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The production of PET blow-molded bottles can be divided into two types, namely, injection stretch blow molding (referred to as injection stretch blow) and extrusion stretch blow molding (referred to as extrusion blow blowing). The process of injection blowing is to inject the bottom preform first, after cooling, the conveyor belt is continuously sent to the heating furnace (infrared or electric heating), heated to the stretching temperature, and then inserted into the blow mold to perform the axis through the stretching rod. Stretching, and finally blowing blow molding. The influencing factors of the reheating and stretch blow molding of the preform preforms are discussed.
1. Most of the domestic equipment used in the country is a two-step internationally advanced blow molding machine manufactured by Germany, France and other countries. This kind of equipment has a high degree of automation, large output, low scrap rate, and easy control of the process. The equipment can hold 4-40 molds, and more than 60 can be produced. The output per hour is about several thousand, and some can reach 20,000. This kind of equipment is generally composed of five parts: blank supply system, heating system, main machine, control system and auxiliary engine. The most important one is the main machine and full computer control, including the opening and closing of the main machine, the opening and closing of the oven lamp, and the adjustment of the molding process. .
2, shadow bottle blowing process as shown in the figure.
The influencing factors in the process are: preforms and their heating, pre-blowing (position, pressure and flow), stretching rods, high-pressure blowing (pressure, position), and molds.
2.1 Preform preforms, also known as parisons, are PET pellets that have been injection molded. It requires that the proportion of recycled material not exceed 10%, and the number of recycling times cannot exceed two. The preform after injection molding or the preform after heating must be cooled for more than 48 hours, and the used preform storage time can not exceed 6 months. Different production dates, especially preforms with long intervals, cannot be mixed. The main reason is that The type of raw materials used in the preform, the proportion of the mixed secondary material and the residual stress in the preform are different, and these factors have a significant impact on the molding process of the blow-molded bottle, and should be specifically treated according to actual conditions.
2.2 The heating of the heating preform is completed by a heating furnace, which is manually set and automatically adjusted. The height of the furnace is about 25mm, which is about 19.6mm from the conveyor wheel. The preform runs continuously on the conveyor wheel through the entire oven, so that the preform is heated more evenly and can be formed better, which overcomes the disadvantages of previous heating of the preform, manual rotation and uneven heating. However, if the heating furnace is improperly adjusted, it will result in uneven distribution of the wall thickness of the blown bottles (such as light weight), defects such as large bottle openings, hard necks, etc., and even torsional failure of mechanical parts.
The preform heating temperature is generally set at 85-120°C, the colorless preform is higher, and the colored preform is lower. Setting the temperature too high or too low can cause product defects such as rags and white fog. The temperature in each area can be specifically adjusted according to the molding conditions of the product, and the opening of the oven lamp should be considered.
The setting of the oven output power also has a great influence on the heating of the preform, which controls the output of the entire oven heat. When it is not turned on for a long time, when it is turned on again, the initial output power should be set higher, and then gradually reduced to the normal state during normal production. The output power is generally about 80%, especially when the ambient temperature is below 5°C. The effect is even more pronounced.
Blowing bottle production process and production environment temperature also have a certain relationship, the ambient temperature is generally at room temperature (around 22 °C) is appropriate. If the temperature is too high, the product is prone to condensate agglomeration; the temperature is too low, the product performance is unstable when the machine is started, and the specific operation must be adjusted according to the actual situation and experience.
2.3 Pre-blowing The role of pre-blow in the blowing process is to make the preform take shape. At the same time, longitudinal stretching of the stretching rod increases the longitudinal strength. The entire process is the pre-blown cam pushes the three-way valve to the pre-blow position during the blowing process and is completed by a one-way valve. Pre-blowing position, pressure and flow can all affect the quality of the bottle.
(1) If the position of the pre-blowing position is advanced, the central point of the bottom of the bottle will be deflected and thinned, the wall thickness of the foot will be uneven and white, the weight will be light, the hard neck, and even the bottom will penetrate; the position of the pre-blowing will be wrong After that, there will be defects such as lighter and heavier weights, thicker center points, and depressions.
(2) Air flow The pre-blowing air flow is controlled by a one-way valve, which is generally 3-4 turns. The air flow is large, the bottom is heavy, the central point is thin, partial, the foot is white, and the wall thickness is uneven; the air flow is small, the center point becomes thicker, and the sectional pieces are overweighted.
