Gas-Assisted Injection Molding (GRIM) is a new type of injection molding process, which has been widely used abroad in recent years, is also used more and more domestically. Principle is to use inert gas with relatively low pressure (nitrogen is commonly used because it is cheap and safe, has function of coolant, pressure is 0.5-300 MPa) to replace part of resin in cavity in traditional molding process to maintain pressure, , so as to achieve purpose of better molding performance of product.

 

Gas-assisted injection molding overcomes limitations of traditional injection molding and foam molding, and has following advantages:

(1) Elimination of pores and depressions. Reasonable air passages are opened in reinforcing ribs and bosses provided at connection of different wall thicknesses of parts, gas is introduced after lack of material injection, which compensates for shrinkage of melt during cooling process, avoids pores and depressions production.
(2) Reducing internal stress and warping deformation. During cooling process of part, a continuous gas channel is formed from gas nozzle to the end of material flow, without pressure loss, and air pressure is consistent everywhere, thus reducing residual stress and preventing warping deformation of part .
(3) Increase strength of parts. Design of hollow ribs and bosses on parts makes strength-to-weight ratio about 5 higher than that of similar solid parts, and moment of inertia of parts is greatly improved, thereby improving use strength of parts.
(4) Improve flexibility of design. Gas-assisted injection can be used to mold products with uneven wall thickness, so that original products that must be divided into several parts to be molded separately can be molded at one time, which is convenient for assembly of parts. For example, an automobile door panel with dozens of metal parts as main body and complex shape originally produced by a foreign company has achieved one-time molding through GAIM technology and using plastic alloy materials.

(1) Saving raw materials. Gas-assisted injection molding forms a cavity in thicker part of product, which can reduce weight of finished product by 10% to 50%.
(2) Reduce equipment cost. Gas-assisted injection requires smaller injection pressure and clamping force than ordinary injection molding (saving 25% to 50%), while saving energy by 30%.
(3) Relatively shorten molding cycle. Due to removal of thicker core material, cooling time can be shortened by 50%. Based on these advantages, gas-assisted injection is suitable for molding large flat products such as desktops, doors, boards, etc.; large cabinets such as household appliance housings, TV housings, office machinery housings, etc.; structural components such as bases, car dashboards, bumpers, car headlights, other automotive interior and exterior trims.

In theory, all thermoplastics that can be used in conventional injection molding methods are suitable for gas-assisted injection molding, including some filled resins and reinforced plastics. Some plastics that are very fluid and difficult to fill, such as thermoplastic polyurethane, have certain difficulties in molding; resins with high viscosity require high gas pressure, which is also technically difficult; glass fiber reinforced materials have certain wear and tear on equipment.
In gas-assisted molding process, since molding wall thickness and surface defects of parts are largely determined by properties of raw materials, changing process parameters has little effect on them, so selection of molding raw materials is extremely important. Table 1 is a list of commonly used plastics for gas-assisted injection molding.
PA (polyamide) and PBT (polysuccinate terephthalate) have unique crystalline stability and are especially suitable for gas-assisted injection molding; PA6, PA66 and PP are also often used for gas-assisted molding; some parts For crystalline resins, cooling rate is relatively slow on inner side near airway during molding, and no obvious amorphous boundary layer is produced.
However, due to rapid cooling of mold wall, an amorphous boundary layer will be generated on outside, which will affect quality of product; for glass fiber reinforced plastics, a slight molecular orientation occurs at mold wall and reaches a maximum along flow direction at a certain distance below mold wall (about 1 mm from outer surface of product). Resins with higher elastic modulus can be selected for molding high-strength parts. In actual production process, appropriate resin materials should be selected according to use requirements of parts and specific molding conditions.

Airway design is one of the most critical design factors in gas-assisted molding technology. It not only affects rigidity of product, but also affects its processing behavior. Since it pre-determines flow state of gas, it also affects melt in initial injection stage. Flow and reasonable airway selection are crucial to molding of higher quality products.

 

For large plates with reinforcing ribs, when gas-assisted injection molding, thickness of substrate is generally 3 to 6 mm. In parts with short gas flow distance or small size, thickness of substrate can be reduced to 1.5 to 2.5 mm; Wall thickness of rib can reach 100% to 125% of wall thickness of connecting part without denting; geometry of air channel should be symmetrical or unidirectional relative to gate, air channel must be continuous, and volume should be less than 10% of the entire part volume.

