Plastic melt from the gate into the cavity of the normal mode of mold filling should be followed by the melt to advance the leading edge of the melt, gradually expanding across the plane of the cavity until it reaches the inner wall of the cavity, filling the entire cavity. Unusual forms of mold filling flow are jet flow and stagnant flow mold filling. Jet flow and stagnant flow are manifested as the melt is injected into the cavity through the gate with large kinetic energy at the beginning of the mold filling, forming melt bead drops and fine filaments that are directly injected into the cavity wall opposite the gate, and the subsequent mold filling process is like the diffusion flow. The abnormal form of flow that occurs during mold filling will cause the melt to separate and fuse, forming more melt fusion joints and adversely affecting part performance.
Factors affecting the melt flow form of mold filling are: melt temperature, mold temperature, injection pressure, injection speed and mold cavity space size, gate size and location.
The color material injection method and transparent mold observation method are used to observe the changes in the form of melt flow under different process conditions. Color filling injection molding method is in the transparent raw material resin mixed with different pigments, injection molding specimens, observation of products on the flow mark pattern, according to the flow mark pattern to determine whether the normal spreading mold filling flow, or abnormal mold filling flow. Transparent mold observation method is the use of transparent molds, direct observation of mold filling flow characteristics of the method.
The working principle of the injection molding machine: with the help of the screw (or plunger) of the thrust, will have been plastified molten state (i.e., viscous flow state) of the plastic injected into the closed cavity, after curing and shaping to obtain the product of the process.
Injection molding is a cyclic process, each cycle mainly includes: dosing - melting and plasticizing - pressure injection - mold filling and cooling - open the mold to take parts. After removing the part, the mold is closed again for the next cycle.
Injection molding machine action program
Nozzle forward→Injection→Pressure preservation→Pre-molding→Inverted shrinkage→Nozzle backward→Cooling→Opening→Ejecting→Returning the needle→Opening the door→Closing the door→Closing the mold→Nozzle forward.
The general injection molding machine includes injection device, mold closing device, hydraulic system and electrical control system and other parts.
The basic requirements of injection molding are plasticization, injection and molding. Plasticization is to achieve and ensure the quality of molded products as a prerequisite, and in order to meet the requirements of molding, injection must ensure that there is enough pressure and speed. At the same time, due to the high injection pressure, the corresponding high pressure in the mold cavity (the average pressure in the mold cavity is generally between 20 ~ 45MPa), so there must be a large enough clamping force. Thus, the injection device and mold closing device are the key components of the injection molding machine.
Pre-molding action selection
Based on the pre-molding before and after the filling of the injection seat back, that is, whether the nozzle leaves the mold, the injection molding machine is generally equipped with three options.
(1) Fixed charging: the nozzle is always pressed into the mold before and after pre-molding, and the seat does not move.
(2) Front filling: the nozzle is pressed against the mold for pre-molding filling, and when pre-molding is completed, the seat backs up and the nozzle leaves the mold. The purpose of choosing this way is: pre-molding the use of mold injection hole against the nozzle, to avoid the melt in the back pressure is higher out of the nozzle, pre-molding can be avoided after a long period of contact between the nozzle and the mold and heat transfer, affecting the relative stability of their respective temperatures.
(3) after the material: injection is completed, the injection seat back, the nozzle away from the mold and then pre-molding, pre-molding after the injection seat and then forward. This action is suitable for processing plastics with particularly narrow molding temperatures. Due to the short contact time between the nozzle and the mold, it avoids the loss of heat and the solidification of the melt in the nozzle hole. Injection Pressure Selection
The injection pressure of the injection molding machine is regulated by a regulator, which controls the high and low injection pressures at the front and rear stages through the on and off switching of the high-pressure and low-pressure oil circuits in the case of the regulated pressure.
