Blow Mould Factory Explains PET Bottle Blowing Process
Polyethylene terephthalate (PET) is referred to as polyester. The production of PET bottles can be divided into injection stretch blow molding (referred to as injection stretch blow) and extrusion stretch blow molding (referred to as extrusion) according to the preform of the parison. Pull blow). 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.
PET blow molding bottles can be divided into two types, one is a pressure bottle, such as a bottle filled with carbonated carbonated drinks; the other is a non-pressure bottle, such as a bottle filled with water, tea, edible oil, etc. Tea beverage bottles are modified PET bottles blended with polyethylene naphthalate (PEN) or composite bottles of PET and thermoplastic polyarylate. They are classified as thermos bottles and can withstand heat above 85 ° C; water bottles are Cold bottles, no requirements for heat resistance. Hot bottles are similar to cold bottles in the forming process. The author mainly discusses the pressurized beverage bottle forming process in cold bottles.
1. Equipment With the continuous advancement of science and technology and the scale of production, the PET blow molding machine is becoming more and more automated and the production efficiency is becoming higher and higher. The production capacity of the equipment has been continuously improved, from the former production of thousands of bottles per hour to the production of tens of thousands of bottles per hour. The operation also developed from the manual button type in the past to the current full computer control, which greatly reduces the difficulty in process operation and increases the stability of the process. At present, manufacturers of PET automatic blow molding machines are mainly imported from France's SIDEL company, Germany's KRONES company, and domestically produced Fujian Quanguan Quinko Company. Although the manufacturers are different, the principles of their equipment are similar, and generally include five parts: the blank supply system, the heating system, the blowing system, the control system, and the auxiliary machinery.
2 Blow molding process PET bottle blow molding process. The important factors that affect the PET bottle blow molding process are preform, heating, pre-blowing, mold and production environment. 2.1 When preparing preforms for blow-molded bottles, PET chips are first injection-molded into preforms. It requires that the proportion of secondary recycled materials should not be too high (below 5%), the number of times of recovery cannot be more than two, and the molecular weight and viscosity must not be too low ( The molecular weight is 31,000-50,000, and the intrinsic viscosity is 0.78-0.85 cm3 / g). According to the National Food Safety Law, secondary recycled materials must not be used for food and pharmaceutical packaging. Preforms for injection molding need to be stored for more than 24 hours before they can be used. The preforms that have not been used up after heating must be stored for more than 48 hours before they can be reheated. Preforms cannot be stored for more than six months.
The quality of preforms depends largely on the quality of PET materials. Materials that are easy to inflate and shape should be selected, and a reasonable preform forming process should be formulated. Experiments have shown that preforms made from PET materials of the same viscosity are easier to blow-mold than domestic materials; while the preforms of the same batch have different production dates, the blow molding process may also be significantly different. The quality of the preform determines the difficulty of the blow molding process. The requirements for the preform are purity, transparency, no impurities, no abnormal color, suitable injection point length and surrounding halo.
2.2 Heating The preform is heated by a heating oven. Its temperature is manually set and automatically adjusted. In the oven, far-infrared light emitted by the far-infrared lamp tube heats the preform radiation, and the fan at the bottom of the oven performs thermal cycling to make the temperature in the oven uniform. The preform rotates while moving forward in the oven, so that the preform wall is evenly heated.
The arrangement of the lamp tube is generally in the shape of a "zone" from top to bottom in the oven, with two ends and less in the middle. The heat of the oven is controlled by the number of lamps turned on, the overall temperature setting, the power of the oven, and the heating ratio of each section. The opening of the lamp should be adjusted in conjunction with the pre-blow bottle.
In order to make the oven work better, the adjustment of its height and cooling plate is very important. If it is not adjusted properly, it is easy to swell the bottle mouth (the bottle mouth becomes larger), hard head and neck (the neck material cannot be pulled), etc. defect.
2.3 Pre-blowing Pre-blowing is a very important step in the two-step bottle blowing method. It refers to the pre-blowing while the stretching rod is lowered during the blow molding process, so that the preform takes shape. The pre-blowing position, pre-blowing pressure and blowing flow rate in this process are three important process factors.
The shape of the pre-blow bottle determines the difficulty of the blow molding process and the quality of the bottle. The normal shape of the pre-blow bottle is a spindle shape, and the abnormal shapes include a sub-bell shape and a handle shape. The causes of abnormal shapes include improper local heating, insufficient pre-blowing pressure or insufficient air flow, and the size of the pre-blowing bottle depends on the pre-blowing pressure and pre-blowing position. In production, the size and shape of all the pre-blow bottles of the entire equipment must be kept consistent. If there are differences, find the specific cause. The heating or pre-blow process can be adjusted according to the pre-blow conditions.
The size of the pre-blowing pressure varies with bottle specifications and equipment capabilities. Generally, the pre-blowing pressure is large and the pre-blowing pressure is small. The equipment has high production capacity and high pre-blowing pressure.
Even if the same equipment is used to produce bottles of the same specification, due to the differences in the properties of PET materials, the required pre-blowing pressures are not the same. Glass fiber reinforced PET material, small pre-blowing pressure can correctly orient macromolecules at the bottom of the bottle; for other preforms with improper materials or improper molding processes, there is a large amount of stress concentration near the injection point, which is not easy to fade. Blow molding often blows off at the injection point or bursts and leaks from the injection point during the stress test. According to the orientation conditions, at this time, the lamp tube can be moved out of 2-3 pieces and turned on above the injection point, and the injection point can be sufficiently heated to provide sufficient heat to promote its rapid orientation.
For preforms that have been heated for secondary use or preforms that have exceeded storage time, due to the effect of time and temperature, the molding processes of the two are similar. Compared with normal preforms, they require less heat and the pre-blowing pressure can be appropriate. reduce.
2.4 Auxiliary machines and molds Auxiliary machines mainly refer to equipment to maintain the constant temperature of the mold. Mold constant temperature plays an important role in maintaining product stability. Generally, the temperature of the bottle body 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 better to control the temperature at 5-8 ° C; while the temperature at the bottom of the hot bottle is much higher.
The mold is an important factor affecting the PET bottle blow molding process. The shape of the mold will reduce or increase the difficulty of process adjustment, such as the ribs, the arc of the transition zone, and the heat dissipation condition at the bottom, which will affect the process adjustment.
2.5 Environment The quality of the production environment also has a greater impact on process adjustments. Constant temperature conditions can maintain process stability and product stability. PET bottle blow molding is generally better at room temperature and low humidity.
3 Other requirements The pressure bottle should meet the requirements of stress test and pressure test at the same time. The stress test is an internal quality control to prevent cracking and leakage of the molecular chain during the contact between the bottom of the bottle and the lubricant (alkaline) during the filling of the PET bottle. The pressure test is to prevent the bottle from filling. Quality control by bursting after a certain pressure of gas is introduced. In order to meet these two requirements, the thickness of the center point must be controlled within a certain range. Generally, the center point is thin, the stress test is good, and the pressure resistance is poor; the center point is thick, the pressure test is good, and the stress test is poor. Of course, the result of the stress test also has a lot to do with the accumulation of material in the transition area around the center point, which should be adjusted based on actual experience.
4 Conclusion The adjustment of the PET bottle blow molding process is based on the corresponding material. If the material is not good, the process requirements are very harsh, and it is even difficult to blow out a qualified bottle.