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temperature control
1, barrel temperature: injection molding process needs to control the temperature of the barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect the plasticization and flow of the plastic, while the latter temperature mainly affects the flow and cooling of the plastic. Each plastic has a different flow temperature, the same plastic, due to different sources or grades, the flow temperature and decomposition temperature are different, due to the difference in the average molecular weight and molecular weight distribution, plastic in different types of injection The plasticization process inside the machine is also different, so the choice of cylinder temperature is not the same.
2, nozzle temperature: nozzle temperature is usually slightly lower than the maximum barrel temperature, this is to prevent the melt flow in the straight-through nozzle may occur "flow phenomenon." The nozzle temperature should not be too low, otherwise it will cause the premature coagulation of the melt and plug the nozzle, or affect the performance of the product due to the early injection into the cavity.
3, mold temperature: mold temperature on the product's intrinsic performance and apparent quality. The mold temperature is determined by the presence or absence of plastic crystallinity, product size and structure, performance requirements, and other process conditions (melt temperature, injection speed and injection pressure, molding cycle, etc.).
Pressure Control: During the injection molding process, the pressure includes plasticization pressure and injection pressure, and directly affects the plasticization and quality of plastics.
1, plasticizing pressure: (back pressure) When using a screw type injection machine, the pressure on the top of the screw when the screw rotates back is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted by the relief valve in the hydraulic system. In the injection, the plasticizing pressure is constant with the rotation speed of the screw. When the plasticizing pressure is increased, the temperature of the melt is increased, but the speed of plasticization is reduced. In addition, increasing the plasticizing pressure can often make the temperature of the melt uniform, the mixing of the colorant and the discharge of the gas in the melt. In general operation, the decision of plasticizing pressure should be as low as possible on the premise of ensuring good quality of the product. The specific value varies depending on the type of plastic used, but it usually rarely exceeds 20 kg/cm2.
2. Injection pressure: In the current production, almost all the injection pressure of the injection machine is based on the pressure exerted by the plunger or the top of the screw on the plastic (converted from the oil pressure). The role of injection pressure in injection molding is to overcome the flow resistance of the plastic from the cartridge to the cavity, to give the rate of filling of the melt, and to compact the melt.
Molding cycle
The time required to complete an injection molding process is called the molding cycle, also called the molding cycle. It actually includes the following sections:
Molding cycle: The molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, all relevant time in the molding cycle should be minimized under the premise of ensuring quality. During the entire molding cycle, the injection time and cooling time are the most important and they have a decisive influence on the quality of the product. The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3-5 seconds.
The pressure holding time in the injection time is the pressure time for the plastic in the cavity, and the proportion of the plastic injection time is relatively large, generally about 20-120 seconds (extra-thick workpieces can be as high as 5 to 10 minutes). Before the melt is frozen at the gate, how much pressure holding time will have an effect on the accuracy of the product size, and if it is later, it has no effect. The dwell time also has the most favorable value. It is known that it depends on the material temperature, mold temperature, and the size of the main channel and gate. If the dimensions and process conditions of the main sprue and the gate are normal, it is usually the pressure that results in the smallest range of fluctuations in the shrinkage of the product. The cooling time is mainly determined by the thickness of the product, the thermal and crystallization properties of the plastic, and the temperature of the mold. The end of the cooling time should be based on the principle of ensuring that the product does not cause changes during demolding. The cooling time is generally about 30 to 120 seconds. It is not necessary to cool it for a long time, not only reducing the production efficiency, but also It is difficult to release the mold, and it may even cause mold release stress when forcibly ejecting the mold. Other times in the molding cycle are related to the continuity and automation of the production process and the degree of continuity and automation.
The general injection molding machine can be adjusted according to the following procedure:
According to the temperature range provided by the material supplier's data, the barrel temperature is adjusted to the middle of the range and the mold temperature is adjusted.
To estimate the amount of glue required, the injection molding machine was adjusted to two-thirds of the estimated maximum shot volume. Adjust the reverse cable (glue) travel. Estimate and adjust the secondary injection time and adjust the secondary injection pressure to zero.
Initially adjust the injection pressure to one-half (50%) of the limit of the injection molding machine; adjust the injection speed to the maximum. Estimate and adjust the cooling time required. The back pressure was adjusted to 3.5 bar. Remove the degraded resin from the barrel. Use semi-automatic injection molding mode; start the injection process and observe the action of the screw.
Need to properly adjust the injection speed and pressure, if you want to shorten the filling time, you can increase the injection pressure. As mentioned before, since there is a process before full filling, the final filling pressure can be adjusted to 100% of the injection pressure of the first stage. The pressure must eventually be adjusted high enough so that the maximum speed that can be achieved is not limited by the set pressure. If there is flash, you can reduce the speed.
