How to improve the heat resistance of pet hot filling bottle
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How to improve the heat resistance of pet hot filling bottle

  • Release date: June 6, 2017 10:01 reading times: 468
  • This paper analyzes some variables that need to be considered in the design and selection of pet hot filling bottle, and puts forward some technological methods to improve the heat resistance during the molding process of pet hot filling bottle.
    1、 Preface
    In the production of low acidity, neutral drinks, such as tea, fruit juice, fruit flavored water, etc., the semi-finished products and bottles after filling must be sterilized to control the microbial contamination of the products. The filling methods available are as follows:
    1. Add preservatives;
    2. Aseptic or cold sterilized filling;
    3. Hot filling (except for aerated drinks);
    4. Pasteurization.
    In recent years, hot filling technology has been widely used in China's beverage industry because of its good safety and economy. The following is the summary and analysis of the working principle of one-step hot filling bottle making machine and hot filling equipment in practical work.  
    2、 Brief introduction of hot filling process
    In the hot filling process, the product is subjected to UHT ultra-high temperature sterilization treatment (instantaneous heating to 1200c-1400c, staying for tens of seconds), and then cooling to the filling temperature (850c-900c). After filling and capping, the bottle body is inverted or lying on its side for about 30 seconds, so as to sterilize the bottle cap and bottle neck with the same temperature as the bottle body. After staying at high temperature for a certain time (30 ~ 120 seconds), the bottles are sent to the cooling channel, and the bottles are cooled to 340c ~ 380c in sections (the time through the cooling channel is about 12 ~ 20 minutes), and then the bottles are labeled, packed and packaged.
    3、 Key points of heat resistant bottle shape design
    The following factors must be considered when designing the heat-resistant bottle shape:
    1. Within 30 seconds after filling, the positive pressure in the bottle rises. This is because:
    (1) After filling, the temperature of residual air in the bottle increased from about 300C to 800C ~ 900C;
    (2) The PET bottle with biaxial orientation shrinks and volume decreases after being heated. At high temperature, the bottle must be able to withstand a positive pressure of 0.1 ~ 0.3 bar without permanent deformation.
    2. The volume shrinkage of PET bottle at high temperature. The shrinkage of ordinary PET bottle can reach 20% at 850c. However, the shrinkage of heat-resistant bottles made of PET particles is usually between 1% and 1.5%;

    3. Change of shrinkage and filling point after filling. The higher the filling temperature. The larger the volume shrinkage of the bottle. The experimental results show that the volume shrinkage between 860c and 900C is particularly sensitive to temperature rise. The lower the filling point height is, the larger the residual air volume in the bottle after filling, and the greater the shrinkage of the bottle. This is because the larger the residual air volume in the bottle, the smaller the ability to resist the shrinkage deformation of the bottle. Generally, the hot filling filling point is at the bottle support ring.

    Figure 1 Relationship between volume shrinkage and filling temperature of hot filling bottle
    4. After blow molding, the cooling time of cycle blowing has an effect on the volume, crystallinity and stiffness of the bottle. The longer the cooling time is, the larger the bottle volume is. Therefore, it is necessary to consider the use of minimum high pressure air consumption to achieve the best performance of the bottle when designing the bottle shape.
    5. When the hot filled beverage drops to room temperature, the liquid volume in the bottle decreases by about 2% due to the change of specific gravity at different temperatures. At the same time, the decrease of temperature also leads to the increase of solubility of residual air in the liquid. All these lead to the volume expansion of the residual air in the bottle, resulting in a negative pressure of 0.2 ~ 0.3 times atmospheric pressure. After filling, the relationship curve between the pressure of the cavity part (residual air) and the bottle temperature and time is shown in the figure below.

    Figure 2 Relationship curve between pressure of cavity part (residual air) and bottle temperature and time after filling. Therefore, heat-resistant bottle must meet the following requirements:
    (1) At high temperature (850c ~ 900C), the volume shrinkage was between 1% ~ 1.5%;
    (2) More than 30% crystallinity ensures good heat resistance and weak water adsorption capacity (the water adsorbed on the bottle wall reduces the mechanical properties of the bottle like the lubricant between molecular chains;
    (3) Reasonable wall thickness distribution can avoid nonlinear shrinkage (deformation) after hot filling;
    (4) The bottle body adopts a special plate and frame structure design, and the bottom of the bottle is provided with a concave reinforcing rib to withstand the negative pressure in the bottle when the bottle is cooled to room temperature.
    4、 Measures to improve the heat resistance of bottles in bottle making process
    (1) Reasonable design of preform. The optimized preform shape design helps to improve the thickness distribution of the bottle and avoid distortion or shrinkage in different regions of the bottle body;
    (2) The process parameters of injection and drawing blow molding as well as the temperature distribution in each region should be strictly controlled to avoid the deformation of bottle caused by the release of residual stress at the glass transition temperature (> 750C);

    (3) Cooling time control of bottle blank injection. Strictly control the injection cooling time of the preform, so that the preform can be demoulded as soon as possible. In this way, the molding cycle can be shortened, the bottle yield can be increased, and spherical crystallization can be induced due to higher residual temperature. The crystal diameter of spherical crystal is very small, only 0.3 ~ 0.7 μ m, which does not affect the transparency.

    Fig. 3 Relationship between crystallinity, volume shrinkage and injection cooling time of thermosensitive bottle
    (4) Application of temperature regulation technology for blow molding mold. The hot oil circulation method is usually used to warm the blow mold. There are three cycles in the temperature regulation of blow molding mould: - hot oil circulation of bottle body. Heat the blow mold to 1200C ~ 1400C. In this way, the temperature difference between the preform and the blow mold cavity decreases, and further crystallization is promoted. By prolonging the blow molding holding time to make the bottle wall contact with the mold cavity for a long time, there is enough time to improve the crystallinity of the bottle body to about 35%, without sacrificing the transparency. The mold temperature below 1000C has little effect on the crystallinity of the bottle body, because the crystallization occurs above 1000C.

    Figure 4 Relationship between crystallinity of heat resistant bottle and temperature of blow molding mold
    ——Cooling water circulation at the bottom of the bottle. Keep low temperature (100C ~ 300C) at the bottom of the bottle to avoid excessive crystallization and whitening of the non stretched bottle bottom< Br > < br > -- bottleneck temperature regulation (optional). After demoulding from the injection mold, the amorphous bottle mouth has been completely cooled. Most of the amorphous bottle mouth is designed to strengthen the bottle mouth (increase the wall thickness of the bottle mouth), so as to improve the sealing performance and avoid the deformation of the bottle mouth during the capping process. Generally, the ovality of the bottle mouth is controlled within 0.2 mm after filling, and the shrinkage rate of the outer diameter of the thread is less than 0.6%.
    (5) Circulation blowing technology. How to control the deformation of bottle after demoulding is very important when using hot blow molding mold. Before opening the blow molding mold, air is blown in and the bottle body is cooled and shaped to control the deformation after demoulding. The inlet air of the circulating cooling air passes through the same channel as the primary blowing and secondary blowing, but exhausts from the small hole in the head of the drawing rod through the rod. The cycle blowing time is about 0.5 ~ 2 seconds. Therefore, the high pressure air consumption of the heat-resistant bottle making machine is much higher than that of the ordinary bottle making machine.
    With the further enrichment of practical experience, the production technology of hot filling bottling is also developing by leaps and bounds. The main development trends include the continuous reduction of bottle weight without lowering the filling temperature and the wide application of amorphous bottle mouth. In order to meet the concept of environmental protection, the new bottle with fashionable and concise appearance is increasingly favored by consumers.
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