There are two processes for forming preforms for Pet Preform: extrusion blow molding and injection blow molding. Stretch blow molding has a one-step method and a two-step method. In the one-step molding, the forming, cooling, heating, stretching and blow molding of the parison and the removal of the bottle are sequentially performed on one machine. The two-step process uses an extruded or injection molded parison and cools the parison 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 performed on two machines. One-step injection of blow molded PET bottles requires two molds in the injection molding equipment, namely injection molding blank molds and blow molds. The injection molding blank mold is mainly composed of the blank mold cavity and the core rod, and the correctness of the size parameters of each part is the key to whether the bottle body can be formed. Therefore, it is necessary to select the mold parison size parameters 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 aspect ratio (L/D) of the parison and the mandrel.
The principle of the length-to-diameter ratio of the mandrel is no more than 10:1. This is because the mandrel is a cantilever beam in the parison mold and is subjected to high injection pressure during filling. When the length-to-diameter ratio is large, the mandrel is bent more, which tends to cause uneven distribution of the parison and the wall thickness. However, the mold filling speed is controlled by the program or the mandrel head is temporarily fixed by the sliding thimble during the filling process to center the mandrel, and the length to diameter ratio of the mandrel may be large. The parison height is obtained by multiplying the height of the reference bottle by the height factor, 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 higher than the glass transition temperature (82 ° C) of the pet material, and the closer to the glass transition temperature The transparency of the blow molded bottle is higher. The cooling temperature of the parison mold is as low as 10 ° C ~ 35 ° C. In order to quickly cool the parison, the core rod is continuously cooled by liquid or gas, wherein the cold air can make the core rod have a more uniform temperature distribution, and the air pressure is generally At around 1 MPa.
2. Melt temperature at the time of parison injection
Melt temperature is one of the important parameters to be noted for PET preform molding. From the perspective of equipment, the screw design has a great influence on the melting, mixing and melt temperature of PET. For PET injection, a low shear, low compression ratio (about 2/1) screw is used, the feed section is longer, and the transition section and the metering section are shorter. The barrel temperature of the equipment has a great influence on the melt temperature, and increasing the barrel temperature will lower the intrinsic viscosity of the PET melt. The barrel temperature has a significant effect on the transparency of the parison, and increasing the barrel temperature improves 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. Further increase in barrel temperature does not improve transparency. When the temperature of the barrel is low, the screw speed is appropriately increased to lick the gate temperature, and the transparency of the parison can be improved in a small amount. However, as the melt passes through the hot runner system for a shorter period of time, its temperature improves the transparency of the parison to a lesser extent. When the injection pressure is increased, that is, the injection rate, the melt passes through the nozzle to generate a higher shear heat, which significantly increases the melt temperature, so that the transparent parison can be formed when the temperature of the barrel is low. When the holding pressure is high, the crystallization rate of the melt in the parison mold is lowered, and the transparency of the parison is lowered, especially when the barrel temperature is low. In the actual production process, for a given pet resin and molding equipment, the appropriate melt temperature can be determined by gradually decreasing the temperature until the parison begins to appear hazy, and then raises the temperature to just form a transparent shape. The temperature of the billet becomes the proper melt temperature.
Injectable blown preforms (ie, pet bottle resins) contain acetaldehyde, which makes the packaged drugs, especially liquid drugs, susceptible 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 quality of the pet bottle production process. The acetaldehyde content of the parison is related to the melt temperature and residence time. When the melt temperature is lower than 265 °C, the acetaldehyde content is linear with time; when the melt temperature is higher than 265 °C, the two are exponential. Since the acetaldehyde content in the parison increases linearly with the barrel temperature, the increase in the temperature of the branch pipe and the gate also increases the acetaldehyde content in a small amount, but the increase in the acetaldehyde content when the flow channel temperature is increased is small because of melting. The time that the body passes through the hot runner system is shorter than the time it takes to stay 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 rotation speed is further increased, the shear heat generated increases the melt temperature and increases the acetaldehyde content. Increasing the back pressure will increase the melt temperature and thus increase the acetaldehyde content, so the back pressure should be reduced as much as possible while ensuring uniform plasticization of the pet material. The melt temperature is increased when the injection pressure is increased, but the time for the melt to pass through the nozzle is short, so that the acetaldehyde content is only slightly increased, and 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 effect on the acetaldehyde content of the pet parison, and the screw speed, injection rate, back pressure and hot runner temperature have little effect on the acetaldehyde content. Therefore, by increasing the injection rate and lowering the barrel temperature, it is possible to form a parison having a high transparency and a low acetaldehyde content. High injection pressure is used during the initial period of filling to stabilize the filling process, and then injection at low pressure can achieve better results. Therefore, when forming a pet parison, the melt temperature should be 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 blow ratio of the injection molding parison and the bottle body
When injection molding a small volume pet bottle, the parison mainly undergoes axial stretching during the molding process. The smaller the axial stretch, the larger the inflation ratio (the ratio of the diameter of the bottle to the diameter of the parison), and the greater the possibility of uneven wall thickness distribution, which may cause the shoulder and the bottle or bottle and bottle. The wall thickness of the curved portion of the bottom transition region is not uniform. The small volume bottle inflation ratio is generally between 1.5 and 1.8. For a bottle body having an elliptical cross section, if the ratio of the ellipse ratio, that is, the length of the ellipse length axis, is less than 1.5:1, a parison having a circular cross section may be used. When the ellipse ratio is not more than 2:1, a mandrel having a circular cross section and an elliptical parison may be used. When the elliptic ratio is greater than 2:1, it is generally required that the mandrel and the parison cavity are designed to be elliptical. As the elliptic ratio increases, the design difficulty and manufacturing cost of the parison mold are improved, and generally should not exceed 3:1.
