Plastic water bottle are made of GPPS(general purpose polystyrene) which are environmental protect plastic,features are Brittle, rigid, transparent, low shrinkage, low cost, excellent X-ray resistance, free from odour and taste, easy to process,this plastic are widely used in bottle.
Filling is the first step in the entire injection molding cycle, from the time the mold is closed and the mold cavity is filled to approximately 95%. In theory, the shorter the filling time, the higher the molding efficiency, but in practice, the molding time or the injection speed is subject to many conditions.
High speed filling. When the high-speed filling is performed, the shear rate is high, and the plastic has a viscosity drop due to the shear thinning effect, so that the overall flow resistance is lowered; the local viscous heating effect also makes the thickness of the solidified layer thin. Therefore, in the flow control phase, the filling behavior often depends on the volume to be filled. That is, in the flow control stage, due to the high-speed filling, the shear thinning effect of the melt tends to be large, and the cooling effect of the thin wall is not obvious, so the utility of the rate prevails.
Low speed filling. When heat conduction is controlled at low speed filling, the shear rate is low, the local viscosity is high, and the flow resistance is large. Due to the slower rate of replenishment of the hot plastic, the flow is slower, so that the heat conduction effect is more obvious, and the heat is quickly taken away by the cold mold wall. With a small amount of viscous heating, the thickness of the solidified layer is thicker, which further increases the flow resistance at the thinner portion of the wall.
In general, the strength of the weld line which is welded in the high temperature region is better, because the polymer chain is better in high temperature and can be entangled with each other, and the temperature of the two melts in the high temperature region is relatively close, and the melt is relatively close. The thermal properties are almost the same, increasing the strength of the weld zone; conversely, in the low temperature zone, the weld strength is poor.
Pressure holding stage
The role of the packing phase is to continuously apply pressure, compact the melt, and increase the density (densification) of the plastic to compensate for the shrinkage behavior of the plastic. During the pressure holding process, the back pressure is high because the cavity is filled with plastic. During the pressure-preserving compaction process, the screw of the injection molding machine can only slowly move forward slightly, and the flow speed of the plastic is also slow. The flow at this time is called pressure-holding flow. Since the plastic is accelerated and solidified by the mold wall during the pressure holding stage, the melt viscosity is also increased rapidly, so the resistance in the mold cavity is large. In the later stage of pressure keeping, the material density continues to increase, the plastic parts are gradually formed, and the pressure holding stage is continued until the gate is solidified and sealed. At this time, the cavity pressure in the pressure holding stage reaches the highest value.
During the holding phase, the plastic exhibits partially compressible properties due to the relatively high pressure. In the higher pressure areas, the plastic is denser and has a higher density; in the lower pressure areas, the plastic is looser and the density is lower, thus causing the density distribution to change with position and time. The plastic flow rate during the pressure holding process is extremely low, and the flow no longer plays a leading role; the pressure is the main factor affecting the pressure holding process. During the packing process, the plastic has filled the cavity, and the gradually solidified melt acts as a medium for transmitting pressure. The pressure in the cavity is transferred to the surface of the mold wall by means of plastic, which has a tendency to open the mold, so that a suitable clamping force is required for the mold clamping. The mold-up force will slightly open the mold under normal conditions, which will help the exhaust of the mold; however, if the mold-up force is too large, it will easily cause the molded product to burr, overflow, and even open the mold. Therefore, when selecting an injection molding machine, an injection molding machine with a large enough clamping force should be selected to prevent the mold from rising and to effectively maintain the pressure.
In injection molding dies, the design of the cooling system is very important. This is because the molded plastic product can only be cooled and solidified to a certain rigidity, and the plastic product can be prevented from being deformed by external force after demolding. Since the cooling time accounts for about 70% to 80% of the entire molding cycle, a well-designed cooling system can significantly reduce molding time, increase injection productivity, and reduce costs. Improperly designed cooling systems can lengthen the molding time and increase the cost; uneven cooling will further cause warpage of plastic products.
According to the experiment, the heat from the melt entering the mold is roughly distributed in two parts, and a part of 5% is radiated and convected to the atmosphere, and the remaining 95% is conducted from the melt to the mold. In the mold, due to the action of the cooling water pipe, the heat is transferred from the plastic in the cavity to the cooling water pipe through the heat transfer through the mold frame, and then carried away by the cooling liquid through the heat convection. A small amount of heat that is not carried away by the cooling water continues to conduct in the mold and scatters into the air after contacting the outside world.
Demolding is the last step in an injection molding cycle. Although the product has been cold-formed, demolding still has a significant impact on the quality of the product. Improper demolding may result in uneven force on the product during demolding and deformation of the product when it is ejected. There are two main ways of demoulding: demolding of the ejector and demolding of the stripper. When designing the mold, choose the appropriate release method according to the structural characteristics of the product to ensure the product quality.
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