The application of the hottest microporous injecti

2022-09-19
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Application of microporous injection molding technology (Part 2)

3 setting of process parameters

3.1 barrel temperature

material temperature is an important factor affecting the fluidity of molten material. Due to the high filling resistance in the microporous area, a higher material temperature is required. For hips, the barrel temperature is generally set to ℃. It must be emphasized that sometimes the temperature of the barrel is quite different from that of the material in the mold. Due to the high injection pressure, the material temperature in the mold will rise rapidly, which may cause plastic decomposition, thus affecting the product quality. Therefore, when setting parameters, ensure that the material temperature in the mold is lower than the plastic decomposition temperature

3.2 mold temperature

mold temperature is one of the key factors affecting the molding of microporous areas. Generally, a higher mold temperature is conducive to the filling of micropores, and the mold temperature imbalance will seriously affect the filling of micropores. If the mold temperature in the microporous area is too low and the mold temperature in other thick wall areas is too high, the molten material in the microporous area will solidify quickly during the filling process. Even if the subsequent filling pressure and injection amount are increased, it can only make more molten material flow to the thick wall area, and even burrs or mold expansion occur in some parts, while there is still a lack of material in the microporous area

to ensure the mold temperature balance, it is necessary to ensure that the cooling water path is unobstructed and the cooling water quality is excellent. In summer, the circulating cooling water is very easy to get dirty. The water contains a lot of flocs such as sand and rust, which is easy to block the mold cooling water path, resulting in poor mold heat exchange. Therefore, the cooling water should be replaced and the mold water path should be cleaned regularly

3.3 clamping force

the method of constant product quality can be used to set the clamping force. First, set a slightly larger clamping force according to the projection area of the product on the parting surface, and then fix other parameters, gradually reduce the clamping force from sole proprietorship production to the establishment of a low clamping force in the middle of research and development until the sudden change in the weight of the product. Multiply the clamping force value at this time by the safety factor (generally 1..3) to be the setting value of the clamping force. If the clamping force is set too large, it is easy to cause poor exhaust in the microporous area, and the products have defects such as lack of material and scorching. However, if the clamping force is set too small, the dynamic and fixed mold may be poorly matched, resulting in defects such as microporous plugging and flash on the parting surface

3.4 pre filling amount

as the gas assisted molding technology and microporous injection molding technology are widely used in the molding of TV shell, the pre filling amount should be greater than 90%, and the microporous area should be fully filled at the same time. If there is a lack of material in the microporous area of the product when the injection is completed, it will be difficult to ensure that the subsequent gas injection is supplemented. On the one hand, because gas assisted molding generally has a delay time of s after the completion of material injection, the fluidity of molten material has become poor during gas injection; On the other hand, because the gas injection pressure is much lower than the injection pressure, it is difficult to promote the melt flow in the microporous area

3.5 injection rate and injection pressure

injection molding of complex products generally requires multi-stage injection rate and injection pressure. Because microporous injection molding needs to solve the problems of exhaust, cooling, injection pressure loss, etc., it generally adopts multi-stage injection rate and injection pressure control

if the mold is well vented, high injection pressure and injection rate can ensure smooth filling of products and good fusion of fusion lines. For example, in the mold trial stage, the microporous area of the product was cracked when opening the mold, and the problem could not be solved after repeated mold repair. After further analysis, the reason is that the strength of the fusion line in the microporous area is not enough, and the product is pulled apart by strong tension when opening the mold. This problem was solved by increasing the injection pressure and injection rate in the final injection stage

in terms of technology, the exhaust problem of microporous molding can be solved by setting multi-stage injection rate and injection pressure. After the material flow passes through the gate, high-speed and medium pressure injection molding is adopted, and the injection rate is 0 92dm3/s, medium speed high-pressure injection molding is used when the material flow reaches the area near the through hole, and the injection rate is 0 72dm3/s, which can effectively prevent the melt temperature from decreasing too much. The last stage adopts low-speed medium pressure injection molding according to the needs of exhaust, and the injection rate is 0 31dm3/s。

for material shortage caused by mold temperature imbalance, when setting parameters, high-speed and medium pressure injection molding can be used in the front section after the material flow passes through the gate, and medium speed and medium pressure injection molding can be used when the material flow is from the middle section to close to the microporous area. Theoretically, the greater the temperature difference is, the faster the heat transfer is. Therefore, properly extending the injection time can balance the material temperature in the mold. The last stage adopts higher injection rate and pressure to make up for the high pressure loss caused by the drop of material temperature and ensure the smooth filling of micropores

after the product is formed by gas assisted molding, the pressure maintaining is completed by gas assisted equipment. It is found that increasing the injection pressure has no obvious effect on the whitening and cracking of the microporous area

3.6 holding pressure and holding time

for the molding of microporous thin-walled products without gas assisted molding technology, too much holding pressure and too long holding time will cause the outer surface of the microporous area to be uneven. This is because the wall thickness of the stiffener area is good for pressure maintaining and shrinkage, while the wall thickness of the microporous area is thin and it is difficult to shrinkage, resulting in the shrinkage difference of each area of the product. In addition, excessive holding pressure and long holding time will also increase the wrapping force of the product on the fixed mold steel needle, making it difficult to demould the product

3.7 cooling time

for the molding of microporous thin-walled products using gas assisted molding technology, the cooling time is mainly determined by the filling time and the cooling time of the main channel. If the mold temperature distribution is uneven or the mold exhaust is poor, increasing the cooling time will be conducive to the filling of the microporous part. Because increasing the cooling time can reduce the mold temperature difference, and at the same time, it can increase the fit gap between mold parts to facilitate exhaust

4 conclusion

adjust the push plate to the inside

(1) in the design of microporous thin-walled products, the pore diameter is generally about 1mm, the pore spacing is generally about 1.7mm, and the wall thickness of the microporous area is about 1.5mm. Stiffeners shall be set on the back of the through-hole area

(2) the steel needle area of the mold is generally designed in the form of inserts, and the demoulding slope of the steel needle should be 10 ° or greater. Industry experts in the product through-hole area also pointed out that the dynamic and fixed mold steel needles can be used for butt molding or fixed mold single-sided steel needles for molding, and the steel needle area must ensure good exhaust. At the same time, the gate size should be appropriately expanded, and the gate should be set away from the through hole

(3) it is necessary to set a higher melt temperature. For hips, the barrel temperature shall be ℃

(4) try to avoid mold temperature imbalance or too low temperature in the mold steel needle area by adjusting the waterway

(5) when the mold is well vented, it should be filled with high injection pressure and injection rate. By optimizing the process parameters, the problems of mold temperature imbalance or poor exhaust can be improved

(6) excessive holding pressure and too long holding time will cause whitening, cracking and uneven surface concave convex defects when demoulding the through-hole area of the product

source: China plastic connect

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