Function introduction of the hottest injection mol

2022-08-18
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Introduction to the functions of injection molding machine (IV)

1.6.1 incomplete plastic molding

this is a common problem, but it is also relatively easy to solve. When it can't be solved by technological means, it can be improved from the perspective of mold design and manufacturing, which can generally be solved

I. equipment:

(1) the plasticizing capacity of the injection molding machine is small. When the product quality exceeds the actual maximum injection quality of the injection molding machine, it is obvious that the supply volume is beyond the income. If the product quality is close to the actual injection quality of the injection molding machine, there will be a problem of insufficient plasticization. The heating time of the material in the barrel is insufficient, and as a result, the appropriate melt can not be provided to the mold in time. This situation can only be solved by replacing the injection molding machine with large capacity. Some plastics, such as nylon (especially nylon 66), have narrow melting range and large specific heat, so an injection molding machine with large plasticizing capacity is needed to ensure the supply of materials

(2) the temperature displayed by the thermometer is not true, and the brightness is high and the reality is low, causing the material temperature to be too low. This is due to the failure of temperature control devices such as thermocouples and their circuits or temperature difference millivoltmeters, or the aging or burning of electric heating rings far away from the temperature measuring points, and the heating failure, which has not been found or has not been repaired and replaced in time

(3) the diameter of the nozzle inner hole is too large or too small. If it is too small, because the flow diameter is small, the specific volume of the strip increases, which is easy to be cooled, blocking the feeding channel or consuming the injection pressure; If it is too large, the flow cross-sectional area is large, and the pressure per unit area of plastic molding is low, resulting in a small shooting force. At the same time, non Newtonian plastics such as ABS can not reduce the viscosity due to the lack of large shear heat, which makes it difficult to fill the mold. The nozzle is poorly matched with the inlet of the main runner, which often leads to overflow outside the mold and insufficient filling in the mold. The nozzle itself has great flow resistance or is blocked by foreign matters, plastic carbonization deposits, etc; The spherical surface of the nozzle or the inlet of the main runner is damaged and deformed, which affects the good cooperation with each other; The mechanical failure or deviation of the injection seat causes the inclination displacement between the nozzle and the axis of the main runner or the separation of the axial compression surface; The nozzle ball diameter is larger than the inlet ball diameter of the main channel. Due to the gap at the edge, gradually increasing the axial pushing force of the nozzle under the pressure of overflow will cause dissatisfaction with the injection of products

(4) plastic frit blocks the feeding channel. Due to the local melting and caking of the plastic in the hopper dryer, the high temperature of the barrel feeding section, the improper selection of the plastic grade, or the excessive lubricant contained in the plastic, the plastic will melt prematurely at the reducing position of the feed inlet or in the deep groove at the screw starting end. The granular material and the molten material bond with each other to form a "bridge", block the channel or wrap the screw, and slide circumferentially with the rotation of the screw without moving forward, Cause supply interruption or irregular fluctuation. This situation can be fundamentally solved only after the channel is chiseled and the block is removed

(5) cold material injection of nozzle. Injection molding machines are usually equipped with straight through nozzles due to pressure loss. However, if the temperature at the front end of the barrel and the nozzle is too high, or there is too much material stored at the front end of the barrel under high pressure, resulting in "salivation", the plastic will accidentally enter the inlet of the main runner first and harden under the cooling effect of the template, which will prevent the molten material from entering the mold cavity smoothly. At this time, the temperature of the front end of the barrel and the nozzle should be reduced, the storage volume of the barrel should be reduced, and the back pressure should be reduced to avoid excessive melt density at the front end of the barrel

(6) the injection cycle is too short. Due to the short cycle, the lack of material will also be caused if the material temperature is too late to keep up, especially when the voltage fluctuation is large. The cycle should be adjusted according to the supply voltage. Generally, the injection and pressure holding time are not considered during the adjustment, and the time from the completion of pressure holding to the return of the screw is mainly considered, which not only does not affect the molding conditions, but also extends or shortens the preheating time of the particles in the barrel

II. Mold aspect

(1) mold gating system is defective. The flow passage is too small, too thin or too long, which increases the fluid resistance. The diameter of the main flow channel should be increased, and the flow channel and shunt channel should be round. The runner or nozzle is too large and the shooting force is insufficient; The runner and gate are blocked by impurities, foreign matters or charcoals; The runner and gate are rough with scars or acute angles, and the surface roughness is poor, which affects the poor material flow; The channel is not provided with a cold charging well or the cold charging well is too small, and the opening direction is wrong; For multi cavity mold, carefully arrange the balance of runner and gate size distribution, otherwise there will be a situation that only the cavity near the sprue or the gate is thick and short can be filled, while other cavities cannot be filled. The runner diameter should be appropriately thickened to reduce the pressure drop of molten material flowing to the end of the runner, and the gate of the cavity far from the main runner should be increased to make the injection pressure and material flow speed of each cavity basically consistent

(2) unreasonable mold design. The mold is too complex, there are many turns, the feed port is improperly selected, the flow channel is too narrow, the number of gates is insufficient or the form is improper; The partial section of the product is very thin, so the thickness of the whole product or part should be increased, or an auxiliary runner or gate should be set near the insufficient filling place; It is not uncommon for inadequate exhaust measures in the mold cavity to cause dissatisfaction with the workpiece. This defect mostly occurs at the corners, deep depressions, thin-walled parts surrounded by thick wall parts, and the bottom of thin bottom shells formed by side gates. The design to eliminate this defect includes opening effective exhaust channels, selecting a reasonable gate position so that the air can be easily discharged in advance, and deliberately making a part of the trapped area of the cavity into an insert when necessary, so that the air overflows from the gap of the insert; For multi cavity mold, gate distribution imbalance is easy to occur. If necessary, reduce the number of injection cavities to ensure that other cavity parts are qualified

