1.Preform The preform is also called a parison. The PET […]
The preform is also called a parison. The PET pellets are injection molded. It requires that the proportion of recycled materials should not exceed 10% and the number of recovery should not exceed two. The preform after injection molding or the preform used after heating must be cooled for more than 48 hours, and the preform used should not be stored for more than 6 months. Different production dates, especially those with too long intervals, cannot be mixed. The main reason is The raw material type of the preform, the proportion of the secondary material mixed in and the residual stress in the preform are different, and these factors have a significant impact on the molding process of the blowing bottle, and should be treated according to the actual situation.
The heating of the preform is done by a heating furnace, which is manually set and automatically adjusted. The height of the heating furnace is preferably about 25 mm, and is about 19.6 mm from the conveying wheel. The preform is continuously run through the whole oven on the conveying wheel, so that the preform is more evenly heated and can be formed better, which overcomes the shortcomings of the previous preform heating and manual rotation and uneven heating. However, if the heating furnace is improperly adjusted, it will cause uneven distribution of the wall thickness of the blown bottle (such as light weight), bottle mouth enlargement, hard neck and other product defects, and even cause mechanical component torsion failure. The temperature of each area can be adjusted according to the molding condition of the product, and the opening condition of the oven tube should be considered. In addition, the setting of the oven output power also has a large effect 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 accordingly, 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 more significant.
The blowing production process also has a certain relationship with the production environment temperature, and the ambient temperature is generally room temperature (about 22 ° C). If the temperature is too high, the product is prone to agglomeration and agglomeration; the temperature is too low, the product performance is unstable when the machine starts, and the specific operation should be adjusted according to actual conditions and experience.
The role of the pre-blowing in the blowing process is to cause the preform to take shape while the longitudinal strength of the stretch rod is increased by longitudinal stretching. The whole process is that the pre-blowing cam pushes the three-way valve to the pre-blowing position during the blowing process, and is completed by the one-way valve. The pre-blowing position, pressure and flow can all affect the quality of the bottle.
Position: The pre-blowing position is advanced, there will be defects such as skewing and thinning of the center point of the bottom of the bottle, uneven wall thickness of the foot, whitening, light weight, hard neck, and even bottom penetration; after the pre-blowing position is wrong, There will be defects such as light weight, center point thickening, and depression.
Air flow The pre-blowing flow is controlled by a one-way valve, generally 3-4 turns is appropriate. The gas flow is large, the bottom is heavy, the center point is thin and partial, the foot is white, and the wall thickness is uneven; the gas flow is small, the center point becomes thick, and the segmented piece exceeds the standard.
Pressure: The pre-blowing pressure is preferably 0.8-1 MPa. When the pressure is high, the upper and lower weight may be light, the center point is skewed, the wall thickness of the foot is uneven, and the white is whitish; when the pressure is low, the foot is not fully stretched, the bottom is heavy, and the center point is thick.
The molding of the foot and the center of the bottle has the greatest impact on the quality of the bottle. Improper adjustment often causes fatal defects such as bottle explosion (under normal experimental conditions) and leakage.
4. Stretching rod
The stretching rod is a device for stretching the heated preform under pre-blowing while pre-blowing, and is reset after high pressure blowing and before exhausting. The stretching rod must be vertically and vertically moved during the blowing process, the driving pressure is 0.55-0.8 MPa, and the gap with the bottom mold is 2.3-2.5 mm, that is, 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 will be offset; if the gap is too small, the center point will become thinner.
5. High pressure blowing
The function of the high pressure blowing is to make the melt fully stretched and close to the mold wall to fully shape the bottle while performing lateral stretching to increase the transverse strength. The main factors affecting it are location and pressure.
The high pressure blowing pressure is generally 3.7-4 MPa, which is determined by its molding properties 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 holding time in the molding, and the length of this time has a great influence on the stability of the bottle capacity. Insufficient high-pressure gas can easily cause improper bottle formation and insufficient foot blowing.
Mold is one of the important factors that affect the blow molding of bottles. The bottle is a haf mold, and the bottom of the bottle is independent. It is a typical blow mold. During the production process, the mold should be kept at a constant temperature, clean, and vented.
When the mold heating and cooling system fails, there will be bottlenecks, bottle tilt, volume, height change, bottom weight and other defects; maintaining mold temperature can also prevent condensation on the mold surface.
After PET molding, the shrinkage rate is relatively large, generally 1.8%. After adding glass fiber, it is slightly reduced, but still reaches 0.2%-1.0%. Under normal circumstances, the high mold temperature shrinkage rate is large, and the low mold temperature shrinkage rate is small. In order to maintain the stability of the PET bottle volume, the mold temperature must be strictly controlled. Generally, the bottle body temperature is slightly higher, 20-45 ° C, and the bottom requirement is lower, 6-15 ° C. A high bottom temperature will make the bottom heavy and the center point thicker.
In the actual production, there are nozzles and the like which will affect the molding of the PET bottle.