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.

Injection Mold Maker for Making Injection Molding Process Easier

Injection Mold Maker for Making Injection Molding Process Easier

Are you looking for high-grade and advanced injection mold machines or complete solutions for injection molds?

Such questions are very common in manufacturing units – mainly those where molding is an important work to give the right shape and design to any machinery, tool or equipment. A selected injection mold maker brings to you a gamut of solutions that are working dedicatedly to bring to you complete solutions. There is no denying the fact that injection mold is so expensive – not only in cost, but also in time. An injection mold maker takes the responsibility of doing everything from very beginning to end.

They provide you complete solutions and injection mold machine – related to mold overall size, location ring, sprue bush, ejection connector tap size with machine and solutions for parts related to gate, parting line and ejection. You will get mold related to waterline layout and nipple size along with cavity layout, runner layout and weight along with mold steel type and hardness.

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Not to mention CAE analysis and know problem a bit early in some case, where CAE is necessary to prevent problems in advanced instead of after mold construction.

You will also get detailed mold test report that assures easy and fast setting in mass production at client site.

Reaching the right company or a selected injection mold maker is important. Online search is one of the time-saving and convenient way to help you in reaching the right one. Among some of the top companies, you will find name of Jazz Mold comes on the top. You have to contact as per your requirement and leave rest of the work on experts working here.

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.

Screw Selection on Injection Molding Machine

Screw Selection on Injection Molding Machine

As we know, “screw plasticizing component” of the injection molding machine is the main core unit of the injection molding machine, which directly reflects the performance of the injection molding machine. Screw is  “heart”, and its performance determines the quality and efficiency of the processed product. With the rapid development of the plastics industry, injection molding equipment has matured. As the designer and user of injection molding equipment, it is necessary to understand which screws are suitable for the processing of materials. Users often use a screw to process plastics with unsuitable physical properties. There is also a misunderstanding: “I think a screw is omnipotent”, but it is not. By this, a brief introduction of several commonly used screws can be used to provide some references when processing products.


(1) PC material (polycarbonate) features:

1. Non-crystalline plastic, no obvious melting point, glass transition temperature 140 ° C -160 ° C, melting temperature 215 ° C -225 ° C, molding temperature 230°C -320°C;

2. High viscosity, high processing temperature, narrow range, easy to decompose, avoiding overheating when using screw;

3. Sensitive to moisture, the resin is easily hydrolyzed at high temperature, the drying temperature is 120 ° C -130 ° C, and the time is about 4 hours.

Parameter selection:

1. L/D is designed to select a large aspect ratio for improving the plasticizing effect for its good thermal stability and high viscosity;

2. Due to its wide melting temperature range, the feeding section is long, the homogenization section is short, and the compression ratio is moderate. The barrier type over-adhesive head can be selected for increasing the plasticizing effect;

3. For example, if you add flame retardant or glass fiber reinforced PC material, you can choose to use corrosion-resistant and wear-resistant high-quality stainless steel tool steel screw and three small pieces.


(2) PMMA (Plexiglas) Features:

1. The glass transition temperature is 105 ° C, the melting temperature is greater than 160 ° C, the decomposition temperature is 270 ° C, and the molding temperature range is 180-250 ° C;

2. High viscosity, poor fluidity, sensitive to shear rate, strict temperature control during processing;

3. The cylinder temperature is 240 ° C -250 ° C, and the residence time exceeds 8 minutes when the temperature exceeds 260 ° C;

4. Strong water absorption, drying temperature 80 ° C -90 ° C, time more than 3 hours.

Parameter selection:

1. L/D selects the aspect ratio 20-22, and the gradient type screw generally takes L1=35-45%, L2=35-40%;

2. The compression ratio ε generally takes 2.3-2.6;

3. For its hydrophilicity, a mixing head structure is used at the front end of the screw.


(3) PA (polyamide), common PA6 (nylon 6), PA66 (nylon 66) characteristics:

1. There are many kinds of crystalline plastics, different types, different melting points, narrow upper melting point, generally PA66 melting point of 260 ° C -270 ° C;

2. Low viscosity, good fluidity, relatively obvious melting point, poor thermal stability;

3. The water absorption rate is high, the dryer temperature is 80 ° C, does not exceed 120 ° C, and the time is 16 hours. If the material is exposed to air for 8 hours, use a vacuum oven at around 105 ° C for 12 hours.

Parameter selection:

1. L/D selects a mutant screw having a length ratio of 18-20;

2. The compression ratio is generally selected from 2.5 to 3.5, which prevents overheating decomposition h3: 0.067-0.089D;

3. Because of its low viscosity, the gap between the rubber ring and the barrel should be as small as 0.05-0.08, and the gap between the screw and the barrel is 0.08-0.12.

