Air-assisted molding techniques

Gas-assisted injection molding is a development from injection molding. This process overcomes an important drawback of injection molding: shrinkage. Due to the inherent insulation of plastics, the cooling rate of the cavity walls of the components is uneven, resulting in shrinkage marks.
Gas-assisted injection molding works by injecting resin into the mold cavity by the underfeed injection method, followed by gas injection into the molten resin. Due to the low pressure and high temperature, the gas is able to enter the various parts of the casting along the path of least resistance. As the gas passes through the molded part, it pushes the resin melt from the wall thickness part evenly to the rest of the casting. After the resin is filled, the gas forms a holding pressure in the mold to compensate for the volume shrinkage of the molded product.
There are two main ways to achieve gas-assisted injection molding. The difference between the two methods is the location of the gas injection cavity, one from the spout (see figure above) and the other directly into the resin delivery pipe or molded part. An important difference between the two methods is that the former requires all gas pipes to start with a nozzle. With the latter, that is, when the gas is injected directly into the mold, the design of the gas pipe is not limited by the position of the gas inlet, as long as the resin is properly filled before the gas is injected.
Advantages of the gas-assisted injection molding process
In the designer's point of view, the most significant advantage of this molding process is that the use of hollow channels and inner cavities makes the molded part stronger without increasing weight or even reducing it. Other advantages include:
Higher stiffness.
Greater design freedom.
Reduce in-mold pressure and warpage.
Reduce/eliminate shrinkage marks.
Increased integration possibilities for injection molded parts.
The use of hollow profiles facilitates filling.
Compared to solid profiles, the production cycle is shortened.
Disadvantages of the gas-assisted injection molding process:
The biggest problem is that many gas injection molding processes are patentable and must be licensed before they can be used. The best approach is to consult with the technology provider to determine whether a licence is necessary. Other disadvantages include:
Gases, gas control systems, and permits that may be required add to the cost.
If in-mold molding technology is used, the mold cost increases.
When designing the mold, consider the design and location of the gas nozzle.
There is a learning curve in early applications.
Wide range of material options
Most thermoplastic materials can be molded with gas-assisted injection. Crystalline materials such as polypropylene, polyamide and PBT resins are ideal because they have precise melting points, low viscosity, and easy gas penetration. Raw materials should be selected according to the requirements for product performance, such as rigidity, strength, performance under special conditions, chemical resistance, etc.
Issues to consider in the design
Manufacturers and designers should consider gas-assisted injection molding when designing molded parts that meet one or more of the following parameters:
Molded parts have many internal structures that can cause shrinkage.
The surface of the molded part must be flat and the pressure in the mold must be low.
The molded part must be stable in shape.
The structure of the molded part is complex and has high structural requirements.
Due to the performance requirements, thick profiles are inevitable.
Using this process requires that the design of the molded part include a gas delivery conduit. These catheters direct gas and resin to areas where shrinkage is likely to occur, or where external pushing is required for filling, or where both of these can occur. Molded parts should be characterized optimally for resin and gas filling, with as few pores as possible and the diameter of the gas line as small as possible. Cavity filling can be analyzed to predict packing flow as a function of component design and process parameters.
Example of a gas-assisted injection molding process:
Automotive parts
Car door handles
Door hardware module
Exterior mirror housing
Exterior trim
Accelerator pedal arm
Passenger armrests
Glass wiper arm
Air filter housing
Grille
Bumper trim
furniture
table
sickbed
Chair (armrests, base, legs)
Lawn and gardening utensils
Office equipment
Copier inserts
Keyboard frame
Paper feed roller
Printer/fax machine insert
Computer border
Computer server embedding
Printer in-line
Recreational facilities
Golf club shaft
backboard
Golf roof panel
Snowmobile bumpers
other
Handles of utensils
Medical analytical instrument cover
Shower room base
TV cabinet
Washing machine wave wheel
Flat rack
Power tool handles
Toilet seat
Vending machine cover
Water cooler panels
As more product designers, engineers, toolmakers and molders become familiar with the gas-assisted injection molding process, the only question is how to use it creatively. As is always the case, every member of the development process of a new product should be involved as early in the development process as possible. In this way, opinions and experiences can converge, helping to significantly shorten the cycle time and reduce costs of new product development.