Important knowledge about "Gas Assist Injection Molding"

Gas-Assisted Injection Molding (GRIM) is a new type of injection molding process that has been widely used abroad in recent years and is increasingly used in China. Its principle is to use relatively low-pressure inert gas (nitrogen is commonly used because of its low cost and safety and both the role of coolant, the pressure is 0.5 to 300 MPa) instead of the traditional molding process in the cavity of part of the resin to maintain pressure, in order to achieve better molding performance of products.
 

I. Advantages of gas-assisted injection molding

Gas-assisted injection molding overcomes the limitations of traditional injection molding and foam molding and has the following advantages.

1、Good performance of parts

(1) Eliminate pores and depressions by opening gas channels in the reinforcement bars and tabs set up at the joints of different wall thicknesses of the parts, and the gas is introduced after the injection of the underfill material, which compensates for the shrinkage of the melt during the cooling process and avoids the generation of pores and depressions.

(2) Reduce internal stress and warpage deformation During the cooling process of the part, a continuous gas channel is formed from the gas nozzle to the end of the material flow without pressure loss, and the air pressure is consistent everywhere, thus reducing residual stress and preventing warpage deformation of the part.

(3) Increase the strength of the part The design of the hollow reinforcement and tabs on the part makes the strength-to-weight ratio higher than that of similar solid parts by about 5, and the moment of inertia of the part increases substantially, thus increasing the strength of the part.

(4) to improve the flexibility of the design of gas-assisted injection can be used to form products with uneven wall thickness, so that the original must be divided into several parts of separate molding products to achieve a single molding, to facilitate the assembly of parts. For example, a foreign company originally produced dozens of metal parts as the main body, the shape of complex car door panels, through GAIM technology and the use of plastic alloy materials to achieve molding.

2、Low cost

(1) save raw materials gas-assisted injection molding in the thicker parts of the product to form a cavity, can reduce the weight of finished products up to 10% to 50%

(2) Reduce equipment costs gas-assisted injection requires less injection pressure and clamping force than ordinary injection molding (can save 25% to 50%) while saving energy up to 30%

(3) Relatively shorten the molding cycle due to the removal of thicker parts of the core material, shorten the cooling time up to 50% It is based on these advantages, gas-assisted injection is suitable for molding large flat products such as tabletops, doors, boards, etc.; large cabinets such as household appliances shells, TV housings, office machinery shells, etc.; structural components such as bases, automotive instrument panels, bumpers, automotive headlight covers, and other automotive interior and exterior trim parts.

II, the choice of molding materials

Theoretically, all thermoplastics that can be used in conventional injection molding methods are suitable for gas-assisted injection molding, including some filling resins and reinforced plastics. Some plastics with very good fluidity and are difficult to fill, such as thermoplastic polyurethane, have some difficulties in molding; resins with high viscosity require high gas pressure and are technically difficult; glass fiber reinforced materials have some wear and tear on the equipment.

In the gas-assisted molding process, the selection of molding materials is extremely important because the wall thickness and surface defects of the part are largely determined by the performance of the raw material, and changing the process parameters does not have a great impact on it. Table 1 shows the common plastics used for gas-assisted injection molding.

PA (polyamide) and PBT (polybutylene terephthalate) have unique crystalline stability and are particularly suitable for gas-assisted injection molding; PA6, PA66, and PP are also often used for gas-assisted molding; some partially crystalline resins are molded without a significant amorphous boundary layer on the internal side near the air channel due to relatively slow cooling rates.

However, the outer side will produce an amorphous boundary layer because of the cooling rate of the mold wall, thus affecting the quality of the products; for glass fiber reinforced plastics, a slight molecular orientation will be produced at the mold wall, and the maximum molding high strength parts at a certain distance under the mold wall (about 1mm from the outer surface of the product) along the direction of material flow can be used with a higher elastic modulus resin, the actual production process should be based on the requirements of the parts and specific molding conditions to select the appropriate resin material.
 

III, the design of air channel in the parts

Gas channel design is one of the most critical design factors in gas-assisted molding technology, which not only affects the rigidity of the product but also influences its processing behavior, and because it pre-defines the flow state of gas, it also affects the flow of melt in the initial injection stage.

1. Geometry of common gas channels

For large plates with reinforcement, the thickness of the substrate is generally 3-6 mm when gas-assisted injection molding and the thickness of the substrate can be reduced to 1.5-2.5 mm in parts with shorter gas flow distance or smaller size; the wall thickness of the reinforcement can reach 100%-125% of the wall thickness of the part it meets without producing depression; the geometry of the gas channel should be symmetrical or unidirectional with respect to the gate, and the gas The geometry of the gas channel should be symmetrical or unidirectional with respect to the gate, and the gas channel must be continuous, and the volume should be less than 10% of the volume of the whole part.

