PET Bottle Processing And Molding Technology

Polyester plastic bottle (PET) is one of the most important packaging containers at present. It is mainly used for the packaging of medicines and carbonated beverages. Packaging with liquids such as detergents. As a pharmaceutical packaging container, polyester bottles have many advantages. First of all, polyester bottles are light in weight, easy to form and strong in body. The strength and elasticity are significantly higher than other plastic materials, and it can withstand considerable impact without damage. It is most suitable for pharmaceutical packaging bottles with thin walls, light weight, and high strength. After the PET bottle came out in the early 1980s in my country, it developed rapidly with an unstoppable momentum. In just two decades, it has become the most important liquid and solid packaging container in the world. In the application in the field of pharmaceutical packaging, under the same drug loading volume, the weight of PET bottles is only about 1/10 of that of glass bottles; for packaging bottles of the same diameter, the capacity of PET bottles is 1.5 times that of glass bottles. Transparent or opaque brown bottles can be made from PET raw materials. Secondly, medicinal PET bottles have good gas barrier properties. Among the commonly used plastic materials, PET bottles have the best water vapor and oxygen barrier performance, which can fully meet the special storage requirements of pharmaceutical packaging. PET has excellent chemical resistance and can be used for packaging of all items except strong alkali and some organic solvents. Third, the recycling rate of PET resin is higher than that of other plastics. When it is burned as waste, it is flammable because of its low calorific value of combustion, and does not produce harmful gases. Fourth, food packaging made of PET meets food hygiene requirements, because PET resin is not only a harmless resin, but also a pure resin without any additives, which has passed quite strict standards including the United States, Europe and Japan. The inspection of the Food Sanitation Law is considered to be a qualified and safe drug and food packaging material. These advantages make polyester bottles the best among plastics. At present, the world's polyester production is growing at a double-digit rate. Pharmaceutical plastic bottles made of polyester are currently one of the most ideal packaging products in terms of appearance, gloss, physical and chemical properties and quality assurance.

Processing technology of medicinal polyester bottle

1. Characteristics of medicinal polyester raw materials

The polyester (PET) raw material used for bottle blow molding is saturated linear thermoplastic polyester. The main application performance indicators are: the intrinsic viscosity (IV) should be controlled at 70-85ml/g, so that the blow-molded bottle has a high mechanical strength. Strength and transparency; for bottles with large volume (more than 2 liters), the intrinsic viscosity of polyester raw materials is 70-75ml/g. Injection blow molding pharmaceutical small-volume bottles are preferably produced with materials with higher intrinsic viscosity indexes. Whether the selection of raw material varieties is correct or not is related to the selection of molding process parameters in the production process and the quality of the bottle body. Therefore, process data such as intrinsic viscosity, crystallization temperature, cooling rate and orientation effect of raw materials are particularly important when producing pharmaceutical PET bottles. This is because the polyester material is a crystallizable polymer, and its crystallization rate is very small, only 6 μm/min (max). That is, through the process of controlling the crystallization temperature and cooling rate and other conditions, the polyester bottle becomes non-crystalline or crystalline, and the type and quantity of crystals can be changed. The required crystallinity of polyester is generally around 30%. At a temperature slightly above the glass transition temperature or slightly below the melting point, the crystallization rate of polyester is very small, and it takes a long time to crystallize. At a temperature around 175°C, the crystallization time is much shorter. In short, only by heating to crystallize polyester, the crystallization process is slow, and it is possible to form spherulites with large grains, which will refract light, making the product white and brittle. In addition, the crystallization rate of polyester decreases with the increase of its IV. Orientation can arrange polyester molecules in an orderly manner and promote crystallization. The formed crystals are called strain-induced crystals. The crystal grains are very small and do not refract light. Therefore, oriented polyester products are transparent. If the polyester sample with oriented partial crystallization is heat-set to make it further crystallized, the sample is still transparent. Therefore, crystallization and orientation are the key factors affecting the guaranteed performance of stretch-blow polyester bottle molding process. If the bottle is hot-filled and pasteurized, it is also necessary to heat-set the partially crystallized polyester to further crystallize it to improve heat resistance.

