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(1)Outline of operation
The GAI molding is a method to obtain molded products by injecting molten
resin into a mold cavity, in which pressurized gas is also injected so
that the molten resin can be cooled while it is pressed against the surface
of the cavity by the pressure of gas. Thus dwell and cooling are simultaneously
proceeded.
The basic pattern of the GAI method is shown in the following figure. Molten
resin is injected into a closed mold, and immediately inert gas such as
nitrogen gas is poured into the inside of molten resin. By this process,
molten resin is pressed against the surface of the cavity. The hollow molding
is obtained by degassing and opening of the mold, after the molten resin
is cooled and solidified.
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(2)Setting of molding conditions in the GAI molding
In addition to the ordinary steps required for usual injection molding,
it is necessary in the GAI molding to newly set [1]Delay time for gas injection,
[2]Gas pressure, [3]Gas retention time , and [4]Gas release time.
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[1]
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Delay time for gas injection means the time from the completion of resin
injection to the start of gas injection. If it is too short, the thickness
of the skin layer can be too thin or even the skin layer can be broken
to form holes in some cases. On the other hand, if it is too long, the
appearance can be poor or the gas can not be injected because the resin
is already solidified.
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[2]
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The gas pressure is usually less than 30MPa and it is not necessary to
apply the pressure as high as 100MPa like in the case of usual injection
molding.
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[3]
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The process of gas retention fills the roles of both dwell and cooling
in usual injection molding. The longer the gas retention time is, the better
the dimensional accuracy becomes,
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[4]
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Gas release time is set in order to release the high pressure gas inside
before opening the mold. If the mold is opened before the release of internal
pressure, the molded products are sometimes broken by the internal pressure.
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(3)Characteristics of GAI molding
Following advantages are generally recognized.
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[1]
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Reduction in operation cost due to the possibility of using injection molding
machines with smaller clamping force.
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[2]
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Shorter molding cycle time especially of thick wall moldings.
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[3]
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Reduction in the weight of moldings, depending on the molding method.
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[4]
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Elimination of sink marks.
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[5]
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Better dimensional accuracy due to reduction in internal strain.
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[6]
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Designing flexibility by neglecting the principle of uniform wall thickness
distribution.
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[7]
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Reduction in assembly cost by making hollow moldings in one step.
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Following disadvantages are generally mentioned.
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[1]
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A hole for gas injection remains.
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[2]
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Difference in surface gloss between solid and hollow portions in some cases.
The defect in appearance so-called "switch over mark" remains
at the tip of molten resin at the time of gas injection.
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[3]
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Difficulty in controlling the position and the shape of the hollow.
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[4]
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Muti-cavity molding is difficult. If higher dimensional accuracy is required,
2-cavity molding is at most.
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[5]
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Poor appearance so-called blush appears on the surface of the channel portion,
which is a thick wall portion installed to lead gas.
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[6]
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Degree of deformation is more sensitive to mold temperature in comparison
to ordinary injection molding.
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[7]
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There is no rule which is applicable before hand to the quantitative estimation
of molding shrinkage. |
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[8]
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To be subject to "The law of high-pressure gas regulation." |
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(4)Classification of GAI molding
Classification by the methods of gas control
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Volume-control method |
Pressure-control method |
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Contents
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The method to meter the gas in a vessel of variable volume, typically a
piston-cylinder for example, and then to send this gas into the inside
of the molding under pressure. Through this process the amount of gas is
decided. |
The gas is compressed and stored in a vessel under high pressure. In the
process of injection, the pressure of the gas is reduced to an arbitrary
level and then the gas is sent to the inside of the molding. Through this
process the amount of gas is decided, depending on the gas pressure. |
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Control ability
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The pressure gradually increases, and then decreases as the volume of the
hollow increases. After all the metered amount of gas is injected, the
further increase of pressure is not possible any more. . |
The pressure rapidly increases and is kept constant after that. It is possible
to obtain pressure profile similar to volume-control method by the stepwise
control of pressure. |
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Applicable resin
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Suitable for commodity plastics. It is easy to prevent the penetration
of gas into unnecessary portions, because the volume of gas is controlled |
It is possible to apply dwell effectively. Suitable for crystalline engineering
plastics. |
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Applicable system
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CINPRES,Engel,
Mannesmann,Krauss-Maffei |
AGI,Battenfeld,
Idemistu GIM,Engel,
Kloeckner,Mannesmann,
Krauss-Maffei |
Classification by the position of gas injection
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Nozzle
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Runner
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Cavity
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| Degree of freedom |
The exclusive nozzle is required.
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The exclusive nozzle is not required.
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The exclusive nozzle is not required.
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The exclusive mold is not required.
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The exclusive mold is required.
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The exclusive mold is required.
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It is difficult to use hot runner.
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It is difficult to use hot runner.
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Hot runner can be used.
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| Control ability |
The resin and the gas flow in the same direction, therefore control is
easy.
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The resin and the gas flow in the same direction, therefore control is
easy.
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It is difficult to send the gas counter currently to the resin flow.
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It is difficult to control multiple cavities.
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It is difficult to control multiple cavities.
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It is easy to control multiple cavities.
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| Appearance |
The gate is also used as a gas inlet.
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The gate is also used as a gas inlet.
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The gas inlet remains as a hole besides the gate.
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Classification by processing method
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Short-shot method
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Full-shot method
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Sub-cavity method
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| Principle |
Molten resin whose volume is smaller than that of the cavity is injected
into the cavity, and then inflated just like a balloon. |
The cavity is filled with molten resin and then high pressure gas whose
volume is equivalent to the shrinkage of the resin caused by cooling and
solidification is introduced into the cavity to eliminate sink marks and
warpage. |
The cavity is filled with molten resin at first and then gas is introduced
to push the excess amount of resin to flow into the sub-cavity. |
| Merit |
The higher ratio of air space can be achieved, |
Good surface appearance. |
Good surface appearance. |
| Demerit |
Not applicable to molten resins which are difficult to be expanded. |
It is difficult to send the gas to the dead end of the molding. |
Reduction in material cost is not achieved, unless the increase in the
ratio of reground is possible. |
| Poor appearance due to switch over marks. |
It is difficult to achieve higher rate of air space. |
The gas inlet remains as a hole besides the gate. |
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Special molding