Cause analysis of common problems of the hottest i

2022-08-15
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Cause analysis of common problems of injection molded products (Part 1)

cause analysis of black spots caused by discoloration and coking of injection molded products

the main reason for black spots caused by discoloration and coking of injection molded products is that plastic or added ultraviolet absorbent, antistatic agent, etc. are overheated and decomposed in the barrel, or they are decomposed and charred after staying in the barrel for a long time, and then form with the molten material injected into the mold cavity. The analysis is as follows:

1 Machine:

(1) due to the heating control system out of control, the material barrel overheated, causing decomposition and blackening

(2) due to the defect of the screw or barrel, the molten material is stuck and stored, which is decomposed by the fixed heating between the spark tester and the take-up device for a long time. Check whether the rubber head kit is worn or whether there are metal foreign matters inside

(3) some plastics, such as ABS, are crosslinked and coked by high heat in the barrel, which is difficult to melt under the condition of almost maintaining the original particle shape, and are entrained into the workpiece after being crushed by the screw

2. Mold:

(1) the mold is not smoothly vented, easy to burn, or the size of the gating system is too small, and the cutting is too severe, resulting in coking

(2) there are inappropriate oil lubricants and release agents in the mold

3. Plastic:

plastic has too much volatile matter, too much humidity, too many impurities, too much recycled materials, and is polluted

4. Processing:

(1) too much pressure, too high speed, too much back pressure and too fast speed will decompose the material temperature

(2) the barrel should be cleaned regularly to remove other additives that are less resistant than plastic

cause analysis of delamination

causes of delamination of injection molded products and Troubleshooting:

1 The material temperature is too low and the mold temperature is too low, resulting in the occurrence of internal stress and fusion joints

2. The injection speed is too low, so it should be slowed down appropriately

3. The back pressure is too low

4. If impurities from different materials are mixed in the raw materials, the different materials should be screened out or replaced with new materials

cause analysis of swelling and bubbling

some plastic parts soon appear swelling or bubbling on the back of metal inserts or particularly thick parts after molding and demoulding. This is because the plastic that has not been completely cooled and hardened releases gas expansion under the action of internal pressure

solutions:

these standards generally have strict provisions on sample preparation and experimental methods. 1 Effective cooling. Reduce the mold temperature, extend the mold opening time, and reduce the drying and processing temperature of the material

2. Reduce the filling speed, forming cycle and flow resistance

3. Increase the holding pressure and time

4. Improve the condition that the wall surface of the workpiece is too thick or the thickness changes greatly

cause analysis of transparent defects

transparent parts of molten spots, crazes, cracked polystyrene and plexiglass can sometimes see some glittering filamentous crazes through the light. These crazes are also called bright spots or cracks. This is due to the stress generated in the vertical direction of the tensile stress, which makes the polymer molecules oriented in the flow direction, so that the refractive index of the oriented part is different from that of the non oriented part, and the light refraction occurs when passing through the interface between the two parts to produce crazing

solution:

(1) eliminate the interference of gas and other impurities, and fully dry the plastic

(2) reduce the material temperature, adjust the material barrel temperature in sections, and appropriately increase the mold temperature

(3) increase the injection pressure and reduce the injection speed

(4) increase or decrease the back pressure of pre molding and reduce the screw speed

(5) improve the exhaust condition of runner and cavity

(6) clean up the possible blockage of nozzle, runner and gate

(7) shorten the molding cycle. After demoulding, the crazing can be eliminated by annealing: keep polystyrene at 78 ℃ for 15 minutes, or keep it at 50 ℃ for 1 hour, and heat polycarbonate above 160 ℃ for several minutes

causes of embrittlement

a large part of the embrittlement of products is caused by internal stress. There are many reasons for the brittleness of products, mainly including:

on the one hand, there are dead corners or obstacles in the machine barrel, which is easy to promote the degradation of molten materials

(2) the plasticizing capacity of the machine is too small, and the plasticization of plastic in the machine barrel is insufficient; The plasticizing capacity of the machine is too large, and the plastic is heated and sheared in the machine barrel for too long. The plastic is easy to age, making the products brittle. (3) The ejector device is inclined or unbalanced, and the cross-sectional area of the ejector rod is small or improperly distributed

in terms of mold

(1) the gate is too small, so we should consider adjusting the gate size or adding an auxiliary gate

(2) if the shunt channel is too small or improperly configured, try to arrange it in a balanced and reasonable manner or increase the size of the shunt channel

(3) abnormal injection cycle caused by poor mold structure

three process aspects

(1) the temperature of the barrel and nozzle is too low, adjust it higher. If the material is easy to degrade, the temperature of the barrel and nozzle should be increased

(2) reduce the back pressure and rotation speed of screw pre molding, make the material slightly loose, and reduce the degradation of plastic caused by shear overheating

(3) mold temperature is too high, and it is difficult to demould; The mold temperature is too low, the plastic cools too early, the fusion joint is poor, and it is easy to crack, especially for high melting point plastics such as polycarbonate

(4) the cavity and core should have an appropriate demoulding slope. When the core is difficult to demould, the cavity temperature should be increased and the cooling time should be shortened; When the cavity is difficult to take off, reduce the cavity temperature and extend the cooling time

(5) try not to use metal inserts. Brittle plastics with large specific capacity of cold and heat, such as polystyrene, cannot be injected into inserts

four aspects of raw materials

(1) when the raw materials are mixed with other impurities or doped with inappropriate or excessive solvents or other additives

(2) some plastics, such as ABS, will undergo catalytic cracking reaction with water vapor when heated under damp conditions, resulting in large strain of parts

(3) too many times of plastic regeneration, too high recycled material content, or too long heating time in the barrel will promote the embrittlement of the workpiece

(4) the quality of plastic itself is poor, such as large molecular weight distribution and excessive occupation of components with uneven structures such as rigid molecular chains; Or contaminated by other plastics, bad additives, dust and impurities are also the reasons for embrittlement. V. product design plastic is one of the most effective and widely used materials at present.

(1) products have sharp corners, notches or parts with great thickness differences that are prone to stress cracking

(2) the product design is too thin or hollowed out too much

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