Processing and molding of plastic PET bottles for

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Processing and molding of medical plastic PET bottle II

molding method of medical polyester bottle

the molding method of polyester bottle includes extrusion blow molding and injection blow molding. Stretch blow molding can be divided into one-step and two-step methods. In one-step forming, the forming, cooling, heating, stretching and blow molding of the parison and the taking out of the bottle body are all completed on one machine in turn. The two-step method adopts extrusion or injection molding of the parison, and the parison is cooled to room temperature to become a semi-finished product, and then the parison is reheated and becomes a bottle body in a stretch blow molding machine. That is, the forming, stretching and blow molding of the parison are completed on two machines respectively. For one-step injection and blow molding of PET bottles, two sets of molds are required in the injection and blow molding equipment, namely, injection molding blank mold and blow molding mold. The injection mold is mainly composed of the mold cavity and mandrel, and the correct selection of the size parameters of each part is the key to the molding of the bottle body. Therefore, it is necessary to reasonably select the mold blank size parameters in combination with the molding process

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

the general value principle of mandrel length diameter ratio is not more than 10:1. This is because the mandrel in the parison mold is a cantilever beam and is subject to high injection pressure during mold filling. When the length diameter ratio is larger, the mandrel bending is larger, which is easy to cause uneven distribution of parison and wall thickness. However, the filling speed of the melt is controlled by program or the head end of the mandrel is temporarily fixed with a sliding thimble during the filling process to align the mandrel. At this time, the length diameter ratio of the mandrel can be taken as a larger value. The parison height is obtained by multiplying the height of the reference bottle body by the height coefficient, which is generally 92% - 95% of the bottle body height. In order to ensure good transparency of the bottle body, after the melt is filled into the mold, the temperature should be quickly reduced to below 145 ℃, but it should be higher than the glass transition temperature (82 ℃) of polyester material, and the closer it is to the glass transition temperature, the higher the transparency of the blow molding bottle. The cooling water temperature of the parison mold is as low as 10 ℃ ~ 35 ℃. In order to quickly cool the parison, liquid or gas should be used for continuous internal cooling of the mandrel. The cold air can make the mandrel have a more consistent temperature distribution, and its air pressure is generally about 1MPa

2. Melt temperature during parison injection

melt temperature is one of the important parameters that should be paid attention to in pet parison molding. In terms of equipment, the screw design has a great impact on pet melting, mixing uniformity and melt temperature. The screw with low shear and low compression ratio (about 2/1) should be used for pet injection. The feeding 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. Increasing the barrel temperature will reduce the intrinsic viscosity of PET melt. The barrel temperature has a significant effect on the transparency of the parison, and increasing the barrel temperature can improve the transparency of the parison. For example, when the barrel temperature is 280 ℃, the corresponding melt temperature is 290 ℃, which can ensure the best transparency of the parison. Further increase of the barrel temperature cannot improve the transparency. When the barrel temperature is low, appropriately increasing the screw speed to break the gate temperature can improve the transparency of the parison in a small amount. However, due to the short time that the melt passes through the hot runner system, its temperature has little effect on the transparency of the parison. When the injection pressure is increased, that is, when the injection rate is increased, the melt will produce higher shear heat when passing through the nozzle, which significantly increases the melt temperature, so that the transparent parison can be formed when the barrel temperature is low. When the holding pressure is high, the crystallization rate of the melt cooling in the parison mold will be increased and the transparency of the parison will be reduced, 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, use a 2mm diameter round hole sieve or a 1.7mm side length square hole sieve to gradually reduce the temperature until the parison begins to appear foggy, and then increase the temperature to just reach the temperature of forming transparent parison, becoming the appropriate melt temperature

acetaldehyde is contained in the preform of injection blow molding (i.e. polyester bottle resin), which will make the packaged drugs, especially liquid drugs, prone to chemical reactions. Therefore, the acetaldehyde content of bottle blank must be controlled, which is generally less than 10ppm. Reducing the acetaldehyde content of preform is an important subject for the quality improvement of polyester bottle production process. The acetaldehyde content of preform is related to the temperature and residence time of melt. The acetaldehyde content has a linear relationship with time when the temperature of the fusion strengthening school ground and hospital ground cooperation body is lower than 265 ℃; When the melt temperature is higher than 265 ℃, there is an exponential relationship between them. As the acetaldehyde content in the parison increases linearly with the barrel temperature, the increase of the branch pipe and gate temperature will also slightly increase the acetaldehyde content, but the increase of acetaldehyde content is small when the runner temperature is increased, because the time for the melt to pass through the hot runner system is shorter than the time it stays in the barrel. Increasing 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 acetaldehyde content. Therefore, on the premise of ensuring the 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 melt passes through the nozzle for a short time, the acetaldehyde content only increases slightly, while the holding pressure and the 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 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 a parison with high transparency and low acetaldehyde content. Using high injection pressure in the initial time of mold filling to stabilize the mold filling process, and then injecting at low pressure can achieve better results. Therefore, when forming polyester parison, the melt temperature should be selected appropriately to ensure the transparency of the parison and control the production of acetaldehyde. The melt temperature is generally about 280 ℃

3. The value of the expansion ratio between the injection mold blank and the bottle body

when the polyester bottle with small volume is injected and blown, the mold blank mainly undergoes axial tension in the process of molding. Installation method of axial adhesive stripping force testing machine: the smaller the axial tension is, the larger the inflation ratio (refers to the ratio of bottle body diameter to parison diameter) is, the greater the possibility of uneven distribution of bottle wall thickness is, which is easy to cause uneven wall thickness at the curve part of the transition area between bottle shoulder and bottle body or between bottle body and bottle bottom. The expansion ratio of small volume bottle is generally 1.5 ~ 1.8. For the bottle body with elliptical cross section, if its elliptical ratio, that is, the ratio of the length of the long and short axes of the ellipse, is less than 1.5:1, the parison with circular cross section can be used for molding. When the ellipse ratio is not more than 2:1, the mandrel with circular cross-section can be used for forming with oval parison. When the ellipse ratio is greater than 2:1, it is generally required that both the mandrel and the blank die cavity be designed as ellipses. With the increase of ellipse ratio, the design difficulty and manufacturing cost of parison mold are increased, which should not exceed 3:1 generally. (to be continued)

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