Four measures to reduce defects in injection molded products

Time:2021-07-01 12:29:50 / Popularity: / Source:

1. Equipment:

(1) Plasticizing capacity of injection molding machine is small

injection molding machine 
If quality of product exceeds actual maximum injection quality of injection molding machine, supply will not make ends meet; if quality of product is close to actual injection quality of injection molding machine, plasticization is insufficient, heating time of material in barrel is insufficient, 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, bright is high and real is low, which causes material temperature to be too low

Due to failure of temperature control devices (such as thermocouple and its wiring or temperature difference millivoltmeter), or aging or burning of electric heating coil far away from temperature measurement point, heating fails. Need to be repaired and replaced in time.

(3) Diameter of inner hole of nozzle is too large or too small

If it is too small, due to small flow diameter, specific volume of strip will increase, which will easily be cooled, block feed channel or consume injection pressure; If it is too large, flow cross-sectional area is large, pressure per unit area of plastic entering 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 other party; failure or deviation of injection molding machinery causes nozzle and main runner axis to produce a tilt displacement or axial compression surface to separate; diameter of nozzle ball is larger than that of main runner inlet. Because of gaps at edge, gradually increasing axial pushing force of nozzle under squeezing of overflow will cause product to be underfilled.

(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, and excessive lubricants contained in plastics, plastic will melt prematurely when it enters reduced diameter position of feed port or in deep groove of screw end, causing pellets and melt to bond to each other to form a "bridge", blocking channel or enveloping screw, sliding circularly along with screw rotation, which cannot move forward, resulting in interruption of supply or irregular fluctuations. This situation can only be fundamentally solved after channel is drilled and material block is eliminated.

(5) Nozzle cold material enters mold

Injection molding machines usually only install straight-through nozzles due to pressure loss. However, if temperature of front end of barrel and nozzle is too high, or there is too much material in the front end 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 opened, 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, material temperature is too late to 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, can extend or shorten preheating time of pellets in barrel.

Second, mold

(1) Mold casting system is defective

injection molded products 
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 mouth is too large, 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, surface roughness is poor, which affects flow of materials; Channel 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 it will happen that only cavity near main runner or thick and short gate can be filled while other cavities cannot be filled.
Diameter of runner should be increased appropriately to reduce pressure drop of melt flowing to the end of runner, gate of cavity farther from main runner should be enlarged to make 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, thickness of entire product or part should be increased, or auxiliary runners or gates should 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 the corners of products, deep depressions, thin-walled parts surrounded by thick-walled parts, 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 when necessary, a certain part of trapped area of cavity is specially made 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.

3. Process

(1) Improper feed adjustment, lack of material or excessive material

Inaccurate feeding metering or abnormal feeding control system operation, abnormal injection cycle caused by injection molding machine or mold or operating conditions, low pre-plastic back pressure, or low particle density in barrel may cause material shortage. For granular materials with large particles, many voids and plastics with large changes in specific volume of crystallinity, such as polyethylene, polypropylene, nylon, etc., as well as plastics with higher viscosity, such as ABS, amount of material should be adjusted higher, and amount of material should be adjusted larger when material temperature is high.
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, makes it difficult to fill product.

(2) Injection pressure is too low, injection time is short, plunger or screw returns too early

Molten plastic has higher viscosity and poor fluidity at lower working temperature, so it should be injected at higher pressure and speed. For example, in production of 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.

(3) Injection speed is slow

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

Temperature of front end of barrel is low, viscosity of melt entering cavity rises to point where it is difficult to flow prematurely due to cooling effect of mold, which hinders filling of far 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 a post-feeding program. Nozzle can maintain necessary temperature; nozzle is too cold when it is just turned on, sometimes a flame gun can be used for external heating to accelerate temperature of nozzle.

Four, 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 plastic, 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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