This blog post will introduce you to gas assisted injection molding, a plastic molding process that combines conventional injection molding with the application of pressurized inert gas. You will learn about the types, stages, materials, applications, advantages, and disadvantages of this process.
Table of Contents
- What is Gas Assisted Injection Molding?
- Types of Gas Assist Injection Molding Processes
- Internal Gas Assisted Molding
- External Gas Assisted Molding
- Stages of Gas Assist Molding
- Plastic Resin Injection
- Primary Gas Penetration
- Secondary Gas Penetration
- Plastic Materials Used for Gas Assist Molding
- Applications of Gas Assisted Molded Plastic Parts
- Advantages of Gas Assist Molding
- Disadvantages of Gas Assist Molding
What is Gas Assisted Injection Molding?
Gas assisted injection molding is a plastic molding process that uses mold cavities in the form of the product’s shape. The mold is partially filled with molten plastic raw material, usually around 70% to 80% of the mold volume. Then, an inert gas, usually nitrogen, is injected into the mold through strategically designed and placed gas inlets, providing pressure that pushes the plastic material against the mold’s walls. This creates hollow channels within the plastic part, resulting in lighter and stronger products.
Gas assisted injection molding is a popular method to manufacture large, hollow molded plastic parts with complex geometries and thin walls. It offers many benefits such as improved surface finish, reduced shrinkage and warpage, lower cycle times, and lower clamping force.
Types of Gas Assist Injection Molding Processes
The gas assisted injection molding process has two main types: internal and external. They differ in the way the gas is injected and the shape of the hollow channels.
Internal Gas Assisted Molding
Internal gas assisted molding is more widely used than external gas assisted molding. In this process, the molten plastic resin enters the mold through sprue channels. Then, the gas enters through a nozzle and forms a bubble inside the molten resin. The pressure of the growing gas bubble forces the resin against the walls of the mold, giving the parts their shape. The path of the bubble is controlled by taking the path of least resistance through the hottest, least viscous plastic, which keeps it centered from the colder walls of the mold.
This process is well-suited to manufacture hollow, lightweight geometries with uniform wall thickness and smooth surfaces. Additionally, the gas exerts constant pressure on the resin, which helps to minimize shrinkage and distortion/warpage. These factors are usually the main reason for the deteriorating dimensional accuracy of thin walls. Thus, thin-walled parts are another special application of this process.
External Gas Assisted Molding
External gas assisted molding is less common than internal gas assisted molding. In this process, the molten plastic resin fills the entire mold cavity. Then, the gas is injected between the mold surface and the plastic part, creating a cushion that pushes the part away from the mold. The gas does not penetrate into the plastic part but forms a thin layer around it.
This process is suitable for producing parts with thick sections or ribs that would otherwise cause sink marks or stress concentrations in conventional injection molding. The gas cushion helps to eliminate these defects by compensating for the differential shrinkage between thick and thin sections. The gas also reduces friction between the part and the mold, facilitating easier ejection.
Stages of Gas Assist Molding
The gas assist molding process follows a specific workflow with three steps: plastic resin injection, primary gas penetration, and secondary gas penetration.
Plastic Resin Injection
In this stage, the molten plastic resin is injected into the mold cavity through a conventional injection molding machine. The injection speed and pressure are controlled to ensure that the resin reaches all parts of the mold cavity without causing excessive shear stress or turbulence. The amount of resin injected depends on whether internal or external gas assist molding is used. For internal gas assist molding, only 70% to 80% of the mold volume is filled with resin to leave space for gas penetration. For external gas assist molding, 100% of the mold volume is filled with resin to create a solid part.
Primary Gas Penetration
In this stage, the inert gas, usually nitrogen, is injected into the mold through a separate gas injection unit. The gas injection point can be located on the mold or on the injection molding machine. The gas pressure and flow rate are controlled to ensure that the gas penetrates into the resin or forms a cushion around the part without causing damage or leakage. The timing and duration of gas injection are also critical to achieve the desired shape and quality of the part. For internal gas assist molding, the gas injection starts before the resin injection is completed to create a bubble inside the resin. For external gas assist molding, the gas injection starts after the resin injection is completed to create a layer between the part and the mold.
Secondary Gas Penetration
In this stage, the gas continues to exert pressure on the resin until the part is fully solidified and cooled. The gas pressure and flow rate are adjusted to compensate for the shrinkage and contraction of the resin as it cools down. The gas also helps to improve the surface finish and dimensional accuracy of the part by eliminating sink marks, warpage, and residual stresses. The gas is then vented out of the mold through a separate gas venting unit. The part is then ejected from the mold using conventional ejection mechanisms.
