Start from four aspects to solve incomplete defects of injection molded products

Time:2022-08-03 09:38:42 / Popularity: / Source:

1. Equipment:

(1) Plasticizing capacity of injection molding machine is small. If weight of product exceeds actual maximum injection weight of injection molding machine, supply will not make ends meet; if weight of product is close to actual injection weight of injection molding machine, plasticization is insufficient, heating time of material in barrel is insufficient, and mold cannot be provided with appropriate melt in time. Need to replace a large-capacity injection molding machine.
(2) Temperature displayed by thermometer is not true, obviously very high but actually very low, causing material temperature to be too low due to malfunction of temperature control device (such as thermocouple and its wiring or temperature difference millivoltmeter), or electric heating ring far away from temperature measurement point is aging or burned, resulting in heating failure. It needs to be repaired and replaced in time.
injection molded products 
(3) If diameter of inner hole of nozzle is too large or too small, specific volume of material strip will increase due to small flow diameter, which is easy to refrigerate, block feed channel or consume injection pressure; if it is too large, flow cross-sectional area will be large, pressure per unit area of plastic into mold is low, resulting in a small injection force. For example, because ABS does not obtain a large shear heat, viscosity cannot be reduced, which makes it difficult to fill mold. Nozzle is not well matched with inlet of main runner, overflow of mold and filling of mold are often unsatisfactory. Nozzle itself has high flow resistance or is blocked by foreign matter, plastic carbonized deposits, etc.;
Nozzle or main runner inlet spherical surface is damaged and deformed, which affects good cooperation with the other party; injection seat mechanical failure or deviation makes nozzle and main runner axis produce tilting displacement or axial pressing surface disengages; nozzle ball diameter is larger than that of inlet of main runner. Because of gap at the edge, gradually increasing axial pushing force of nozzle under squeezing of overflow will cause product to be dissatisfied.
(4) Plastic frit blocks feeding channel. Due to partial melting and agglomeration of plastics in hopper dryer, excessively high temperature in feed section of barrel, improper selection of plastic grades, excessive lubricants contained in plastics will cause plastic to melt prematurely when it enters reduced diameter position of feed port or deep groove of screw end, causing pellets and melt to bond to each other to form a "bridge", blocking passage or enclosing screw, sliding along circumference of screw with rotation of screw. This situation can only be fundamentally resolved after channel is drilled through and material block is eliminated.
injection molded products 
(5) Nozzle cold feed injection molding machines usually only install straight-through nozzles due to pressure loss. However, if temperature of front of barrel and nozzle is too high, or there is too much material at the front of barrel under high pressure, "salivation" will occur, which will cause plastic to accidentally enter main runner entrance and harden under cooling action of mold plate before injection is started and mold is open, thereby preventing melt from entering cavity smoothly. At this time, temperature of front end of barrel and nozzle should be lowered, storage capacity of barrel should be reduced, back pressure should be reduced to avoid excessive melt density at the front end of barrel.
(6) Injection cycle is too short. Due to short cycle time, material temperature cannot keep up, which will also cause material shortage, especially when voltage fluctuates greatly. Period should be adjusted accordingly according to supply voltage. Generally, injection and holding pressure time are not considered during adjustment. Main consideration is to adjust time from completion of holding pressure to return of screw, which does not affect filling and molding conditions, and can extend or shorten preheating time of pellets in barrel.

2. Mold

injection molded products 
(1) Mold gating system has defects. Runner is too small, too thin or too long, which increases fluid resistance. Main runner should increase diameter, runner and branch runner should be round. Runner or gate is too large, and injection force is insufficient; runner and gate are blocked by impurities, foreign objects or carbonized materials; runner and gate are rough with scars, or have sharp angles, and surface roughness is poor, which affects flow of materials; Runner does not have a cold slug well or cold slug well is too small, opening direction is wrong; for multi-cavity molds, runner and gate size distribution should be carefully arranged, otherwise there will be a type with only main runner or thick and short gate. Cavity can be filled but other cavities cannot be filled. Diameter of runner should be increased appropriately to reduce pressure drop of melt flowing to the end of runner, and gate of cavity farther from main runner should be enlarged to the injection pressure and material flow velocity of each cavity basically same.
(2) Unreasonable mold design. Mold is too complicated, with many turns, improper selection of feed inlet, too narrow runner, insufficient number of gates or improper form; partial section of product is very thin, and thickness of the whole product or part should be increased, or auxiliary runners or gates be set near insufficient filling; it is not uncommon for parts to be underfilled due to inadequate venting measures in cavity. This kind of defects mostly occur in corners of products, deep recesses, thin-walled parts surrounded by thick-walled parts, and bottom of thin bottom shells formed by side gates. Design to eliminate this defect includes opening effective vents, selecting a reasonable gate position to make air easy to discharge in advance, and if necessary, specially make a certain part of trapped area of cavity into an insert, so that air overflows from gap of insert; for multi-cavity molds that are prone to unbalanced gate distribution, number of injection cavities should be reduced if necessary to ensure that other cavity parts are qualified.
injection molded products 

3. Process

(1) Improper feed adjustment, lack of material or multi-material feeding, inaccurate metering or abnormal operation of feeding control system, abnormal injection cycle caused by limitation of injection molding machine or mold or operating conditions, low pre-plastic back pressure, or low particle density in barrel may cause material shortage. For large particles, large voids, plastics with large crystalline specific volume changes, such as polyethylene, polypropylene, nylon, etc., and high viscosity plastics such as ABS, higher materials should be adjusted. When material temperature is high, material volume should be adjusted larger. When there is too much material at the end of barrel, screw will consume extra injection pressure to compress and push excess stock in barrel during injection, which greatly reduces effective injection pressure of plastic entering mold cavity and makes it difficult to fill product.
(2) Injection pressure is too low, injection time is short, and plunger or screw is retracted too early. Molten plastic has a higher viscosity and poor fluidity at a lower working temperature, so it should be injected at a higher pressure and speed. For example, when making ABS color parts, high temperature resistance of colorant limits heating temperature of barrel, which has to be compensated by higher injection pressure and longer injection time than usual.
injection molded products 
(3) Slow injection speed. Injection speed is of great significance for some products with complex shapes, large thickness changes, and long processes, as well as plastics with high viscosity such as toughened ABS. When product cannot be filled with high pressure, high-speed injection should be considered to overcome problem of underfilling
(4) Material temperature is too low, and temperature at the front end of barrel is low. Melt entering cavity will cause viscosity to rise too early to the point where it is difficult to flow due to cooling effect of mold, which hinders filling of distal end; temperature in back section of barrel is low, plastic with high viscosity is difficult to flow, which hinders forward movement of screw. As a result, it seems that pressure indicated by pressure gauge is sufficient, but melt actually enters cavity at low pressure and low speed. If nozzle temperature is low, it may be that nozzle is in contact with cold mold for a long time during fixed feeding and loses heat, or nozzle heating ring is insufficiently heated or has poor contact resulting in low material temperature, which may block mold's feeding channel; if mold does not have a cold slug well, use a self-locking nozzle and adopt post-feeding procedure. Nozzle can maintain necessary temperature; nozzle is too cold when it is just turned on, and sometimes a flame gun can be used for external heating to accelerate temperature of nozzle.

4. Raw materials

injection molded products 
Mainly because of poor fluidity of plastics. Many plastic factories often use recycled scrap, and recycled scrap tends to reflect a tendency to increase viscosity. In order to improve 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). Addition of lubricants not only improves fluidity of plastic, but also improves stability and reduces air resistance of gaseous substances.

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