After one cycle of observation, the amount of injection and the switching point are adjusted. The procedure is set so that the filling of 95-98% of the mold can be obtained by the shot weight at the first injection molding stage.
When the injection volume, transfer point, injection speed, and pressure of the first injection molding are all properly adjusted, the second-stage pressure maintenance calibration procedure can be performed.
Adjust the packing pressure as required, but do not overfill the mold cavity.
The screw speed was adjusted to ensure that the melt was completed just before the cycle was completed and the injection cycle was not limited.
Shorten cycle time to increase productivity
For most injection molding companies, the injection molding cycle can directly affect the following two main purposes: 1. Get more parts from the machine every day; 2. The parts meet the requirements of the guests.
The injection cycle consists of the following:
The cycle begins - the screw begins to advance, injection;
Component gate cooling
The screw starts to turn - Plasticizing stroke begins
Screw return completed - screw rotation stopped
Glue extraction if necessary
Mold open (possibly including core pulled out)
The parts can be ejected with sufficient cooling
Top Out
Mold closing (possibly including core return)
The mold closes - the cycle restarts.
The auto-injection cycle is in the same continuous sequence, and the same thing repeats again and again. There are three main parts of the cycle:
Mold opening time; filling time; mold closing time; holding pressure time
The goal of increasing productivity is to complete all necessary actions in a very short period of time, complete the ejection, and ensure that the mold is protected (including pulling and retracting the slider and the lateral core). Therefore, any mold or injection molding machine that delays the opening of the mold must be repaired. In addition, if the mold opening time for each injection is different, the parts will not be the same.
Injection Molding Cavity (from 1-2)
For materials with good flowability, the filling time should be 1/10 to 1/8 of the entire cycle. The most critical time for filling is the fast and stable screw advancement time, and the lowest and stable injection pressure.
When the screw advances, the solution flows from the feed pipe through the nozzle, the vertical flow channel, the horizontal flow, the gate, and then enters the cavity, where it encounters resistance. This resistance is affected by the nozzle diameter, runner size, gate size, product thickness, and mold exhaust design.
The flow resistance should be improved and reduced in the mold to reach the balance and stability of the filling. Otherwise, due to uneven filling, different mold cavity injection molded articles have different sizes, insufficient strength, or poor appearance.
Holding time (2-3)
When molding crystalline materials, dwell time is the most important process. This period of time starts from 99% of the melt-filled cavity until the gate freezes. The strength and toughness of a module depend on whether or not the pressure is maintained on the melt after injection until the part/gate solidifies. Keep a small amount of melt in advance of the screw while maintaining pressure. This section of holding pressure is the key to preventing the strength of the module from being insufficient due to the void after the solidification and shrinkage of the melt, or the weak position of the gate.
Cooling time (4-7)
When the melt enters the mold cavity and hits the metal surface, the melt cooling step has already begun. Due to the high solidification temperatures of semi-crystalline materials such as polyoxymethylene, nylon, and poly-carbon, little cooling is required. If a typical module is used, the module should have sufficient cooling time after the melt is completed. If problems are found while ejecting the module, the cooling time can be extended slowly until the problem is resolved.
Opening time (8-11)
The open time of the mold is an important part of the entire cycle, especially for molds with inserts. Even in relatively standard molds, the mold opening time is often higher than 20% of the entire cycle.
Factors affecting mold opening:
The first thing to consider is the speed and travel distance of the mold. The distance the mold moves during opening and ejecting the part should be reduced to avoid wasting time. Of course, the mold movement must be sufficient to allow the part to exit the mold before the mold is closed again. Therefore, the shorter the moving distance needed to release the parts, the less the room they spend. When the injection molding machine is in good condition, the transition from high-speed to low-speed ejection can be quite smooth. The equipment needs some maintenance to accomplish these speed changes, but these costs can be reduced from the molding time, save time and get multiple returns. In order to achieve the minimum mold movement time, adjust the deceleration limit switch so that the mold does not touch or break the part too much during the pre-ejection process, and optimize the high speed section of the stroke. Again, proper periodic maintenance ensures that this deceleration can be repeated each time. Producing the clamping pressure time is another delay in the entire mold opening time. This time may be affected by mechanical wear and failure of the hydraulic valve, so periodic mechanical maintenance can maintain a good operating state.
note:
Shortens the opening stroke of the mold to the minimum necessary so that parts and runners fall off
Exclude any factors that make the ejection difficult, like the flash edge around the thimble
Shorten the eject stroke to the minimum required
With the fastest mold opening and closing speeds, at the same time to slowly suspend and close slowly to prevent damage to the mold to find all closed molds and produce mold clamping pressure in the delay, they indicate the mechanical or hydraulic valve failure in the mold a large number The mounting and inserting activities also increase the mold opening time. Slight consideration of product design (reduction of undercuts) can often automate or semi-automate ejector actions.