2、Strength analysis of the part

The traditional parts with reinforcement are often dented and warped, while the panels with various cross-sectional geometries shown in Fig. 1 are formed by gas-assisted injection molding, which not only ensures the strength of the products but also overcomes the shortcomings of traditional injection molding. Generally, under the same substrate thickness, the strength of the part with hollow wide T-shaped reinforcement similar to Figure 1(e) is higher than that of the part with hollow narrow T-shaped reinforcement, which in turn is higher than that of the part with hollow semicircular reinforcement similar to Figure 1(a) in the same cross-section.

The strength of the part varies greatly with the size of the force and its form, although the use of reinforcement can increase the stiffness of the product, if the local concentration of stress is applied to it, it will greatly weaken the strength of the product.

3、Gas channel size

The size design of the gas channel is closely related to the flow direction of the filling gas, the gas in the flow channel always flows along the direction of least resistance. Stable 0 fixed Newtonian fluid through the diameter of the circular tube D, the pressure drop formula is ΔP = 32μVL/D, where μ is the fluid viscosity, V is the average flow rate, L is the length of the fluid section, D is the diameter of the tube, because the gas full viscosity is very small, less than 0.1% of the resin and the pressure drop in the length direction can be ignored, and therefore only need to consider the resistance generated by the resin pressure drop.

Pseudoplastic fluid flow in a circular tube pressure drop formula and Newtonian fluid form similar, so the use of the above formula without considering the actual fluid and gas conditions, compared based on the gas near the pouring point of different directions of the pressure drop ΔP (i.e., compare the size of each section of L and D), can qualitatively solve the problem of gas Zhu filling direction ΔP small direction that is the preferred direction of gas flow.

Changing the size of the runner directly leads to the change of the pressure drop in different directions, thus changing the flow direction of the gas and affecting the molding quality of the part.
 

IV. Mold design

Since gas-assisted injection molding adopts relatively low injection pressure and clamping force, the mold can be made of zinc-based alloy, forged aluminum, and other light alloy materials, in addition to the general mold steel.

The mold design of the gas-assisted injection molding process is similar to that of ordinary injection molding, the defects caused by the design of the mold and part structure cannot be compensated by adjusting the parameters in the molding process, but the design of the mold and part structure should be modified in time:

(1) To absolutely avoid the phenomenon of injection Although there is a trend towards thin-walled products and the production of special-shaped bends, the traditional gas-assisted injection is still mostly used for the production of large cavity volume parts, and the material flow through the gate is subject to high shear stress, which is prone to melt rupture phenomena such as injection and creep. The design can appropriately increase the size of the inlet gate, set the gate at the thin products to improve the situation.

(2) cavity design due to gas injection in the amount of underfill, gas injection pressure, time, and other parameters difficult to control consistent, so gas injection generally requires a mold cavity, especially when the product quality requirements should be high. In actual production, there are examples of four cavities in one mold, and when using a multi-cavity design, it is required to use the balanced pouring system arrangement.

(3) Gate design generally uses only one gate, and its position should be set to ensure that the melt of the under-injected part is evenly filled with the cavity and to avoid jetting. If the gas needle is installed in the injector nozzle and the pouring system, the gate size must be large enough to prevent the melt from condensing here before gas injection.

One of the most common problems in the gas-assisted injection is that the gas penetrates through the intended gas channel into the thin-walled part of the part, forming finger-like or leaf-like gas fingering on the surface, even a few such "fingerprints" can be fatal to the product and should be avoided at all costs.

Research [s], the main reason for the formation of such defects is due to the inappropriate gate size and gas delay time settings, and these two factors often interact, for example, when using a smaller shallow mouth and a shorter delay time, it is very easy to produce such adverse consequences, not only affect the appearance of the product quality and greatly reduce the strength of the part. Generally, we can use the method of shortening the length of the gas channel, increasing the size of the inlet gate, and reasonably controlling the gas pressure to avoid this unfavorable situation.

(4) The geometry of the runner should be symmetrical or unidirectional with respect to the gate, and the direction of gas flow and the direction of molten resin flow must be the same.

(5) The overflow space to regulate the flow balance should be designed in the mold to get the ideal hollow channel.

In recent years, gas injection molding technology has been widely used in household appliances, automobiles, furniture, office supplies, and other industries, and it is developing in the direction of improving the dimensional stability of products, manufacturing thin-walled products with excellent surface properties, producing special-shaped pipes, replacing metal parts in the automotive industry, etc. It is believed that gas injection technology will still play an important role in future industrial production.