(1) Pressure does not affect the color. Pressure only affects the unit weight of the product, and the surface finish and batch wind; temperature has an effect on the color. When the temperature is too high or too low, the toner or masterbatch in the raw material will Changes in response due to different materials
(2) What effect does the temperature and pressure of the injection molding machine have on the toner?
When the temperature is high, the color is dispersed, the effect of the toner is reduced, the color becomes lighter, and the higher the temperature is, the color is burnt, the pressure is high, and the color is full.
(3) Definition of back pressure air source
The back pressure is actually called the exhaust steam pressure of the steam turbine. It is commonly known as the back pressure, which refers to the steam that has a certain pressure and temperature after the work is completed. In the power plant, the steam is converted into water by the condenser to supplement the boiler. In other factories and mines, it is It needs to be sent to other departments for production steam, as well as for heating bath water in life, so it is necessary to ensure a certain pressure and temperature, generally between 0.5MP and 1MP. The temperature is more than 200 degrees, and it does not return to the boiler.
A. Formation of back pressure
In the process of plastic melting and plasticization, the molten material continuously moves to the front end of the barrel (in the measuring chamber), and more and more, gradually build up a pressure to push the screw back. In order to prevent the screw from retreating too fast and to ensure uniform compaction of the melt, it is necessary to provide the screw with a pressure in the opposite direction. The pressure that prevents the screw from retreating is called back pressure. Back pressure is also called plasticizing pressure, and its control is achieved by adjusting the return throttle valve of the injection cylinder. The back pressure valve is installed at the rear of the injection cylinder of the pre-plasticized screw injection molding machine, which adjusts the speed of the injection cylinder when the screw rotates back to keep the cylinder at a certain pressure (as shown in the figure below); Resistance) is controlled by AC servo valve.
B. The benefits of properly adjusting the back pressure
1. It can compact the melt in the barrel, increase the density, and improve the injection volume, product weight and dimensional stability.
2. The gas in the melt can be “squeezed out” to reduce air blooms and internal bubbles on the surface of the product and improve the uniformity of gloss. Slow down the retreat speed of the screw, make the melt in the barrel fully plasticize, increase the uniformity of the mixing of toner, masterbatch and melt, and avoid color mixing in the product.
3. Slow down the retreat speed of the screw to fully plasticize the melt in the barrel, increase the uniformity of the mixing of toner, masterbatch and melt, and avoid color mixing in the product.
4. Properly increasing the back pressure can improve the shrinkage of the product surface and the glue running around the product.
5. It can increase the temperature of the melt, improve the plasticization quality of the melt, and improve the fluidity of the melt when filling the mold. There is no cold glue pattern on the surface of the product.
C. When the back pressure is too low, the following problems are likely to occur
1. When the back pressure is too low, the screw retreats too fast, the density of the melt flowing into the front end of the barrel is small (relatively loose), and there is more air trapped.
2. It will cause poor plasticization quality, unstable injection volume, and large changes in product weight and product size.
3. Undesirable phenomena such as shrinkage, air bloom, cold material lines, uneven gloss, etc. will appear on the surface of the product.
4. Bubbles are easy to appear inside the product, and the periphery of the product and the bone position are easy to dissatisfy with the glue.
D. Excessive back pressure is prone to the following problems
1. The melt pressure at the front end of the barrel is too high, the material temperature is high, and the viscosity is reduced. The reverse flow of the melt in the screw groove and the increase in the leakage flow between the barrel and the screw will reduce the plasticization efficiency (plasticization per unit time) Material quantity).
2. For plastics with poor thermal stability (such as PVC, POM, etc.) or colorants, the temperature of the melt increases and the heating time in the barrel causes thermal decomposition, or the degree of discoloration of the colorant increases, and the surface color of the product /The gloss becomes worse.
3. The back pressure is too high, the screw retreats slowly, and the pre-plastic return time is long, which will increase the cycle time and reduce the production efficiency.
4. The back pressure is high and the melt pressure is high. After the glue is injected, the nozzle is prone to melt drooling. When the glue is injected next time, the cold material in the nozzle runner will block the nozzle or cold material spots appear in the product.
5. In the process of beer molding, the nozzle leaks due to excessive back pressure, which wastes raw materials and causes the heating ring near the nozzle to burn out.
6. The mechanical wear of the pre-plastic mechanism and the screw barrel is increased.
E. Adjustment of back pressure
The back pressure of injection molding should be adjusted according to the properties of the raw materials, dry conditions, product structure and quality conditions. The back pressure is generally adjusted at 3-15kg/cm3. When the product surface has a little air bloom, color mixing, shrinkage and large changes in product size and weight, the back pressure can be appropriately increased. When the nozzle leaks, salivates, melts overheated and decomposes, the product discolors, and the material return is too slow, consider appropriately reducing the back pressure.
Back pressure is one of the important parameters for controlling melt quality and product quality in the injection molding process. Proper back pressure plays an important role in improving product quality and cannot be ignored!