If this delay is caused by the loss of the mold, the mold should be repaired to reduce the delay.
Develop a good operating habit of injection molding machines
Developing good injection molding machine operation habits will greatly benefit machine life and production safety.
Before booting:
(1) Check if there is water or oil entering the electrical control box. Do not turn on the appliance if it is wet. The maintenance personnel should blow the electrical parts dry before starting up.
(2) Check whether the supply voltage meets, generally should not exceed 15%.
(3) Check the emergency stop switch, whether the front and rear safety door switch is normal. Verify that the motor and oil pump are in the same direction of rotation.
(4) Check whether the cooling pipes are unblocked and pass cooling water to the oil cooler and the cooling water jacket at the end of the barrel.
(5) Check if there is any lubricant (fat) in each active part and add enough lubricant.
(6) Turn on the electric heat and warm the various sections of the barrel. When the temperature of each section reaches the required temperature, it is kept for a period of time to stabilize the temperature of the machine. The holding time varies according to the requirements of different equipment and plastic materials.
(7) Add enough plastic in the hopper. According to the requirements of injection molding different plastics, some raw materials are best dried.
(8) To cover the heat shield on the barrel, this can save energy, and can extend the life of the electric heating coil and contactor.
During operation:
(1) Do not arbitrarily cancel the function of the security door for the sake of convenience.
(2) Observe the temperature of the pressure oil. The oil temperature should not exceed the specified range. The ideal working temperature of hydraulic oil should be kept between 45~50°C, and it is generally suitable in the range of 35~60°C.
(3) Pay attention to adjust each stroke limit switch to avoid the impact of the machine during the action.
At the end of work:
(1) Before stopping the machine, the plastic in the barrel should be cleaned to prevent oxidation of leftovers or long-term thermal decomposition.
(2) The mold should be opened so that the toggle mechanism is locked for a long time.
(3) The workshop must be equipped with lifting equipment. Care should be taken when installing or removing bulky components such as molds to ensure production safety.
Some knowledge in the use of injection molding machine
Back pressure function
The application of back pressure ensures that the screw can generate enough mechanical energy to melt and mix the plastic during a rotary reset. Back pressure also has the following uses:
Discharge volatile gases, including air, out of the shot tank; uniformly mix adjuncts (such as toners, colors, antistatic agents, talc, etc.) with the melt; homogenize the melt through the length of the screw Provide uniform and stable plasticized material for accurate finished product weight control.
The selected back pressure value should be as low as possible (for example 4-15 bar, or 58-217.5 psi), as long as the melt has the appropriate density and uniformity, there is no air bubble, volatile gas and incomplete plastic in the melt. Plastic can be.
The use of back pressure increases the pressure and temperature of the injection molding machine. The magnitude of the rise is related to the set back pressure value. Larger injection molding machines (screw diameters in excess of 70mm/2.75in) can have up to 25-40 bar (362.5-580psi) backpressure. However, it should be noted that too high back pressure causes the temperature of the melt in the shot barrel to be too high, which is detrimental to the production of heat sensitive plastics.
And too high back pressure also causes the screw to be oversized and irregularly offside, which makes the amount of injected rubber extremely unstable. The number of offsides is affected by the viscoelastic properties of the plastic. The more energy stored in the frit, the abrupt backlash occurs when the screw stops rotating. The beating of some thermoplastics is more severe than other plastics, such as LDPE, HDPE, PP, EVA, PP/EPDM composites, and PPVC. Compared with GPPS, HIPS, POM, PC, PPO-M and PMMA, it is more prone to beating.
In order to obtain the best production conditions, it is important to set the correct back pressure so that the melt can be properly mixed and the offside range of the screw will not exceed 0.4 mm (0.016 in).
Mold opening and closing
In general, the mold opening and closing time used by most injection molding machines is slower than the reference time (about 100-359%). This difference is related to the weight, size, and complexity of the mold, and also to the safety protection of the mold (on It is related to the prevention of mold damage during the operation.
The typical mold opening and closing time is as follows (tcm: time unit quoted by the injection molding machine):
Traditional double board mold: 1-2tcm
Composite molds (including use of side mold cores and screw-out devices) and multi-plate molds: 2-3.5 tcm
If the mold opening and closing time is 15% longer than the actual operation time, then you need to modify the mold or use another injection molding machine to shorten the time. Newer injection molding machines can provide faster opening and closing speeds and use low mold opening and closing (mold sensing) pressures to initiate clamping force to close the mold.
Injection molding machine operators often do not pay attention to the board speed or time of a particular injection molding machine, but set the mold opening and closing time based on personal experience, which often results in long operation time. A one-second reduction in the operation of a ten-second operation immediately results in a 10% improvement. This improvement is often the difference between profit and loss.
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