1. Screw speed and stroke settings
The setting of the screw rotation speed and stroke discussed in this section means that the plastic is pushed to the front of the injection cylinder by the rotation of the screw during the plasticizing process. The forward speed of the screw (injection speed) is discussed in a later section.
1.1 Screw rotation speed
Part of the heat needed to soften the plastic comes from the rotation of the screw. The faster the rotation, the higher the temperature. Although the rotation speed of the screw can reach a very high value, this does not mean that we should use such a high rotation speed. It is better to adjust the rotation speed of the screw according to the type of construction plastics and the length of the production cycle.
1.1.1 Selection of screw rotation speed
The rotational speed of the screw significantly affects the stability of the injection molding process and the amount of heat applied to the plastic. When the screw rotates at high speed, the friction (shear) energy transmitted to the plastic increases the plasticizing efficiency, but at the same time, it also increases the non-uniformity of the melt temperature. This is extremely undesirable for the stability of the production, as it may cause local overheating of the melt. Moreover, the use of high screw speed also increases the consumption of energy (electric energy), on the contrary, the lower the screw speed, the better the temperature uniformity of the melt, because there is no local overheating phenomenon; Moreover, from an economic point of view, less energy is required for the manufacture of the product, from which it can be seen how important it is to select the correct screw rotation speed in order to specify the production capacity of a certain injection molding process, and the value to be set must be able to take into account the natural changes in the parameters of the various conditions during production.
When we refer to the rotational speed of the screw, the most important parameter is the speed of the screw. The maximum screw surface speed allowed by different plastic materials is also different (please refer to Table 6.2). The unit of this speed is millimeter/second (mm/s), or fat scale/second (m/s), or British scale/second (ft/s). Because the rotational speed of the screw (rpm) and its surface speed are linear, different plastic materials have different speeds. Their maximum allowable rotational speeds are also unequal.
1.1.2 Calculation of screw speed
The screw rotation speed of a large injection molding machine should be less than that of a small injection molding machine. The reason is that the shear heat energy generated by a large screw is much higher than that of a small screw at the same rotation speed. Mathematically, we can use the following formula to express the relationship between the surface speed of the screw and the diameter of the screw and the speed of the screw per minute.

Screw surface speed (mm/s) = screw diameter (mm) × screw speed (rpm) × 0.0524 (where 0.0524 is a conversion constant between mm and rpm). The optimum and maximum screw surface speeds for each plastic are detailed in Table 1.1.
The maximum value in the table can help the injection molding personnel to determine whether the production problem is caused by the screw speed. As a matter of fact, many injection molding personnel unconsciously use too high screw speed when adjusting the injection molding machine, and do not realize the need to reduce the screw speed until the thermal degradation of the plastic occurs (the thermal degradation of the plastic can be known from the shortcomings of the injection parts). When the screw speed is appropriate, the temperature of the melt is relatively stable, and the wear of the screw and the injection cylinder device of the injection molding machine is relatively slight.
1.2 Screw reset
The reset of the screw means that after the plasticizing process of the injection cycle is finished, the screw moves backwards to the original position before injection, the rotating speed of the screw when the screw moves backwards is a preset value, a preset back pressure acts on the screw at the same time, and the reason why the screw can move backwards stably is that the plastic in the injection cylinder is pushed forwards at a stable speed. At the same time, as the melt continues to increase in the front of the shooting cylinder, a pressure is generated on the screw, forcing it to move back to its original position.
1.2.1 Importance of accurate reset of screw
The accuracy of the reset of the screw per cycle is not important. This position determines the actual distance the screw needs to advance (screw stroke) in the next cycle so that the cavity can be fully filled before the packing switch, which affects the subsequent injection time, the length of the screw pad and the weight of the injection part, especially when the mode of packing switch is stroke-dependent or time-dependent, so the accuracy of the screw reset is less accurate. The greater the instability of the production process. The reset change of the screw is usually due to the fact that the screw runs through the set weight (metering position) when it retreats; Ideally, the screw should stop at the set weight position when it retracts. However, the fact that the screw often recedes a little causes a change in the melt volume at the front end of the screw. The larger the diameter of the screw is, the more the offside degree of the screw needs to be controlled. A good injection molding practice is to control the offside range within 0.4 mm (0.16 in), preferably within 0.2 mm (0.008 in).