III. process aspect

(1) improper feeding adjustment, lack of material or excessive material. Incorrect feeding measurement or abnormal operation of feeding control system, abnormal injection cycle caused by injection molding machine or mold or limited operating conditions, small pre plastic back pressure or small particle density in the barrel may cause material shortage. For granular materials with large particles and many voids, plastics with large specific volume change of crystallinity such as polyethylene, polypropylene, nylon, etc., and plastics with large viscosity such as ABS, the material volume should be adjusted higher, and when the material temperature is high, the material volume should be adjusted higher

when there is too much material stored at the end of the barrel, the screw will consume extra injection pressure to compress and push the excess stock in the barrel during injection, which greatly reduces the effective injection pressure of the plastic entering the mold cavity and makes the products difficult to fill

(2) the injection pressure is too low, the injection time is short, and the plunger or screw returns too early. Molten plastic has high viscosity and poor fluidity at low working temperature, so it should be injected at high pressure and speed. For example, when making ABS color parts, the non high temperature resistance of colorants limits the heating temperature of the barrel, which should be compensated by higher injection pressure and longer injection time than usual

(3) slow injection speed. Injection speed is of great significance for some products with complex shape, large thickness changes and long process, as well as plastics with high viscosity, such as toughened ABS. When the product cannot be filled with high pressure, high-speed injection should be considered to overcome the problem of insufficient injection

(4) the feed temperature is too low. The temperature at the front end of the barrel is low, and the viscosity of the molten material entering the cavity rises too early to flow due to the cooling effect of the mold, which hinders the mold filling at the far end; The plastic with low temperature and high viscosity in the rear section of the barrel has difficulty in flowing, which hinders the forward movement of the screw. As a result, it seems that the pressure shown by the pressure gauge is sufficient, but in fact, the molten material enters the cavity at low pressure and low speed; If the nozzle temperature is low, it may be that the nozzle loses heat by contacting with the cold mold for a long time during fixed feeding, or the material temperature is low due to insufficient heating or poor contact of the nozzle heating ring, which may block the feeding channel of the mold; If the mold does not have a cold charging well, use a self-locking nozzle, and use the post charging procedure, so that the nozzle can maintain the necessary temperature; When starting up, the nozzle is too cold. Sometimes you can use a flame gun for external heating. The process between them is very troublesome, and this is the process of the user. You have to pay a certain amount of money to open and cancel to speed up the temperature rise of the nozzle

four raw materials

poor liquidity of plastics. Plastic factories often use recycled crumbs, which often reflect the tendency of viscosity increase. The experiment points out that the unit volume density of molecular chain breaking generated by oxidative cracking increases, which increases the viscosity of flow in the barrel and mold cavity, and the regenerated scrap contributes to the production of more gaseous substances, which increases the loss of injection pressure and makes it difficult to fill the mold. In order to improve the fluidity of plastics, it should be considered to add external lubricants such as stearic acid or its salts, preferably silicone oil (viscosity 300~600cm2/s). The addition of lubricant not only improves the fluidity of plastics, but also improves the stability and reduces the gas resistance of gaseous substances

1.6.2 overflow (flash)

overflow, also known as flash, overflow, drape, etc., mostly occurs at the parting position of Yongxing special steel, such as the parting surface of the mold, the sliding part of the slider, the gap of the insert, the hole of the ejector rod, etc. If the overflow is not solved in time, it will be further expanded, resulting in local collapse of the stamping die, causing permanent damage. The overflow of insert gap and ejector pin hole will also make the product stuck on the mold and affect demoulding

on the one hand, the equipment

(1) the real clamping force of the machine is insufficient. When selecting an injection molding machine, the rated clamping force of the machine must be higher than the tension formed by the longitudinal projection area of the injection molded product during injection. Whether the low temperature will also affect the blades of the mixer will cause mold expansion and flash

(2) the mold closing device is not adjusted well, the elbow bar mechanism is not straightened, resulting in uneven mold closing, left and right or up and down, and the parallelism of the mold cannot be achieved, resulting in the situation that one side of the mold is closed tightly and the other side is not closely attached, and flash will appear during injection

(3) the parallelism of the mold itself is poor, or the installation is not parallel, or the template is not parallel, or the force distribution and deformation of the pull rod are uneven, which will cause the clamping of the mold is not tight and flash

(4) the check ring is seriously worn; Failure of spring nozzle spring; Excessive wear of barrel or screw; The failure of the cooling system at the inlet causes the phenomenon of "bridging"; Insufficient injection volume of the barrel and too small buffer pad may cause repeated flash, which must be repaired or replaced in time

second, the accuracy of mold parting surface is poor. The movable formwork (such as the middle plate) is deformed and warped; There are foreign matters on the parting surface or protruding skid marks and burrs around the mold frame; The old die caused fatigue collapse around the cavity due to the previous flash extrusion

(2) unreasonable mold design. The opening position of the mold cavity is too biased, which will cause tension on one side of the mold during injection and cause flash; Plastics with too good fluidity, such as polyethylene, polypropylene, nylon, etc., have low viscosity in the molten state, and are easy to enter the movable or fixed gap, requiring high manufacturing accuracy of the mold; On the premise of not affecting the integrity of the product, it should be placed on the symmetrical center of quality as far as possible, and the material should be fed at the thick part of the product, which can prevent the situation of material shortage and flash at the same time; When there is a molding hole in or near the center of the product, it is customary to open a side gate on the hole. Under a large injection pressure, if the clamping force is insufficient, and the supporting force of this part of the mold is insufficient, slight warping will occur, resulting in flash. For example, when the side of the mold is equipped with a movable member, the projected area of its side will also be affected

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