Plastic Materials Used for Gas Assist Molding
Gas assist molding can use a variety of plastic materials, depending on the desired properties and applications of the part. However, some general criteria for selecting suitable materials are:
- High melt strength and viscosity to withstand gas pressure and prevent gas leakage
- High thermal conductivity and low specific heat to facilitate faster cooling and shorter cycle times
- Low moisture absorption and low sensitivity to environmental factors to prevent degradation or contamination of the material
- Good compatibility with nitrogen gas to prevent chemical reactions or corrosion
Some examples of plastic materials that are commonly used for gas assist molding are:
- Polyamide (PA)
- Polypropylene (PP)
- Polyethylene (PE)
- Polycarbonate (PC)
- Acrylonitrile butadiene styrene (ABS)
- Polybutylene terephthalate (PBT)
- Polyphenylene sulfide (PPS)
Applications of Gas Assisted Molded Plastic Parts
Gas-assisted injection molding, also known as gas-assisted molding or gas-assisted injection molding, is a specialized plastic molding process that involves injecting a controlled amount of nitrogen gas into the mold cavity after the initial plastic injection. This process creates hollow sections or channels within the plastic part, reducing material usage and achieving specific design objectives. Gas-assisted molded plastic parts offer a wide range of applications across various industries due to their unique characteristics and benefits.
One of the primary applications of gas-assisted molded plastic parts is in the automotive industry. Manufacturers often utilize this process to produce automotive components with complex shapes and reduced weight, leading to improved fuel efficiency and overall vehicle performance. For example, automotive seat frames, door panels, and instrument panels can be designed with hollow sections using gas-assisted injection molding, resulting in lightweight yet durable components. Additionally, the process enables the integration of functional features, such as airflow channels for ventilation or reinforcement ribs for added strength, directly into the plastic part.
The appliance industry also benefits significantly from gas-assisted molded plastic parts. Household appliances like washing machine tubs, refrigerator components, and dishwasher racks can be manufactured using this process to achieve cost-effective, lightweight, and structurally robust parts. The hollow sections created through gas-assisted molding provide excellent insulation properties, contributing to energy efficiency in appliances that require temperature control. Furthermore, the ability to incorporate design features directly into the plastic part reduces assembly steps, streamlining the manufacturing process and enhancing product reliability.
In the consumer goods sector, gas-assisted molded plastic parts are widely used to produce products with improved functionality and aesthetics. Furniture components, such as chair backs and arms, can be manufactured using this process to create ergonomic designs with hollow sections that offer enhanced comfort and reduced material usage. Outdoor recreational products, such as bicycle frames, sports equipment, and toys, can also benefit from gas-assisted molding, as it allows for the creation of lightweight and durable components that meet the demands of active lifestyles.
In the medical and healthcare industries, gas-assisted molded plastic parts find applications in producing products that require precise design features and specific performance characteristics. Medical device housings, hospital equipment, and laboratory components can be manufactured using this process to achieve lightweight and hygienic parts with integrated features like handles, grips, and fluid channels. The reduced material usage and improved aesthetics of gas-assisted molded parts make them suitable for medical devices that require strict compliance with regulatory standards and user-friendly design.
Moreover, gas-assisted molding is employed in the production of furniture, office equipment, and other industrial products. Chair bases with internal channels for cable management, office equipment housings with reinforced sections, and industrial containers with improved structural integrity are examples of products that can be efficiently manufactured using gas-assisted molding.
In conclusion, gas-assisted molded plastic parts offer a wide range of applications in various industries due to their unique characteristics and benefits. The automotive, appliance, consumer goods, medical, and industrial sectors all benefit from the lightweight, cost-effective, and functional designs achievable through gas-assisted injection molding. The ability to create hollow sections or channels within plastic parts reduces material usage, improves product performance, and allows for the integration of design features directly into the molded component. As technology and material advancements continue, gas-assisted molding will likely continue to expand its applications and contribute to innovative and sustainable product designs in the manufacturing landscape.
Advantages of Gas Assist Molding
Gas assist molding offers many advantages over conventional injection molding, such as:
- Reduced part weight and material consumption by creating hollow channels within the part Reduced clamping force and energy consumption by using lower injection pressure and temperature Reduced cycle time and cooling time by using less material and faster heat transfer Improved surface quality and appearance by eliminating sink marks, flash, weld lines, etc.
- Improved dimensional stability and accuracy by reducing shrinkage, warpage, and residual stresses Increased design flexibility and complexity by enabling thin walls, ribs, bosses, undercuts, etc.
Disadvantages of Gas Assist Molding
Gas assist molding also has some disadvantages compared to conventional injection molding, such as:
- Higher initial cost and maintenance cost by requiring additional equipment and components for gas injection and venting Higher technical complexity and difficulty by requiring precise control and synchronization of resin injection and gas injection Higher risk of defects and failures by involving high-pressure gas that can cause damage or leakage if not handled properly ited material selection and compatibility by requiring specific properties and characteristics of the plastic material Limited aesthetic options and transparency by creating hollow channels that can affect the color or clarity of the part.
Gas assisted injection molding is a novel plastic molding process that combines conventional injection molding with pressurized inert gas. It can produce hollow molded plastic parts with improved quality, performance, and efficiency. It has two main types: internal gas assisted molding and external gas assisted molding. It follows three stages: plastic resin injection, primary gas penetration, and secondary gas penetration. It can use various plastic materials depending on the application. It has many advantages but also some disadvantages compared to conventional injection molding.