1. Residual stress of parts
Effect of mold and injection molding conditions on residual stress:
|
serial number |
condition |
increase stress |
reduce stress |
|
1 2 3 4 5 6 7 8 |
Material temperature Mold temperature Part cooling speed injection speed Injection pressure Injection time Part thickness Gate size |
Low Low quick slow high long Thin big |
high high slow quick Low short thick Small |
2. Inspection of residual stress cracking of parts
There are several ways to diagnose in advance whether parts with residual stress will crack in the use environment:
For polystyrene, use kerosene at room temperature;
For high-density polyethylene, use a 2% detergent solution at 80°C;
For polypropylene, use a mixture of 63% (weight percent) chromium trioxide and water at 80°C.
3. Annealing treatment
Some parts with a tendency to crack can use annealing heat treatment to eliminate internal stress, thereby reducing the formation of cracks. First, put
The molded product is heated (usually near the glass transition temperature) and kept for a certain period of time, and then allowed to slowly cool naturally to cause cracking.
The macromolecules in the wrinkles can move freely and return to their original state. This annealing heat treatment method is generally performed immediately after forming.
4.Humidity control treatment
For nylon plastic parts, in order to improve the internal stress distribution and crystal structure in the plastic, improve the toughness of the parts and keep the dimensions relatively stable
If it is determined, humidity control treatment can be carried out, and the effect is better than annealing treatment. The method is to immerse the parts in boiling water or potassium acetate solution (the ratio is 1.25:100, boiling point 121℃), the soaking time depends on the maximum wall thickness of the part, ranging from 2 hours to 16 hours.
Plastic viscosity
1.Definition:
The friction between macromolecules when molten plastic flows is called the viscosity of the plastic. The coefficient of this viscosity is the viscosity, which is a direct reflection of the fluidity of the molten plastic.
2. What conditions affect viscosity?
(1) Effect of molecular weight
The same plastic can have different molecules and molecular weight distributions. The larger the molecular weight, the stronger the intermolecular force, and the greater the viscosity reflected.
(2) Influence of low molecular additives
Low molecular weight additives can reduce the force between macromolecular chains, thereby reducing the viscosity and making it easier to mold filling.
(3) Influence of external temperature
Different plastics have different sensitivity to temperature. The viscosity of PC, PA, PS, etc. decreases significantly when the temperature rises. It is effective to use high temperature to reduce the viscosity during processing, but such as PE and PP, we should not focus on it. To achieve the purpose of reducing viscosity by raising temperature.
(4) Effect of shear speed
Effectively increasing the shear speed of the plastic can reduce the viscosity of the plastic melt, but different plastics are affected by the shear speed in different ways. For example, the viscosity of PC is hardly affected by the screw speed, while the viscosity of PS is very much affected. big.
(5) Effect of pressure
Although high injection pressure can increase the injection speed and obtain a large shear effect during the injection process, which seems to be beneficial to reducing the viscosity, from the physical meaning of pressure, pressurization will increase the viscosity of the molten plastic. The reason is simple. Plastic macromolecular chains maintain a rock distance between themselves. This is due to the force between molecules. The increase in pressure means the reduction of the molecular distance, so the dislocation between the molecular chains becomes more difficult, and the overall flow viscosity is increased. The following table lists the multiples of viscosity increase of several plastics under 1372Mpa pressure and 17228Mpa pressure:
|
Plastic name |
Viscosity increase times under two pressures |
|
ON PP PS |
4-9 7.3 100 |
It can be seen from the table that although the viscosity of PE and PP is not greatly affected by pressure, the impact on PS is considerable. Therefore, when injection molding PS, the impact of pressure on viscosity cannot be ignored. Pay attention to the use of high-speed injection, that is, high shear. The function of cutting speed should not blindly increase the pressure.
5.Defect treatment of transparent parts
1. Silver streaks (cracks, flash spots)
(1) Eliminate pollution (including water).
(2) Reduce the material temperature.
(3). Increase injection pressure.
(4). Increase or decrease the back pressure and reduce the return speed.
(5). Improve the exhaust conditions of the runner and cavity.
(6). Clean possible blockages in the nozzle, runner and gate.
(7). Shorten the forming cycle.
(8). Eliminate silver streaks by annealing: hold polystyrene at 78°C for 15 minutes, or at 50°C for 1 hour. polycarbonate
Keep the acid fat at 160℃ or above for 3-10 minutes.
2. Bubbles (vacuum bubbles)
(1). Increase injection energy: pressure, speed, time, and measurement to make the mold fully filled.
(2). Adjust the material temperature (increase the material temperature to smooth the flow, lower the material temperature to reduce shrinkage).
(3). Increase the mold temperature.
(4). Shorten the cooling time and put the product in hot water to cool slowly if necessary.
(5). Improve the mold exhaust condition.
(6). Set the gate at the wall thickness of the part to improve the flow conditions of the nozzle, runner and gate.
3. Low smoothness and poor surface gloss
(1). Increase material temperature, injection pressure, and injection speed.
(2). Increase the mold temperature.
(3). Extend the cooling time.
(4). Improve the gate settings.
4. Shock ripples (ripples)
(1). Increase the material temperature, especially the nozzle temperature.
(2). Increase injection pressure and speed.
(3). Increase the mold temperature.
(4). Improve the size of the runner and gate, and polish the nozzle hole and runner.
(5). Improve the exhaust condition of the mold and set up a large enough cold slug well.
5. Whitening and fogging
(1). Remove pollution including water vapor.
(2). Increase the material temperature.
(3). Increase injection pressure and back pressure.