Ordinary medium-sized injection molding machine above the setting of three pressure options, namely, high-pressure, low pressure and first high-pressure and then low-pressure. High-pressure injection is realized by passing high-pressure pressure oil into the injection cylinder. Due to the high pressure, the plastic enters the mold cavity at high pressure and high speed from the very beginning. High-pressure injection plastic into the mold quickly, the injection cylinder pressure gauge readings rise quickly. Low-pressure injection is realized by the injection cylinder through the low-pressure pressure oil, the injection process pressure gauge readings rise slowly, the plastic in the low-pressure, low-speed into the mold cavity. First high pressure and then low pressure is based on the type of plastic and the actual requirements of the mold from the time to control the pressure into the cylinder pressure oil pressure to achieve.
In order to meet the requirements of different plastics have different injection pressure, can also be used to replace the screw or plunger of different diameters, so as to meet the injection pressure, but also give full play to the production capacity of the machine. In the large injection molding machine often has a multi-stage injection pressure and multi-stage injection speed control function, so as to better ensure the quality and precision of the products.
Selection of injection speed
General injection molding machine control board are fast - slow knob to meet the requirements of the injection speed. In the hydraulic system there is a high-flow oil pump and a small-flow pump running at the same time to supply oil. When the oil circuit is connected to the high flow, the injection molding machine realizes fast opening and closing of the mold, fast injection, etc. When the hydraulic oil circuit only provides a small flow, the injection molding machine will be slow to carry out various actions.
Selection of ejector form
The ejector form of injection molding machine has two kinds of mechanical ejector and hydraulic ejector, and some of them are also equipped with pneumatic ejector system, and the number of ejector is equipped with single and multiple ejector two kinds. Ejector action can be manual or automatic. The ejector action is activated by the mold opening and stopping limit switch.
Mold closing control
Mold closing is the process of closing the mold by applying a large mechanical thrust to resist the high pressure injection of molten plastic during the injection process and the large opening force that occurs in the mold as a result of filling the mold.
The mold clamping structure of the injection molding machine has all-hydraulic type and mechanical linkage type. Regardless of the form of structure, the final is by the linkage completely straighten to implement the mold closing force. The straightening process of the connecting rod is the process of the movable plate and the tail plate being held open, and also the process of the four tie rods being stretched by the force.
Mold opening control
When the molten plastic is injected into the mold cavity and cooled, the mold is opened to remove the product. The mold opening process is also divided into three stages. The first stage is a slow mold opening to prevent the part from tearing in the mold cavity. The second stage is fast mold opening to shorten the mold opening time. The third stage is slow mold opening to reduce the shock and vibration caused by mold opening inertia.
Control of injection molding process conditions
Programmed control of injection speed
Programmed control of injection speed is to divide the screw's injection stroke into 3 to 4 stages, using the appropriate injection speed for each stage. The injection speed is slowed down when the molten plastic first passes through the gate, high speed injection is used during the mold filling process, and the speed is slowed down at the end of mold filling. In this way, overflow can be prevented, flow marks can be eliminated, and residual stresses in the product can be reduced.
Low speed mold filling flow rate is smooth, the product size is more stable, less fluctuation, low stress within the product, the product inside and outside the stress tends to be the same (for example, a polycarbonate parts immersed in carbon tetrachloride, high-speed injection molding of the parts have a tendency to crack, low-speed does not crack). In the slower mold filling conditions, the temperature difference between the material flow, especially before and after the gate material temperature difference is large, to help avoid the occurrence of shrinkage and depression. However, due to the long duration of mold filling time, it is easy to make the parts appear layered and poorly bonded fusion marks, which not only affects the appearance, but also greatly reduces the mechanical strength.
High-speed injection, the material flow rate is fast, when the high-speed mold filling is smooth, the molten material is quickly filled with the cavity, the material temperature drops less, the viscosity decreases less, you can use a lower injection pressure, is a hot material mold filling situation. High-speed mold filling can improve the gloss and smoothness of the parts, eliminating the seam line phenomenon and delamination phenomenon, shrinkage depression is small, the color is uniform and consistent, the larger part of the parts can ensure that the fullness. However, it is easy to produce products fat blisters or yellow parts, or even burned and scorched, or cause demolding difficulties, or the phenomenon of uneven mold filling. For high viscosity plastics may lead to melt rupture, so that the surface of the parts produced cloudy spots.