4. The nozzle can be equipped with a spring self-locking type nozzle.


(4) PET (Polyester) Chemical Name: Polyethylene terephthalate Characteristics:

1. Melting point of 250 ° C -260 ° C, blow molding PET molding temperature is wide, about 255 ° C -290 ° C, enhanced grade GF-PET up to 290 ° C -315 ° C;

2. High melting point, low viscosity, high adhesion to metal, easy to hydrolyze;

3. Hydrophilic, the binder is sensitive to water at high temperature, the drying temperature is 150 ° C -160 ° C, and the time is more than 4 hours.


1.L/D generally takes 20-22, three-segment distribution L1=50-60%, L2=20-30%;

2. Using low shear, low compression ratio screw compression ratio ε = 1.8-2.2, because shear overheating leads to discoloration or opacity h3=0.09-0.10D;

3. When using recycled material (sheet material), increase the conveying capacity of the feeding section and adopt the forced feeding plasticizing system.


(5) PVC (polyvinyl chloride)

Thermally sensitive materials are generally classified into hard and soft, the difference being the amount of plasticizer added to the raw material, less than 10% being hard, and more than 30% being soft.


1. No obvious melting point, softening at 60 °C, viscoelastic state at 100 °C-150 °C, narrow melting temperature range at 140 °C, poor thermal stability, easy decomposition, rapid decomposition at 170 °C, softening point close to decomposition point, decomposition release For HC1 gas.

2. Due to the high viscosity of UPVC melt, poor thermal stability, temperature and time will lead to decomposition, so it has a corrosive effect on general steel;

3. The temperature control is strict, the screw design should be as low as possible to prevent overheating, the flow channel must be smooth, there must be no dead angle, and the accumulation of material can be prevented from decomposing;

4. The screw barrel should be protected from corrosion;

5. The injection molding process needs to be strictly controlled.


1.L/D=18-21, h3=0.067-0.08D, ε=1.6-2, L1=40-45%, L2=35-45%, in order to prevent the hiding of the material without apron, head 20 ° C -30 ° C, using a hard chrome plated screw;

2. The choice of temperature control;

1 melted cylinder multi-stage air cooling device;

2 Low shear high mixing UPVC special screw barrel device.

Reach right plastic mold factory to get services and processes

Reach right plastic mold factory to get services and processes

Plastic molds are important industrial products in order to shape and mold plastic before it results to a more viable and feasible product in the market. It commonly undergoes injection molding in various forms but the most popular technique is forcing a plastic liquid into a metal mold. The molding is made through the use of molten plastic that cools while it is inside the mold.

Plastic molds are used in order to lessen down the costs related to various manufacturing processes. This technology is favored because of the quick turnaround time in the procedure. A variation known as reaction injection molding is also used for fiberglass, epoxy and polymer plastic. Here are ideas on the services, processes and equipment design employed in the manufacture of these products.

Plastic molding services result from rotational molding, blow molding, thermoforming and compression molding. Other services include mold prototyping, insert molding, micro-molding, two- shot injection molding, machining and hot stamping. These services have characteristics that make them distinct from one another.

For instance, machining is that phase where the molds are removed through drilling, milling or turning. Micro-molding is known to use special and mini molding machines in order to produce small parts out of the process. The parts produced out of these services are commonly less than two grams in weight. In a two- shot injection, a plastic product is produced from two sets of raw materials through the use of only one operation.

Plastic molding processes

Plastic molds cannot be completed without undergoing some processes. First on the list is the development of a three-dimensional or 3D CAD model. The making of a computer model is that point when a manufacturer may ask a consumer for concepts and prototypes to see if they are good enough for the production of plastics.

Machines for injection molding, blow molding and thermoplastic injection are a few examples of plastic mold factory equipment designs. The basic outputs from these machines include insulated runners as well as CAM, encapsulating, stack and insert molds. This important aspect in manufacturing plastic molds is the fabrication of machineries to create the output. Barrels, liners, component and feeders are also created out of the equipment design. Along the process, special forms of equipment for different plastic mold services are formed.

Plastic molding, one of the most vital process required, is required to taken into utilize to shape plastic using a rigid frame or mould. Not to mention the way of using technique that allows for the creation of objects of all shapes and sizes with huge design flexibility for both simple and highly complex designs.

Reaching the right and reputed plastic mold factory is important where you will gt complete solutions for this process that is a popular manufacturing option – responsible for many car parts, containers, signs and other high volume items. There are a number of added features and benefits associated with it that will surely persuade you to reach the right factory that is involved in offering you complete and comprehensive solutions.