Table 1.1 Typical Screw Speed (mm/s)
Most ideal maximum
　　OPTIMUM　MAXIMIM　　
ABS　　550　　650　　
BDS　　700　　750　　
GPPS　　800　　950　　
HIPS　　850　　900　　
LDPE　　700　　750　　
NYLON11／12　　400　　500　　
NYLON66　　400　　500　　
PC　　400　　500　　
PEI　　400　　500　　
PES/PSU　　150　　250　　
PMMA　　350　　400　　
POM(HO)　　100　　300　　
PP/EPDM blend 550 650
PPS　　200　　300　　
PSU　　150　　250　　
PC/ABS blend 450 550
PC/PBT blend 350 400
ASA　　600　　650　　
EVA　　500　　550　　
GDPE　　750　　800　　
HIPS (Fast Cycle) 950 1000
LLDPE　　700　　750　　
NYLON6　　400　　500　　
PBT　　300　　350　　
PEEK　　300　　400　　
PETP　　250　　350　　
PETG　　300　　400　　
POM(CO)　　200　　500　　
PP　　750　　850　　
109
PPO-M　　400　　500　　
PPVC　　150　　200　　
SAN　　400　　450　　
UPVC　　150　　200　　
TPU/PUR　　250　　400　　
Convert to ft/sec divided by 304.8i.e. 300mm/s = 0.98ft/s
Convert to m/sec divided by 1,000i.e 300mm/s = 0.3m/SCC
1.2.2 Deceleration when the screw is reset
If we can use the fast/slow double-speed method in the reset action of the screw, the degree of the screw back offside will be greatly reduced, which can be controlled within the range of 0.2mm (0.008 in), and the plasticizing process can be stabilized.
Having found the maximum allowable screw speed for the construction plastic (see Table 6.2), we should start with a lower optimum screw speed, choose the optimum speed as the first speed, and switch to a slower second speed with about 15% of the return stroke remaining. A second speed, which is about 60-70% of that optimal speed, is selecte so that the reset action is completed 1-2 second before the mold cooling stage is completed. If such a fast/slow screw speed does not allow the reset action to be complete before the cooling stage, it is necessary to slightly increase the retreat speed of the preceding section of the screw, O that the screw can be reset before the end of the cooling stage (including decompression or screw backward movement of screw reset).
1.3 Screw stroke
The screw stroke refers to the linear movement distance of the screw in the injection cylinder of the screw direct injection molding machine. The movement distance is from the position where the screw stops after the plasticizing process is completed to the switching position of the holding pressure (mold holding pressure).
The melt volume injected into the cavity during the mold filling phase is largely determined by the linear stroke of the screw.
1. Screw rotation speed (as described above);
2. Backward movement at the end of screw reset;
3. The value of the back pressure used when the screw is reset.
1.4 Retreat of screw
The need for screw retraction (often referred to as "backing off" or "pressure relief") during the injection cycle is based on several reasons. Initially, the screw is reset to prevent melt leakage from the nozzle, avoiding the need for a closed nozzle. Now the application of screw reset is extended to improve the stability of the production process, and the speed and distance of screw retraction should be accurately controlled to achieve the purpose of stabilizing the production process.
1.4.1 Retreating speed and distance of the screw
For the traditional hydraulic injection molding machine; The screw reset is that the hydraulic cylinder pulls the screw backward for a preset distance. For the all-electric injection molding machine, the backward action of the screw is the reverse rotation of the AC servo motor.
Like the screw rotation speed, the screw retreating speed can be selected at will, and the faster the retreating speed is, the less controllable the retreating distance is. So we should use 20-30% of the fastest possible retreat speed of the screw. If the screw is based on
A controlled and slower speed of retreat; The flow stop valve thread sleeve at the front end of the screw rod can return to the same position in each injection molding cycle, and the change of the length of the screw gasket is reduced. The distance that the screw is retracted depends on the diameter of the screw and the design movement range of the stop valve thread sleeve. Typical values are 4-10 mm (0.160-0.394 in). In applications where high injection speed is required, the screw reset and distance may be 12-18 mm (0. 472-0.709in)。 Some plastics (such as polyolefin) often require a long screw reset distance to ensure that the stop valve thread sleeve can return to the same position. However, if the screw reset distance is too long, the phenomenon of oxygen absorption (air is absorbed into the melt from the nozzle or feed inlet) can be formed, which is manifested by silver wires or pouring marks on the finished product.
1.4.2 Optimum setting of screw reset distance
In order to set the optimal stroke of the screw reset distance, we need to know the actual movement range of the stop valve thread sleeve. Different injection molding machines have different stop valve designs. In general, however, the stop valve insert is designed to travel about 1-1.2 times the crater depth of the screw injection section.
