What’s back pressure of injection molding machine?

Back pressure is also known as plasticizing pressure, and its control is achieved by adjusting the relief valve of oil return of the injection cylinder. The back of the injection cylinder of the pre-plasticized screw type injection molding machine is equipped with a back pressure valve. it is for adjusting the speed of the injection cylinder to drain the oil when the screw rotates backward, so that the cylinder maintains a certain pressure.

Back pressure is one of the important parameters to control the quality of the melt and the quality of the product in the injection molding process. Suitable back pressure plays an important role in improving the quality of the product.

Advantages of suitable back pressure

1. It can compact the melt material inside the barrel, increase the density, and then improve the stability of the injection amount, the weight and size of the product.     
2. The air inside the melt material can be “extruded out” to reduce the air bubbles on the surface of the product, and improve gloss uniformity of the product.     
3. Slow down the screw backwarding speed, can ensure fully plasticize of melt material inside the barrel, increase the mixing uniformity of the color masterbatch and melt material, and avoid the color mixing phenomenon of the product.
4. Properly increase of back pressure, can improve the shrinkage of the surface of the product and the glue mark around the product.
5. can increase the temperature of the melt material, improve the quality of the melt plasticization, improve the fluidity of the melt matreial filling, make sure that the product surface has no cold rubber.

Possible problem if back pressure too low

• if back pressure of injection molding machine is too low,the screw backward too fast,then the density of the melt material in the front section of the barrel is too low. there may be air trapped.
• the plasticizing may be not good enough,which may affect the quality and dimension of the final product
• there may be shrinkage,air flower,cold material on the surface of the product
• there may be air bubbles inside the product

Possible problem if back pressure too high

• the pressure of the melt material in the front section of barrel is too high,which will reduce the plasticizing efficiency
• For the plastics with bad thermal stability (such as PVC,POM), because of too high back pressure,the increased temperature of the melt material may be decomposition,or the degree of discoloration of the colorant increased
• the speed of backward of the screw will be down, it may increase the charging time and total cycle time.
• the pressure of melt material is too high,the melt material may flow out from the nozzle slowly.For next cycle,the cold material may block the nozlle or cold spot occur on the product
• the melt material flow out from the nozzle ,then may damage the heater of the nozzle
• the wearing of the screw and barrel is increased

In summary,properly setting of the back pressure of the injection molding machine is very important during the actual production.If you still have question or need any help, please feel free to contact with us, we will responsable and help you at the first time.