一、Preface
Gas assisted injection molding process is on the success of the eighties of the foreign and the 1990s to get practical application of a practical injection molding process and its principle is using the high pressure inert gas is injected into the molten plastic forming vacuum section and promote melt forward, injection, holding pressure and cold but process. Because the gas with high pressure transmission can be consistent throughout the airway internal pressure. Thus, it is possible to eliminate internal stress and prevent deformation of product, at the same time, it can reduce the pressure in the die cavity, so in the molding process does not require very high clamping force, in addition, gas assisted injection molding also has to reduce the weight of the product, to eliminate shrinkage mark, improve production efficiency, the advantages of improving the product design degrees of freedom. In recent years, in the home appliances, automobiles, furniture and other industries, gas assisted injection molding has been more and more widely used, promising. Kelon Group in the 1998 the introduction of a set of gas assisted equipment for the production of refrigerators, air conditioning parts.
二、Gas assisted equipment
Gas assisted equipment including gas control unit and nitrogen generating device. It is a system independent of the injection molding machine, injection molding machine and its only interface is the injection signal connection line. Injection molding machine will be the beginning of a signal injection of injection or screw position is passed to the gas auxiliary control unit after, began to a gas injection process, etc. the next injection process begins when a given another injection signal to start another cycle, and so forth.
Gas assisted injection used in the gas must be inert gas (usually nitrogen), the maximum gas pressure is 35MPa, special up to 70MPa, nitrogen gas purity is higher than 98%.
Gas assisted control unit control device for gas injection time and injection pressure of gas, it has multiple sets of gas path design, can simultaneously control multiple sets of injection molding machine of gas assisted production gas assisted control unit is provided with the function of gas recovery, as far as possible to reduce gas consumption.
In the future, the development trend of the auxiliary equipment is to be built in the injection molding machine, as a new function of the injection molding machine.
三、Gas assisted process control
1．Gas injection parameter
The gas assistant control unit is a device for controlling the gas pressure in each stage, and the auxiliary parameters are only two values: gas injection time (SEC) and gas injection pressure (MPa). Typical gas auxiliary parameters control as shown below:
Figure (1) gas assisted process parameter curve Note: the time of 0~2 segment is delay time
2．Gas assisted injection molding process is in mold injection plastic melt into high pressure gas, between melt and gas exist complex phase effect. Therefore, controlling the process parameters is very important below discuss the method to control the parameters:
a．Injection volume
Gas assisted injection molding is the so-called "short shot method (short size), that is, first in the mold cavity injection of a certain amount of material (usually for surjective 70 - 95%), then injected into a gas, to achieve full process. Melt injection volume and mold airway size and cavity structure relationship. The airway section is larger, more penetrating gas, tunneling rate is high, suitable for short shot in the larger rate". At this time, if the use of excessive amount of material, it is very prone to melt accumulation, where there will be a number of shrinkage marks. If the material is too small, it will lead to blow through.
If the airway is fully consistent with the flow direction, so the most favorable for gas penetration, airway tunneling rate. As a result, it is possible to keep the airway in the mold design in the same direction as the flow direction.
b．Injection speed and holding pressure
In ensuring that the products do not appear defective, as far as possible, the use of high injection speed, melt material as soon as possible to fill the cavity, when the melt temperature is still high, is conducive to the gas penetration and mold filling. Gas promote molten material to fill the mold cavity remains a certain pressure, equivalent to preserving the traditional injection molding pressure, so generally speaking gas assisted injection molding process can save injection moulding machine is used to ensure the process of pressing. But some products due to structural reasons still need to use a certain injection molding to ensure the quality of product performance. But not use high pressure maintaining, because the holding pressure is too high will make the gas needle is sealed, the air in the cavity can not be recycled, when the mould is opened easily produce blow. The high pressure gas penetration will also be blocked, increasing pressure injection may make the product appear more shrinkmarks.
c．Gas pressure and gas injection speed
The relationship between gas pressure and material flow is the most. A good flow of material (such as PP) using a lower gas injection pressure. Several materials recommended pressure are as follows:
Type 熔紙（g /10min） 使用氣壓（MPa）
PP 20~30 8~10
HIPS 2~10 15~20
ABS 1~5 20~25
