analysis of gas-尊龙凯时人生就是搏

analysis of gas-assisted injection molding technology

gas-assisted injection molding has the advantages of improving appearance, saving materials, shortening cycle and reducing internal stress. however, due to the short development time of gas-assisted process, the difficulty of gas control in debugging, and the lack of experience of many first contact technicians, it often leads to the increase of waste products in production.

in this paper, the gas assisted forming process, structural characteristics and the measures to deal with forming defects are emphatically described.

technological principle

gas assisted molding (gim)

gas-assisted moulding (gim) is a new injection moulding technology that injects high-pressure inert gas when the plastic fills the cavity properly (90%~99%). gas drives the melted plastic to continue filling the cavity and replaces the plastic holding process with gas holding.

it injects molten resin into the cavity at high pressure and high speed, and then introduces high pressure gas into the wall thickness of the workpiece to produce a hollow section, which drives the melt to complete the filling process and keep pressure (as shown in figure 1).

when debugging the gas-assisted forming process, the following factors should be paid attention to:

1. for the air needle type panel die, when the air needle is pressed into the air outlet, the air intake imbalance is most likely to occur, which makes debugging more difficult. the main phenomenon is shrinkage. the solution is to check the gas flow smoothness 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 the temperature of rubber compound. excessive temperature of nozzle material will cause the phenomena of material splashing and burning, and too low temperature of nozzle material will cause the phenomena of cold glue, cold nozzle, plugging air needle, etc. the main phenomena reflected in the products are shrinkage and material flower. the solution is to check whether the temperature of the compound is reasonable.

3. check whether there is spill in the return of sealing needle nozzle under manual condition.

if this phenomenon occurs, the gas-assisted sealing needle fails to seal the nozzle. when gas is injected, the high-pressure gas will flow back into the feeding pipe. the main phenomena are large area burning and material splashing at the nozzle position, and the material return time is greatly reduced. when the sealing needle is opened, gas will be discharged. the main solution is to adjust the length of the sealing pin tie rod.

4. check whether the gas-assisted induction switch is sensitive or not, otherwise it will cause unnecessary losses.

5. gas-assisted products are maintained by gas pressure and can be reduced properly when shrinkage occurs. the main purpose is to reduce the pressure and space inside the product, so that the gas can be more easily punctured to the place where the rubber level is thick to compensate pressure.

advantages of gas-assisted molding

1. reduce residual stress and warpage.

in traditional injection moulding, sufficient high pressure is needed to push plastics from the main channel to the most peripheral area; this high pressure will cause high flow shear stress, while residual stress will cause product deformation. gas channel in gim can effectively transfer pressure and reduce internal stress so as to reduce warpage of finished products.

2. eliminate depression marks.

traditional injection moulding products will form sink mark behind thick areas such as rib and boss, which is due to uneven shrinkage of materials. gim can be pressurized by the air pipe to make the product shrink from the inside to the outside, and there will be no such trace in the appearance after solidification.

3. reduce clamping force.

traditional injection moulding requires high clamping force to prevent plastic overflow, but the pressure required by gim is not high, which can reduce the clamping force requirement by 25%~60%.

4. reduce the length of the runner.

the larger thickness design of gas flow pipeline can guide and help the plastic flow without special design of external runner, thus reducing the cost of die processing and controlling the position of welding line.

5. save material.

the products produced by gas-assisted injection moulding can save up to 35% of materials compared with traditional injection moulding, and the savings depend on the shape of the product. in addition to saving material in the interior hollow, the material and quantity of gate (nozzle) are also greatly reduced.

6. shorten the production cycle time.

traditional injection moulding, because of its thick ribs and many pillars, often requires a certain injection and pressure keeping to ensure product shape. gas-assisted forming (gaf) products seem to have a very thick glue level on the outside, but due to the hollow inside, the cooling time is shorter than that of traditional solid products, and the total cycle time is shortened due to the reduction of holding pressure and cooling time.

