Important factors affecting PET bottle blow molding

Plastics are closely related to us in life and are everywhere. For example, mineral water bottles and beverage bottles. The materials of mineral water bottles and beverage bottles are mainly PET.

PET bottles are manufactured with PET as raw material. The production of blow-molded bottles can be divided into injection stretch blow molding (abbreviated as injection stretch blow molding) and extrusion stretch blow molding (abbreviated as extrusion stretch blow molding) according to the different preforms of the preforms.

① Bottle blanks: When preparing blow-molded bottles, PET slices are first injection molded into bottle blanks. It requires that the proportion of secondary recycled materials cannot be too high (less than 10%), the number of recycling times cannot exceed two times, and the molecular weight and viscosity cannot be too low (molecular weight 31000-50000, intrinsic viscosity 0.78-0.85cm3/g). Injection molded bottle blanks need to be stored for more than 48 hours before they can be used. Bottle blanks that are not used up after heating must be stored for more than 48 hours before they can be reheated and used. The storage time of bottle blanks cannot exceed six months. The quality of the preform depends largely on the quality of the PET material. Materials that are easy to blow and easy to shape should be selected, and a reasonable preform molding process should be formulated. Experiments show that for preforms of the same viscosity of PET materials, imported raw materials are easier to blow mold than domestic materials; and the same batch of preforms may have different production dates and blow molding processes. The quality of the preform determines the difficulty of the blow molding process. The requirements for the preform are purity, transparency, no impurities, no different colors, and the length of the injection point and the surrounding halo are appropriate. 

② Heating: The heating of the preform is completed by the heating oven, and its temperature is set manually and adjusted automatically. The far-infrared lamp in the oven emits far-infrared rays to radiate and heat the preform, and the fan at the bottom of the oven performs heat circulation to make the temperature in the oven uniform. The preform moves forward in the oven while rotating, so that the wall of the preform is heated evenly. In order to make the oven play a better role, the adjustment of its height, cooling plate, etc. is very important. If the adjustment is improper, defects such as bulging bottle mouth (bottle mouth becomes larger) and hard neck (neck material cannot be pulled open) are prone to occur during blow molding.

③ Pre-blowing: Pre-blowing is a very important step in the two-step bottle blowing method. It refers to the pre-blowing process that starts at the same time as the stretching rod descends during the blow molding process to give the bottle blank an initial shape. In this process, the pre-blowing position, pre-blowing pressure and blowing flow rate are three important process factors.The size of the pre-blowing pressure varies with the bottle specifications and equipment capacity. Generally, the larger the capacity, the smaller the pre-blowing pressure; the higher the equipment production capacity, the higher the pre-blowing pressure.For bottle blanks that have been heated for secondary use or bottle blanks that have been stored for an extended period of time, due to the time-temperature isotropic effect, the molding processes of the two are similar. Compared with normal bottle blanks, they require less heat and the pre-blowing pressure can be appropriately reduced.

④ Auxiliary machines and molds: Auxiliary machines mainly refer to equipment that maintains a constant temperature of the mold. Constant temperature of the mold plays an important role in maintaining the stability of the product. Generally, the temperature of the bottle body is high and the temperature of the bottle bottom is low. For cold bottles, since the cooling effect at the bottom determines the degree of molecular orientation, it is best to control the temperature at 5-8℃; while the temperature at the bottom of hot bottles is much higher.

⑤ Environment: The quality of the production environment also has a great impact on process adjustment. Constant conditions can maintain the stability of the process and the stability of the product. PET bottle blow molding is generally best at room temperature and low humidity.

⑥ Other requirements: Pressure bottles should meet the requirements of stress testing and pressure resistance testing at the same time. Stress testing is an internal quality control to prevent the degradation of molecular chains during the contact between the bottom of the bottle and the lubricant (alkaline) when the PET bottle is filled with beverages, resulting in cracking, leakage, etc.; pressure resistance testing is a quality control to prevent the bottle from bursting after a certain pressure gas is filled. In order to meet these two needs, the thickness of the center point should be controlled within a certain range. Generally, if the center point is thin, the stress test is better and the pressure resistance is poor; if the center point is thick, the pressure resistance test is better and the stress test is poor. Of course, the results of the stress test are also closely related to the accumulation of materials in the transition area around the center point, which should be adjusted according to actual experience.

As discussed, the success of PET bottle blow molding is influenced by several critical factors — including preform quality, temperature control, stretch-blow ratio, and mold precision. Even minor deviations in any of these areas can lead to defects such as uneven wall thickness, bottle deformation, or poor sealing performance.

To consistently achieve high-quality output, it's not just about controlling process parameters — it's also about using the right equipment. That’s why investing in a professional-grade blowing system is crucial.

If you're looking for a reliable solution tailored for bottled water production, we recommend the PET mineral water bottle blow molding machine. This machine is designed for high efficiency, stable operation, and consistent bottle quality across a range of sizes. Whether you're starting a new bottling line or upgrading an existing one, it offers excellent value and long-term performance.

Choose precision, choose stability — choose the PET mineral water bottle blow molding machine for your production line.