The PET bottle blow molding process
Take you to understand the PET bottle blow molding process
The production of polyethylene terephthalate (PET) blow-molded bottles can be divided into injection stretch blow molding (referred to as injection stretch blow) and extrusion stretch blow molding (referred to as extrusion pull blow) according to the preform of the parison. . In these two molding methods, the injection stretch-blow process is easy to control, the production efficiency is high, and the number of waste products is small, which is more common.
The production of polyethylene terephthalate (PET) blow molded bottles can be divided into injection stretch blow molding (abbreviated as injection blow molding) and extrusion stretch blow molding (called extrusion) according to different preforms. Out blow molding). In these two molding methods, the injection blowing process is easy to control, the production efficiency is high, and fewer and fewer waste products.
PET blow molding bottles can be divided into two types, one is a pressure bottle, such as a bottle filled with carbonated beverages; the other is a non-pressure bottle, such as a bottle filled with water. Tea tea beverage bottles are made with polyethylene naphthalate Ester (PEN) or PET and thermoplastic polyarylate compound bottle modified PET bottle. It is classified as a hot bottle and can withstand heat to 80 ° C or higher; cold bottles do not require heat resistance. Hot bottles are similar to cold bottles during the molding process.
The following mainly discusses the forming process of pressurized beverage bottles in cold bottles.
With the continuous progress of technology and the expansion of production scale, PET blow molding machines are becoming more and more automated and the production efficiency is getting higher and higher. The production capacity of the equipment has been continuously improved, from thousands of bottles per hour to tens of thousands of bottles per hour. This operation has also evolved from the type of manual button in the past to the current full computer control, which greatly reduces the difficulty of process operation and increases the stability of the process.
At present, the manufacturers of injection molding equipment mainly include SIRON of France and KRONES of Germany. Although the manufacturers are different, the principle of the equipment is similar, generally including the blank system heating system blowing system control system and auxiliary machinery five parts.
Blow molding process
PET bottle blow molding process. The important factors affecting the PET blow molding process are the preform heating the pre-blow mold and the environment.
Preform: When preparing blow-molded bottles, PET chips are first injection molded into preforms. This requires that the proportion of secondary recycled materials should not be too high (less than 5%), the number of recycling times should not exceed 2 and the molecular weight and viscosity should Too low (Molecular weight 31,000-50000, Intrinsic viscosity 0.78-0.85 cm3 / g). Injection molded preforms can be stored for 48 hours or more. Preheated preforms must be stored for another 48 hours before being reheated. Preforms should not be stored for more than six months. The quality of the preform is largely determined by the quality of the PET material. You can choose materials that swell easily and establish a reasonable preform molding process.
Experiments have shown that PET materials of the same viscosity are easier to blow-mold than domestic materials; although the same batch of pre-molds have different production dates, the blow molding process may also be very different. The quality of the preform determines the difficulty of the blow molding process. The requirements of the preform are pure. The length of the transparent, no impurities and no color spots and the surrounding vignetting are suitable.
Heating: The heating of the preform is completed by a heating furnace. The temperature of the heating furnace can be set manually and automatically adjusted. In the oven, far-infrared rays are emitted from a far-infrared tube to radiate heat to the preform, and a fan at the bottom of the oven performs a thermal cycle to make the temperature in the oven uniform. The preform is rotated while moving forward in the oven, so that the wall of the preform is evenly heated. The arrangement of lights in an oven is usually "area". Top-to-bottom shape, more ends and less middle. The heat of the oven is turned on by the number of lights. The total temperature setting oven power is controlled together with the heating ratio of each part. The opening of the lamp should be adjusted together with the pre-blow bottle.
For the oven to work better, it is important to adjust the height of the cooling plate. If it is not properly adjusted, defects such as hard necks (necks cannot be opened) at the mouth of the bottle (the mouth of the bottle becomes larger) during blow molding can easily occur.
Pre-blowing: Pre-blowing is a very important step in a two-step blowing method. It refers to the preblowing of the preform when the stretch rod is lowered during the blowing process, so that the preform has a shape. The pre-blowing position and pre-blowing pressure in this process are three important process factors. The advantages and disadvantages of the shape of the pre-blow bottle determine the difficulty of the blow molding process and the performance of the bottle. The normal pre-blow bottle shape is a spindle shape, and the abnormal shape is a bell-shaped handle shape, as shown in Figure 2. The abnormal shape is caused by improper local heating, insufficient pre-blowing pressure or air flow, and the size of the pre-blowing bottle depends on the pre-blowing pressure and pre-blowing position. In production, the same size and shape must be maintained for all pre-blow bottles throughout the plant. If there are differences, you need to find the specific cause, and the heating or pre-blowing process can be adjusted according to the pre-blowing conditions. Pre-blowing pressure varies with bottle size. The overall ability is great. The pre-blowing pressure is small; the equipment has high production capacity and high pre-blowing pressure.
Even if bottles of the same specifications are produced by the same equipment, the pre-blowing pressure required is different due to the differences in the properties of the PET material. Glass fiber reinforced PET material, a small pre-blowing pressure can correctly orient the polymer at the bottom of the bottle; for other pre-forms that are incorrectly used or incorrectly shaped, a large amount of stress concentration near the injection point is not easy to subside. Blows out or bursts from injection point during stress test
Depending on the orientation conditions, the lamp can be removed from the 2-3 rods as shown above to turn on above the injection point, and the point is sufficiently heated to provide enough heat to promote its orientation. For preforms that have been heated for secondary use or preforms that have exceeded storage time, the molding process is similar due to the time-temperature difference effect. Compared with ordinary preforms, less heat is required and the pre-blowing pressure is also appropriate. reduce.
Auxiliary machines and molds: Auxiliary machines mainly refer to equipment that keeps the mold at a constant temperature. Mould constant temperature plays an important role in maintaining product stability. Usually, the temperature of the bottle is high and the temperature of the bottom of the bottle is low. For cold bottles, because the cooling effect at the bottom determines the degree of molecular orientation, it is best to control the temperature at 5-8 ° C; the temperature at the bottom of the hot bottle is much higher. Mold is an important factor affecting the blow molding process of PET bottles. The advantages and disadvantages of the mold shape will reduce or increase the difficulty of process adjustment. For example, the ribs, the curvature of the transition area, and the heat dissipation at the bottom all have an effect on process adjustments.
Environment: The quality of the production environment also has a greater impact on process adjustments. Constant conditions can maintain process stability and product stability. PET bottle blowing is generally preferably performed at room temperature and low humidity.
The pressure bottle shall meet the requirements of pressure test and pressure test. The pressure test is an internal quality control to prevent cracks and leaks of the molecular chains during the contact of the bottom of the PET bottle with the lubricant (alkaline); the pressure test is to prevent the bottle from filling up. Quality control after explosion into a certain pressure gas.
In order to meet these two requirements, the thickness of the center point should be controlled within a certain range. Generally, the center point is thin, the stress test is good, and the withstand voltage is poor; the center point is thick, the voltage test is good, and the stress test is poor. Of course, the results of the stress test are also closely related to the accumulation of material in the transition zone around the center point, which is adjusted based on actual experience.
Adjust the PET blowing process for the corresponding material. If the material is not good, the process is very demanding and it is even difficult to blow out a qualified bottle.