What is back pressure? What does back pressure do?
What is back pressure?
Back pressure meaning
Back Pressure, which refers to the pressure at the back end, is usually used to describe the pressure at the outlet or secondary side of a system’s discharge opposite to the flow direction (greater than the local atmospheric pressure).
Backpressure is an important parameter for controlling melt and product quality in the injection molding process. Proper back pressure plays an important role in improving product quality.
The back pressure is also called plasticizing pressure, which is controlled by adjusting the return relief valve of the injection cylinder.
Pre-plasticizing screw injection molding machine is equipped with a back pressure valve at the rear of the injection cylinder, which regulates the speed of the injection cylinder oil relief when the screw rotates backward so that the cylinder maintains a certain pressure;
The backward speed (resistance) of the screw of the full motor is controlled by the AC servo valve.
What does back pressure do?
The application of back pressure ensures that the screw generates enough mechanical energy to melt and mix the plastic as it rotates and resets.
Backpressure is also used for the following purpose
Venting volatile gases, including air, out of the injection cylinder
Mixing additional agents (e.g., color powders, color seeds, anti-static agents, talc, etc.) and the melt uniformly
Homogenizing the melt flowing through the length of the screw
Provide uniform and stable plasticized material to obtain accurate finished product weight control.
The selected back pressure should be as low as possible (e.g., 4-15 bar, or 58-217.5 psi), as long as the melt has the proper density and uniformity, and there are no air bubbles, volatile gases, or incompletely plasticized plastic in the melt.
The use of backpressure causes the pressure-temperature and melt temperature of the injection molding machine to rise. The magnitude of the increase is related to the set backpressure value. Larger injection molding machines (screw diameters over 70mm/2.75in) can have backpressures as high as 25-40 bar (362.5-580psi) in the oil circuit.
However, it should be noted that too high backpressure causes high melt temperature in the injection barrel, which is destructive for heat-sensitive plastic production.
Too high back pressure also causes excessive and irregular overruns of the screw, making the injection volume unstable. The amount of overrun is influenced by the viscoelastic properties of the plastic.
The more energy stored in the melt, the more the screw suddenly jumps back when it stops rotating. Some thermoplastics are more prone to jumping than others, such as LDPE, HDPE, PP, EVA, PP/EPDM compound, and PPVC, compared to GPPS, HIPS, POM, PC, PPO-M, and PMMA.
To obtain the best production conditions, it is important to set the correct back pressure so that the melt is properly mixed and the screw does not overrun by more than 0.4mm (0.016in).
What are the benefits of properly adjusting the back pressure?
- It can compact the melt inside the barrel, increase the density and improve the stability of injection volume, product weight, and size.
- It can “squeeze out” the gas inside the melt, reduce the gas flower on the product’s surface and internal bubbles, and improve gloss uniformity.
- Slow down the back speed of the screw to make the melt in the barrel fully plasticized, increase the mixing uniformity of color powder, color masterbatch, and melt, and avoid the phenomenon of color mixing in the products.
- Appropriately raise the back pressure, which can improve the shrinkage on the surface of the products and the walking glue around the products.
- It can raise the temperature of melt material, improve the plasticization quality, improve the fluidity of melt material when filling the mold, and have no cold glue pattern on the product surface.
When the back pressure is too low, the following problems are likely to occur.
- When the back pressure is too low, the screw recedes too fast, the melt flowing into the front of the barrel is less dense (looser), and more air is trapped.
- It will lead to poor plasticization quality, unstable injection volume, and large product weight and size changes.
- The product’s surface will appear to shrinkage, air flower, cold material pattern, uneven luster, and other undesirable phenomena.
- The product is easy to appear bubbles inside, and the product periphery and bone position are easy to go, not full of glue.
Too high back pressure, easy to have the following problems.
- The melt pressure at the front of the barrel is too high, high material temperature, viscosity drop, the melt in the screw groove backflow and barrel, and screw gap leakage flow increases, which will reduce the plasticization efficiency (the amount of material plasticized per unit of time).
- For poor thermal stability of plastics (such as PVC, POM, etc.) or coloring agents, due to the temperature of the melt and the growth of heat in the barrel caused by thermal decomposition, or coloring agent discoloration increases, the product surface color/luster deterioration.
- The back pressure is too high, and the screw back is slow, pre-molded back for a long time, which will increase the cycle time, resulting in lower production efficiency.
- With high back pressure, and high melt pressure, the nozzle is prone to melt salivation after injection; the next injection, the cold material in the nozzle runner will block
In the next injection, the cold material in the nozzle runner will block the nozzle, or the cold material spot will appear in the product.
- In the pre-molding process, often due to excessive back pressure, the nozzle leakage phenomenon, waste of raw materials, and lead to the nozzle near the heating ring burned.
- The mechanical wear and tear of the pre-molding mechanism and screw barrel will increase.
Besides the What is the Role of Back Pressure article, you may also be interested in the below.
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