Secondly, we also need to know the offside distance when the screw retreats. This offside distance must be stable, and then we measure the value of this distance. The correct screw reset distance is the sum of the movement distance of the stop valve thread sleeve and the screw offside distance plus 0.5mm (0.020in).
The calculation method is as follows:
If the screw position set by the stock = 85mm (3.347 in)
Actual screw stop = 86.2mm (3.394 in)
That is, screw offside distance = 1.2mm (0.047 in)
Movement distance of stop valve thread sleeve = 4.5mm (0.177in)
Therefore, the screw should be reset to the selected distance = 86.2 + 4.5 + 0.5mm = 91.2mm (3.59 in)
In many cases, the injection molding personnel just casually add 3mm (0.118 in) to the set position of the stock when setting the reset (reverse cable) position of the screw. They don't know that the screw will be offside. In this case, the production process must be unstable, and the variation range of the screw stop position exceeds the specified 0.4 mm (0.016 in), resulting in the variation of the length of the screw spacer exceeding 0.4 mm (0.016 in). Table 1.2 shows the difference between correct and incorrect screw position settings for the injection molding machine.
2. Back pressure
As the screw rotates, the plastic, which is softened (plasticized) by heat, is pushed forward past the stop valve to the front of the screw. As the melt is constantly pushed forward, pressure is created in this area and acts on the screw and the stop valve, pushing them back so that there is more room for more melt. When the screw retreats, the piston of the connected hydraulic cylinder is also pushed back, and the pressure oil in the rear chamber of the hydraulic cylinder flows back to the oil cylinder of the injection molding machine through the oil pipe. If we control the backflow speed of the pressure oil, a pressure will be generated in the rear chamber of the hydraulic cylinder (the pressure provides the backward resistance of the screw), and the greater the speed limit of the backflow pressure oil is, the greater the pressure generated in the hydraulic cylinder is; We call this pressure back pressure.
2.1 Types of back pressure
There are two kinds of back pressure, which are called oil circuit back pressure and melt back pressure respectively. Usually, the back pressure we say mostly refers to the oil circuit back pressure, and its application is necessary to maintain the quality of the finished product (the pressure range can be adjusted to 25% of the maximum oil circuit pressure). The back pressure of the oil circuit is generated from the oil pressure cylinder for injection, which acts on the screw during the storage stage and slows down the retreat speed of the screw. Therefore, the higher the oil circuit back, the longer the reset time of the screw, and the pressure generated by the molten material in front of the screw must be greater than the oil circuit back pressure to make the screw move backward.
The increasing melt at the front end of the injection cylinder generates the pressure that makes the screw retreat, which is called melt back pressure and has a direct relationship with the back pressure of the oil circuit. This relationship is related to the configuration of the injection molding machine (such as the screw diameter and the piston diameter of the injection oil pressure cylinder), and the general design practice is that the oil circuit back pressure is one tenth of the melt back pressure generated.
Most injection molding machines are powered by oil pressure, so it is very easy to adjust the back pressure during the storage process, and different back pressure values can be used at different screw positions. However, for all-electric injection molding machines, the control of back pressure is more complex. The setting of back pressure (through the load device or converter) when the screw rotates produces resistance on the pressure bearing. The value of this resistance is a function of the rotation speed of the AC servomotor, that is, the higher the value of the back pressure, the greater the resistance, and the lower the rotation speed of the servomotor. For an all-electric injection molding machine, the back pressure can be called a resistance-induced back pressure.
Table 1.3 Screw position before consistent and inconsistent injection molding
Consistent screw Non-consistent screw
Backward position Backward position
　　mm　　 in　　　　   mm　　in　　
　　70.6　　2.780　　　　70.2　　2.764　　
　　70.6　　2.780　　　　70.4　　2.772　　
　　70.6　　2.780　　　　70.5　　2.776　　
　　70.6　　2.780　　　　70.6　　2.78　　
　　70.6　　2.780　　　　70.5　　2.776　　
　　70.7　　2.784　　　　70.3　　2.768　　
Table 1.4
　　70.6　　2.780　　　　70.2　　2.764　　
　　70.6　　2.780　　　　70.6　　2.780　　
　　70.7　　2.784　　　　70.5　　2.776　　
　　70.6　　2.780　　　　70.6　　2.780　　
　　70.6　　2.780　　　　70.4　　2.772　　
　　70.7　　2.784　　　　70.3　　2.768　　
　　70.6　　2.780　　　　70.5　　2.776　　
　　70.7　　2.784　　　　70.6　　2.780　　
　　70.6　　2.781　　　　70.4　　2.722　　
　　0.1　　0.004　　　　 0.4　　0.016　　
2.2 Function of back pressure
The application of back pressure can ensure that the screw can generate enough mechanical energy to melt and mix the plastic when it is rotated and reset. The back pressure also has the following uses:
1. Discharging volatile gas including air out of the injection cylinder;
2. Mixing the additive (such as toner, color seed, antistatic agent, talcum powder, etc.) And the melting material uniformly;