(4). Increase the mold temperature.
6. White smoke and black spots
(1). Reduce the material temperature.
(2). Screen out the material powder, which is prone to oxidative degradation due to its high heat sensitivity.
(3). Remove different plastic pollution in the barrel and raw materials.
6.The main causes and solutions of defects in injection molded products
|
Anomalies |
cause |
Solution |
|
Lack of glue |
1. The temperature of the barrel and nozzle is low 2. Mold temperature is too low 3. Measurement is too small 4. Injection pressure and speed are too small 5. Injection time is too short 6. Poor mold exhaust 7. Debris blocks the nozzle or gate 8. The gate is too small |
1. Increase the temperature of the barrel and nozzle 2. Increase mold temperature 3. Increase measurement 4. Increase injection pressure and speed 5. Extend the injection time 6. Add exhaust to the mold 7. Clean the nozzle or gate 8. Design the pouring system correctly |
|
Burr |
1. The injection pressure is too high or the speed is too fast 2. The clamping force is too small or the force is applied in one direction 3. The pressure-holding switching position is too small 4. The material temperature is too high 5. Debris falling into the mold room |
1. Reduce injection pressure and speed 2. Adjust the clamping force 3. Maintain pressure in advance 4. Reduce material temperature 5. Wipe the mold clean |
|
clamp line |
1 The material temperature is too low. 2. Low mold temperature 3. Low injection pressure 4.Slow injection speed 5. Measurement is too small 6. Poor mold exhaust |
1. Increase the material temperature 2. Increase mold temperature 3. Increase injection pressure 4.Speed up the injection speed 5. Increase measurement 6. Mold exhaust |
|
Black spots and stripes |
1. The material temperature is too high, causing decomposition. 2. The barrel or nozzle is not tightly connected 3. Poor mold exhaust 4. Uneven dyeing 5. The material is mixed with dark matter |
1. Lower the material temperature 2. Repair joints and remove dead corners 3. Mold exhaust 4.Re-dye 5. Remove dark matter from the material |
|
Silver Thread (Qi Flower) |
1. The material temperature is too high and the decomposed materials enter the mold cavity. 2. The raw material contains high moisture content and vaporizes during forming. 3. The raw materials are mixed with other rubber particles 4. The material contains volatile substances 5. Caused by cold glue entering 6. The back rope is too large and air enters, causing 7. Injection speed is too fast |
1. Rapidly lower the material temperature 2. Preheating or drying of raw materials 3. Clear or change materials 4. Preheat and dry raw materials 5. Remove glue chips, increase the temperature of the nozzle, and increase the rewinding cable 6. Reduce the back rope 7. Reduce injection speed |
|
deformation |
1.Short cooling time 2. Uneven force on ejection 3. The mold temperatures of the front and rear molds are not well controlled. 4. The internal stress of the product is too large 5. Poor water transportation and uneven cooling 6. The thickness of the product is uneven |
1. Extend the cooling time 2. Change the ejection position 3.Adjust mold temperature 4. Eliminate internal stress 5. Change the mold water path 6. Correctly design products and molds |
|
ripple |
1. The material temperature is too low and the viscosity is high 2. Mold temperature is too low 3. Injection pressure is too small 4. Injection speed is too slow 5. The gate is too small |
1. Increase the material temperature 2. Increase mold temperature 3. Increase injection pressure 4. Increase injection speed 5. Increase the gate size appropriately |
|
peeling, layering |
1. Impure raw materials 2. Mixing of different grades or brands of the same plastic 3. Add too much lubricant 4. Uneven plasticization 5. Mixed foreign matter 6. The gate is too small and the friction is high 7. The pressure holding time is too short |
1. Purification of raw materials 2. Use materials of the same grade or brand 3. Reduce the amount of lubricant used 4. Increase back pressure and return speed 5. Remove foreign matter 6. Increase the gate size 7. Extend the holding time |
|
crack |
1. The mold is too cold 2. Cooling time is too long 3. Plastic and metal inserts have different shrinkage rates 4. The ejection device is tilted or unbalanced, and the ejection cross-sectional area is small or improperly distributed. 5. Rough mold surface 6. The demoulding slope is not enough, making demoulding difficult |
1. Adjust the mold temperature 2. Reduce cooling time 3. Metal insert preheating 4. Adjust the ejector device or reasonably arrange the number and position of ejector pins 5. Ministry 6. Correctly design the draft angle |
|
decrease in intensity |
1. The material temperature is too high and the plastic decomposes 2. Excessive internal stress in plastics and inserts 3. Plastics are reused many times 4. Plastic contains water |
1. Reduce the material temperature and control the residence time of the material in the barrel 2. Preheat the insert to ensure there is a certain thickness of plastic around the insert. 3. Control the ratio of nozzle materials 4. Preheating and drying of raw materials |
|
Difficulty in demolding |
1. The mold ejection device has poor structure 2. The demoulding slope is not enough and the mold surface is not smooth. 3. The mold cavity temperature is inappropriate 4. There are seams or material deposits in the mold cavity 5. The cycle is too short or too long 6. The mold core has no air inlet holes |
1. Improve the ejection device 2. Correctly design molds and save molds 3. Properly control mold temperature 4. Clean the mold 5. Properly control the injection cycle 6. Modify the mold |
references:
1. Zhong Zhixiong: Plastic Injection Molding Technology, Guangdong Science and Technology Press, 1995.
2. Gong Liucheng, Qin Lijie, Xu Dingyu, Gao Chunrong: Practical Handbook of Plastic Forming Processing, Beijing Science and Technology Press, 1990.