high.
  • The inner wall of the thread changes with the change of the outer wall thread.
  • There is a seam at the bottom of the container.

Most HDPE bottles are formed by extrusion blow molding, such as shampoo bottles, shower bottles, and the like.

 

2. INJECTION BLOW MOLDING

Injection blow molding is another common way to produce hollow products. It is also the most common way to process plastic products except extrusion, injection molding and molding, and it has a rapid development trend and belongs to the category of secondary molding technology.

Injection blow molding is to use the injection molding machine to inject the parison onto the mandrel. After the parison is properly cooled, the mandrel and the parison are simultaneously fed into the blow mold, the blow mold is closed, and the mandrel is guided. The gas is compressed to inflate the parison to form the desired product, and the product is removed after cooling molding.

The two steps of making and inflating the parison can be done separately.

ADVANTAGES AND DISADVANTAGES OF THE INJECTION BLOW MOLDING METHOD:

  • The product has no stitching seam and does not need to be trimmed later.
  • The thread and the mouth are of high dimensional accuracy and the inner wall of the neck is a smooth cylindrical surface.
  • Production can be enormous.
  • There are fewer auxiliary machines.
  • The bottom end of the product is high in hardness.
  • Less raw material consumption.
  • The wall thickness is even.
  • Production is fast.
  • The investment in machinery and equipment projects is large.
  • Production, processing and manufacturing cycle time is longer.
  • High requirements for operating technicians.
  • The shape can’t be too complicated.
  • Container specifications are limited (the height of the container should not be too high).

Suitable for the production of small high-precision containers. For example, medicinal plastic bottles and jars are often formed in this way.

 

3. STRETCH BLOW MOLDING

Stretch blow molding is a molding method in which a preform bottle preform has been formed, a bottomed parison heated below the melting point is placed in a mold, and a stretched rod is used for axial stretching and then blown immediately. At the same time of blow molding, large molecules are aligned and fixed in the wall of the product, thereby greatly improving the performance of the plastic product.

ADVANTAGES AND DISADVANTAGES OF THE STRETCH BLOW MOLDING METHOD:

  • The scrap rate is small.
  • Production speed is fast.
  • The weight is easy to control.
  • The product has high impact toughness and high rigidity.
  • Transparency and brightness are improved.
  • Good barrier and sealing.
  • Stretch temperature control is required to be high.
  • The investment in machinery and equipment projects is large.

Most PP and PET bottles are formed by stretch blow molding, such as cola bottles.

 

4. MULTI-LAYER BLOW MOLDING

Multi-layer blow molding is a multi-layer composite parison that is produced by a blow molding process to produce a multilayer container. Through the reasonable matching of various plastic layers, the performance of each layer of plastics can be effectively complemented, thereby avoiding some inherent defects of single-layer plastic containers.

ADVANTAGES AND DISADVANTAGES OF MULTI-LAYER BLOW MOLDING METHODS:

It can improve the physical properties of plastic products and improve the performance of containers, such as light blocking, heat insulation, flammability, adaptability and so on.

 

BLOW MOLDING HISTORY

  • In the 1930s, automated blow molding machines based on glass blow molding technology were introduced and patented for inventions.
  • In 1949, the Normmt and Reinold Hagen brothers in Germany produced processing methods and equipment for the production of plastic bottles from thermoplastics. In May 1950, they obtained German patents. Since then, extrusion molding technology began to develop in Europe; During the Second World War, blow molding began to produce low-density polyethylene vials.
  • In the late 1950s, with the advent of HDPE and the development of blow molding machines, blow molding technology has become more widely used.
  • In the 1980s, the use of blow molding was rapidly developed. The volume of hollow containers reached several thousand liters, and some manufacturing has taken computer control.

RAW MATERIALS FOR BLOW MOLDING

The most common raw materials for blow molding include engineering plastics such as PE, PP, PVC, PA, PC and ABS, including elastomers. Among them, HDPE is the most widely used, and the obtained hollow container is widely used as an industrial plastic packaging container. The application of products involves automotive, office supplies, home appliances, medical equipment and other fields.

Recently, ABS has been widely accepted in the automotive industry. It is because of its mechanical strength, weather resistance, electrical properties, optical properties and other properties. Among them, automotive plastic fuel tanks are currently the largest production of hollow blow molding automotive parts. Since 1973, production has accounted for 90% of the total automotive fuel tanks.

 

BLOW MOLDING PRODUCTS

Blow molded products are widely known as bottles, barrels, cans, boxes and packaging containers for various foods, beverages, make-up products, medicines and household items. Large blow molded containers are commonly used on the packaging of chemicals, lubricants and bulk materials. Other blow molding products include balls, plastic bellows, children’s toys, baby products, and sports supplies.

The blow molding production process can be used not only to shape liquid containers of various sizes and shapes in daily life and industrial production. It can also be used to form a wide range of hollow industrial parts, especially in the automotive industry. For the automotive industry, fuel tanks, car shock absorbers, seat seats, center brackets, and handrails and headrest covers are blown. Plastic.

For machinery manufacturing and furniture manufacturing, blow molding parts have plastic casings, door frames, storage shelves, tables and chairs, boxes and so on.

Blow molding products are used in various fields such as automobiles, office supplies, home appliances, medical equipment, cultural and entertainment equipment, and home furnishing.