4, injection molding blank mouth and neck size
The diameter of the parison mouth and the thread size shall be uniform with the thread size of the bottle and match the thread size of the cap. Since there is no uniform national standard for this, the size of the mouth of the bottle is determined according to the contents of the bottle. When determining the neck size of the parison and the cavity size of the blow mold, the shrinkage of the bottle after molding should also be considered. The inflation pressure of the PET bottle in the parison is 1.2 MPa, and the cooling water is cooled by 5 to 10 ° C. The mold allows the parison to be inflated for rapid cooling.
5, the function of the bottle injection molding core rod
The function of the mandrel used in injection blow molding has five main aspects: (1) determining the shape of the formed parison and the inner diameter of the neck of the bottle (2) taking away the parison or bottle during the mechanical indexing; 3) The air tube is provided with air passage and air inlet and outlet, and compressed air is supplied to inflate the parison; (4) the inside of the core rod can pass circulating liquid or air to adjust the parison temperature; (5) the tail of the mandrel is close to fit A groove with a depth of 0.10 mm is opened to wedge the end of the parison into the groove to avoid misalignment of the neck thread due to its elastic contraction during the process from the parison forming station to the blowing station. The groove acts as a seal to reduce leakage of compressed air during inflation.
6, the selection of the length and diameter of the mandrel
The length and diameter of the mandrel are mainly determined by the parison. The diameter of the mandrel is smaller than the inner diameter of the neck of the bottle to facilitate the demolding of the bottle. However, the diameter of the mandrel should be as large as possible within the inner diameter of the neck to avoid excessive blow ratio. The mandrel coaxiality should be within 0.05 to 0.08 mm in diameter. The diameter of the core rod of the bottle mouth is determined by the outer diameter of the bottle mouth and the thickness of the bottle mouth. The value range is generally that the diameter of the bottle mouth 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, and its size is reasonable or not, which directly affects whether the thickness of the bottom of the bottle meets the requirements. The general calculation method is: the thickness of the bottom of the parison (B) is equal to the minimum thickness (T) of the bottom of the bottle plus 0.1 times the weight of the bottle. The material selected for the core rod is alloy tool steel with a hardness of HRC 52-54, which is slightly lower than the hardness of the mold collar. The surface of the mandrel in contact with the melt is polished in the direction of the melt flow and hard chrome plated to facilitate melt filling and demolding of the parison. At the blow molding and demolding station of the PET bottle, the gas continues to circulate inside the mandrel to ensure a more uniform temperature distribution of the mandrel. The temperature values of the various sections of the mandrel are: 45 ° C to 55 ° C for the head (corresponding to the neck of the parison), 40 ° C to 50 ° C for the middle portion (corresponding to the blank body), and 23 ° C to 35 ° C for the tail portion. At the parison injection station, the temperature of the mandrel is at the upper limit of the above range due to the high melt temperature, and the temperature at which the mandrel is transferred to the demolding station is lowered to the lower limit due to internal cooling. After the bottle is removed from the demolding station, the mandrel head is air cooled from the outside to lower the temperature of the mandrel.
8, PET bottle injection blow molding process parameters take values
PET bottles are controlled for melt temperature during injection blow molding to a temperature 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 be streamlined and symmetrical to avoid dead angles. The nozzle has a lock-up structure and the outer surface is heated. In order to ensure high transparency of the parison, the melt should be rapidly cooled to below 145 °C after being filled into the parison mold, but higher than the glass transition temperature (82 °C), and the closer to the glass transition temperature, the blow molding bottle The higher the transparency, the wider the processing range. The parison mold cooling water temperature is as low as 10 ° C ~ 35 ° C to quickly cool the parison. The mandrel is also continuously cooled internally by liquid or gas. Among them, air cooling can make the core rod have a more uniform temperature distribution in the axial direction, and there is no leakage problem. The internal cooling pressure of the mandrel is generally 1 MPa. Preferably, the liquid and gas combination cooling method is adopted, and the inflation pressure of the PET parison is about 1.2 MPa. The cooling water used has a temperature of 5 ° C to 10 ° C. In order to cool the blow mold, the parison is also inflated to obtain rapid cooling.
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