2. Dehumidification and drying of polyester materials

When processing polyester, the moisture contained in it will react with the polyester melt during the hydrolysis process and be consumed rapidly, that is, the moisture will generate bubbles in the product. Hydrolysis will reduce the intrinsic viscosity of polyester melt, and also affect the mechanical strength and performance of products, and polyester raw materials are hygroscopic polymers, so they should be strictly dried before processing to make the residual moisture content less than 0.005%. Polyester raw materials should be dried by dehumidification drying system. Because the dry hot air provided by the drying device enters the raw material from the bottom of the hopper, absorbs the moisture contained in the raw material and returns to the drying device. In order to prolong the life of the dehumidification bed in the drying device and maintain its efficiency, the humid air from the hopper should pass through the post-cooling device (circulating water cooling can be used) to keep the air temperature below 65°C. The aftercooler should be thermostatically controlled to prevent overheated air from entering the dehumidification bed. Cooled air passes through filters to remove airborne particles. The air is dehumidified by the dehumidification bed, and finally returns to the drying hopper after being heated by the heater. Conditions for drying raw materials in the drier: drying temperature is 140°C to 180°C, air dew point is -40°C, and air volume is 0.06 cubic meters/min (kg.h). The drying time is 4h. Pay attention to the above conditions: (1) When the air volume is higher than 0.06 cubic meters/min (kg.h), the operating range can be widened, the drying temperature can be lowered, and the energy consumption will be too high. (2) It is very important to ensure that the drying dew point is as low as possible, but there will be no problem when the dew point is as low as -10°C; but the air dew point should be strictly monitored, and it should be lowered in time if it is found to be too high. (3) Drying temperature is a key parameter. The optimum drying temperature can be determined by drying at various temperatures and measuring the intrinsic viscosity of the parison, generally 150°C to 163°C. When shutting down, the drying temperature should drop to about 120°C. (4) Prolonging the drying time will reduce the intrinsic viscosity of polyester raw materials. Reasonable control of drying temperature becomes the key parameter. A small increase in temperature will lead to a large decrease in the intrinsic viscosity of polyester. Therefore, the drying time should be as short as possible to ensure Broaden the operating range. During the drying process, due to the high drying temperature of polyester, the hopper of the equipment should have good heat insulation performance and use glass fiber as the heat insulation layer. The contact between the dry polyester raw material and the outside air should be avoided, because the polyester raw material will Quickly absorbs moisture from the air. For example, after the completely dry polyester raw material is in contact with air with a relative humidity of 35% to 40% for 12 minutes, the moisture content will reach 0.005%.

Molding method of medicinal polyester bottle

There are two methods of forming polyester bottles: extrusion blow molding and injection blow molding. Stretch blow molding is divided into one-step method and two-step method. In one-step molding, the molding, cooling, heating, stretching and blow molding of the parison and the removal of the bottle body are all completed sequentially on one machine. The two-step method uses extrusion or injection molding of a preform, which is allowed to cool to room temperature to become a semi-finished product, which is then reheated and formed into a bottle in a stretch blow molding machine. That is, the forming, stretching and blow molding of the parison are completed on two machines respectively. One-step injection blow molding of PET bottles requires two sets of molds in the injection blowing equipment, namely the injection parison mold and the blow mold. The injection molding parison mold is mainly composed of a parison cavity and a mandrel, and whether the size parameters of each part are selected correctly is the key to whether the bottle body can be formed. Therefore, it is necessary to reasonably select the size parameters of the mold parison in combination with the molding process.

1. The ratio of the height of the PET bottle to the diameter of the neck thread can determine the length-to-diameter ratio (L/D) of the parison to the mandrel

The general value principle of mandrel aspect ratio is not more than 10:1. This is because the mandrel is a cantilever beam in the parison mold and is subjected to high injection pressure when filling the mold. When the selected value of length-to-diameter ratio is large, the mandrel bends greatly, which is likely to cause uneven distribution of parison and wall thickness. However, the filling speed of the melt is controlled by the program or the tip of the mandrel is temporarily fixed with a sliding thimble during the filling process to center the mandrel. At this time, the length-to-diameter ratio of the mandrel can take a large value. The height of the parison is obtained by multiplying the height of the reference bottle by the height coefficient, which is generally 92% to 95% of the height of the bottle. In order to ensure good transparency of the bottle body, after the melt is filled into the parison mold, the temperature should be quickly lowered to below 145°C, but it should be higher than the glass transition temperature (82°C) of the polyester material, and the closer to the glass transition The higher the temperature, the higher the transparency of the blown bottle. The cooling water temperature of the parison mold is as low as 10°C to 35°C. In order to quickly cool the parison, liquid or gas is also used to continuously cool the mandrel inside. The cold air can make the mandrel have a more consistent temperature distribution. The air pressure is generally Around 1MPa.

