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This page will summarize solutions to various problems in plastic molding!

Images of plastic pellets

Mechanism of gas burning and countermeasures

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Problem 1: Gas burning

Agenda

What is Gas?

What is gas burn?

Countermeasures (1)

Countermeasures (2)

Countermeasure (iii)

glossary

injection molding

 mold

nesting

gas

gas vent

gas burn

ガスとは

ガスとは? What is Gas?

Image in which gas is generated by thermal decomposition

Gas is the vaporization or gassification of volatile components and decomposition products contained in resins at high temperatures. (Since this gas contains volatile components, there is a possibility of ignition depending on the situation.

Gas generation (Gas is generated from the nozzle tip along with molten resin at high temperature)

ガス焼けとは

ガス焼けとは? What is Gas burn?

Mechanism of gas burning

When gas generated from resin fills the mold during the injection molding process, the volume of the gas is compressed rapidly as the resin fills the mold because the inside of the mold is basically a sealed space. At this time, the compressed gas becomes hot, causing ignition and explosion. This causes the resin to burn and the surface of the molded product to become discolored. This is called gas burning.
*** Translated with www.DeepL.com/Translator (free version) ***

Gas burning in plastic products

Gas scorching (resin is scorched by ignition and explosion, discoloring the surface of the molded product)

Countermeasures (1)

対策方法①

If the cylinder temperature at which the molding material is melted is too high, the material will thermally decompose and discolor. If the residence time in the cylinder is too long, the material will also thermally decompose and discolor. At the molding site, high-viscosity plastics tend to be molded at higher cylinder temperatures and speeds to make them flow more easily.

Factors of gas burning due to molding conditions
対策方法②

Countermeasures (2)

Gas vents are usually built into a part of the mold and provide a path for the gases in the mold to escape. When the mold is closed or material is injected, the gases are vented out of the mold through the gas vent. The design of the gas vent is optimized for the mold geometry and material properties. Without sufficient gas vents, gases can become trapped inside the mold. trapped in the mold. Conversely, excessive gas venting can lead to material leakage and uneven molding.
*** Translated with www.DeepL.com/Translator (free version) ***

List of gas vent depth by resin used
Gas vent installation location
SG-WIND

Countermeasure (iii)

Most gas vents are installed at the final filling point, but gas vents can also be installed in the area of the molded product during the filling process. In molds with slits or holes, weld lines and air traps do not always occur at the final filling point, so gas venting nests can be set at arbitrary locations to make weld lines and air traps less noticeable. Therefore, weld lines and air traps can be made inconspicuous and eliminated. Molding results from setting the gas venting nets in the area where weld lines and air traps occur. The gas venting nets are set in a wide area so that weld lines are not noticeable and air traps do not occur. This time, the product is a palm-sized molded product, but for large molded products such as automobile bumpers, the shape becomes more complex and the amount of resin used increases, so the amount of gas generated will inevitably increase. The larger the molded product, the more important it is to vent the gas.
*** Translated with www.DeepL.com/Translator (free version) ***

Example of gas venting receptacle (harmonica type)
View of resin flow in the mold
Location of gas venting nets

Advance①

Gas venting nest: SG-WIND

Gas venting insert designed by a molding shop that also makes molds

sg.png

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Advance②

CAE: Prediction by flow analysis

Pioneering introduction and accumulation of know-how in the molding industry

Accurately predicts weld lines and air traps

☞ Click here☜

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