Molding traditional parts with reinforcing ribs often have depressions, warping deformation, etc., while plates with various cross-sectional geometric shapes of reinforcing ribs as shown in Figure 1 are gas-assisted injection molding, which not only ensures strength of product, but also overcomes shortcomings of traditional injection molding. Generally, under condition of same base plate thickness, parts with hollow wide T-shaped stiffeners like Figure 1(e) have higher strength than those with hollow narrow T-shaped stiffeners, which in turn is stronger than hollow semi-circular ribbed panel of same cross-section like Figure 1(a).
Strength of part varies greatly with size of force and its form. Although use of reinforcing ribs can increase rigidity of product, if a local concentrated stress is applied to it, strength of product will be greatly weakened.

Size design of air channel is closely related to flow direction of filling gas, and gas always flows in direction of least resistance in flow channel. A stable Newtonian fluid passes through a circular tube with a diameter of D, and its pressure drop formula is ΔP=32μVL/D, where μ is viscosity of fluid, V is average flow rate, L is length of fluid section, and D is diameter of pipe, because gas viscosity is extremely small, lower than 0.1% of resin, and pressure drop in length direction can be is ignored, so only resistance due to resin pressure drop is considered.
Pressure drop formula of pseudoplastic fluid flowing in a circular tube is similar to that of Newtonian fluid. Therefore, using above formula without considering actual fluid and gas conditions, compare pressure drop ΔP in different directions based on gas near pour point (that is, comparing magnitudes of L and D in each section), gas charging direction problem can be solved qualitatively. Direction of small ΔP is preferential flow direction of gas.
Changing size of flow channel directly leads to change of pressure drop in different directions, thereby changing flow direction of gas and affecting molding quality of part.

Because gas-assisted injection molding uses relatively low injection pressure and clamping force, in addition to general mold steel, it can also be made of light alloy materials such as zinc-based alloy and forged aluminum.
Mold design of gas-assisted injection molding process is similar to that of ordinary injection molding. Defects caused by structure design of mold and part cannot be compensated by adjusting parameters in molding process, but design of mold and structure of part should be modified in time. Design principles required in ordinary injection molding are still applicable in process of gas-assisted injection molding. Following are main points that should be paid attention to when designing different parts:
(1) Jet phenomenon should be absolutely avoided. Although gas-assisted injection has a trend of developing towards thin-walled products and producing special-shaped elbows, traditional gas-assisted injection is still mostly used to produce parts with relatively large cavity volumes. Flow through gate is subjected to high shear stress, which is prone to melt fracture such as jetting and creep. In design, size of inlet gate can be appropriately increased, and gate can be set at thinner part of product to improve this situation.
(2) Cavity design. Since it is difficult to control parameters such as amount of material shortage, gas injection pressure, time and other parameters in gas-assisted injection, one mold and one cavity are generally required for gas-assisted injection, especially when product quality requirements are high. There is an example of one mold with four cavities in actual production. When using multi-cavity design, a balanced gating system layout is required.
(3) Gate design. In general, only one gate is used, its position should be set to ensure that melt in under-feed injection part evenly fills cavity and avoids injection. If gas needle is installed in injection machine nozzle and gating system, gate size must be large enough to prevent melt from condensing there before gas is injected.
One of the most common problems in gas-assisted injection is that gas penetrates predetermined airway and enters thin-walled part of workpiece, forming a finger-like or leaf-like gas flow pattern (Gas fingering) on the surface. Even a few such "fingerprint" effects are fatal to product and should be avoided.

 

Studies have shown that main cause of such defects is due to improper setting of inlet gate size and gas delay time, and these two factors often interact. For example, when a small shallow mouth and a short delay time are used, such adverse consequences are easily produced, which not only affects appearance quality of product, but also greatly reduces strength of product. Generally, method of shortening length of air passage, increasing size of gate, and reasonably controlling gas pressure can be used to avoid this unfavorable situation.
(4) Geometry of runner should be symmetrical or unidirectional relative to gate, and gas flow direction must be same as molten resin flow direction.
(5) Overflow space for adjusting flow balance should be designed in mold to obtain an ideal hollow channel.
Gas-assisted injection molding technology has been widely used in household appliances, automobiles, furniture, office supplies and other industries in recent years, and it is moving towards improving dimensional stability of products, manufacturing thin-walled products with excellent surface properties, producing special-shaped pipes, and replacing in automotive industry. It is believed that gas-assisted injection technology will still play an important role in industrial production in the future.