Basic knowledge of bottle blowing

What does blow molding mean in plastic processing? It's like blowing a balloon, but a mold is added to the outside of the balloon when blowing it. Of course, this intuitive explanation is too far fetched.

PET blow molded bottles can be divided into two categories, one is pressure bottles, such as bottles filled with carbonated beverages; Another type is pressureless bottles, such as those filled with water, tea, oil, etc. Tea beverage bottles are modified PET bottles mixed with polyethylene naphthalate (PEN) or composite bottles of PET and thermoplastic polyaromatic esters. They are classified as hot bottles and can withstand temperatures above 80 ℃; Water bottles are classified as cold bottles and do not require heat resistance. Hot bottles are similar to cold bottles in terms of molding process. The author mainly discusses the molding process of pressure beverage bottles in cold bottles.

1 Equipment

With the continuous advancement of technology and the scale of production, the automation level of PET blow molding machines is becoming higher and the production efficiency is also increasing. The production capacity of equipment continues to improve, from producing thousands of bottles per hour in the past to now producing tens of thousands of bottles per hour. The operation has also evolved from manual button based in the past to fully computer-controlled, greatly reducing the difficulty of process operation and increasing the stability of the process.

2 Blow molding process

PET bottle blow molding process flow.

The important factors affecting the PET bottle blow molding process include preform, heating, pre blowing, mold, and environment.

2.1 Bottle preform

When preparing blow molded bottles, PET slices are first injection molded into preforms, which require that the proportion of secondary recycled materials should not be too high (less than 5%), the number of recycling times should not exceed two, and the molecular weight and viscosity should not be too low (molecular weight 31000-50000, intrinsic viscosity 0.78-0.85cm3/g). Injection molded preforms need to be stored for at least 48 hours before they can be used. Bottles that have not been used up after heating must be stored for at least 48 hours before they can be reheated and reused. The storage time of bottle preforms cannot exceed six months.

The quality of bottle preforms largely depends on the quality of PET materials. Materials that are easy to blow and shape should be selected, and a reasonable bottle preform forming process should be developed. Experiments have shown that bottle preforms made of PET material with the same viscosity are easier to blow mold from imported raw materials than from domestic materials; However, for the same batch of preforms, there may be significant differences in the blow molding process due to different production dates. The quality of the bottle blank determines the difficulty of the blow molding process, and the requirements for the bottle blank are purity, transparency, no impurities, no discoloration, appropriate injection point length, and appropriate surrounding halo spots.

2.2 Heating

The heating of the preform is completed by a heating oven, and its temperature is manually set and automatically adjusted. The far-infrared lamp emits far-infrared radiation to heat the preform in the oven, and the bottom fan of the oven performs thermal circulation to ensure uniform temperature inside the oven. The preform rotates while moving forward in the oven, ensuring even heating of the preform wall.

The arrangement of light tubes in the oven generally forms a "zone" shape from top to bottom, with more at both ends and less in the middle. The heat of the oven is controlled by the number of light tubes turned on, the overall temperature setting, the oven power, and the heating ratio of each section. The opening of the lamp tube should be adjusted in conjunction with the pre blowing bottle.

To make the oven function better, it is important to adjust its height, cooling plate, etc. If not adjusted properly, defects such as swollen bottle mouth (bottle mouth becoming larger) and hard neck (neck material cannot be pulled open) may occur during blow molding.

2.3 Pre blowing

Pre blowing is an important step in the two-step bottle blowing method, which refers to the process of starting pre blowing air while the stretching rod is lowered during the blow molding process, so that the bottle blank takes shape initially. The pre blowing position, pre blowing pressure, and blowing flow rate are three important process factors in this process.

The quality of the pre blown bottle shape determines the difficulty of the blow molding process and the performance of the bottle. The normal shape of a pre blown bottle is spindle shaped, while abnormal shapes include sub bell shaped, handle shaped, etc., as shown in Figure 2. The reasons for the abnormal shape include improper local heating, insufficient pre blowing pressure or blowing flow rate, etc. The size of the pre blowing bottle depends on the pre blowing pressure and pre blowing position. In production, it is necessary to maintain consistency in the size and shape of all pre blown bottles on the entire equipment. If there are differences, specific reasons should be identified and the heating or pre blowing process can be adjusted according to the pre blown bottle situation.

The size of the pre blowing pressure varies with the bottle specifications and equipment capabilities. Generally, larger capacities require lower pre blowing pressure; The equipment has high production capacity and high pre blowing pressure.

Even if the same equipment is used to produce bottles of the same specifications, the required pre blowing pressure may vary due to differences in PET material properties. Fiberglass reinforced PET material can achieve correct orientation of large molecules at the bottom of the bottle with minimal pre blowing pressure; For other bottle preforms with improper materials or molding processes, there is a large amount of stress concentration near the injection point that is not easy to dissipate. If blown, it often blows through at the injection point or bursts or leaks from the injection point during stress testing. According to the orientation conditions, at this time, the lamp tube can be moved out 2-3 times as shown and turned on above the injection point to provide sufficient heating at the injection point, providing enough heat to promote rapid orientation.

For bottle preforms that have been heated for secondary use or stored for too long, due to the time temperature difference effect, the forming process of the two is similar. Compared with normal bottle preforms, they require less heat and the pre blowing pressure can be appropriately reduced.

2.4 Auxiliary Machinery and Molds

Auxiliary equipment mainly refers to the equipment that maintains 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 bottle bottom is low. For cold bottles, the cooling effect at the bottom determines the degree of molecular orientation, so it is best to control the temperature at 5-8 ℃; The temperature at the bottom of the hot bottle is much higher.

Mold is an important factor affecting the PET bottle blow molding process. The shape of the mold can reduce or increase the difficulty of process adjustment, such as reinforcing ribs, the curvature of the transition zone, and the heat dissipation condition at the bottom, all of which have an impact on process adjustment.

2.5 Environment

The quality of the production environment also has a significant impact on process adjustments, and constant conditions can maintain process stability and product stability. PET bottle blow molding is generally preferred at room temperature and low humidity.

3 Other requirements

Pressure bottles should meet the requirements of both stress testing and pressure testing simultaneously. Stress testing is an internal quality control carried out to prevent cracking, leakage, and other issues caused by the degradation of molecular chains during the contact between the bottom of PET bottles and lubricants (alkaline) when filling beverages; The pressure test is a quality control carried out to prevent the bottle from bursting after being filled with a certain pressure gas. To meet these two needs, the thickness of the center point should be controlled within a certain range. Generally, the center point is thin, with good stress testing and poor pressure resistance; The center point is thick, with good pressure resistance testing and poor stress testing. Of course, the results of stress testing are also closely related to the accumulation of materials in the transition area around the center point, which needs to be adjusted based on actual experience.

4 Conclusion

The adjustment of PET bottle blow molding process is based on the corresponding materials. If the materials are poor, the requirements for the process will be very strict, and it may even be difficult to blow mold qualified bottles.