Factors affecting the process quality of bottle blowing machines

The factors that affect the quality of bottle blowing process include: preform and its heating, pre blowing (position, pressure, and flow rate), stretching rod, high-pressure blowing (pressure, position), and mold, etc

1. Bottle preform

Bottle preforms, also known as preforms, are PET pellets that are injection molded. They require a recycling ratio of no more than 10% and a recycling frequency of no more than two times. The preform after injection molding or the preform after heating must be cooled for more than 48 hours, and the storage time of the preform used cannot exceed 6 months. The preforms with different production dates, especially those with long intervals, cannot be mixed. The main reasons are the different types of raw materials used in the preform, the proportion of secondary materials mixed in, and the residual stress inside the preform. These factors have a significant impact on the blowing molding process and should be treated specifically according to the actual situation.

2. Heating up

The heating of the preform is completed by a heating furnace, which is manually set and automatically adjusted. The height of the heating furnace should be around 25mm, and it should be around 19.6mm away from the conveyor wheel. The preform runs continuously through the entire oven on the conveyor wheel, so that the preform is heated more evenly and can be better formed, overcoming the previous disadvantage of stationary heating and uneven heating caused by manual rotation. However, if the heating furnace is not adjusted properly, it can cause uneven distribution of the wall thickness of the blown bottles (light above and heavy below), excessive enlargement of the bottle mouth, hard neck and other product defects, and even torque failure of mechanical components. The company mainly produces fully automatic bottle blowing machines.

The heating temperature for bottle preforms is generally set at 85-120 ℃, higher for colorless preforms and lower for colored preforms. Setting the temperature too high or too low can cause product defects such as tearing, white mist, etc. The temperature in each area can be adjusted according to the molding situation of the product, while considering the opening of the oven light tube. In addition, the setting of the output power of the oven also has a significant impact on the heating of the preform, as it controls the overall heat output of the oven. When the device is not turned on for a long time and restarted, the initial output power should be set higher accordingly. During normal production, it should gradually decrease to the normal state. The output power is generally around 80%, especially when the ambient temperature is below 5 ℃, the effect is more significant.

The production process of blow molding is also related to the production environment temperature, and the ambient temperature is generally suitable at room temperature (around 22 ℃). If the temperature is too high, the product is prone to condensation and clumping; The temperature is too low, and the product performance is unstable when the machine starts. The specific operation should be adjusted according to the actual situation and experience.

3. Pre blowing

The role of pre blowing in the blowing process is to shape the preform and increase its longitudinal strength by stretching it longitudinally with a stretching rod. The entire process involves the pre blowing cam pushing the three-way valve to the pre blowing position during the bottle blowing process, and the one-way valve cooperating to complete it. The pre blowing position, pressure, and flow rate can all affect the quality of the bottle.

Pre blowing the position in advance may result in defects such as the center point of the bottle bottom being skewed, thinning, uneven and whitish foot wall thickness, heavy top and light bottom, hard neck, and even bottom penetration; If the pre blowing position is incorrect, defects such as light up and heavy down, thickening of the center point, and indentation may occur.

The pre blowing air flow rate is controlled by a one-way valve, and it is generally recommended to open it 3-4 times. Large airflow, heavy bottom, thin and skewed center point, white feet, uneven wall thickness; The airflow is small, the center point becomes thicker, and the weight of the segmented parts exceeds the standard.

The optimal pre blowing pressure is 0.8-1MPa. When the pressure is high, it may cause the top to be heavy and the bottom to be light, the center point to be skewed, uneven foot wall thickness, whitening, etc; When the pressure is low, it cannot be fully stretched, the bottom is heavy, and the center point is thick.

The forming condition of the bottle feet and center point has a significant impact on the quality of the bottle, and improper adjustment can often cause fatal defects such as bottle bursting (under normal experimental conditions) and leakage.

4. Stretching rod

The stretching rod is a device that stretches the heated preform while pre blowing, and is reset after high-pressure blowing and before exhaust. The stretching rod must be able to move vertically and smoothly up and down during the blowing process, with a driving pressure of 0.55-0.8MPa and a gap of 2.3-2.5mm with the bottom mold, which is 1/3-1/2 of the thickness of the bottle blank. Excessive gap can cause the center point of the bottle bottom to shift; The gap is too small, and the center point becomes thinner.

5. High pressure blowing

The function of high-pressure blowing is to fully stretch the melt, tightly adhere to the mold wall, and fully shape the bottle. At the same time, it performs lateral stretching to improve its lateral strength. The main influencing factors are location and pressure.

The high-pressure blowing pressure is generally 3.7-4MPa, which is determined by its molding characteristics and the properties of the filled beverage (carbon dioxide). There are two positions for it, namely the high-pressure gas position and the exhaust position. The time between the two is the holding time during molding, and the length of this time has a significant impact on the stability of the bottle capacity. Insufficient high-pressure air can lead to improper bottle molding and insufficient foot blowing.

6. Mold

Mold is one of the important factors affecting bottle blowing molding. The bottle body is a Haff mold, and the bottom of the bottle is independent, which is a typical blow molding mold. During the production process, the mold should be kept at a constant temperature, clean, and the exhaust holes should be unobstructed.

When the heating and cooling system of the mold malfunctions, defects such as skewed bottlenecks, tilted bottles, changes in volume and height, and heavier bottoms may occur. Maintaining a constant temperature of the mold can also prevent condensation on its surface.

The shrinkage rate of PET after molding is relatively high, usually 1.8%. After adding fiberglass reinforcement, it slightly decreases, but still reaches 0.2% -1.0%. In general, the shrinkage rate is higher at high mold temperatures and lower at low mold temperatures. In order to maintain the stability of PET bottle volume, the mold temperature must be strictly controlled. Generally, a slightly higher temperature of 20-45 ℃ is required for the bottle body, and a lower temperature of 6-15 ℃ is required for the bottom. High bottom temperature will make the bottom heavier and the center point thicker. Experts in fully automatic bottle blowing machines summarize that in actual production, nozzles and other factors can affect the formation of PET bottles.