 

BLOW MOLDING STEP

80% of blow molded products are produced by extrusion blow molding.

The extrusion blow molding production process consists of 5 steps:

  1. Extruding a tubular plastic parison;
  2. Put the parison into the mold to close, lock the mold and cut the parison;
  3. Injecting compressed air into the cavity to blow the parison and maintaining a certain air pressure during cooling.
  4. Open the mold and take out the blown finished product;
  5. Repair the flash of the product to make it smoother without obvious burrs.

P CONTROL OF BLOW MOLDING PRODUCTS

In the production and processing of blow molded products, the control of the wall thickness of the parison is very important for improving the quality of blow molding products and reducing the cost of blowing materials.

Therefore, early blow molding equipment has used two-stage oil pressure to adjust the two positions of the core shaft clearance, but only thick or thin.

In order to meet the increasingly high precision and stability requirements of blow molding products, the modern blank wall thickness control system has been equipped with servo oil pressure system and computer to establish multi-stage adjustment, which greatly improves the quality of blow molding products. .

The blow molding products are precisely controlled during the blow molding process, and the wall thickness of the blank of the finished product can be made uniform, resulting in a blow molded product having higher impact resistance.

The test found that the blow molding products with the same wall thickness not only reduced the amount of raw materials used, but also had higher impact rigidity.

Therefore, by adding a wall thickness control system to the blow molding machine, the blow molding raw material consumption can be reduced, the production volume can be increased, and the cooling time of the finished product can be shortened, thereby improving the overall manufacturing profit.

If the blow molding product does not have a controlled wall thickness during the blow molding process, the thickness of the blank wall after cooling may be uneven, and those thinner portions during the test may be easily cracked.

The reason is that the stresses generated by the walls of different thicknesses during cooling are different, thus reducing the quality of the product.

If the core shaft clearance can be varied with different parison positions to produce a product with a relatively uniform wall thickness, the cracking caused by the different wall thicknesses is avoided.

The purpose of the blown wall thickness control parameter adjustment is to make the wall thickness of the blow molded product as uniform as possible on the basis of satisfying the minimum wall thickness requirement of the product, and the weight of the single blow molded product is as small as possible (reducing the use amount of the blow molding material).

An effective way to set the blow thickness control parameters is to combine experience with numerical analysis techniques.

THE MAIN PROCESS IS:

  1. Simulate the blow molding working state by using the built computer model;
  2. input parameters of the influence of each link on the wall thickness distribution of the parison;
  3. Analyze the obtained simulation conclusions, and show through computer simulation which parts of the wall thickness are not up to the requirements, and which parts of the wall thickness exceed the thickness requirements;
  4. With the help of manual experience, adjust the input parameters and repeat the process of 1~3 several times to ensure that the wall thickness of each part of the blow molding product is minimized under the premise that the parts of the blow molding product reach the minimum wall thickness.
  5. analysis and comparison of the conclusions of multiple blow molding schemes, and finally the optimized blow molding parameters.

BLOW MOLDING INDUSTRY DEVELOPMENT PROSPECTS

In recent years, blow molding technology has developed rapidly, from single-layer blow molding to double-layer and multi-layer blow molding.

Blow molding production processes have emerged from extrusion blow molding, injection blow molding, and extrusion or injection stretch blow molding.

The blow molding equipment has adopted the closed-loop control system of microcomputer and solid-state electronics. The computer simulation technology has also played an increasingly important role in the blow molding process. The computer CAE/CAM technology has become more mature, making the blow molding machinery more systematic and more Unique.

The blow molding process is heading towards the production of larger and more diverse hollow articles as well as faster production speeds and higher quality.

Blow molding technology will have the following development trends in raw materials, mechanical equipment, auxiliary equipment, control systems, and software as the market demands its products.

  1. Raw materials In order to meet the functions and performances (medicine, food packaging) of blow molding products, the raw materials of blow molding grade will be more abundant, and the production and processing performance will be better. For example, PEN raw materials not only have high rigidity, good heat resistance, strong gas barrier property, transparency, and ultraviolet radiation resistance, but can be used for blowing various plastic bottles, and have high filling temperature and excellent barrier properties against carbon dioxide gas and oxygen. And resistant to chemicals.
  2. Plastic packaging containers and industrial products will have a great growth, and injection blow molding and multi-layer blow molding will have faster development.
  3. Precision and high efficiency of blow molding machinery and equipment; automatic control of auxiliary equipment. The efficient precise are not only high production speed & high pressure resistance of blow molding machine in it‘s production process. but also requires the products produced to achieve high stability in terms of dimensional fluctuations and weight fluctuations. That is to say, the dimensions and shape geometry of each part of the produced product are high, the deformation and shrinkage are small, and the appearance, internal quality and yield of the product are all at a high level. Auxiliary operations include deflashing, cutting, weighing, drilling, leak detection, etc., and the whole process of mechanical automation is one of the development trends.
  4. The research on the blow molding mechanism of blow molding is more in-depth, and the mathematical model of blow molding is rationally established. The fast and accurate numerical algorithm is the key to simulation. The blow molding simulation will be in the prediction and control of product quality prediction. Play an increasingly important role.

CONTAMINANTS PRODUCED BY BLOW MOLDING

During the blow molding process, the plastic particles will generate a small amount of organic waste gas due to high temperature volatilization, and the main component thereof is non-methane total hydrocarbon.

 


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