1 equipment

With the continuous progress of science and technology and the scale of production, the automation degree of PET bottle blowing machine is getting higher and higher, 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. The operation has also developed from manual button type in the past to full computer control now, which greatly reduces the difficulty of process operation and increases the stability of process.

2 Blow molding process

The important factors affecting the blow molding process of PET bottles are bottle blank, heating, pre-blowing, mold and environment.      

 2.1 bottle blank

When preparing blow-molded bottles, firstly, PET chips are injection molded into bottle blanks, which requires that the proportion of secondary recycled materials should not be too high (below 5%), the recycling times should not exceed two times, and the molecular weight and viscosity should not be too low (molecular weight 31,000-50,000, intrinsic viscosity 0.78-0.85 cm3/g). Injection molded bottle blanks can only be used after being stored for more than 48h. Bottle blanks that have not been used up after heating must be stored for more than 48 hours before they can be reheated. The storage time of bottle blanks should not exceed six months.  

The quality of bottle blank depends largely on the quality of PET material, so we should choose materials that are easy to inflate and set, and make reasonable bottle blank forming process. The experiment shows that the bottle blank made of PET material with the same viscosity is easier to blow mold with imported raw materials than with domestic materials. And the same batch of bottle blanks, different production dates, blow molding process may also be quite different. The advantages and disadvantages of the bottle blank determine the difficulty of the blow molding process, and the requirements for the bottle blank are purity, transparency, no impurities, no color, appropriate injection point length and surrounding halo spots.  

2.2 heating

The heating of the bottle blank is completed by a heating oven, and its temperature is set manually and automatically adjusted. In the oven, far infrared rays emitted by far infrared tubes radiate and heat the bottle blanks, and the fan at the bottom of the oven carries out thermal circulation to make the temperature in the oven uniform. The bottle blank rotates while moving forward in the oven, so that the bottle blank wall is heated evenly.  

The arrangement of lamps in the oven is generally in the shape of "zone" from top to bottom, with more than two ends and less in the middle. The heat of the oven is controlled by the number of lamps opened, the overall temperature setting, the oven power and the heating ratio of each section. The opening of the lamp tube should be adjusted in combination with the pre-blowing bottle.  

In order to make the oven work better, it is very important to adjust its height and cooling plate. If it is not adjusted properly, it will easily appear during blow molding.

Defects such as expanding the bottle mouth (the bottle mouth becomes bigger) and hard head and neck (the neck material cannot be pulled open).  

2.3 Pre-blowing

Pre-blowing is a very important step in the two-step bottle blowing method, which means that pre-blowing is started while the stretching rod descends during the blow molding process, so that the bottle blank takes shape. Pre-blowing position, pre-blowing pressure and blowing flow rate are three important technological factors in this process.  

The shape of pre-blown bottle determines the difficulty of blow molding process and the performance of bottle. Normal pre-blown bottles are spindle-shaped, while abnormal ones are bell-shaped and handle-shaped, as shown in Figure 2. The causes of abnormal shape are improper local heating, insufficient pre-blowing pressure or blowing flow, etc., while the size of pre-blowing bottle depends on pre-blowing pressure and pre-blowing position. In production, it is necessary to maintain the same size and shape of all pre-blown bottles of the whole equipment. If there are differences, it is necessary to find out the specific reasons, and the heating or pre-blowing process can be adjusted according to the pre-blown bottles.  

Pre-blowing pressure varies with bottle specifications and equipment capacity, generally with large capacity and low pre-blowing pressure; The production capacity of the equipment is high, and the pre-blowing pressure is also high.

Even if bottles of the same specification are produced by the same equipment, the required pre-blowing pressure is different due to the different properties of PET materials. Glass fiber reinforced PET material can make the macromolecules at the bottom of the bottle orient correctly with a small pre-blowing pressure; Other bottle blanks with improper materials or molding process have a lot of stress concentration near the injection point, which is not easy to fade. If they are blown, they will often burst at the injection point or leak from the injection point during stress testing. According to the orientation conditions, at this time, the lamp tube can be moved 2-3 branches above the injection point as shown, and the injection point can be fully heated to provide enough heat to promote its rapid orientation.  

Because of the time-temperature difference effect, the molding process of the heated bottle blank or the bottle blank with excessive storage time is similar. Compared with the normal bottle blank, it requires less heat and the pre-blowing pressure can be reduced appropriately.