Understanding the Molding Parameters

There are over 200 different parameters that must be established and controlled to achieve proper action molding of a plastic part. These parameters fall within four major areas :

4 parameter areas [1]
Reference : A guide for injection molders , by D.M. Bryce

To the troubleshooter, all the 4 areas are important, but the pressure and temperature areas are the ones most commonly considered during the troubleshooting process. Based on the requirements of any particular plastic material, the must be sufficient to inject the plastic material and to hold the mold closed. In addition, the temperature of the injected plastic and mold must be correctly maintained.

1 . PRESSURE

Pressure is found primarily in the injection area, but there is also pressure found in the clamp unit of the molding machine.

1.1 Back Pressure

The first pressure to consider is back pressure. This is pressure that is created during t he return action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of screw. This pressure is used for better mixing of the plastic (especially if colors are added to the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of tthe plastic and result in thermally degraded plastic.

The first pressure to consider is back pressure. This is pressure that is created during the returns action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of screw. This pressure is used for better mixing of the plastic (especially if colors are added to the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of the plastic and result in thermally degraded plastic.

1.2 Injection Pressure

The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and the fastest fill rate are the best conditions. However, high pressure will increase molded-in stress. And the stress will be released at some time. There is no question as to its being released, only as to when it will be released. And remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.

The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and the fastest fill rate are the best conditions. However, high pressure will increase molded-in stress. And the stress will be released at some time. There is no question as to its being released, only as to when it will be released. And remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.

 

 

 

 

 

 

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