Gas assisted injection moldinghas the advantages of improving appearance, saving materials, shortening cycle time and reducing internal stress. However, due to the short development time of gas assisted process, the gas is not easy to control during commissioning, and many process personnel who are in contact for the first time are inexperienced, which often leads to the increase of waste products in production.
This paper focuses on the gas assisted molding process, structural characteristics and the measures to deal with molding defects.
Process principle
Gas assisted molding (GIM)
Gas assisted molding (GIM) is a new injection molding technology, in which high pressure inert gas is injected when the plastic is filled into the mold cavity (90% ~ 99%), and the molten plastic is driven by the gas to continue to fill the cavity, and the gas packing is used to replace the plastic packing process.
It injects the molten resin into the cavity through high pressure and high speed, and then introduces the high-pressure gas into the wall thickness part of the workpiece to produce a hollow section, which drives the melt to complete the filling process and maintain the pressure (as shown in Fig. 1).
The following factors should be paid attention to during the commissioning of gas assisted molding process:
1. For the air needle panel mold, when the air needle is pressed into the air outlet, the air intake imbalance is most likely to occur, which makes the debugging more difficult. The main phenomenon is shrinkage. The solution is to check the gas flow when venting.
2. The temperature of rubber compound is one of the key factors affecting the normal production.
The quality of gas assisted products is more sensitive to rubber temperature. If the material temperature of the nozzle is too high, it will result in the phenomenon of material blooming and burning; if the material temperature is too low, it will cause the phenomenon of cold glue, cold nozzle and blocking air needle. The product reflects the phenomenon of shrinkage and material. The solution is to check whether the temperature of the compound is reasonable.
3. In manual mode, check whether there is overflow phenomenon when the sealing needle type nozzle returns.
If there is such a phenomenon, it means that the gas assisted sealing needle fails to seal the nozzle. During gas injection, the high pressure gas will flow back into the feed pipe. The main phenomenon is that the nozzle position is large area of coke and material bloom, and the material return time is greatly reduced, and gas will be discharged when the sealing needle is opened. The main solution is to adjust the length of needle sealing rod.
4. Check whether the gas auxiliary induction switch is sensitive, otherwise it will cause unnecessary loss.
5. The gas auxiliary product is maintained by gas, and the glue can be reduced properly when the product shrinks. It is mainly to reduce the pressure and space inside the product, so that the gas can easily puncture to the place with thick glue position to make up pressure.
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Advantages of gas assisted molding
1. Reduce residual stress and warpage.
In traditional injection molding, enough high pressure is needed to push the plastic from the main channel to the most peripheral area; this high pressure will cause high flow shear stress, and the residual stress will cause product deformation. Gas channel formed in GIM can effectively transfer pressure and reduce internal stress, so as to reduce warpage of finished products.
2. Eliminate the dent marks.
Traditional injection products will form sink marks in thick areas such as rib & boss, which is the result of uneven shrinkage of materials. GIM can be pressed by hollow gas pipeline to make the product shrink from inside to outside, so there will be no such trace on the appearance after curing.
3. Reduce the clamping force.
In traditional injection molding, high packing pressure needs high clamping force to prevent plastic overflow, but GIM needs low packing pressure, which can reduce the locking force by 25% ~ 60%.
4. Reduce the length of flow passage.
The larger thickness design of the gas flow pipe can guide and help the plastic flow, without special external runner design, so as to reduce the mold processing cost and control the welding line position.
5. Save materials.
The product produced by gas assisted injection molding can save up to 35% material compared with traditional injection molding, and the amount of saving depends on the shape of the product. In addition to the internal hollow material saving, the gate (nozzle) material and quantity of the product are also greatly reduced.
6. Shorten the production cycle time.
Due to the thick reinforcement and many column positions in traditional injection molding, a certain amount of injection and pressure maintaining are needed to ensure the product shape setting. For gas assisted forming (gaif) products, the surface appears to be very thick, but due to the hollow interior, the cooling time is shorter than that of traditional solid products, and the total cycle time is shortened due to the reduction of pressure holding and cooling time.
7. Prolong the service life of the mould.
In traditional injection molding process, high injection speed and pressure are often used to produce "peak" around the gate (nozzle), and the mold often needs to be repaired; after using gas assisted injection, the injection pressure, injection holding pressure and mold locking pressure are reduced at the same time, and the pressure borne by the mold is correspondingly reduced, and the mold maintenance times are greatly reduced.
8. Reduce the mechanical loss of injection molding machine.
Due to the reduction of injection pressure and clamping force, the pressure on the main stressed parts of the injection molding machine, such as coring column, hinge and plate, is also reduced accordingly. Therefore, the wear and tear of the main parts are reduced, the service life is prolonged, and the times of maintenance and replacement are reduced.
Model
It has characteristics
1. The cross section of the air passage is generally semicircular, and the design requirements of its diameter are as small as possible and consistent, which is generally 2-3 times of the wall thickness. Too large or too small will be detrimental to the end of the airway penetration. There should be a large arc transition at the corner of the airway; the air duct can be arranged at the root of the stiffener, self tapping screw column and other structures, so that the structural parts can be used as the sub airway for feeding.
2. The fit clearance of the gas needle should be less than 0.02mm to prevent molten material from entering the air needle gap; the seal between the outer circumference of the gas needle and the mold must be good, and high temperature resistant sealing ring is required.
3. The structure of the gas needle is required to prevent nitrogen from flowing from the needle and the product during cooling