3. Homogenizing the melt flowing through the different auxiliary screw lengths, and
4. Provide uniform and stable plasticized material for precise weight control of the finished product.
Many injection molders use only a single value of back pressure throughout the entire storage process. The value of back pressure selected 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 is free of bubbles, volatile gases, and incompletely plasticized colloidal particles. For all-electric
The maximum resistance induction back pressure of the dynamic injection molding machine is also set to be equivalent to 15 bar (217.5 psi) of the oil circuit back pressure. The selected value is in direct proportion to the force acting on the motor pressure bearing. In order to facilitate the conversion of the resistance of the melt back pressure bearing, it can be found from the chart.
The use of back pressure increases the pressure temperature of the injection molding machine and the melt temperature. The magnitude of the rise is related to the value of the back pressure that is set. For larger injection molding machines (screw diameter over 70 mm (2.75 in)), the oil circuit back pressure can be as high as 25-40 bar (362.5-580 psi).
However, it should be noted that too high oil circuit back pressure or resistance induction back pressure causes too high melt back pressure, which also indicates that the melt temperature in the injection cylinder is too high, which is destructive to the production of heat-sensitive plastics.
Moreover, too high back pressure also causes the screw to be too large and irregular offside, which makes the injection amount extremely unstable. The amount of offside is affected by the viscoelastic properties of the plastic; The more energy stored in the melt, the greater the offside distance of the screw. This stored energy causes a sudden backward jump of the screw when it stops rotating, which is more pronounced for some thermoplastics than for others, such as LDPE, HDPE, PP, EVA, PP/EPDM composites, and PPVC. GPPS, HIPS, POM, PC, PPO-M, and PMM are all relatively prone to jitter.
In order to obtain optimum production conditions, it is important to set the back pressure correctly so that the melt is properly mixed and the screw offsetting range does not exceed 0.4 mm (0.016 in).
2.3 Application of multiple back pressure
Due to the backward movement of the screw during the storage phase, the effective length of the plastic passing through the screw is not the same, which means that the shear energy acting on the plastic is also different. Therefore, the longer the screw stroke is, the greater the change of the effective plasticizing length of the screw is, and the greater the unstable effect is. If we constantly change the value of the back pressure during the storage, we can offset the change of the effective length of the screw plasticization, and have a more stable effect on the offside phenomenon of the screw (sometimes called the screw runout phenomenon).
The effective plasticizing length of the screw is further explained here. Since the screw rotates and retreats at the same time during the storage stage, we can imagine that the length of the screw from the end to the feed inlet is different at the beginning and end of the storage. The length of the screw is the longest at the beginning of the storage and the shortest at the end of the storage. This means that the plastic that falls into the screw pit at different times has to flow through the screw for different lengths and absorbs different shear energy. This phenomenon causes the nonuniformity of the melt temperature (that is, viscosity), so the quality of the finished product obtained is also unstable.
The use of different and increasing back pressure values at different screw retreat positions can greatly reduce the effects of the above phenomena and stabilize the production process. For example, by increasing the back pressure and decreasing the screw rotation speed in the last 10-15% of the storage stroke, the crossing distance of the screw can be successfully controlled within 0.2mm (0.008in). Of course, the best match between the screw rotation speed and the back pressure can only be obtained after some tests. Screw parameter setting as in the following example;
1. The screw is initially rotated at the optimum surface speed. The melt back pressure is a value of 50-7 bar (752-1,015 psi). The optimum screw surface speed for a particular plastic can be found from Table 6.2.
2. The melt back pressure rises to 100 bar (2,450 psi) and 20 bar (1,740 psi) at 25% and 60% of the screw stock stroke, respectively; The rotational speed of the screw is constant to reduce the variation caused by different effective lengths of the screw.
