Gas Assisted Injection Molding
Gas-assist injection molding is based on injection an inert gas such as nitrogen into the injected melt to create one or more hollow channels within an injection-molded plastic part. At the end of the filling stage, the gas is injected into the still liquid core of the molding. The injected gas follows the path of the least resistance and replaces the thick molten sections with gas-filled channels. In addition, the gas pressure packs the plastic against the mold cavity surface, compensating for volumetric shrinkage until the part solidifies. Finally, the gas is vented to atmosphere or recycled.(RDC 3/19/2007) (based on http://www.bayerone.com/gas-assist/ 3/19/2007)
The plastic melt is introduced into the cavity as a “short shot” (less than full volume of the cavity). An inert gas such as nitrogen is then introduced as the plastic injection finishes. The gas can be introduced through the nozzle, sprue, runner or through the cavity itself. The gas takes the path of least resistance, which is typically in center section of thicker channels that are at a relatively high temperature towards the area of low pressure which is at the end of flow. As a result, the gas pushes the plastic melt from the thick section of the part to the unfilled extremities of the cavities, thereby filling the part. The gas filling phase is known as primary gas penetration. This creates the hollow section in the thicker channel(s) of the part. The gas pressure is also used to pack the part during volumetric shrinkage phase of the process as the melt cools from the molten to solid phase. This portion of the process is known as secondary gas penetration. Because of the distribution of the gas channels, the gas applies a very uniform pressure throughout the part, improving shrinkage uniformity, reducing stresses, warpage and sink marks. After the part is completely cooled, the gas is then vented before the tool opens. The gas assist process steps can be summarized as: mold filling by plastic melt, melt/gas transition, gas injection phase (primary gas penetration), gas pack phase (secondary gas penetration), cooling, gas venting phase, and opening of the tool.
S. Shah, Delphi, Journal of Injection Molding Technology, 1, #2 (96-103) 1997
GAIM LLDPE
Injection Molding
Processing
Recent Journal Articles
Recent Journal Articles To get to the abstract or article, copy the citation and paste it into favorite search engine (ex. Google). The date is the date the entry was found.
11/12/2010
Stress relaxation behavior of high density polyethylene (HDPE) articles molded by gas-assisted injection molding
(866-871) Polymer Testing 29 #7 (2010)
Lu et al of Sichuan University, China showed that resistance to stress relaxation is better at higher gas pressure, smaller short-shot size and an appropriate delay time. This stress relaxation behavior is dependent on crystallinity, grain size and grain number of the crystals. (RDC 11/12/2010)
11/6/2010
Study on the packing effects of external gas-assisted injection molding on part shrinkage in comparison with conventional injection molding
(2085–2092)Polymer Engineering & Science 50 #11 (2010)
Chen, Lin and Huang found that applying gas pressure on the reverse side of the part during gas assisted injection molding can reduce or eliminate part shrinkage during the molding process. This process requires only 9 MPA to reduce shrinkage,as opposed to 100 MPA in conventional injection molding. (RDC 11/5/2010)
7/17/2009
Effect of Melt and Mold Temperatures on the Solidification Behavior of HDPE during Gas-Assisted Injection Molding: An Enthalpy Transformation Approach
(p 336-344) Macromolecular Materials and Engineering 294 #5 (2009)