Both gas-assisted injection molding and water-assisted injection molding belong to hollow auxiliary injection molding processes. Both processes can solve common industry problems such as thick-walled plastic part shrinkage marks, large internal stress, long molding cycle and excessive material consumption. The two processes have similar principles, adopting medium-assisted hollow molding. The difference lies in the different physical characteristics of auxiliary media, resulting in obvious differences in cooling capacity, molding quality, mold requirements, adapted materials and product application scope. Reasonably distinguishing the advantages and disadvantages of the two processes can accurately select the molding scheme in the product development stage, reduce production costs and stabilize the molding quality of plastic parts.
1. Introduction to Molding Principles of Two ProcessesGas-assisted injection molding adopts high-pressure inert nitrogen as the auxiliary medium. In the injection stage, seventy percent to ninety percent of plastic melt is injected into the mold cavity first, and then high-pressure nitrogen is injected into the melt through a special air needle. The gas pushes the melt to fill the cavity continuously under pressure and form a stable hollow channel inside the plastic part. The gas compensates melt shrinkage during the pressure maintaining stage, and demolds after cavity cooling. The gas is compressible with strong penetration and fluidity, which can adapt to irregular curved flow channels, and the medium will not cause corrosion and hydrolysis impact on plastic materials.
Water-assisted injection molding adopts high-pressure pure water as the auxiliary medium. Quantitative melt injection is completed in the early stage of molding, and high-pressure water is injected into the melt. Relying on the incompressible physical characteristics of water to push melt filling and molding. The thermal conductivity of water is far higher than that of gas, which can realize rapid forced cooling on the inner wall of plastic parts. After molding, compressed air is used to discharge residual internal water, and demolding is completed after pressure maintaining and shaping. Water has extremely high cooling efficiency with uniform molded wall thickness, which is more suitable for hollow plastic parts with simple and straight structure.
2. Core Performance Comparison of Two ProcessesIn terms of cooling performance, gas-assisted injection molding relies on nitrogen to complete internal pressure maintaining. The thermal conductivity of gas is low, the internal cooling speed of plastic parts is slow, and the optimization range of overall molding cycle is limited. Water-assisted injection molding has excellent water thermal conductivity, which greatly improves the cooling speed of the inner wall of plastic parts, and the overall molding cycle is further shortened on the basis of gas-assisted, especially the production efficiency of thick-walled products is significantly improved.
In terms of molding quality of plastic part inner wall, the inner wall formed by gas-assisted injection molding is relatively rough, prone to wave lines and gas retention marks, with average uniformity of hollow wall thickness, unable to form smooth inner wall for pipeline products. The inner wall formed by water-assisted injection molding is flat and smooth without wave and concave-convex defects, with stable and uniform hollow wall thickness, which can effectively reduce fluid transmission resistance and adapt to the production of precision pipeline plastic parts.
In terms of material adaptation scope, gas-assisted injection molding has stable chemical properties of medium, compatible with most general plastics and engineering plastics, including special plastic materials that are not resistant to hydrolysis, with wide material compatibility. Plastic parts are directly in contact with water during water-assisted injection molding, which is not suitable for plastics that are easy to absorb water and hydrolyze and deteriorate, only adapting to crystalline plastics with strong chemical stability, with stricter material restrictions.
In terms of mold and equipment requirements, gas-assisted injection mold has simple structure, low sealing requirements, ordinary mold steel can meet production, with low equipment cost, mature industry technology and low maintenance difficulty. Water-assisted injection molding requires high-strength sealing structure, high precision requirements for internal mold water channel needle valve, and corrosion-resistant coating treatment on cavity surface. The equipment manufacturing cost is higher, and the later maintenance process is more complicated.
3. Application Production Scenarios of Gas-Assisted Injection MoldingGas-assisted injection molding process has strong versatility, suitable for the production of hollow plastic parts with complex structure and irregular shape. Prioritize gas-assisted injection molding for curved special-shaped structural parts and handle products with uneven thickness. The strong fluidity of gas can smoothly penetrate complex flow channels without medium retention and jamming in the molding process. Home appliance shells and automotive interior decorative parts with high appearance grade requirements adopt gas-assisted process to avoid water flow mark defects and keep the plastic part surface flat and clean. Large support structural parts with coexisting thick and thin ribs can eliminate surface shrinkage marks and reduce internal stress to avoid later deformation and cracking by using gas-assisted injection molding. Special plastics that are not resistant to hydrolysis and easy to absorb water must adopt gas-assisted molding to prevent performance attenuation of materials caused by water contact.
4. Application Production Scenarios of Water-Assisted Injection MoldingWater-assisted injection molding is more suitable for hollow plastic parts with simple straight structure and thick wall and large diameter. Various fluid transmission pipelines are the mainstream application products of this process. The formed pipeline inner wall is smooth and transparent, with smaller fluid transmission resistance and stable pressure resistance. Industrial structural parts with large wall thickness can use the rapid cooling characteristics of water-assisted injection molding to shorten the molding cycle, and ensure uniform wall thickness and higher dimensional precision. Crystalline plastic materials are suitable for water-assisted processing, with uniform material molding crystallization, higher rigidity and strength of plastic parts and lower comprehensive production cost. Long straight hollow tubular civil products and industrial straight flow channel plastic parts have better molding effect by selecting water-assisted injection molding with much higher production efficiency than gas-assisted process.
5. Process Selection and Judgment PrinciplesPrioritize gas-assisted injection molding if the product structure is complex with curved special-shaped design. Prioritize water-assisted injection molding if the product structure is simple and straight with large wall thickness. Gas-assisted process with lower cost can be adopted if the inner wall finish of plastic parts is not high and only shrinkage mark defects need to be eliminated. Water-assisted injection molding must be selected if the product is used as fluid pipeline and requires smooth inner wall without resistance. Gas-assisted injection molding can only be adopted if the processing material is not resistant to hydrolysis and easy to absorb water and deteriorate. Mass production of thick-walled general plastic products pursuing extreme production efficiency is suitable for investing in water-assisted equipment for mass production.
6. Process SummaryGas-assisted injection molding technology is mature and stable with wide material compatibility and low mold investment cost, suitable for complex special-shaped parts and high appearance plastic parts, and is the most widely used process in hollow injection molding at present. Water-assisted injection molding has outstanding cooling capacity, excellent inner wall molding quality and higher production efficiency, only adapting to thick-walled straight plastic parts with simple structure and stable materials, with higher threshold of mold and equipment investment. Production enterprises should comprehensively judge the process scheme combined with product structure, material used, appearance precision and mass production demand, reasonable selection can optimize product quality, reduce molding defects, and effectively compress injection molding production costs.
