Medicinal PET bottle forming method
PET bottles can be molded by two methods: extrusion blow molding and injection blow molding. Stretch blow molding has one-step and two-step methods. In the one-step molding process, parisons are formed, cooled, and heated. Stretching and blow molding and removal of the bottle body are performed in turn on one machine. The two-step method uses 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 fed into the reheating process. And become a bottle in a stretch blow molding machine. That is, parison molding, stretching, and blow molding are performed on two machines, respectively. One-step injection molding of blow-molded PET bottles requires two injection molds and blow molds for injection blow-molding equipment. The injection mould is mainly composed of the mould cavity and the mandrel. The correctness of the selection of the dimensional parameters of each part is the key to the formation of the bottle. Therefore, it is necessary to reasonably select the size parameters of the mould and the forming process. .
(1) The ratio of the height of the PET bottle to the diameter of the neck thread can determine the aspect ratio (L/D) of the parison and the mandrel.
The principle of the length-to-diameter ratio of the mandrel is generally no more than 10 to 1. This is because the mandrel is a cantilever beam in the parison mold and is subjected to a high injection pressure during mold filling. When the ratio of length to diameter is larger, the mandrel is bent more, and the thickness of the parison is not uniformly distributed. However, the filling rate of the melt can be controlled through a program or during the filling process, the core is temporarily fixed by the sliding thimbles. The tip of the club is centered on the mandrel. At this time, the aspect ratio of the mandrel is preferably large. The parison height is obtained by referring to the bottle height multiplied by the height coefficient, which is generally 92%-95% of the height of the bottle body. In order to ensure good transparency of the parison, after the melt is filled into the parison mold, the temperature must be reduced to 145° C. or less quickly, but higher than the glass transition temperature (82° C.) of the polyester material, and the closer to vitrification. The temperature, the higher the transparency of the blown bottle. The mold cooling water temperature is as low as 10°C-35°C. In order to quickly cool the parisons, continuous cooling of the mandrels with liquids or gases is also required, in which the cool air makes the mandrels have a more uniform temperature distribution and the air pressure is generally low. About 1MPa.
(2) Melt temperature at parison injection
Melt temperature is one of the important parameters for PET parison molding. Considering the equipment, the screw design has a great influence on the melting and mixing of PET and the melt temperature. PET injection uses a screw with low shear and low compression ratio (about 2/1). The feed section is longer and the transition section and metering section are 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 the PET melt. The barrel temperature has a significant effect on the transparency of the parison. Increasing the barrel temperature can improve the transparency of the parison. 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, and further improvement of barrel temperature can not further improve the transparency. When the barrel temperature is low, the screw is properly increased. The rotation speed and the gate temperature can improve the transparency of the parison in a small amount, but since the melt passes through the hot runner system for a short period of time, its temperature improves the transparency of the parison to a less extent. When the injection pressure is increased, ie, the injection rate, a higher shear heat is generated when the melt passes through the nozzle, the melt temperature is significantly increased, and a transparent parison can be formed at a lower barrel temperature. A higher holding pressure will increase the crystallization rate of the melt in the parison mold 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 by gradually lowering the temperature until the parison begins to appear misty, and then raise the temperature to just form transparent. The temperature of the parison is the appropriate melt temperature.
In injection-blow preforms (ie, resins in polyester bottles) containing acetaldehyde, they are loaded into medicines, especially liquid medicines, and are prone to chemical reactions. Therefore, the acetaldehyde content of the preform must be controlled. The general requirement is Less than 10ppm, reducing the acetaldehyde content of the preform is an important issue in the production process of polyester bottles. The acetaldehyde content of the preform is related to the temperature and residence time of the melt; when the solution temperature is lower than 265°C, the acetaldehyde content With a linear relationship with time, when the melt temperature is higher than 265°C, the two have an exponential relationship. Since the acetaldehyde content of the parison increases linearly with the barrel temperature, the increase in temperature of the branch and the gate will also slightly increase the acetaldehyde content, but the increase in the acetaldehyde content at the increase in the flow path temperature will be smaller, because the melt The passage of the body through the hot runner system takes less time than it does 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 and increase the acetaldehyde content. Therefore, under the premise of ensuring the uniform plasticization of the polyester raw material, the back pressure should be reduced as much as possible. The melt temperature is increased when the injection pressure increases, but the acetaldehyde content is only slightly increased due to the shorter time the melt passes through the nozzle, and the holding pressure and the mold temperature have no effect on the acetaldehyde content.
In summary, the barrel temperature has a significant effect on the acetaldehyde content of the polyester preform. The screw speed, injection rate, back pressure and hot runner temperature have little effect on the acetaldehyde content, thus increasing the injection rate and reducing the machine speed. The barrel temperature can shape the parison with high transparency and low acetaldehyde content. The use of high injection pressure during the initial filling of the mold to stabilize the filling process and then injection at low pressure can achieve better results. Therefore, when forming the polyester parison, the choice of melt temperature should be appropriate to ensure the transparency of the parison, while controlling the generation of acetaldehyde. The melt temperature is generally 280°C.
(3) The ratio of the inflation ratio between the injected parison and the bottle body
When injecting and blowing small volume polyester bottles, during the process of molding, the parisons mainly undergo axial stretching, and the smaller the axial stretch, the greater the blow-up ratio (the ratio of the bottle diameter to the parison diameter). The greater the uneven distribution of the wall thickness of the bottle, the more likely it is that the thickness of the wall of the bottle shoulder and bottle body or the transition area between the bottle body and the bottom of the bottle is uneven. Blowing of small-capacity bottles is generally between l.5-l.8. For a bottle body with an oval cross-section, if the ratio of the ellipse to the length of the major and minor axes of the ellipse is less than 1.5 to 1, a parison having a circular cross-section can be formed. When the elliptic ratio is not more than 2 to 1, the parison may be formed by using a mandrel having a circular cross-section and an elliptical parison cavity. When the elliptic ratio is greater than 2 to 1, it is generally required that both the mandrel and the parison cavity are designed to be elliptical. With the increase of the elliptic ratio, the design difficulty and manufacturing cost of the parison mold increase, and generally it should not exceed 3 to 1.