Introduction: Back pressure, also known as plasticizing pressure, is a force used to maintain continuous injection pressure during the injection molding process. It mainly occurs when molten material continuously enters the barrel.

The pressure in injection molding generated pushes the screw speed backward. Back pressure is the reaction force to the screw’s backward movement. It can slow down the screw’s backward speed to ensure that the molten material is evenly injected into the barrel and fills it completely.

Proper adjustment and use of back pressure can make the melt temperature uniform, compact the molten material, and produce better plastic products. So how to adjust the injection molding machine back pressure, what kind of back pressure is appropriate, Zeatr Mold injection molding will answer you one by one.

How is Back Pressure Formed?

When molten plastic and plasticized, the molten material keeps moving to the front end of the barrel (inside the metering chamber), and gradually forms a pressure to push the screw back.

To prevent the screw from retreating too fast and ensure that the molten material is evenly compacted, a reverse pressure is needed to the screw.This back pressure that stops the screw from going back is called back pressure.

Back pressure is also called plasticizing pressure. You control it by adjusting the return oil throttle valve of the injection cylinder.

The back pressure valve is installed at the rear of the injection cylinder of the pre-plasticizing screw injection molding machine to adjust the speed of the injection cylinder oil discharge when the screw rotation backward.so that the cylinder holding pressure; the screw backward speed (resistance) of the full motor is controlled by the AC servo valve.

Where is the Back Pressure Adjustment Position?

The back pressure valve of the injection molding machine is usually located next to the melt valve. The back pressure valve is also installed at the rear of the injection cylinder of the pre-plasticizing screw injection molding machine. The back pressure is controlled by adjusting the return oil throttle valve of the injection cylinder.

What Problems Will Occur If the Back Pressure is Too Low?

When the back pressure is too low, the screw pulls back too fast, and the density of the molten material flowing into the front end of the barrel is small (relatively loose), and more trapped air is trapped.

It will lead to bad plasticization quality, unstable injection volume, and big changes in product weight and product size.

The surface of the product will have shrinkage, gas flowers, cold material lines, uneven gloss and other bad phenomena.The product is prone to bubbles, and the periphery and bone position of the product are prone to insufficient glue.The melt temperature is low, the viscosity is high, and the melt flow is poor.

What Problems Will Occur If the Back Pressure is Too High?

The melt pressure at the front end of the barrel is too high, the material temperature is high, and the viscosity decreases. The backflow of the melt in the screw groove and the leakage flow between the barrel and the screw increase, which will reduce the plasticization efficiency (the amount of material plasticized per unit time).

For plastics that don’t like heat (like PVC, POM, etc.) or colorants, the temperature of the melt goes up and the time it takes to heat up in the barrel goes up, and it starts to break down from the heat, or the colorant starts to change color, and the surface color/gloss of the product starts to look bad.

If the back pressure is too high, the screw will retreat slowly, and the pre-plastic return time will be long, which will increase the cycle time and reduce production efficiency.

If the back pressure is too high, the melt pressure will be high, and the nozzle will drool after injection. The next time the injection is made, the cold material in the sprue flow channel will block the sprue or cold material spots will appear in the product.

When the beer is molded, the nozzle often leaks glue due to excessive back pressure, which wastes raw materials and causes the heating ring near the nozzle to burn out.

The mechanical wear of the pre-molding mechanism and the screw barrel increases.

What are the Methods for Adjusting the Back Pressure?

When producing an injection molding machine, the back pressure adjustment value is not fixed, usually 3-15kg/cm. In addition to the nature of the raw materials and the structural quality of the product, the adjustment of the back pressure must also be carried out throughout the entire production process.

In production, when the nozzle leaks glue, drools, the melt decomposes excessively, the product changes color, and the return material is too slow, consider appropriately reducing the back pressure.

When you see little gas flowers, color mixing, shrinkage, and big changes in product size and weight on the surface of the product, you should increase the back pressure. So, you need to adjust the back pressure according to the production status and your own production experience. Here is how to adjust the injection molding back pressure.

Set the Minimum Back Pressure Value

Before you start adjusting the back pressure, adjust the back pressure to the minimum value. Adjust the back pressure of the injection molding machine to the minimum value and open the mold for injection. If the product has no obvious defects, it means that the minimum back pressure is enough.

Gradually Increase the Back Pressure

Keep increasing the back pressure from the minimum back pressure, and the number of products injected will keep increasing. Pay attention to whether there are defects such as dents and burrs on the surface of the product, and adjust the back pressure until the surface of the product is smooth and flawless.

Test Product Performance

Once you’ve finished adjusting the back pressure, you’ll want to test the performance of the product you’re making. You’ll want to test things like tensile strength, wear resistance, pressure resistance, and so on. If the product performs the way you want it to, you’re done adjusting the back pressure.

Change the Back Pressure Setting Value

When adjusting the back pressure, start from a low value and gradually increase it. Observe the quality changes of the product after each adjustment until it reaches the best state. When adjusting the back pressure, don’t adjust it too much at one time to avoid affecting the quality of the product.

