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Polyplastics' new testing method prevents gas burns during injection molding
Source:Adsale Plastics Network    Editor:JK    Date:27.Sep.2021

Polyplastics Co., Ltd. has identified several solutions to help processors limit the occurrence of gas burns during the injection molding process. The company used its Gas Investigation Method in Injection Molding (GIMIM) test method to analyze the phenomenon and help pose preventative measures.


Gas burns are caused by adiabatic compression occurring when resin decomposition gases and air get trapped inside the mold. Normal molds have gas vents and parting lines, hence gas burns do not often occur. However, continuous molding can cause gas vents to clog, and adiabatic compression will similarly result in gas burns.


Illustration of gas burns from trapped air & gas.

The origin of the substances that cause the vents to clog cannot be identified, which means that the cause remains unclear, and the phenomenon is difficult to reproduce.

Polyplastics' study firstly examined gas resulting from energy loss at the time of injection. The nozzles of typical injection machines are comprised of a drastically contracted structure, and energy loss arises due to the shape. Potential solutions include lowering the injection speed, using multiple gates or cavities, and increasing the size of the gate and nozzle.


Illustration of gas burns with clogged vents.

To prevent uneven thermal conduction within the injection barrel, the findings suggest avoiding injection machines that are excessively large relative to the molded articles, reduce cylinder temperature, and avoid mold-contact molding so that heat loss from the mold does not occur.


The GIMIM method also determined that vent clogging is caused by the workings of a complex relationship between the volumes of low boiling and high boiling substances, and the total volume of substances within decomposition gas. Corrective measures include lowering cylinder temperature and using a rag or similar object to frequently wipe away mold deposits that are lightly stuck to the gas vents.


Comparison of gas burns from different purging conditions.


GIMIM comparison of gas substances under each set of conditions.

Lastly, it was determined that the GIMIM testing method can verify the purging process. Until now, there was no method to verify the effects of the purging process other than visual confirmation after molding many shots of material. Since GIMIM can detect substances in units of ppm, it can be used to verify whether the purging process was conducted properly through only a small number of machine shots.


Polyplastics will be conducting further research and studies using GIMIM. It also continues providing new solutions to gas-related and mold deposit problems while developing materials and grades that reduce the occurrences of these problems.


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