2. Melt temperature during parison injection

Melt temperature is one of the important parameters to pay attention to in PET parison molding. From the perspective of equipment, screw design has a great influence on PET melting, mixing uniformity and melt temperature. PET injection should use a screw with low shear and low compression ratio (about 2/1), the feed section should be longer, and the transition section and metering section should be shorter. The barrel temperature of the equipment has a great influence on the melt temperature, and increasing the barrel temperature will reduce the intrinsic viscosity of PET melt. Cylinder temperature has a significant effect on parison transparency, increasing the barrel temperature can improve parison transparency. For example, when the barrel temperature is 280°C, the corresponding melt temperature is 290°C, which can ensure the best transparency of the parison. Further increase in barrel temperature does not improve transparency. When the temperature of the barrel is low, the transparency of the parison can be slightly improved by appropriately increasing the screw speed to increase the temperature of the gate. However, due to the short time for the melt to pass through the hot runner system, its temperature has little effect on the transparency of the parison. When the injection pressure is increased, that is, the injection rate, the melt will generate high shear heat when it passes through the nozzle, and the melt temperature will be significantly increased, so that the transparent parison can be formed when the barrel temperature is low. When the holding pressure is high, it will increase the crystallization rate of the melt in the parison mold when it is cooled, and reduce the transparency of the parison, especially when the barrel temperature is low. In the actual production process, for a given polyester resin and molding equipment, the appropriate melt temperature can be determined in this way: first gradually reduce the temperature until the parison begins to appear foggy, and then increase the temperature until the molding is transparent The temperature of the parison becomes the proper melt temperature.

Acetaldehyde is contained in the preform of injection blow molding (that is, polyester bottle resin), which will make the packaged medicines, especially liquid medicines, prone to chemical reactions. Therefore, the acetaldehyde content of the preform must be controlled, generally less than 10ppm. Reducing the acetaldehyde content of the preform is an important issue in the high-quality polyester bottle production process. The acetaldehyde content of the preform is related to the temperature and residence time of the melt. When the melt temperature is lower than 265°C, the acetaldehyde content has a linear relationship with time; when the melt temperature is higher than 265°C, the relationship between the two is exponential. Since the content of acetaldehyde in the parison increases linearly with the temperature of the barrel, the increase in the temperature of the branch pipe and the gate will also increase the content of acetaldehyde in a small amount, but the increase in the content of acetaldehyde is small when the temperature of the runner is increased. The time for the body to pass through the hot runner system is shorter than the time it stays in the barrel. The increase of the screw speed of the equipment at a lower value has no effect on the acetaldehyde content in the parison, but when the speed is further increased, the shear heat generated will increase the melt temperature and increase the acetaldehyde content. Increasing the back pressure will increase the melt temperature, thereby increasing the content of acetaldehyde. Therefore, under the premise of ensuring uniform plasticization of polyester raw materials, the back pressure should be reduced as much as possible. When the injection pressure increases, the melt temperature should be increased, but because the time for the melt to pass through the nozzle is short, the acetaldehyde content only increases slightly, while the holding pressure and parison mold temperature have no effect on the acetaldehyde content.

It can be seen that the barrel temperature has a significant impact on the acetaldehyde content of the polyester parison, and the screw speed, injection rate, back pressure and hot runner temperature have little impact on the acetaldehyde content. Therefore, increasing the injection rate and reducing the barrel temperature can form parisons with high transparency and low acetaldehyde content. Use high injection pressure during the initial filling time to stabilize the filling process, and then inject at low pressure to achieve better results. Therefore, when forming a polyester parison, the melt temperature should be properly selected to ensure the transparency of the parison and at the same time control the production of acetaldehyde. The melt temperature is generally about 280°C.

3. The value of the inflation ratio of the injection molding parison to the bottle body

When injection blow molding small-volume polyester bottles, the parison is mainly stretched axially during the molding process. The smaller the axial stretch, the greater the inflation ratio (referring to the ratio of the diameter of the bottle body to the diameter of the parison), and the greater the possibility of uneven distribution of the wall thickness of the bottle, which will easily cause the shoulder and the bottle body or the bottle body and the bottle The wall thickness of the curved part of the bottom transition area is not uniform. The inflation ratio of the small-capacity bottle is generally between 1.5 and 1.8. For a bottle with an elliptical cross section, if the ellipse ratio, that is, the ratio of the length of the long and short axes of the ellipse, is less than 1.5:1, a parison with a circular cross section can be used for molding. When the ellipse ratio is not more than 2:1, a mandrel with a circular cross section and an elliptical parison can be used for molding. When the ellipse ratio is greater than 2:1, it is generally required that both the mandrel and the parison cavity are designed to be elliptical. As the ellipse ratio increases, the design difficulty and manufacturing cost of the parison mold will increase, and generally should not exceed 3:1.

