Back Pressure Control of Injection Molding Machine

Back Pressure Control of Injection Molding Machine

The formation of back pressure

During the melting and plasticizing process of the plastic, the melt is continuously moved to the front end of the barrel (measuring chamber), and more and more, gradually forming a pressure to push the screw backward. In order to prevent the screw from retreating too fast, and to ensure that the melt is evenly compacted, it is necessary to provide a reverse pressure to the screw. This reverse pressure, which prevents the screw from retreating, is called back pressure.

Back pressure, also known as plasticizing pressure, is controlled by adjusting the oil return throttle valve of the injection cylinder. The back of the injection cylinder of the pre-plasticized screw injection molding machine is equipped with a back pressure valve to adjust the speed of the injection cylinder when the screw rotates backward, so that the cylinder maintains a certain pressure (as shown in the figure above); the screw backward speed of the whole motor (Resistance) is controlled by an AC servo valve.

Adjustment of back pressure

The adjustment of the injection back pressure should be based on the properties of the raw materials, the drying conditions, the product structure and the quality conditions. The back pressure is generally adjusted at 3-15kg/cm3. When the surface of the product has air, mixed color, shrinkage, and product size and weight change, the back pressure can be increased. When the nozzle leaks, flows, melts overheating, product discoloration, and screw charge too slowly, consider reducing the back pressure.

Benefits of back pressure

1. It can compact the melt in the barrel, increase the density, and improve the stability of the amount of shot and the weight and size of the product.

2. The gas in the melt can be “extruded” to reduce the air bubbles on the surface of the product, internal bubbles, and improve gloss uniformity. Slow down the screw retreat speed, fully plasticize the melt in the barrel, increase the mixing uniformity of the toner, color masterbatch and melt material, and avoid the color mixing phenomenon of the product.

3. Slow down the screw retreat speed, fully plasticize the melt in the barrel, increase the mixing uniformity of the toner, color masterbatch and melt material, and avoid the color mixing phenomenon of the product.

4. appropriate increase of back pressure, can improve the shrinkage of the surface of the product and the rubber around the product.

5. It can raise the temperature of the melt, improve the plasticization quality of the melt, improve the fluidity of the melt during filling, and there is no cold rubber on the surface of the product.

Problems with too low back pressure

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

2. will lead to poor plasticization quality, unstable injection volume, product weight, product size changes.

3. the surface of the product will appear shrinkage, air flower, cold material, uneven gloss and other undesirable phenomena.

4. the product is prone to bubbles inside, the product around the bone and easy to walk away from the glue.

Problem with too high back pressure.

1. The melt pressure at the front end of the barrel is too high, the material temperature is high, the viscosity is decreased, the flow of the melt in the screw groove and the leakage flow between the barrel and the screw gap are increased, which will reduce the plasticizing efficiency (plasticization per unit time).

2. For plastics with poor thermal stability (such as: PVC, POM, etc.) or colorants, the thermal decomposition of the melt due to the increase in temperature of the melt and the increase of the heating time in the cylinder, or the degree of discoloration of the colorant increases, the surface color of the product / Gloss deteriorates.

3. The back pressure is too high, the screw is slowed down, and the pre-plasticizing time is long, which will increase the cycle time, resulting in a decrease in production efficiency.

4. The back pressure is high, the melt pressure is high, and the nozzle is prone to melt the glue after the glue is injected. When the glue is injected next time, the cold material in the nozzle flow channel will block the cold spot in the nozzle or the product.

5. In the process of beer plasticization, the back pressure is often too large, and the nozzle leaks, which wastes the raw materials and causes the heating ring near the nozzle to burn out.

6. The mechanical wear of the pre-plasticizing mechanism and the screw barrel is increased.

UPVC Injection Molding Setting

UPVC Injection Molding Setting

UPVC, also known as hard PVC, is an amorphous thermoplastic resin made by polymerizing vinyl chloride monomer with certain additives (such as stabilizers, lubricants, fillers, etc.).

In addition to the use of additives, the method of blending and modifying with other resins has been adopted, which has obvious practical value. These resins are CPVC, PE, ABS, EVA, MBS, etc.

UPVC has a high melt viscosity and poor fluidity. Even if the injection pressure and melt temperature are increased, the fluidity does not change much. The molding temperature is very close to the thermal decomposition temperature, so the temperature range is narrow. It is a difficult material of molding.

1. Plastic processing

After 24 hours at room temperature, the amount of water absorption is less than 0.02%, so there is no need to dry. If it must be dried, it can be placed in dryer at 60-70 °C for 3 hours, or at 80 ° C for 1-1.5 hours.

Recycled materials can be mixed with up to 20% of the new material, depending on the end use of the product. Abuse of recycled materials can cause injection molding to fail, not only because there are too many recycled components, but also because the materials are reprocessed many times.

2. Injection molding machine selection

UPVC has a high viscosity and easy decomposition, and the decomposition products have a corrosive effect on iron. The screw and barrel is special designed, and extra fan on barrel may be required.

3. Mold and gate design

The mold temperature can be set 40°C. The length of the flow path is short and the diameter is large to reduce pressure loss and keep holding pressure. The shorter the gate, the better. The gate cross section should be round, the diameter is at least 6 mm, and the shape is conical, and the inner angle is 5° degree.

The runner cross section is also round and 7mm diameter.

The mold is stainless steel with a chromium content of at least 13%, preferably 16%, and a Rockwell hardness of at least 55. The mold can be chrome-plated to form protection against corrosion.

4. Melt temperature

It varies from 185-205 °C. The exact melt temperature of the UPVC can be known from the smoothness of the injected material. If the material is rough,  the material is not uniform (insufficient plasticization), indicating that the temperature is too low; if the foam is emitted after the injection and a large amount of smoke is emitted, the temperature is too high.

5. Injection speed

The injection speed need to be slow, otherwise excessive shearing will degrade the material. When UPVC is used to produce extremely smooth thick-walled products, multi-stage injection molding speed should be used.

6. Screw speed

Should be in accordance with the molding cycle. The screw surface speed should not exceed 0.15-0.2m/s

7. Back pressure

Up to 150 bar, but the lower the better. The common is 5 bar.

8. Material residence time in barrel

At 200 °C, the residence time can not exceed 5 minutes. When the temperature is 210 ° C, the residence time can not exceed 3 minutes.