Causes of and Countermeasures for Gas Burns

In plastic injection molding, a phenomenon can occur in which gas burns occur at the tip parts of thin ribs, etc., and a part of the molded product becomes black because of being carbonized.

The mechanism of generation of gas burns is that, at the time that the air inside the cavity of the mold is exhausted by the molten plastic flowing into the mold, if some of the air gets locked in a closed space, that air is compressed and gets heated by its own heat generation, thereby causing burning of the plastic.

Although air being a gas gets compressed, it generates heat when compressed. This is the same phenomenon as the pump becoming hot when it is used for filling air into a bicycle tire.

The compression of the air remaining inside the cavity occurs in a very short time of about 0.1 to 0.5 second, and since it is also compressed to a very high pressure such as 200 to 500 kgf/cm2, the temperature easily reaches the burning temperature of the plastic material. (See Fig.)

The following countermeasures are effective for preventing gas burns.

	1.	If the part into which plastic flows is in the closed state, use a divided structure and insert a core pin. Since air is exhausted between the cavity and the core pin, gas burns are not generated. It is more effective if an air vent is provided on the side surface of the core pin. However, since in this method, parting line will appear on the surface of the molded item, it is necessary to take care because this method cannot be used in the case of molted items that do not permit such parting lines on the surface.
	2.	In the molding conditions, make the injection speed as low as possible and fill slowly. Although this may improve the situation in the case of light gas burns, it is necessary to be aware that this is not a fundamental countermeasure.
	3.	Carry out the preliminary drying of the molding material sufficiently, and put it in a condition in which it is difficult for air to get mixed inside the molten plastic. Even in this case, it is necessary to be aware that this is not a fundamental countermeasure.
	4.	Change the flow pattern of the molten plastic by changing the wall thickness of the molded product, or by changing the gate position, thereby changing the position where air gets trapped. Although this method is effective, since the shape of the molded product and the weld position change, it is necessary to obtain permission in advance from the designer of the molded product.
	5.	Change the position where the air gets trapped by changing the position of changing the injection speed of the screw. This may improve the situation in some cases if the gas burns are light.