Principles of injection molding and preparation conditions for injection molding process

2021-03-23 15:25:15 admin

Principles of injection molding and preparation conditions for injection molding process




The core process of injection molding in an injection molding machine is mold filling. The flow model (flow state) of the plastic melt when filling the mold cavity determines the condensed structure and apparent structure of the part (such as crystallization, molecular orientation, fusion uniformity, etc.), and ultimately affects the performance of the part.


The normal filling method of plastic melt entering the cavity from the gate should be that the subsequent melt advances the melt front edge and gradually expands across the plane of the cavity until it reaches the inner wall of the cavity and fills the entire cavity. The abnormal form of filling flow is jet flow and stagnant filling form. Jet flow and stagnant flow show that the melt is injected into the cavity through the gate with larger kinetic energy at the beginning of the mold filling, and the melt bead and filament are respectively formed and sprayed directly on the wall of the cavity opposite the gate. The filling process is like diffusion flow. The abnormal flow during mold filling will cause the melt to separate and fuse, and form more melt weld seams, which will adversely affect the performance of the part.


The factors that affect the form of melt filling flow are: melt temperature, mold temperature, injection pressure, injection speed, and the size of the mold cavity, gate size and location.


Adopt the pigment filling injection method and the transparent mold observation method to observe the changes in the form of melt filling flow under different process conditions. The pigment filling injection method is to mix different pigments in the transparent raw material resin, inject the sample, observe the flow mark pattern on the product, and judge whether it is normal spreading mold filling flow or abnormal mold filling flow according to the flow mark pattern . The transparent mold observation method uses a transparent mold to directly observe the characteristics of the filling flow.


The working principle of the injection molding machine: with the help of the thrust of the screw (or plunger), the plasticized molten state (ie viscous fluid state) plastic is injected into the closed mold cavity, and the product is obtained after solidification and shaping .


Injection molding is a cyclic process, each cycle mainly includes: quantitative feeding-melting plasticization-pressure injection-mold filling and cooling-mold opening and taking parts. After taking out the plastic part, the mold is closed again for the next cycle.


Motion program of injection molding machine


Nozzle advances→injection→hold pressure→preform→reverse→nozzle back→cool→open mold→extrude→needle withdrawal→open door→close door→close mold→nozzle advance.


General injection molding machine includes injection device, mold clamping device, hydraulic system and electrical control system.


The basic requirements of injection molding are plasticization, injection and molding. Plasticization is a prerequisite for realizing and ensuring the quality of molded products, and in order to meet the requirements of molding, injection must ensure sufficient pressure and speed. At the same time, due to the high injection pressure, a correspondingly high pressure is generated in the cavity (the average pressure in the cavity is generally between 20 and 45 MPa), so there must be a sufficient clamping force. It can be seen that the injection device and the clamping device are the key components of the injection molding machine.


Pre-molding action selection


According to whether the injection seat retreats before and after the pre-plastic feeding, that is, whether the nozzle leaves the mold, the injection molding machine generally has three options.


(1)Fixed feeding: The nozzle is always stuck into the mold before and after pre-molding, and the injection seat does not move.


(2) Front feeding: the nozzle is pressed against the mold for pre-plastic feeding. After the pre-plasticizing is completed, the injection seat will move back and the nozzle will leave the mold. The purpose of choosing this method is to use the injection hole of the mold against the nozzle during pre-molding to prevent the melt from flowing out of the nozzle when the back pressure is high. After pre-molding, it can prevent the nozzle and the mold from contacting for a long time to generate heat transfer and affect them. The relative stability of their respective temperatures.


(3) Post-feeding: After the injection is completed, the injection seat moves backwards, the nozzle leaves the mold and then preforms, and the injection seat advances after preforming. This action is suitable for processing plastics with a particularly narrow molding temperature. Because the nozzle and the mold have a short contact time, the loss of heat and the solidification of the melt in the nozzle hole are avoided. Injection pressure selection


The injection pressure of the injection molding machine is adjusted by the pressure regulating valve. Under the condition of adjusting the pressure, the high and low pressure oil circuits are turned on and off to control the injection pressure in the early and late stages.