3. At 85% completion of the screw stock stroke, the melt back pressure is increased to 150 bar (2,175 psi); The screw speed is halved to reduce the degree of screw offside.
4. When the screw stops rotating, back off the screw by 5 mm (0.197 in) (reverse cable or depressurize) (see Figure 6.4).
When the plastic of construction is nylon 11/12, the melt back pressure is often 11410 bar (145 psi) and is constant throughout the stock phase, and in fact many injection molding handbooks recommend such a back pressure value in order to achieve the stability of the nylon plastic described above
Produce.
6.5.4 Recommended back pressure values
The following are some recommended melt back pressure values for thermoplastics for general reference purposes.

(Note: 1Kg/cm2 = 14.3P Si, 1bar = 1.02Kg/cm2)

The most stable production state can be obtained if different combinations of screw rotation speed and back pressure values can be properly used in the storage stage.
6.6 Starting the mold filling phase
In order to get the best cavity filling state before packing switching, we need to correctly set the value of each point on the injection speed curve and control the injection volume.
The actual injection volume of the plastic as the screw advances is influenced by:
1. The position where the screw stays before injection.
2. Offside distance after screw storage.
3. The value of the back pressure used.
4. Screw rotation speed.
5. Screw stroke used.
6. Operation of the stop valve.
6.6.1 Stop valve device
The stop valve at the front end of the screw is also called the backflow valve. Its function is to prevent the melt at the front end of the screw from flowing backward before the injection stage.
The closing action of the stop valve is very important. It must be accurate and stable. The closing time required by the stop valve is also important. If this time changes, the amount of melt flowing back through the stop valve is also unstable, resulting in different injection amount per cycle. This is evidenced by the change in the length of the screw pad plastic.
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In order to achieve the constant closure of the stop valve, the stop valve thread sleeve must stay at the same position after the screw is reset, so the speed and stroke of the screw must be adjusted properly every cycle, and the best stroke of the screw should be more than the actual closure of the stop valve.
The stroke length shall be 0.5mm (0.020in), and the reversing speed shall be 20-30% of the maximum speed.
6.6.2 Mold filling speed
Mold filling rate is the linear rate at which melt is injected into the mold cavity. During the filling phase of injection molding, the rate at which melt enters the cavity must be controlled to suit the characteristics of the product plastic and the mold. For example, the injection speed should be set as fast as the product design, wall thickness design and plastic characteristics allow, and thin-walled injection parts will require extremely fast injection speed to ensure that the finished product is full; Whereas wall casting requires a slow injection speed to prevent the formation of voids and scars. No matter what value of injection speed is used, it should be the maximum value within the possible range. Generally, the setting of injection speed depends on the experience of injection molding personnel, and there is usually only one set value, such as 120 mm/s (4.724 in/s), but the actual injection speed is not constant during the whole production period. Its value is influenced by the different homogeneity of the melt and the temperature of the pressure oil.
6.6.2.1 Pressure oil temperature
One of the reasons for the change of mold filling is the pressure oil temperature of the injection molding machine, which can significantly affect the performance of the injection molding machine. Therefore, the operating oil temperature of the injection molding machine is generally recommended to be 40-50 ℃. Some injection molding machines are equipped with heating devices, which can raise the oil temperature to the appropriate operating temperature in advance before production. If the temperature has not reached the set value, The injection molding machine cannot be started.
6.6.2.2 Injection pressure and speed
In order to ensure that the injection speed of the injection molding machine reaches and maintains the required value, the injection pressure value must be set enough. The injection pressure is one of the important production parameters, which determines the forward speed of the screw and the subsequent filling of the mold.
If the injection pressure is too low, the resistance generated when the melt enters the mold will be relatively large, so that the injection speed can not be maintained at the set water level, resulting in a long injection time. Therefore, when setting the injection pressure, a certain value should be 15 bar (217 psi) higher than the resistance caused by the melt. The resistance value produced by the melt can be obtained from the pressure gauge of the injection molding machine or from the database of the screen of the advanced injection molding machine (the system is equipped with a stress converter to detect the resistance of the melt). The pressure gauge and/or the stress converter are installed near the oil supply throttle of the injection oil pressure cylinder.
Since too low an injection pressure causes a change in the injection cycle time, we can apply this principle to check whether the injection pressure is sufficient. At the optimum injection pressure setting, the injection cycle time should not vary by more than 0.08 seconds, preferably within 0.04 seconds.
