Taizhou Fupusi Machinery Co., Ltd.
2. Poor heat resistance.
◆ Hot-filled bottles are heat-resistant in this way:
(1) Use a special mold design to withstand the negative pressure in the bottle:
① The bottle body has a rectangular concave block (which can be moved in and out on the mold) to absorb the negative pressure generated in the bottle after the liquid is cooled.
② Bottle design, use neck and waist (concave ring) to prevent the bottle from becoming oval.
③ Use bottle bottom design (usually petal-shaped) to withstand stress or carbon dioxide pressure (concave bottom design is used for high temperature sterilization bottles at room temperature).
(2) Use the high temperature oil of the hot oil machine to increase the mold temperature (the mold temperature is between 120°C and 145°C) to eliminate the internal stress caused by the stretching of the preform, increase the crystallinity of the bottle body plastic, and resist the high temperature hydrothermal fluid. The bottle is not deformed.
◆ Measures to improve the heat resistance of the bottle:
① Choose a reasonable preform and bottle design. The optimized preform shape design and bottle mold design help to improve the wall thickness distribution of the bottle and avoid distortion or shrinkage in different areas of the bottle body;
② Control the cooling time of preform injection. Strictly control the cooling time of the preform injection to allow the preform to be demolded as soon as possible. In this way, the molding cycle can be shortened and the output can be increased, and the spherical crystallization can be induced due to the higher residual temperature. The crystal diameter of the spherical crystal is very small (only 0.3mm～0.7mm), which does not affect the transparency;
③ Strictly control the injection and stretch-blow molding process parameters and the temperature distribution in each area to avoid the release of residual stress at the glass transition temperature of PET (>75°C) and lead to bottle deformation.
④The application of temperature adjustment technology for blowing molds. The hot oil circulation method is usually used to heat the bottle blowing mold. There are three kinds of cycles for the temperature adjustment of the bottle blowing mold: The bottle body hot oil cycle. Heat the blowing mold to 120°C to 145°C. In this way, the temperature difference between the preform and the blowing cavity is reduced, which promotes further crystallization. Extend the pressure-holding time of the bottle blowing, make the bottle wall and the cavity contact for a long time and have enough time to increase the crystallinity of the bottle body, reaching about 35%, but without destroying the transparency. The mold temperature below 100°C has little effect on the crystallinity of the bottle body, because the bottle body crystallization occurs above 100°C. Cooling water circulation at the bottom of the bottle. Keep the bottom of the bottle at a low temperature (10℃～30℃) to avoid excessive crystallization and whitening of the unstretched bottom part. Bottleneck temperature adjustment (optional). The non-crystalline bottle mouth part has been completely cooled after being demolded from the injection mold. Most of the non-crystalline bottle mouth adopts a strengthened bottle mouth design (increasing the wall thickness of the bottle mouth) to improve the sealing performance and avoid deformation of the bottle mouth during the capping process. Generally, the ovality of the bottle mouth after filling is controlled within 0.2mm, and the shrinkage rate of the outer diameter of the thread is less than 0.6%.
⑤ Cycle blowing technology. When using hot blow molding, how to control the deformation of the bottle after demolding is very important. Before opening the mold, air is blown into the blowing mold and the cycle is exhausted to cool and shape the bottle body to control the deformation after demolding. The air intake of the circulating cooling air passes through the same passage as the primary blowing and secondary blowing, but exhausts from the small hole in the head of the drawing rod through the drawing rod. The cycle blowing time is about 0.5 seconds to 2 seconds. Therefore, the high-pressure air consumption of the heat-resistant bottle making machine is much higher than that of the ordinary bottle making machine.
3. Large fluctuations in capacity.
Biaxially stretched PET bottles have a certain shrinkage rate, and the maximum shrinkage rate is about 2%. The main factors affecting the capacity of PET bottles are as follows:
(1) Influence of mold The capacity of PET bottles is mainly affected by mold size and shape. The size of each bottle mold is usually fixed. The shrinkage rate of bottles of different shapes will be different when designing the shrinkage rate. The less the ribs on the bottle body and the thinner the bottle thickness, the greater the shrinkage rate of the bottle.
(2) The influence of environmental factors The environmental temperature and humidity have a greater influence on the capacity of the bottle. The higher the ambient temperature and the greater the humidity, the greater the shrinkage of the bottle’s capacity.
(3) The influence of the production process. When blowing bottles with complex shapes, a higher blowing pressure is required. If the blowing pressure is insufficient, the bottle will be poorly formed and the capacity will be small; higher mold temperature will also cause the capacity to be small.
(4) Natural shrinkage of the bottle As the PET bottle shrinks naturally, the size of the bottle mold should be designed in an adjustable form (plus or minus gasket). Take a 1.5L PET bottle as an example. The average capacity of the newly produced bottle is about 1508ml. After 3 days of storage at room temperature, the bottle capacity will decrease by 5ml~6ml; as the bottle storage time increases, the bottle capacity will shrink and become difficult to control. At present, more and more production lines use in-line blowing, that is, blowing and filling, to avoid the attenuation of bottles (capacity and heat resistance).
(5) The influence of filling methods Different filling methods have different influences on volume control. The quantitative filling method has the least influence on the capacity, and the self-weight filling has the greatest influence on the capacity. For 1.5L PET bottles, the difference may be up to 20 ml to 25 ml. Therefore, to solve the bottle capacity problem, the mold (gasket) can be appropriately adjusted, the production process can be controlled, and the storage conditions should be improved. The most important thing is to shorten the bottle’s storage period as much as possible.