There are three pressure options for ordinary medium-sized injection molding machines, namely, high pressure, low pressure, and first high pressure and then low pressure. High-pressure injection is realized by passing high-pressure oil into the injection cylinder. Due to the high pressure, the plastic enters the cavity under high pressure and high speed from the beginning. During high-pressure injection, the plastic enters the mold quickly, and the pressure gauge of the injection cylinder rises quickly. Low-pressure injection is realized by the injection of low-pressure pressure oil into the injection cylinder. The reading of the pressure gauge rises slowly during the injection process, and the plastic enters the cavity at low pressure and low speed. The first high pressure and then the low pressure is achieved by controlling the pressure of the pressure oil flowing into the cylinder from the time point according to the actual requirements of the plastic type and the mold.


In order to meet the requirements of different plastics with different injection pressures, the method of replacing screws or plungers with different diameters can also be used, which not only satisfies the injection pressure, but also gives full play to the production capacity of the machine. In large-scale injection molding machines, there are often multi-stage injection pressure and multi-stage injection speed control functions, which can better ensure the quality and accuracy of the product.


Selection of injection speed


Generally, there is a fast-slow knob on the control panel of an injection molding machine to meet the requirements of injection speed. In the hydraulic system, a large flow oil pump and a small flow pump are provided for supplying oil at the same time. When the oil circuit is connected to a large flow, the injection molding machine realizes rapid mold opening and closing, rapid injection, etc., when the hydraulic oil circuit only provides a small flow, various actions of the injection molding machine proceed slowly.


Choice of ejection form


There are two types of ejection modes for injection molding machines: mechanical ejection and hydraulic ejection, and some are equipped with pneumatic ejection system, and the ejection times are divided into two types: single ejection and multiple ejection. The ejection action can be manual or automatic. The ejection action is started by the mold opening stop limit switch.


Clamping control


Clamping is a huge mechanical thrust to close the mold tightly to resist the huge opening force of the mold caused by the high pressure injection of molten plastic during the injection molding process and the filling of the mold.


The clamping structure of the injection molding machine has a full hydraulic type and a mechanical linkage type. Regardless of the structure, the clamping force is implemented by the connecting rod being completely straightened. The straightening process of the connecting rod is the process in which the movable plate and the tail plate are stretched apart, and it is also the process in which the four tie rods are stretched under force.


Mold opening control


When the molten plastic is injected into the mold cavity and cooling is completed, the mold is opened and the product is taken out. The mold opening process is also divided into three stages. In the first stage, the mold is opened slowly to prevent the part from tearing in the mold cavity. The second stage is to open the mold quickly to shorten the mold opening time. The third stage is to open the mold slowly to reduce the impact and vibration caused by the inertia of the mold opening.


Control of injection molding process conditions


Program control of injection speed


The program control of the injection speed is to divide the injection stroke of the screw into 3~4 stages, and use respective appropriate injection speeds in each stage. Slow down the injection speed when the molten plastic first passes through the gate, use high-speed injection during the filling process, and slow down the speed at the end of the filling process. Using this method can prevent flashing, eliminate flow marks and reduce residual stress of products.


During low-speed filling, the flow rate is stable, the size of the product is relatively stable, the fluctuation is small, the internal stress of the product is low, and the internal and external stress of the product tends to be the same (for example, a polycarbonate part is immersed in carbon tetrachloride, and high-speed injection molding is used. The product has a tendency to crack, and it does not crack at low speed). Under relatively slow mold filling conditions, the temperature difference of the material flow, especially the large temperature difference between the material before and after the gate, helps to avoid the occurrence of shrinkage holes and dents. However, due to the long filling time, it is easy to cause the parts to appear delamination and poorly bonded weld marks, which not only affects the appearance, but also greatly reduces the mechanical strength.


