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Table of contents
Gas-Assisted Injection Molding is a process where an inert gas is introduced at pressure, into the polymer melt stream at the end of the polymer injection phase.
The gas injection displaces the molten polymer core ahead of the gas, into the as yet unfilled sections of the mold, and compensates for the effects of volumetric shrinkage, thus completing the filling and packing phases of the cycle and producing a hollow part.
Traditionally, injection molded components have been designed with a relatively constant wall thickness throughout the component. This design guideline helps to avoid major flaws or defects such as sink marks and warpage. However, apart from the simplest of parts, it is impossible to design a component where all sections are of identical thickness. These variations in wall thickness result in different sections of the part packing differently, which in turn means that there will be differentials in shrinkage throughout the molding and that subsequently distortion and sinkage can often occur in these situations.
By coring out the melt center, gas injection molding enables the packing force (which compensates for differential shrinkage) to be transmitted directly to those areas of the molding which require attention. This dramatically reduces differentials in shrinkage and thus the sinkage. In addition, the internal stresses are kept to a minimum, considerably reducing any distortion that may otherwise have taken place.
Maximum clamp pressures are normally required during the packing phase of a molding cycle. This is due to the force which has to be exerted at the polymer gate in order to pack melt into the extremities of the mold cavity in an effort to compensate for the volumetric shrinkage of the solidifying melt. In comparison to compact injection molding, gas injection molding typically has considerably shorter distance over which the solidifying melt is required to be packed because of the gas core. This means that proportionally lower packing pressures are required to achieve the same results and in turn, lower machine clamp forces are required.
Gas injection allows cost effective production of components with:
- Thick section geometry.
- No sink marks.
- Minimal internal stresses.
- Reduced warpage.
- Low clamp pressures.
Gas-assisted Fill+Pack analysis benefits
Gas-assisted Fill+Pack analysis provides you with the ability to study polymer and gas flow behavior within a part model and examine the influence that design modifications make on both the polymer and gas flow paths.
Using this information, the design engineer will be able to optimize product design and accurately position polymer and gas injection points. Also to ensure that the product specifications are met, utilizing the full capabilities of the gas injection molding process. Expensive tool modifications, long lead times and trial and error will also be kept to a minimum.
The process engineer will benefit from the program's capacity to examine the effects that varying processing conditions will have on the component and enable optimum processing conditions to be established prior to mold commissioning.
Topics in this section
- Gas-assisted injection molding analysis types and analysis technologies
The following table shows the available analysis technologies for a Gas-assisted injection molding analysis type.
- Preparing the model for Gas-assisted injection analysis
There are modeling tasks required for a Gas-assisted Fill+Pack analysis.
- Modeling gas channels
Compared to modeling for traditional injection molding, modeling for Gas-assisted Fill+Pack analysis requires more attention to the mesh detail in regions where gas is expected to penetrate.
- Gas entrances
A gas entrance is the position where compressed gas is injected into the mold cavity.
- Gas injection methods
During Gas-assisted injection molding, the gas can be injected into the polymer melt either through the nozzle of the molding machine, or by direct injection into the mold or into a runner.
- Gas injection control
The injection of pressurized gas (usually nitrogen) is achieved by the use of a gas control unit that compresses and then releases the gas into the polymer melt.
- Overflow wells-gas injection molding
Ideally, the placement and extent of gas channels should be controlled by suitable modifications to the part geometry.
- Venting locations
Vents are tiny channels at the end of the flow path that allow air and/or polymer to escape the cavity during a Gas-assisted Fill+Pack analysis.