The Application and Process of Gas-Assisted Injection Molding Technology

1. Principles of Gas-Assisted Injection Molding

The use of high-pressure inert gas (nitrogen) injected into the molten plastic to create a hollow section and push the molten material forward, facilitating the injection, pressure holding, and cooling processes.

Because gases efficiently transmit pressure, they maintain consistent pressure throughout the gas channel, eliminating internal stress, preventing product deformation, and significantly reducing mold cavity pressure.

Therefore, high mold clamping forces are not required during the molding process, and it can also reduce product weight and eliminate sink marks.

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2. Gas-Assisted Equipment

Gas-assisted equipment comprises a gas-assisted control unit and a nitrogen gas generation device.

It is a separate system from the injection molding machine, with the only interface being the injection signal connection line.

After the injection molding machine sends an injection signal to the gas-assisted control unit, it initiates a gas injection process. Another injection signal is given when the next injection process begins, starting another cycle, and this process repeats.

The gas used in gas-assisted injection must be an inert gas, typically nitrogen, with a maximum pressure of 35 MPa, or up to 70 MPa in special cases, and a nitrogen purity of ≥98%.

The gas-assisted control unit is responsible for controlling the injection time and pressure. It has multiple gas circuit designs and can simultaneously control gas-assisted production for multiple injection molding machines.

The control unit also features gas recovery functionality to minimize gas consumption.

3. Gas-Assisted Process Control

(1) Venting through Venting Channels

The gas-assisted control unit is the device that controls the magnitude of gas pressure at different stages.

There are only two values for gas-assisted parameters: gas injection time (seconds) and gas injection pressure (MPa).

The gas-assisted injection molding process involves injecting molten plastic into the mold while simultaneously injecting high-pressure gas.

There is a complex two-phase interaction between the molten material and the gas, making process parameter control quite important.

The control methods for each parameter are as follows:

(2) Injection Volume

Gas-assisted injection molding uses the so-called ‘short shot’ method, which involves injecting a certain amount of material into the mold cavity first (typically 70-95% of a full shot) and then injecting gas to achieve complete filling.

The amount of molten resin injection is primarily related to the size of the mold’s gas channels and cavity structure. The larger the cross-section of the gas channel, the easier it is for gas to penetrate, resulting in a higher void ratio, making it suitable for a larger ‘short shot’ rate.

In this case, if too much material is used, molten material accumulation can occur, leading to sink marks where there is excess material. If there is too little material, it can lead to blow-through.

If the gas channel is completely aligned with the material flow direction, it is most conducive to gas penetration, and the void ratio of the gas channel is maximized.

Therefore, in mold design, it is advisable to align the gas channels with the material flow direction as much as possible.

(3) Injection Speed and Holding Pressure

While ensuring that the product does not exhibit defects, it is advisable to use a higher injection speed whenever possible to ensure that the molten material fills the mold cavity as quickly as possible. At this point, the molten material temperature remains relatively high, which is beneficial for gas penetration and mold filling.

The gas maintains a certain pressure after pushing the molten material to fill the mold cavity, which is equivalent to the holding pressure stage in traditional injection molding.

Therefore, gas-assisted injection molding processes generally eliminate the need for the injection machine to apply additional holding pressure.

However, for some products due to structural reasons, it is still necessary to use a certain amount of injection holding pressure to ensure product quality. However, high holding pressure should be avoided because excessive holding pressure can block the gas needle, prevent gas recovery inside the cavity, and easily cause blowouts when the mold is opened. Excessive holding pressure can also hinder gas penetration and may result in more significant sink marks in the product.

(4) Gas Pressure and Injection Speed

Gas pressure is closely related to the flowability of the material. Materials with good flowability (such as PP) use lower gas injection pressure.

  • High gas pressure is prone to penetration but may cause blowouts.
  • Low gas pressure may result in insufficient mold filling, incomplete filling, or sink marks on the product surface.

High injection speed can fill the mold cavity at higher molten material temperatures.

For molds with long flow paths or small gas channels, increasing the injection speed is advantageous for proper molten material filling and can improve the quality of the product surface.

However, excessively high injection speed may lead to blowouts, while products with large gas channels may exhibit surface flow marks and gas marks.

(5) Delay Time

Delay time is the time interval from when the injection molding machine starts injecting resin to when the gas-assisted control unit starts injecting gas. It can be understood as a parameter that reflects the “synchronicity” of resin injection and gas injection.

A short delay time, that is, starting gas injection when the resin is still at a relatively high temperature, is obviously advantageous for gas penetration and mold filling.

However, if the delay time is too short, the gas is prone to disperse, leading to poor cavity shape and insufficient hollowing.

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4. Gas-Assisted Molds

Gas-assisted molds are not significantly different from traditional injection molds, except for the addition of an air injection component (referred to as the gas pin) and the design of gas channels.

The term ‘gas channels’ can be simply understood as the pathways through which gas flows after entering, some of which are part of the product, and some are specially designed locations to guide the airflow.

The gas pin is a critical component of gas-assisted molds, directly affecting process stability and product quality.

The core part of the gas pin consists of numerous tiny gaps. If these gaps are too large, they may get clogged by molten resin, resulting in reduced gas flow.

5. Gas-Assisted Injection Molding Process

The gas-assisted injection molding process consists of four steps.

Step 1 Resin Filling

A portion of the mold is filled with molten resin.

Step 2 Gas Filling

Nitrogen gas is injected into the hot molten resin as required. The gas flows rapidly in the high-temperature, low-pressure area.

The direction of gas flow is typically the path of least resistance.

According to the design, gas channels should be placed in locations that facilitate the guidance of gas to the low-pressure area.

Pressure gas is used to displace the molten material at thick cross-sections in the plastic part, completing the filling of the plastic.

Step 3 Gas Pressure Holding

Due to the combined action of molten resin and gas, after the mold is filled, nitrogen gas remains in the gas channels of the plastic part. It exerts enough pressure to compact the plastic part.

Subsequently, as the resin cools and shrinks, the gas pressure forces the uncured resin into the voids created by shrinkage.

The pressure holding is used to eliminate sink marks on the surface of the plastic part and ensures that in the next molding cycle, the mold has a better surface quality for producing high-quality plastic parts.

Step 4 Gas Venting

All the gas required during the entire process must be vented before opening the mold.

Failure to vent pressure gas in a timely manner can cause the plastic part to expand or even burst.

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