During high-speed injection, the material flow rate is fast. When the high-speed mold filling is smooth, the molten material fills the cavity quickly, the material temperature drops less, and the viscosity drops less. Lower injection pressure can be used, which is a kind of hot material filling. Modal situation. High-speed mold filling can improve the gloss and smoothness of the part, eliminate the phenomenon of seam line and delamination, have small shrinkage and depression, uniform color, and can ensure fullness for the larger part of the part. However, it is easy to produce fat, bubbling or yellowing of the product, or even burn zoom, or cause difficulty in demolding, or uneven filling of the mold. For high-viscosity plastics, it may cause melt fracture, causing cloud spots on the surface of the part.


High-speed and high-pressure injection can be considered in the following situations: (1) Plastics have high viscosity, fast cooling, and long-process parts that use low pressure and slow speed can not completely fill all corners of the cavity; (2) Parts with too thin wall thickness, melting material When it reaches the thin wall, it is easy to condense and stay, and a high-speed injection must be used to make the molten material enter the cavity immediately before a large amount of energy is consumed; (3) Plastics reinforced with glass fiber or plastics with a large amount of filling materials are due to poor fluidity , In order to get a smooth and uniform surface, high-speed and high-pressure injection must be used.


For high-end precision products, thick-walled parts, parts with large wall thickness changes and thicker flanges and ribs, it is best to use multi-stage injection, such as two-stage, three-stage, four-stage or even five-stage.


Program control of injection pressure


Generally, the control of injection pressure is divided into the control of primary injection pressure, secondary injection pressure (holding pressure), or three or more injection pressures. Whether the timing of pressure switching is appropriate is very important to prevent excessive pressure in the mold, prevent flash or lack of material, etc. The specific volume of the molded product depends on the melt pressure and temperature when the gate is closed during the pressure holding stage. If the pressure and temperature are the same every time when switching from the holding pressure to the product cooling stage, the specific volume of the product will not change. At a constant molding temperature, the most important parameter that determines the product size is the holding pressure, and the most important variables that affect the product dimensional tolerance are the holding pressure and temperature.


Program control of screw back pressure and speed


The high back pressure can make the melt material get strong shear, and the low speed can also make the plastic get longer plasticizing time in the barrel. Therefore, the control of simultaneous programming of back pressure and speed is used more frequently. For example: in the screw metering full stroke, firstly high speed and low back pressure, then switch to lower speed and higher back pressure, then switch to high back pressure and low speed, and finally plasticize under low back pressure and low speed. In this way, most of the pressure of the melt at the front of the screw is released, and the rotational inertia of the screw is reduced, thereby improving the accuracy of screw measurement. Excessive back pressure often causes the degree of discoloration of the colorant to increase; the mechanical wear of the pre-plasticizing mechanism and the barrel and screw is increased; the pre-plasticizing cycle is prolonged, and the production efficiency is reduced; the nozzle is prone to salivation and the amount of recycled material increases; even if the self-locking type is used If the back pressure of the nozzle is higher than the designed spring locking pressure, it will also cause fatigue damage. Therefore, the back pressure must be adjusted appropriately.


With the advancement of technology, it has become possible to incorporate a small computer into the control system of an injection molding machine and use a computer to control the injection molding process.


Preparation before injection molding


The preparation before molding may include many contents, such as: inspection of material processing performance (determination of plastic fluidity, moisture content, etc.); dyeing and selection of raw materials before processing; preheating and drying of pellets; inserting Cleaning and preheating; mold trial and barrel cleaning, etc.


Pretreatment of raw materials


According to the characteristics of the plastic and the supply of materials, the appearance and process performance of the raw materials should generally be tested before molding. If the plastic used is in powder form, such as: polyvinyl chloride, it should also be batched and dry mixed; if the product has color requirements, you can add an appropriate amount of colorant or color masterbatch; the supplied pellets often contain varying degrees of moisture , Solvents and other volatile low-molecular-weight substances, especially some plastics with hygroscopic tendency, always exceed the allowable limit for processing. Therefore, it must be dried before processing and the water content must be measured.


