Gas-assisted injection molding is a plastic molding process that combines traditional injection molding principles with the application of pressurized inert gas. This is a very popular method of manufacturing large, hollow plastic parts with excellent surface finish and geometric accuracy. In this guide, we’ll explain what gas-assisted injection molding is, how it works, what its advantages and disadvantages are, which materials and applications are suitable for it, and how to design parts for this process.
First, let’s understand what gas-assisted injection molding is. It is an advanced plastic molding technology that injects molten plastic into the mold by injecting high-pressure inert gas (such as nitrogen) during the injection process. This method can effectively fill complex mold structures and improve the surface quality and geometric accuracy of molded parts.
Next, let’s explore how gas-assisted injection molding works. During the injection process, molten plastic is injected into the mold, while high-pressure inert gas is also injected. These gases form a bubble that pushes the molten plastic to all corners of the mold, ensuring that the entire mold is fully filled. As the plastic cools and solidifies, the gas is released, leaving a fully filled part with excellent surface finish and geometric accuracy.
Now, let’s take a look at what are the advantages and disadvantages of gas-assisted injection molding. Advantages include: 1) Ability to manufacture large parts with complex shapes and details; 2) Improved surface finish and geometric accuracy of parts; 3) Reduced internal stress and warpage; 4) Thinner part wall thicknesses can be used, thereby reducing weight and reduce costs. Disadvantages include: 1) higher equipment cost; 2) longer cooling time required; 3) additional post-processing steps may be required, such as removal of gas residues or heat treatment.
So, which materials and applications are suitable for gas-assisted injection molding? Gas-assisted injection molding is suitable for various thermoplastics, such as polypropylene (PP), polyethylene (PE), polycarbonate (PC), etc. This technology is widely used in automobiles, aerospace, medical, home appliances and other fields to manufacture various large and complex parts, such as car bumpers, seat backs, medical equipment casings, etc.
Finally, let’s discuss how to design parts for gas-assisted injection molding. In order to ensure the successful molding of parts, designers need to consider the following points: 1) Select the appropriate material, considering its fluidity, shrinkage and thermal stability; 2) Design a simple and easy-to-demould mold structure; 3) Optimize pouring port system to ensure that the molten plastic can flow into the mold smoothly; 4) Consider the wall thickness distribution of the part to achieve uniform filling and reduce internal stress; 5) Fully consider the characteristics of gas-assisted injection molding during the design process to ensure the quality of the part .
How Does Gas Assisted Injection Molding Work?
Gas assisted injection molding follows a specific workflow with three stages: plastic resin injection, primary gas penetration, and secondary gas penetration.
Plastic Resin Injection: In this stage, the molten plastic resin enters the mold through sprue channels. The mold is partially filled with plastic, leaving some space for the gas injection. The amount of plastic injected depends on the part geometry and design.
Primary Gas Penetration: In this stage, 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.
Secondary Gas Penetration: In this stage, the gas continues to penetrate into the plastic until it reaches a balanced pressure with the mold cavity. The gas also compensates for any volume shrinkage of the plastic as it cools down. The gas remains inside the part until it solidifies completely.
What are the Advantages and Disadvantages of Gas Assisted Injection Molding?
Gas assisted injection molding offers many benefits over conventional injection molding, such as:
- Reduced part weight and material consumption
- Reduced cycle time and energy consumption
- Reduced clamping force and mold wear
- Improved surface quality and dimensional stability
- Elimination or reduction of sink marks, warpage, and residual stresses
- Ability to create complex shapes and features with uniform wall thickness
- Ability to integrate functions such as hinges, clips, or handles
However, gas assisted injection molding also has some drawbacks, such as:
- Higher initial investment in equipment and tooling
- Higher complexity and difficulty in process control and optimization
- Higher risk of cosmetic defects such as bubbles, blisters, or burn marks
- Limited suitability for clear or transparent materials
- Limited applicability for multi-cavity molds or dissimilar cavity sizes
What Materials and Applications are Suitable for Gas Assisted Injection Molding?
Gas assisted injection molding can be used with most thermoplastic materials that are compatible with conventional injection molding. However, some materials are more suitable than others based on their melt viscosity, thermal conductivity, shrinkage rate, crystallinity, and aesthetic properties. Some of the common materials used for gas assisted injection molding are:
- Polypropylene (PP)
- Polyethylene (PE)
- Polystyrene (PS)
- Acrylonitrile Butadiene Styrene (ABS)
- Polyamide (PA)
- Polycarbonate (PC)
- Polybutylene Terephthalate (PBT)
- Polyethylene Terephthalate (PET)
Gas assisted injection molding can be used for a wide range of applications that require large, hollow, or thin-walled parts with high strength-to-weight ratio, complex geometries, or integrated functions. Some of the typical applications are:
- Automotive parts such as bumpers, spoilers, door handles, or instrument panels
- Medical devices such as syringes, catheters, or inhalers
- Furniture parts such as chair backs, armrests, or table legs
- Consumer products such as toys, sports equipment, or luggage
- Industrial products such as pipes, valves, or containers
How to Design Parts for Gas Assisted Injection Molding?
