一. Temperature control
1. Barrel temperature
The 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 plastics, while the second temperature mainly affects the flow and cooling of plastics.
Each kind of plastic flow with different temperature, the same kind of plastic, due to different sources or brand, the flow temperature and decomposition temperature is little different, this is due to the different average molecular weight and molecular weight distribution, plastic in different type of injection machine of plasticizing process is also different, so choose the cylinder temperature is not the same.
2. Nozzle temperature
The nozzle temperature is usually slightly below the maximum barrel temperature to prevent the "salivating phenomenon" that may occur in the straight-through nozzle.The nozzle temperature should not be too low, otherwise it will cause the early coagulation of the melt and the nozzle will be blocked, or because of the early coagulation injection mold cavity and affect the performance of the product.
3. Mold temperature
The mold temperature has great influence on the intrinsic properties and apparent quality of the product.The mould temperature depends on the crystallinity of the plastic, the size and structure of the product, the performance requirements, and other technological conditions (melt temperature, injection speed and pressure, molding cycle, etc.).
二. Pressure control
In the process of injection molding pressure includes two kinds of plastic pressure and injection pressure, and directly affect the plastic and product quality.
1. Plasticizing pressure
(back pressure) When the screw injection machine, the top of the screw melt in the screw rotation back when the pressure is called plasticizing pressure, also known as back pressure.The pressure can be adjusted by means of a relief valve in the hydraulic system.
In the injection, the plasticizing pressure is constant with the rotating speed of the screw, so increasing the plasticizing pressure will increase the melt temperature, but decrease the plasticizing speed.In addition, increasing the plasticizing pressure can often make the melt temperature uniform, the color mixture uniform and discharge the gas in the melt.
In general operation, the plasticizing pressure should be determined as low as possible on the premise of ensuring good product quality. The specific value varies with the type of plastic used, but it is rarely more than 20 kg/cm2.
2. Injection pressure
In the current production, almost all injection machine injection pressure is based on the top of the plunger or screw pressure applied to the plastic (converted from the oil line pressure).The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, to give the rate of filling the molten material and to compacting the molten material.
三.Three, molding cycle
The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle.It actually consists of the following parts:
The molding cycle
Molding cycle directly affects labor productivity and equipment utilization rate.Therefore, in the production process, should be under the premise of ensuring quality, as far as possible to shorten the molding cycle of each relevant time.
In the whole molding cycle, the injection time and cooling time are the most important, which have a decisive influence on the quality of the product.The mold filling time in the injection time is directly inversely proportional to the mold filling rate. In production, the mold filling time is generally about 3-5 seconds.
The pressure holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (the extra thick parts can be as high as 5-10 minutes).Before the melting material at the gate is frozen, the amount of holding time has an influence on the accuracy of product size. If it is later, it has no influence.
The pressure holding time also has an optimal value, which is known to depend on the feed temperature, the mold temperature, and the size of the main channel and gate.If the dimensions and process conditions of the main runner and gate are normal, the pressure value with the smallest fluctuation range of shrinkage rate of the product is usually taken as the criterion.The cooling time depends on the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature.The end point of the cooling time should be in order to ensure that the product will not change when stripping, the cooling time is generally between 30~120 seconds, the cooling time is too long is not necessary, not only reduce the production efficiency, but also will cause the difficulty of stripping on the complex parts, even will produce the stripping stress when forced stripping.The rest of the molding cycle depends on whether the production process is continuous and automated and how continuous and automated it is.
四. Matters needing attention
1. Before starting:
(1) Check whether there is water and oil in the electrical control box. If the electrical appliance is damp, do not start the machine.The maintenance personnel should blow dry the electrical parts before starting the machine.
(2) Check whether the supply voltage is consistent, generally not exceeding ±15%.
(3) Check the emergency stop switch and whether the front and rear safety door switches are normal.Verify the rotation direction of motor and oil pump is consistent.
(4) Check whether all cooling pipes are unblocked and pass the oil cooler and the cooling water jacket at the end of the cylinder into the cooling water.
(5) Check whether there is lubricating oil (grease) in each moving part and add enough lubricating oil.
(6) Turn on the electric heat and heat each section of the cylinder.When the temperature of each section reaches the requirement, keep heat for a period of time, so that the machine temperature tends to be stable.The insulation time varies according to the requirements of different equipment and plastic materials.
(7) Add enough plastic into the hopper.According to different plastic injection requirements, some of the raw materials had better be dried first.
(8) To cover the heat shield on the cylinder, which can save energy and extend the life of the heating coil and current contactor.
2. Operation Process:
(1) Do not arbitrarily cancel the function of the safety door for convenience.
(2) Pay attention to the temperature of the pressure oil. The oil temperature should not exceed the prescribed range.The ideal working temperature of hydraulic oil should be kept between 45 ℃ and 50℃, generally within the range of 35 ℃ to 60℃.
(3) Pay attention to the adjustment of each stroke limit switch to avoid the impact of the machine during the operation.
3. At the end of work:
(1) Before stopping the machine, the plastic inside the machine barrel should be cleaned up to prevent the oxidation of the remaining material 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.Great care should be taken when assembling and removing heavy parts such as molds to ensure production safety.