Volumes of production
Strictly a high-volume production process.
Unit price vs. capital investment
Like standard injection molding (see p.196), it combines low unit costs with high investment.
Speed
Due to the fact that material is injected only once and cools more quickly than it would in standard injection molding, cycle times are reduced.
Surface
One of the key advantages of this form of injection molding is the superior finish. During standard injection molding, stress usually occurs along the flow line inside the mold, resulting in warping. The introduction of gas helps to distribute the pressure evenly and eliminate the stress and flow lines in the plastic at specific points. Types/complexity of shape
Injection molding is one the best methods of producing complex shapes, and gas-assisted injection molding is no exception. Depending on how much you want to spend on tooling and the number of parts in the mold, you can achieve some highly complex shapes.
Scale
From casings for small electronic components to large pieces of furniture.
Tolerances
With greater control over the material and less shrinkage than in standard injection molding, tolerances are higher.
Relevant materials
Most thermoplastics, including high-impact polystyrene, talc-filled polypropylene, acrylonitrile butadiene styrene (ABS), rigid PVC, and nylon, and also composites.
Typical products
Virtually all moldable parts can be made using gas-assisted injection molding. External gas-assisted injection molding is often used for components with large surface areas, such as car-body panels, furniture, refrigerator doors, and high-end plastic garden furniture.
Similar methods
Injection molding (p.196) and reaction injection molding (RIM) (p.199).
Sustainability issues
Gas-assisted injection molding can allow for significant reductions in material use as hollow, lightweight parts can be produced. Cycle times are also much faster, with significant savings in energy consumption in comparison to traditional injection molding. In addition, the reduction in weight can be beneficial in reducing fuel consumption during transportation of the product.
Further information
www.magisdesign.com
www.gasinjection.com
MuCell® Injection Molding
The traditional injection molding technique has been used to mass-produce plastic components for everything from cell-phone casings to shoes for more than 50 years. With the evolution of new composite materials, it is no surprise that MuCell® injection molding has evolved along with them. MuCell® is a process that can be applied to both injection molding and extrusion, but introduces a new substance—microcellular foam—into the mix. On average the process allows for a weight reduction of 10 percent and reduced molding cycle times of 35 percent.
Product |
HVAC valve component |
Materials |
Talc-filled, PP (black plastic molded with MuCell®; second sealing gasket molded in solid with thermoplastic elastomer |
Manufacturer |
Valeo |
Country |
Germany |
MuCell® technology is used in this product to create a strong but lightweight (7 percent) component. The material is dispersed equally, which results in less shrinkage of the part.
A polymer mixed with a foaming agent is forced into the mold cavity under very high pressure. Once the polymer has spread throughout the cavity, nitrogen gas is shot into the mold at a very high heat at which it reaches a point between being a vapor and a liquid. This is called the “critical temperature” and is crucial to the behavior of the polymer. Once the gas reaches a higher temperature than this it dissolves into the molten polymer. However, as the pressure inside the mold begins to decrease the gas changes state again, and separates to form a consistent cell structure within the polymer. The cells are microscopic in size but they have incredible strength while being very low in weight. The polymer mix has now formed a microcellular foam.
Because the structure of the microcells is consistent and uniform, the stress within the mold cavity is dispersed equally, which results in less shrinkage than there would be in traditional plastics, which do not have a uniform structure. The molded product is significantly lighter in weight and has less viscosity than if it were conventionally molded. There is also a closer compliance to mold shape and dimensions because of the expanding nature of the foam. The uniform structure provides the molded product with excellent stiffness in addition to thermal and conductive insulation.
The technique is not suitable for everyday molded products, but has been designed for engineered plastic components that require high precision and accuracy. It is a single-phase process—the polymer and gas are injected within the same cycle—and increase in material flow provides the potential for producing thinner parts. To give an idea of the scale the process works with, the parts it produces typically have a wall thickness of no more than 1/8 inch.
– Weight of molded parts is significantly reduced.
– Increased dimensional stability because of the uniform cell structure.
– Cycle time is reduced due to the reduction in weight and viscosity.
– No shrinkage during cooling.
– Limited number of manufacturers.