Preheating of inserts


In order to meet the requirements of assembly and strength of injection molded products, it is necessary to embed metal inserts in the products. During injection molding, when the cold metal insert placed in the mold cavity and the hot plastic melt are cooled together, due to the significant difference in shrinkage of metal and plastic, large internal stress is often generated around the insert (especially like polystyrene Polymers with rigid chains such as ethylene are more prominent). The existence of this internal stress causes cracks to appear around the insert, which greatly reduces the performance of the product. This can be achieved by selecting metals with large thermal expansion coefficients (aluminum, steel, etc.) as inserts, and preheating the inserts (especially large metal inserts). At the same time, measures such as arranging a larger thick wall around the insert when designing the product.


Cleaning of the barrel


Before the newly purchased injection molding machine is used for the first time, or when the product needs to be changed, the raw material is changed, the color is changed or the plastic is found to be decomposed in the production, the injection molding machine barrel needs to be cleaned or disassembled.


Selection of release agent


Release agent is a substance that can easily release plastic products. Zinc stearate is suitable for general plastics except polyamide; liquid paraffin is better for polyamide plastics; silicone oil is expensive, troublesome to use, and less used.


The use of release agent should be controlled in an appropriate amount, and used as little or not as possible. Excessive spraying will affect the appearance of the product and adversely affect the color decoration of the product.


Causes of defects in injection molded products and their treatment methods


During the injection molding process, it may be due to poor raw material processing, unreasonable product or mold design, *workmanship not mastering the appropriate technology* as conditions, or due to mechanical reasons, often resulting in dissatisfaction, depression, and flashing of the product. , Bubbles, cracks, warpage deformation, dimensional changes and other defects.


There are three main aspects to the evaluation of plastic products. The first is the appearance quality, including completeness, color, gloss, etc.; the second is the accuracy between the size and relative position; the third is the mechanical properties, chemical properties, and Electrical performance, etc. These quality requirements are different depending on the use occasions of the products, and the required scales are also different.


The plasticizing capacity of the screw refers to the amount of melt that can be provided per unit time when the back pressure is zero and the screw speed is maximum.


The design level of the screw can be evaluated by detecting its plasticizing ability and the sensitivity of the screw speed, back pressure and power consumption to the plasticizing ability. When designing the screw, it is hoped that the diameter of the screw can be as small as possible, and the rotating speed that the screw can withstand is as high as possible, so as to achieve high plasticization capacity and good plasticization quality.


The plasticizing capacity of the injection molding machine determines the production capacity and production efficiency of the injection molding machine. According to the plasticization mechanism of the injection screw, due to the intermittent work of the screw, the axial movement of the screw during plasticization and the movement of the material in the groove during injection, the melting process of the plastic in the groove is an unsteady process, showing The axial temperature difference of the melt is large, and the plasticizing ability and power consumption of the screw are unstable.


When the screw is plasticized, the back pressure has a significant effect on the plasticizing ability. During the screw plasticization process, when the return resistance of the injection cylinder is increased (the back pressure increases), the front part of the screw homogenization section is increased. The pressure of the melt increases the reverse flow rate and reduces the plasticizing capacity accordingly.


As the back pressure increases, the screw driving power will also increase. The screw speed is proportional to the plasticizing capacity, and the screw driving power is directly proportional to the plasticizing capacity, so the screw driving power is proportional to the screw speed.


The mold temperature is uniform. When the mold temperature is increased, the injection molding process and product performance will be affected as follows:


Facilitate the flow of melt filling, and the filling pressure is slightly reduced;


The cooling time is prolonged, the required pressure holding time is prolonged, and the molding cycle is also prolonged;


Product demolding is difficult, the crystallinity of the crystalline polymer increases (the product density increases), the post-shrinkage decreases, and the product shrinkage rate increases;


The surface brightness of the product is improved, the orientation of macromolecules in the product is reduced, and the internal stress is reduced;


The impact strength decreases and the mold temperature is not uniform: the product shrinks unevenly, which causes the product to produce internal stress, warping deformation and stress cracking. Too low mold temperature leads to lower melt fluidity, dissatisfaction with mold filling or low strength of weld lines. If there is a large internal stress in the product, it is prone to warping deformation or stress cracking.