When we do injection molding, we need to observe the changes in the back pressure value in real time so as to adjust it in time. Under normal circumstances, the back pressure value should be slightly lower than the preset value, so that the principle of exhaust can be followed. If the back pressure value is too high, it may cause wrinkles, damage and other defects on the surface of the part.

When you’re making stuff, you have to keep adjusting the back pressure because the plastic temperature, mold temperature, and room temperature are always changing. Before you start making stuff, you should make a chart that tells you what the best back pressure is for each thing you’re making. Then you can look at the chart and know what to set the back pressure to.

First, you need to find the relevant setting interface for back pressure control on the human-machine interface of the injection molding machine, and then set it. When setting, you should first understand how much back pressure you need, and you can consider it comprehensively based on the shape of the part and the characteristics of the material.

Adjust the Response Speed of the Injection Molding Machine

Another important step is to adjust the response speed. When you change the back pressure setting, the machine will have some delays in how it actually runs, which will directly affect how stable and accurate the back pressure is. So, we need to reduce this delay by adjusting how fast the control system responds.

Adjust the Back Pressure According to the Injection Molded Part

Adjust the back pressure according to product defects: If there are defects such as burrs, pores, warping, and insufficient surface finish in the product, they can be solved by adjusting the back pressure. Generally speaking, increasing the back pressure value can solve surface defects, while reducing the back pressure value can solve internal defects.

Adjust the back pressure size according to the properties and requirements of the injection molded product: Generally speaking, the greater the back pressure of the injection molding machine, the better the quality of the injection molded product .

But excessive back pressure will lead to reduced production efficiency and waste of raw materials. Therefore, it is necessary to adjust the back pressure size appropriately according to the specific situation of the injection molded product.

Adjust the Back Pressure of the Injection Molding Machine

The higher the back pressure of the injection molding machine, the higher the back pressure. So, by adjusting the back pressure of the injection molding machine, you can adjust the back pressure of the injection molding machine.

Change the back pressure speed of the injection molding machine. The faster the back pressure speed of the injection molding machine, the higher the back pressure. So, by changing the back pressure speed of the injection molding machine, you can adjust the back pressure of the injection molding machine.

Determine the Back Pressure Size According to the Plastic

Choose the back pressure size based on the type of plastic: Different plastics have different back pressure requirements. In general, plastics with low melting points need less back pressure, while plastics with high melting points need more back pressure.

So, you need to adjust the back pressure based on the specific type of plastic you’re using when you’re injection molding.

Here’s how you figure out what back pressure to use based on the type and color of plastic you’re using: Different types and colors of plastic need different back pressure settings.

If you don’t set the back pressure value right, it’ll cause shrinkage, warping, and other problems in your part. As a general rule, transparent or glossy materials have higher back pressure values, while non-transparent or matte materials require lower back pressure values.

Adjust Mold Temperature

Mold temperature is one of the important factors affecting the size of back pressure. When adjusting the back pressure, you can adjust the mold temperature first, and then make corresponding modifications based on the effect of the adjusted back pressure.

Adjust the Injection Molding Machine Pressure Setting

The pressure setting of the injection molding machine will also affect the size of the back pressure. Generally speaking, a higher pressure setting requires a higher back pressure, and a lower pressure setting requires a lower back pressure.

When adjusting the back pressure, you can first adjust the pressure setting of the injection molding machine appropriately, and then make corresponding modifications based on the effect of the adjusted back pressure. In addition, injection molding pressure settings typically start with the injection unit and pressure settings.

What are the Benefits of Properly Adjusting the Back Pressure?

It can compress the molten material in the barrel, increase the density, and improve the stability of the injection volume, product weight and size.

It can “squeeze out” the gas in the molten material, reduce the gas flowers and internal bubbles on the surface of the product, and improve the uniformity of gloss. Slow down the screw retreat speed to fully plasticize the molten material in the barrel, increase the mixing uniformity of the color powder, masterbatch and molten material, and avoid the phenomenon of “mixed color” in the product.

Slow down the screw retreat speed to fully plasticize the molten material in the barrel, increase the mixing uniformity of the color powder, masterbatch, and molten material, and avoid the phenomenon of “mixed color” in the product.

Increasing the back pressure properly can improve the shrinkage of the product surface and the glue flow around the product.

It can increase the temperature of the molten material, improve the plasticization quality of the molten material, improve the fluidity of the molten material when filling the mold, and there is no cold glue mark on the surface of the product.

It is good to take out the gas in the barrel and reduce the gas flowers and bubbles on the surface of the product;Slow down the screw retreat speed to fully plasticize the molten material in the barrel;

Properly increase the back pressure to improve the shrinkage of the product surface (applicable to molds and products without sequential valve gates) .

It can heat up the molten stuff, make the molten stuff more melty, and make the molten stuff flow better when it goes into the mold.

Conclusion

In short, adjusting the back pressure of the injection molding machines correctly can improve the injection molding quality and production efficiency, reduce the scrap rate and production cost.

Therefore, during the injection molding manufacturing process, we need to adjust according to the specific situation and find the best back pressure adjustment method to achieve the best injection molding effect.