The following cases can be considered to use high-speed high-pressure injection: (1) high viscosity plastic, cooling speed, long process parts using low-pressure slow speed can not be completely filled with all corners of the cavity; (2) wall thickness is too thin parts, the melt to reach the thin wall is easy to condensate and stagnation, must be used in a high-speed injection, so that a large amount of molten material before the consumption of energy to immediately enter the cavity; (3) glass fiber reinforced plastic, or containing a large number of filler materials. (3) glass fiber reinforced plastics, or plastics containing a large number of filler materials, due to poor fluidity, in order to get a smooth and uniform surface of the parts, must be used in a high-speed high-pressure injection.
The advanced precision products, thick-walled parts, wall thickness varies greatly and has a thicker flange and tendon parts, it is best to use multi-stage injection, such as secondary, tertiary, quaternary and even five-stage.
Program control of injection pressure
Control of injection pressure is usually divided into a primary injection pressure, secondary injection pressure (holding pressure) or more than three times the control of injection pressure. Whether the timing of pressure switching is appropriate or not is very important to prevent overpressure in the mold, overflow or lack of material. The specific volume of the molded product depends on the melt pressure and temperature when the gate is closed during the holding pressure stage. If the pressure and temperature are the same every time you switch from the holding pressure to the cooling phase of the molded part, the specific volume of the molded part will not change. At a constant molding temperature, the most important parameter in determining the product size is the holding pressure, and the most important variables affecting the dimensional tolerances of the product are the holding pressure and temperature.
Program control of screw backpressure and speed
High backpressure gives strong shear to the melt, and low speed gives long plasticizing time in the barrel. Therefore, the control of back pressure and rotational speed at the same time program design is more often used. For example, in the full stroke of screw metering first high speed, low back pressure, then switch to lower speed, higher back pressure, then switch to high back pressure, low speed, and finally in the low back pressure, low speed for plasticizing, so that the pressure of the melt in the front of the screw to get most of the pressure release, reduce the rotational inertia of the screw, thus improving the accuracy of the screw metering. Excessive back pressure often results in increased colorant discoloration; pre-molding mechanism and barrel screw mechanical wear increases; pre-molding cycle is extended, decreased productivity; nozzles are prone to salivation, the amount of regenerated material increases; even with self-locking nozzles, if the back pressure is higher than the design of the spring locking pressure, it will also cause fatigue damage. Therefore, the back pressure must be properly adjusted.
With the progress of technology, the small computer into the injection molding machine control system, the use of computers to control the injection molding process has become possible.
Preparation work before injection molding
Preparation work before molding may include a lot of content, such as: material processing performance test (determination of plastic fluidity, moisture content, etc.); raw materials before the processing of dyeing and selection of particles; granules of preheating and drying; inserts of cleaning and preheating; test molds and barrel cleaning and so on.
Preprocessing of raw materials
Depending on the characteristics of the plastic and the supply situation, the appearance and process performance of the raw material should generally be tested before molding. If the plastic used for the powder, such as: polyvinyl chloride, should also be batching and dry mixing; if the products have coloring requirements, it can be added to the appropriate amount of coloring agent or masterbatch; the supply of granules often contain varying degrees of moisture, solvents, and other volatile low molecular matter, in particular, some of the hygroscopic tendency of the water content of the plastic is always more than the processing of the permissible limits. Therefore, before processing must be drying treatment, and determine the moisture content.
Preheating of inserts
Injection molded products in order to assembly and strength requirements, the need to embed metal inserts in the product. Injection molding, placed in the cavity of the cold metal inserts and hot plastic melt cooling together, due to the significant difference between the shrinkage of metal and plastic, often make the inserts around a large internal stress (especially like polystyrene and other rigid chain of polymer more significant). The presence of such internal stresses causes cracks to appear around the insert, leading to a significant reduction in the performance of the product. This can be achieved by selecting metals with a high coefficient of thermal expansion (aluminum, steel, etc.) for inserts and by preheating the inserts (especially large metal inserts). At the same time, the design of products in the insert around the arrangement of larger thick walls and other measures.
