What is Gas-assisted injection molding
Gas-assisted injection molding is a manufacturing process that involves injecting nitrogen gas alongside molten plastic, resulting in a part with hollow sections. The primary objective is to address common challenges in traditional injection molding, such as plastic flow and shrinkage. Various categories of gas-assisted applications exist, but applying this process to produce hollow parts is a logical choice. While it may not be as effective for large diameters as blow molding, which can achieve a remarkable 75% weight reduction, gas assist can still offer a substantial 30 to 40% weight reduction in hollow areas.
Gas assist becomes particularly relevant in applications where injection-molded details surpass the capabilities of blow molding. Its key advantage in hollow parts lies in the ability to integrate a hollow component with an otherwise flat piece or incorporate details resembling those achievable through injection molding.
Advantages of Gas-assisted injection molding
Gas-assisted injection molding demonstrates its true potential when applied to thin-walled structural parts, offering designers the capability to create components with the cost efficiency of thin walls combined with the strength typically associated with thick walls. Employing a short-shot technique involves coring out oversized ribs using a stream of gas, resulting in the formation of hollow tubes within the molded article, thereby achieving an impressive strength-to-weight ratio. Compared to parts relying on tall ribs for stiffness, this technique can yield a notable 25 to 40% increase.
The critical challenge in design and processing lies in containing the gas bubble within the rib pattern. An optimized design should eliminate any margin of error that might allow the bubble to penetrate the wall section, a phenomenon known as fingering. Thick-walled structural parts can be likened to structural foam components, where the foam is replaced by an interconnected web of hollow sections. The concept behind structural foam strength lies primarily in solid skins. Gas assist eliminates the blowing agent and completes the short shot with a burst of gas, eliminating swirl. In this concept, the gas webs act as an internal cushion, similar to foam.
Achieving a density reduction greater than what foam achieves proves challenging, and from a structural standpoint, the wall design must accommodate the worst-case web scenario. Structural foam tends to have more uniform physical properties. While gas-assist parts derive stiffness from oversized ribs, increasing wall thickness diminishes the inherent low weight and cost benefits associated with thin-walled gas assist. Thick-walled gas assist becomes a sensible choice when the application necessitates a thicker wall, whether due to existing mold constraints or ergonomic considerations.
Full-shot injection molding can benefit from incorporating a gas cushion in lieu of the conventional plastic cushion. In this approach, the gas is introduced after the resin is fully injected, serving to compensate for any subsequent resin shrinkage. Frequently, this gas injection is directed precisely to a designated thick spot or problematic area within the molded article.
Upon injection into molten resin, the gas promptly seeks the path of least resistance. It naturally gravitates toward the thickest area of the part, effortlessly navigating corners—an occurrence known as race tracking. The gas bubble undergoes profiling, maintaining a consistent section through which it flows. Specifically, the gas bubble initiates with a larger diameter and gradually reduces in size as it progresses toward the end of the flow.
Gas-assisted injection molding process
The Gas-assisted injection molding process can be elucidated through five key steps in short-shot molding. In Figure 2.16a, molten plastic is injected into a sealed mold under high pressure. Moving to Figure 2.16b, the gas injection process is initiated, causing the simultaneous flow of gas and molten plastic into the mold cavity. Transitioning to Figure 2.16c, plastic injection halts, allowing the continuous flow of gas into the cavity. The gas effectively propels the plastic forward, completing the cavity filling process. It naturally gravitates towards areas with the highest temperature and lowest pressure.
Proceeding to Figure 2.16d, once the cavity is completely filled, the gas maintains its force, pushing the plastic against the cooler surfaces of the mold. This action significantly reduces the cooling cycle duration, mitigates the occurrence of sink marks, and enhances dimensional reproducibility. Finally, in Figure 2.16e, the plastic part has sufficiently cooled to retain its shape. The gas nozzle is retracted to release the trapped gas, allowing for the ejection of the finished part.
Among various structural plastic processes, gas assist stands out as having the most potential for leveraging a designer’s insight into the molding process. The designer assumes dual roles as both the mold designer and the process engineer, wielding control over the flow of both plastic and nitrogen. This integrated approach enhances the precision and efficiency of the gas-assisted injection molding process.
Ribs play a crucial role in defining the gas passage within the design. The gas, inherently following the path of least resistance, tends to navigate toward thicker areas in the part due to their greater volume and subsequently lower pressures. This characteristic attracts the gas bubble to these regions. Establishing these thicker areas effectively involves considering the aspect ratio concerning wall thickness.
In essence, these thicker regions evolve into manifolds or gas passages that connect to a centralized gas injection point. It is advisable for these gas passages to maintain an aspect ratio ranging from three to six times the thickness of the wall section. Lower aspect ratios prove inefficient and may lead to undesired phenomena like fingering, while higher aspect ratios increase susceptibility to gas breakthrough. Gas breakthrough occurs when the gas stream advances ahead of the resin flow front during the filling process. Achieving an optimal aspect ratio is key to ensuring the effectiveness and reliability of the gas-assisted injection molding process.
Gas passages are accommodated within gas runner ribs, wherein intentional variations in wall thickness, resembling ribs, are regarded as projections. It is imperative for gas passages to extend to the extremities of the part. The foundational geometry for the gas passage comprises oversized stiffening ribs. Diverse designs for ribs are conceivable, and practical solutions for deeper ribs involve stacking a conventional rib onto a gas passage rib, maintaining proper aspect ratios. This addresses the challenge of achieving appropriate thickness throughout the rib, preventing issues of being too thin at the top and too thick at the bottom, commonly known as the deep rib draft problem.
Above figure illustrates several variations of rib designs, showcasing the adaptability of the approach. A pivotal aspect of successful product development lies in maximizing the potential of molded components. Particularly in gas-assisted injection molding, the piece-part design takes precedence. The rib pattern emerges as the path of least resistance, serving as a conduit for both plastic (during filling) and gas. Computerized mold filling simulations enhance rib placement, streamlining the process.
The remainder of the part design adheres closely to established practices, with a focus on maintaining a uniform wall section, facilitating the creation of an accurate computer model. The success of any gas-assist program is ultimately under the control of the part designer. Adhering to established design principles eliminates unnecessary variables, reinforcing the importance of a meticulous and strategic approach.
Achieving optimal control over the gas bubble is accomplished through the use of spillovers or overflow cavities. The removal of excess plastic involves displacing the incoming gas volume, representing an advanced stage in gas-assisted injection molding. This enhanced process is available for licensing from various gas assist equipment suppliers. Noteworthy advantages include precise regulation of the injected gas volume, leading to meticulous control over the gas passage profile. The initial mold filling involves a complete plastic shot, offering greater ease of control compared to a short shot.
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