If the injection cycle time varies by more than 0.08 seconds, it may indicate, in addition to insufficient pressure, poor melt uniformity or a problem in controlling the injection speed 116 of the injection molding machine.
6.6.2.3 Consideration of gate
In many cases, the setting of the injection speed depends on the size and position of the gate, especially when the diameter of the gate is in the range of 0.5-1.5 mm (0.020-0.060 in). When the melt passes through the fine gate at a high speed, a large shear stress is generated in the melt. The higher the velocity, the greater the shear stress. This huge shear stress makes the temperature of the molten material rise sharply, which can cause the degradation of the plastic and make the surface of the finished product have ugly marks.
If the diameter of the gate is slightly increased, the shear stress can be greatly reduced, and the defects of the casting can be eliminated. The following example illustrates the effect of increasing the gate diameter; The cross-sectional area of a 0.6 mm (0.024 in) diameter gate is 0.283 mm2 (0.00045 in2), while the area of a 0.7 mm (0.028 in) diameter gate is 0.385 mm2 (0.00062 in2). If we can increase the diameter of the gate from 0. 6 mm (0.024) to 0.7 mm (0.028 in), the increase in area I
0.103mm2 (0.00017in2) is equivalent to a 26% increase in area. Increasing the cross-sectional area of the gate greatly reduces the shear stress of the melt, which is likely to sweep away the related problems caused by injection molding.

In fact, I have introduced the back pressure in my previous post, and it is also necessary for us to discuss it further today. The so-called back pressure is the pressure that the melt at the head of the screw is subjected to when the screw rotates and retreats, which can be adjusted by the back pressure control circuit in the hydraulic system. During premolding, the screw can retreat only after the melt pressure at the head of the screw overcomes the system resistance when the screw retreats. It is well known that the back pressure during injection molding is related to the plasticizing quality, driving power, leakage flow and plasticizing capacity.

    The effect of back pressure on the melt temperature is very obvious: for different plastics, the melt temperature increases with the increase of back pressure under certain process parameters. The reason is that the back pressure increases the pressure in the melt, enhances the shear effect and forms the shear heat. The heat energy of that macromolecule is increase, thereby increasing the temperature of the melt.

    The increase of the back pressure contributes to the compaction of the plastic in the spiral groove and drives away the gas in the material; The increase of back pressure increases the resistance of the system, slows down the screw withdrawal speed, prolongs the thermal process of the material in the screw, and improves the plasticizing quality. However, excessive back pressure will increase the backflow and leakage of the melt in the metering section of the screw, reduce the melt conveying capacity, reduce the amount of plasticization, and increase the power consumption. If the back pressure is too high, the shear heat will be too high or the shear stress will be too large, which will degrade the polymer materials and seriously affect the quality of products.

    Injection molding thermosensitive plastics, such as PVC, POM, etc., with the increase of back pressure and melt temperature, the surface quality of products is better, but it may cause discoloration, performance deterioration and degradation of products. Injection of plastics with higher melt viscosity, such as PC, PSF, PPO, PPS, PSU, etc., with too high back pressure. It is easy to cause power overload; Plastics with very low melt viscosity, such as PA, have too high back pressure, on the one hand, they are easy to cast, on the other hand, their plasticizing ability is greatly reduced. In the above cases, the back pressure should not be too high.

    Some plastics with good thermal stability and moderate melt viscosity, such as PE, PP, PS, ABS, etc., can increase the back pressure appropriately. In general, the back pressure shall not exceed 15 kg/cm2. Now many back pressures are controlled in a computer by means of proportional adjustment. After further development, there is now a plasticizing system with self-generated stable melt adhesive back pressure. It can eliminate the uneven density of plastic melt at the front end caused by the uneven stacking density of plastic particles, the pause in the working process, the change of movement resistance, the change of the effective length of the screw in the glue melting process and other factors to the maximum extent. After the back pressure is controlled by the computer, it is easy to adjust and control the multi-level back pressure.

The back pressure is related to the type of nozzle and the feeding mode: when the straight-through nozzle or the post-feeding mode is selected, the back pressure should be low to prevent the casting caused by the increase of the back pressure; The back pressure can be slightly increased by the self-locking nozzle or the pre-feeding and fixed feeding modes. Some people always think that the adjustment of back pressure should be determined according to the on-site situation of the product, which is right, but after knowing the principle of back pressure, if you adjust the back pressure without an upper limit, you are not a potential craftsman, because you may have been deeply misled and unable to extricate yourself!