The core of Gas-Assisted Injection Molding lies in injecting high-pressure nitrogen gas into the molten plastic during the injection molding process. Nitrogen gas filling is an advanced injection molding process that optimizes the production workflow, enhances product quality, and reduces overall costs by “replacing plastic with gas.” Although it increases process complexity and initial investment, it offers irreplaceable advantages in solving persistent issues of traditional injection molding and achieving product lightweighting and integrated design.
I. What is Nitrogen Gas Filling?
Nitrogen gas filling is an innovative injection molding technology. It adds a “nitrogen injection” stage after the conventional “plastic injection” stage. Using specialized gas-assist equipment and control units, high-pressure, pure nitrogen gas is injected into the core of the plastic melt. The gas propels the melt to fill the mold cavity and forms hollow gas channels within the product.
II. Process Steps
- Short Shot (Partial Plastic Melt Injection):
- First, a predetermined amount of molten plastic is injected into the mold cavity, but it does not completely fill it (typically 70%-95% of the cavity volume). This shot volume must be precisely controlled.
- Gas Injection:
- Immediately after the plastic injection is completed, high-pressure nitrogen is injected into the core of the melt through special gas pins installed in the mold.
- As the melt surface skin has formed and the exterior begins to cool and solidify, the gas does not penetrate the melt. Instead, it advances along the path of least resistance (usually thicker sections or rib areas), pushing the melt towards the end of the cavity to ensure complete filling.
- Gas Packing:
- After the cavity is completely filled, the gas pressure is maintained for a period, serving as a “packing” phase. This internal gas pressure is more uniform and effective than the external hydraulic packing pressure in traditional injection molding. It continuously applies pressure to the product shell, compensating for plastic shrinkage, preventing sink marks, and ensuring the appearance surface fully conforms to the mold surface.
- Gas Venting and Recovery:
- After the product has cooled and solidified, the internal high-pressure nitrogen is safely vented and recovered via the control system before mold opening. This results in the pre-designed hollow structure inside the product.
- Mold Opening and Ejection:
- After cooling is complete, the mold opens and the finished product is ejected.
III. Why Use Nitrogen?
- Inert Gas: Nitrogen is chemically stable, unlikely to react with the plastic, making it safe and reliable.
- Dry and Pure: It is readily available as a dry gas source, avoiding the negative effects of moisture on the molding process and product quality.
- Low Cost: Nitrogen is a primary component of air, making it inexpensive to obtain.
IV. Core Advantages
- Eliminates Sink Marks and Improves Surface Quality:
- This is one of the most significant advantages. Traditional injection molding often causes sink marks on the opposite side of thick areas like reinforcing ribs. Internal gas packing pressure effectively eliminates these defects, resulting in smooth, flat appearance surfaces.
- Significantly Reduces Product Weight and Cost:
- The product’s interior is hollowed out by the gas, potentially saving 30% or more in plastic material usage.
- Reduces Internal Stresses and Warpage:
- Gas packing pressure is uniform, avoiding the molecular orientation and internal stresses generated by traditional packing. This leads to less warpage and higher dimensional stability.
- Shortens Molding Cycle Time:
- Due to the reduced wall thickness and because the gas cavity acts as an excellent thermal insulator, cooling efficiency is higher. This shortens cooling time and increases production efficiency.
- Increases Design Freedom:
- Allows for the design of large, complex hollow structural components molded in one shot, integrating multiple parts and simplifying product architecture. It enables the design of products with significant wall thickness variations without concerns about defects.