What to do if plastic injection molding is incomplete?

Time:2021-09-24 08:50:32 / Popularity: / Source:

This is a frequently encountered problem, but it is also relatively easy to solve. When it is really not solved by technological means, it can be improved from design and manufacture of molds, which can generally be solved.

1. Equipment:

(1) Plasticizing capacity of injection molding machine is small. When quality of product exceeds actual maximum injection quality of injection molding machine, it is obvious that amount of material supplied will not make ends meet. If quality of product is close to actual injection quality of injection molding machine, there is a problem of insufficient plasticization. Heating time of material in barrel is insufficient, and as a result, appropriate melt cannot be provided to mold in time. In this case, the problem can only be solved by replacing injection molding machine with a large capacity. Some plastics such as nylon (especially nylon 66) have a narrow melting range and a large specific heat. An injection molding machine with a large plasticizing capacity is required to ensure supply of materials.
(2) Temperature displayed by thermometer is not true, bright high is low, material temperature is too low. This is because temperature control device such as thermocouple and its wiring or temperature difference millivoltmeter fails, or because heating coil far away from temperature measurement point is aging or burned, heating failure has not been found or has not been 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 will be large, pressure per unit area of plastic will be low, resulting in a small injection force. At the same time, non-Newtonian plastics such as ABS cannot reduce viscosity due to lack of large shear heat, which makes it difficult to fill mold.
Nozzle is not well matched with inlet of main runner, it often happens that mold overflows and mold is not filled. Nozzle itself has high flow resistance or blocked by foreign matter, plastic carbonized deposits, etc.; spherical surface of nozzle or main runner inlet is damaged or deformed, which affects good cooperation with the other party; injection seat mechanical failure or deviation causes nozzle and main runner axis to produce an inclination displacement or axial pressing surface to separate; nozzle ball diameter is larger than main runner inlet ball diameter. Because of gap at the edge, gradually increasing nozzle axial pushing force under squeeze of overflow will cause product to be dissatisfied.
(4) Plastic frit blocks feeding channel. Because plastic is partially melted and agglomerated in hopper dryer, or temperature in feed section of barrel is too high, or plastic grade is not selected properly, or plastic contains too much lubricant, plastic will melt prematurely when it enters reduced diameter position of feed port or deep groove of screw end of screw, pellets and melts. They are bonded to each other to form a "bridge", which blocks passage or envelops screw, slides circularly along with rotation of screw, cannot move forward, resulting in interruption of feeding or irregular fluctuations. This situation can only be fundamentally solved after passage is cut through and material blocks are 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 at 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, which prevents 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.
 manufacture of molds 

2. Mould:

(1) Mold casting system is defective. 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 matter or carbide; runner and gate are rough with scars, or have sharp angles, surface roughness is poor, which affects material flow;
Runner does not have a cold slug well or cold slug well is too small, and 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 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) Mold design is unreasonable. Mold is too complex, 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 defect mostly occurs in the corners of product, deep recesses, 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, when necessary, deliberately making 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.

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 large particles, large voids, 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, amount of material should be adjusted 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 and 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, 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.
(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 molten material entering cavity rises too early to point where it is difficult to flow due to cooling effect of  mold, which hinders filling of far end; temperature in back section of barrel is low, and 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 temperature of material, which may block feeding channel of mold; if mold does not have a cold slug well, use a self-locking nozzle and adopt a post-feeding procedure, nozzles 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.

4. Raw materials:

Plastic has poor fluidity. Plastic factories often use recycled scrap, recycled scrap tends to reflect a tendency to increase viscosity. Experiment pointed out that unit volume density of molecular chain scission generated by oxidative cracking increased, which increased viscosity of flow in barrel and cavity, regenerated scraps promoted production of more gaseous substances, which increases injection pressure loss and makes it difficult to fill mold.
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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