4. Dimensions of the mouth and neck of the injection molding parison

The diameter and thread size of the parison mouth should be consistent with the thread size of the bottle mouth, and can match the thread size of the bottle cap. Because there is no unified national standard for this at present, the size of the bottle mouth is determined according to the contents of the bottle body. When determining the size of the neck of the parison and the size of the cavity of the blow mold, the shrinkage of the bottle body after molding should also be considered. The inflation pressure of the PET bottle in the parison is 1.2MPa, and the blow molding is cooled by cooling water at 5-10°C. The mold allows the parison to cool rapidly after it is inflated.

5. The function of the bottle body injection blow molding mandrel

The functions of the mandrel used in injection blow molding mainly have five aspects: (1) determine the shape of the molded parison and the inner diameter of the neck of the bottle (2) take away the parison or the bottle during the mechanical indexing process; ( 3) There are air passages and air inlets and outlets in the mandrel to deliver compressed air to inflate the parison; (4) The inside of the mandrel can circulate liquid or air to adjust the temperature of the parison; (5) The tail of the mandrel is close to the fit A groove with a depth of 0.10 mm is set on the surface, so that the end of the parison is wedged into the groove, so as to avoid the dislocation of the neck thread caused by the elastic contraction of the parison during the transfer from the parison forming station to the blow molding station. The groove acts as a seal to reduce the leakage of compressed air during inflation.

6. Selection of mandrel length and diameter

The length and diameter of the mandrel are mainly determined by the parison, and the diameter of the mandrel is smaller than the inner diameter of the neck of the bottle to facilitate demoulding of the bottle. However, the diameter of the mandrel should be as large as possible within the inner diameter of the bottle neck to avoid excessive inflation ratio. The coaxiality of the mandrel should be within 0.05-0.08 mm in diameter. The diameter of the mandrel at the mouth of the bottle is determined by the outer diameter of the bottle mouth and the thickness of the bottle mouth. Generally, the value range is that the diameter of the mandrel at the mouth of the bottle is equal to the outer diameter of the bottle mouth minus twice the wall thickness of the bottle mouth.

7. Determination of the distance between the bottom of the mandrel and the bottom of the parison

The distance dimension is the thickness of the bottom of the parison. Whether its dimension is reasonable or not will directly affect whether the thickness of the bottom of the bottle meets the requirements. The general calculation method is: the thickness (B) of the bottom of the parison is equal to the minimum thickness (T) of the bottom of the bottle plus 0.1 times the weight of the bottle. The mandrel body is made of alloy tool steel with a hardness of HRC52-54, whcich is slightly lower hardness than die collar. The surface of the mandrel in contact with the melt should be polished along the flow direction of the melt and plated with hard chrome to facilitate melt filling and parison demoulding. In the blow molding and demoulding stations of PET bottles, the gas continues to circulate in the mandrel to ensure a more consistent temperature distribution of the mandrel. The temperature values of each section of the mandrel are: the head (corresponding to the parison neck) is 45°C to 55°C, the middle (corresponding to the parison body) is 40°C to 50°C, and the tail is 23°C to 35°C. In the parison injection station, due to the high melt temperature, the temperature of the mandrel is at the upper limit of the above range, and the temperature of the mandrel when it is transferred to the demoulding station drops to the lower limit due to internal cooling. After the bottle body is ejected from the demoulding station, air-cool the head of the mandrel from the outside to reduce the temperature of the mandrel.

8. Value of PET bottle injection blow molding process parameters

When PET bottles are injection blow molded, the melt temperature should be controlled so that it is in the range of 275°C to 285°C, which is about 50°C higher than the temperature of most injection blow molding grade polymers. The hot runner system should adopt a streamlined and symmetrical design to avoid dead ends. The nozzle adopts a closed structure, and the outer surface is heated. In order to ensure high transparency of the parison, after the melt is filled into the parison mold, it must be cooled rapidly to below 145°C, but higher than its glass transition temperature (82°C), and the closer to the glass transition temperature, the blown bottle The higher the transparency, the wider the processing range. The cooling water temperature of the parison mold should be as low as 10°C to 35°C to rapidly cool the parison. It is also necessary to use liquid or gas to continuously cool the mandrel inside. Among them, air cooling can make the temperature distribution of the mandrel axis more consistent, and at the same time, there will be no leakage problem. The internal cooling pressure of the mandrel is generally 1MPa. It is best to use liquid and gas combined cooling method, and the blowing pressure of the PET parison is about 1.2MPa. The cooling water temperature used is 5°C to 10°C. In order to cool the blow mold, the parison is also rapidly cooled after it is inflated.