7. extend die life.

the traditional injection moulding process often uses high injection speed and pressure to produce "spikes" around the gate (nozzle), and the mould often needs to be repaired. after gas-assisted injection moulding pressure, injection holding pressure and clamping pressure are reduced at the same time, the pressure of the mould is also reduced correspondingly, and the times of mould maintenance are greatly reduced.

8. reduce the mechanical loss of injection moulding machine.

due to the decrease of injection pressure and clamping force, the pressure on the main parts of the injection machine, such as colin pillar, hinge and machine 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.

die characteristics

1. the cross-section of the airway is generally semi-circular, and the diameter of the airway should be designed as small and consistent as possible, usually 2-3 times 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, and the airway can be arranged at the root of the structures such as reinforcing bars and self-tapping screw columns to make use of the structural parts as the diversion channel feeding.

2. the mating clearance of the air needle should be less than 0.02mm to prevent the melt from entering the air needle clearance. the sealing of the air needle periphery and the die must be good, and the high temperature resistant sealing ring should be used.

3. the structure of the air needle requires that nitrogen escape from the gap between the air needle and the product during cooling.

4. the position of air needle should not be too close to the gate. because the material temperature near the gate is the highest and the viscosity is low when filling, it is easy to make the melt enter the air needle gap, resulting in defects such as shrinkage marks and blown cracks.

5. in the design of runner and gate, because the injection compensation phase is cancelled in gas-assisted moulding, fewer runners and gates can be set up. in order to ensure a faster filling speed, the runner and gate should be enlarged. the diameter of latent gate is generally about 1.5mm. excessive gate size will increase the solidification time of gate, affect the production efficiency, and may also cause the danger of nitrogen entering the barrel after passing through the gate and runner.

equipment characteristics

gas-assisted injection moulding system usually consists of low-pressure nitrogen gas source generation system, nitrogen pressurization and control system. the system includes air compressor, freeze-dryer, nitrogen generator, nitrogen buffer tank, pipeline filter, diaphragm nitrogen supercharger, high pressure nitrogen storage tank and gas-assisted console.

common defects and elimination methods:

1. gas penetration.

the defects can be solved by increasing the degree of pre-filling, accelerating the injection temperature, increasing the melt temperature, shortening the gas delay time or selecting materials with high fluidity.

2. no chamber or too small chamber.

it can be solved by reducing the degree of pre-filling, increasing melt temperature and gas pressure, shortening gas delay time, prolonging gas holding and pressure relief time, selecting materials with high fluidity, increasing gas passage and using side chamber method. in addition, it can check whether the air needle is broken or blocked, and whether the gas pipeline is leaking.

3. shrinkage marks.

the methods for eliminating shrinkage marks include reducing pre-filling degree and melt temperature, increasing melt holding pressure, shortening gas delay time, increasing gas pressure, prolonging gas pressure relief time, reducing die temperature, increasing gate diameter, runner mouth and airway, etc. in addition, the pressure curve of gas injection can be adjusted to check whether the pipeline and gas needle are working properly.

4. the weight is not stable enough.

these defects can be overcome by reducing injection speed, increasing back pressure, improving die exhaust, changing gate position and increasing gate.

5. the airway wall is too thin.

the defects can be overcome by reducing injection speed, reducing barrel temperature and gas pressure, prolonging gas delay time and increasing airway.

6. fingerprint effect.

when this phenomenon occurs, we can consider improving filling degree, reducing injection speed, reducing barrel temperature and gas pressure, prolonging gas delay time, shortening gas and pressure relief time, resetting injection pressure curve, selecting materials with low fluidity, reducing die temperature and wall thickness, etc. in addition, the change of gate position and the increase of airway can also help to improve this defect.

7. gas enters the screw barrel.

when this phenomenon occurs, it can be considered to increase the holding pressure and holding time of melt, reduce the nozzle temperature and gas pressure, shorten the holding time and pressure relief time of gas, reset the pressure curve of gas injection, select materials with low fluidity, reduce gate diameter and change gate position.

8. cracking occurs after demoulding.

when this phenomenon occurs, we can consider reducing the gas pressure, prolonging the holding time, resetting the pressure curve of gas injection, reducing the gas volume, and check whether the gas needle is blocked or not.