(3) The pressure pre-blowing pressure is preferably 0.8-1 MPa. When the pressure is high, it may cause the weight to be lighter, the center point may be skewed, the foot may be uneven in wall thickness, white, etc. When the pressure is low, it cannot be fully stretched, the bottom is heavy, and the center is thick.
The molding conditions of the bottle's feet and center point have the greatest impact on the quality of the bottle. Improper adjustment often leads to fatal defects such as bottle burst P (normal experimental conditions) and leakage.
2.4 Stretch rod Stretch rod is a device that stretches the heated preform under pre-blowing together with pre-blowing. It is reset after high pressure blowing and before venting. The stretching rod must be able to move up and down vertically and smoothly during the blowing process. The driving pressure is 0.55-0.8 MPa, and the clearance with the bottom mold is 2.3-2.5 mm, ie the thickness of the preform is 1/3-1/2. If the gap is too large, the center point of the bottom of the bottle may be shifted. When the gap is too small, the center point becomes thin.
2.5 The role of high-pressure blowing high-pressure blowing is to make the molten material fully stretched, close to the mold wall, so that the bottle is fully formed, while the transverse stretching, to increase its lateral strength. The main factors affecting are location and pressure.
The high pressure blowing pressure is generally 3.7-4 MPa, which is determined by its molding characteristics and the nature of the filled beverage (carbon dioxide). There are two positions, namely, the position of the high pressure gas and the position of the exhaust gas. The time between the two is the pressure holding time during the molding. The length of time has a great influence on the stability of the bottle capacity. Insufficient high-pressure gas makes it easy to improperly shape the bottle and insufficiently blow the foot.
2.6 mold mold is one of the important factors affecting the bottle blow molding. The bottle body is Huff mould and the bottom of the bottle is independent. It is a typical blow mold. During the production process, it is necessary to keep the mould constant temperature, cleanliness, and vents open.
When the mold heating and cooling system fails, bottleneck defects, bottle tilt, volume, height variation, and bottom-heavy defects are maintained; maintaining the mold temperature also prevents condensation on the surface of the mold.
The shrinkage of PET after molding is relatively large, generally 1.8%. After adding glass fiber, the shrinkage slightly decreases, but it still reaches 0.2%-1.0%. In general, high mold temperature shrinkage rate and low mold temperature shrinkage rate are small. In order to maintain the PET bottle volume stability, the mold temperature must be strictly controlled. The bottle body temperature is generally required to be slightly higher at 20-45°C and the bottom requirement is lower at 6-15°C. A high bottom temperature will make the bottom heavier and the center point thicker.
In actual production, there are nozzles and other factors that affect the molding of PET bottles.
3. Quality standards for bottles PET bottles of different volumes have standards for height, wall thickness, weight of each piece, volume error, and center point thickness. A good PET bottle should have a smooth, transparent appearance, appropriate wall thickness, straight bottle necks, centered center points, uniform wall thickness on the feet, no hard necks, bottle necks, split bottle mouths, pits, white fog, and hard rings. Such defects, and the bottle does not burst under the specified conditions, no leakage.
The specified condition is that a gas with a pressure of about 0.6 MPa is put in the bottle, sealed and put into a 20% NaOH solution for about 20 minutes, or a gas with a pressure of about 0.4 MPa is generated in the bottle, and the gas is sealed after being capped and sealed. , Put it in 20% NaOH solution for more than 30 minutes, no burst, no leakage. Whether the bottle bursts or leaks under the above conditions is a key indicator to check the quality of the bottle. In order to maintain the invisibility and impermeability of the bottles, it is also required that the storage facilities of the beverage manufacturers should be kept in a ventilated, dry and cold state.
According to the author's practice, the thickness of the center point is 1.4-1.9mm, the center is not biased, the wall thickness of the foot is uniform, and there is no white fog. Under the condition that the weight of the bottom part is appropriate, the popping and leakage phenomenon will not occur. If there is still a defect, The issue of raw materials for preforms should be considered.
In recent years, PET blow-molded bottles have been widely used in beverage packaging due to their light weight (1/10 of that of equivalent glass bottles), high strength, transparency, and non-toxicity, especially in the packaging of carbonated beverages (such as Coke, etc.) It is a sudden rise. In addition, due to the repeated accidents of explosions and injuries caused by the bundled glass beer bottles, foreign countries have already turned to PET bottles for packaging. Therefore, the market for PET blown bottles is promising.