Designing parts for gas assisted injection molding requires careful consideration of the part geometry, material selection, gas injection location, gas channel size and shape, and mold design. Some of the general design guidelines are:
- Avoid sharp corners, undercuts, or thin sections that may cause stress concentration or gas leakage
- Provide adequate draft angles and ejector pins to facilitate part ejection
- Provide sufficient venting and cooling channels to prevent gas entrapment or overheating
- Choose a material that has low melt viscosity, high thermal conductivity, low shrinkage rate, and high crystallinity
- Choose a gas injection location that is close to the thickest section or the center of gravity of the part
- Choose a gas channel size and shape that is proportional to the part size and shape, and avoid abrupt changes in cross-section
- Choose a mold design that allows for easy access and maintenance of the gas injection system
Conclusion
Gas assisted injection molding is an innovative plastic molding process that uses pressurized inert gas to create hollow parts with high quality and efficiency. It has many advantages over conventional injection molding, such as reduced weight, material, cycle time, clamping force, and defects. It also has some disadvantages, such as higher cost, complexity, and risk. It can be used with most thermoplastic materials and for various applications that require large, hollow, or thin-walled parts with complex shapes or functions. It requires careful design of the part geometry, material selection, gas injection location, gas channel size and shape, and mold design.
What is Gas Assisted Injection Molding?
Gas assisted injection molding is a plastic molding process that combines principles of conventional injection molding with the application of pressurized inert gas. It uses mold cavities in the form of the product’s shape. The mold is partially filled with ~70%-80% molten plastic raw material. Injecting Nitrogen gas fills the remaining volume to pack the plastic material against the mold’s walls. It is a very popular method to manufacture large, hollow molded plastic parts. It enjoys numerous advantages such as remarkable surface finish and geometric accuracy.
Types of Gas Assist Injection Molding Processes
The gas assisted injection molding process has two main types, internal and external. In the following sections, we distinguish between the two types.
Internal Gas Assisted Molding
Internal gas assisted molding is more widely used than the external process. Initially, molten plastic resin enters the mold through spruce 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. This process is well-suited to manufacture hollow, lightweight geometries. 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. Cycle times are also significantly lower for internal gas assisted molding. Due to the geometries having hollow portions and thin cross-sections, they cool down quickly in comparison to solid parts.
External Gas Assisted Molding
External gas assisted molding is a less common technique that involves injecting gas into a separate chamber outside the mold cavity. The gas then pushes on a movable wall or a piston that transfers the pressure to the molten resin inside the mold cavity. This process is suitable for creating solid parts with thick sections or ribs that would otherwise cause sink marks or voids. The gas pressure helps to eliminate these defects by compensating for the shrinkage of the resin during cooling. However, external gas assisted molding requires more complex molds and equipment than internal gas assisted molding.
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 nozzle or a gate. The injection speed and pressure are controlled to ensure that the resin fills about 70%-80% of the mold volume. The resin should reach all corners and edges of the mold cavity without creating any air pockets or bubbles.
Primary Gas Penetration
In this stage, high-pressure nitrogen gas is injected into the molten resin through a separate nozzle or a gate. The gas forms a bubble or a channel inside the resin that pushes it against the mold walls. The gas pressure and volume are adjusted to achieve the desired shape and thickness of the part. The gas should follow the path of least resistance through the hottest and least viscous resin, avoiding contact with the colder and solidified resin near the mold walls.
Secondary Gas Penetration
In this stage, the gas pressure is maintained or increased to compensate for the shrinkage of the resin during cooling. The gas also helps to cool down the resin by transferring heat to the mold walls through conduction. The gas pressure is released after the part has solidified completely and then vented out of the mold.
Plastic Materials Used for Gas Assist Molding
Gas assist molding can use various types of plastic materials, such as thermoplastics and thermosets. However, some factors need to be considered when selecting the material, such as the viscosity, the melting point, the thermal conductivity, the shrinkage rate, and the compatibility with the gas. Some of the common plastic materials used for gas assist molding are:
- Polypropylene (PP)
- Polyethylene (PE)
- Polystyrene (PS)
- Acrylonitrile Butadiene Styrene (ABS)
- Polyamide (PA)
- Polycarbonate (PC)
- Polyethylene Terephthalate (PET)
Applications of Gas-Assisted Molded Plastic Parts
Gas-assisted molded plastic parts are widely used in various industries and sectors, such as automotive, medical, furniture, electronics, and consumer goods. Some of the examples of gas-assisted molded plastic parts are:
- Door handles
- Steering wheels
- Bumpers
- Instrument panels
- Housings
- Handles
- Tubes
- Pipes
- Chairs
- Tables
Advantages of Gas Assist Molding
Gas assist molding offers many benefits over conventional injection molding, such as:
- Reduced part weight and material consumption by creating hollow sections
- Improved surface quality and appearance by eliminating sink marks, voids, and warpage
- Increased design flexibility and complexity by allowing thin walls, ribs, and bosses
- Enhanced mechanical strength and stiffness by creating uniform wall thickness and internal support structures
- Reduced cycle time and energy consumption by facilitating faster cooling and lower injection pressure
- Lower clamping force and mold wear by reducing the internal stress and friction
Disadvantages of Gas Assist Molding
Gas assist molding also has some drawbacks and limitations, such as:
- Higher initial cost and maintenance of gas injection equipment and molds
- Difficulty in controlling the gas flow and distribution inside the resin
- Risk of gas leakage or contamination during the process
- Incompatibility with some plastic materials or colors that may degrade or discolor due to gas exposure
- Unsuitability for producing clear or transparent parts that may show gas marks or bubbles
Conclusion
Gas assist molding is a novel plastic molding process that uses pressurized inert gas to create hollow or solid parts with high quality and efficiency. It has many advantages over conventional injection molding, such as reduced weight, improved surface finish, increased design flexibility, enhanced mechanical strength, reduced cycle time, and lower clamping force. However, it also has some disadvantages and challenges, such as higher initial cost, difficulty in controlling the gas flow, risk of gas leakage or contamination, incompatibility with some materials or colors, and unsuitability for producing clear or transparent parts. Therefore, it is important to consider the factors such as the part design, the material selection, the gas injection parameters, and the mold design when applying this process.