Barrel cleaning
Newly purchased injection molding machine before the initial use, or in the production of the need to change the product, change the raw material, change the color or found in the plastic decomposition phenomenon, need to be cleaned or dismantling of the barrel of the injection molding machine.
Selection of release agent
Removal agent is a substance that can make plastic products easy to release. Zinc stearate is suitable for general plastics except polyamide; liquid paraffin is more effective for polyamide plastics; silicone oil is expensive and troublesome to use, less used.
The use of mold release agent should be controlled in the right amount, as little as possible or not used. Spraying too much will affect the appearance of the product, the color of the product will also have a negative impact on the decoration.
Reasons for defects in injection molded products and their treatment
In the injection molding process may be due to poor raw material processing, products or mold design is unreasonable, * workers do not grasp the appropriate technology * conditions, or for mechanical reasons, often make the products are not injected into, depression, flying edge, bubbles, cracks, warping and deformation, dimensional changes and other defects.
There are three main aspects of the evaluation of plastic products, the first is the appearance of quality, including integrity, color, gloss, etc.; the second is the size and relative position between the accuracy; the third is with the use of the corresponding mechanical properties, chemical properties, electrical properties. These quality requirements and according to the different occasions of product use, the scale of requirements are different.
Screw plasticizing capacity refers to when the back pressure is zero, screw speed is the maximum amount of melt that can be provided per unit of time.
The level of screw design can be evaluated by examining its plasticizing capacity and the sensitivity of the effects of screw speed, back pressure and power consumption on plasticizing capacity. In the design of the screw, it is hoped that the screw diameter can be as small as possible, the screw can withstand the speed as high as possible, so as to achieve high plasticizing capacity and good plasticizing quality.
The plasticizing ability of the injection molding machine determines the production capacity and productivity of the injection molding machine. According to the injection screw plasticizing mechanism, due to the intermittent work of the screw and plasticizing screw axial movement and the injection of the screw groove of the material movement and other roles, the formation of the plastic in the screw groove of the melting process for the non-steady state process, the performance of the molten material axial temperature difference, the screw's plasticizing capacity and power consumption is not stable.
Screw plasticizing, back pressure on the plasticizing capacity of the impact is significant, in the screw plasticizing process, when increasing the injection cylinder back to drain resistance (back pressure increases), that is, increase the pressure of the melt in front of the screw homogenization section, so that the reverse flow increases, the plasticizing capacity is reduced accordingly.
Back pressure increases, the screw drive power will also increase, screw speed and plasticizing capacity is proportional to the screw drive power is proportional to the plasticizing capacity, so the screw drive power is proportional to the screw speed.
The mold temperature is uniform, increase the mold temperature, the injection molding process and product performance has the following effects:
Favorable to the melt filling flow, mold filling pressure is slightly reduced;
Cooling time is prolonged, the required holding time is prolonged, the molding cycle is also prolonged;
Products are difficult to release the mold, the crystalline polymer crystallinity increases (product density increases), after the Shrinkage is reduced, the product shrinkage rate increases;
Product surface brightness increased, the degree of macromolecular orientation within the product is reduced, the internal stress is reduced;
Decrease in the impact strength of the mold temperature is not uniform: uneven contraction of the product, resulting in the product to produce internal stresses, warping and deformation and stress cracking. Mold temperature is too low resulting in reduced melt fluidity, mold filling is not full or produce fusion marks low strength. The existence of large internal stress in the product is prone to warping and deformation or stress cracking.
In the mold opening process, the speed of the injection molding machine is required to change the order of: slow, fast, slow three sections of the three speed levels to avoid mold impact and ensure molding efficiency.
Originally, mold opening was also a term for mechanical production or process production, referring to the manufacture of mold sets. Now the term is used in industrial design to refer to the tooling that forms the design of a product, including machinery and molds. At the same time, molding is a production process that accounts for a high total investment, production technology and materials and other factors may add costs to molding, so molding is a fairly important production process.