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11-16-2015

Constant Mold Breathing

As clamping force builds up, the mold deforms and the volume of the cavity decreases. During injection, the deformation is offset in part by the force generated by the incoming melt. The increase in cavity volume is called mold breathing.

During injection molding, the melt pressure forces the two mold halves apart by a few thousandths or hundredths of a millimeter. This process is called mold breathing. Since mold breathing is difficult to measure, simply the maximum clamping force is applied in actual practice. This can, however, cause improper mold venting, increased wear and unnecessary use of energy.

Using the existing sensors on the injection molding machine, newly developed software permits calculation of the mold breathing during the molding process. This requires that the entire clamping system, including the mold, be viewed as a spring. During injection, the stress in the spring increases slightly due to mold breathing; the clamping force increases minimally beyond the set value.

To determine this increase as accurately as possible, the injection molding machine is dry-cycled several times at the start of production. During these, reference curves for the clamping force are recorded and saved. During subsequent production, the current clamping force curve is compared with the reference cycles. With the known spring stiffness, it is then possible to calculate the clamping force difference into the mold breathing.

Based on the calculated mold breathing, the software establishes the required clamping force fully automatically within the specified limits. Optimization can be activated during the ongoing molding process. At the push of a button, the machine automatically changes the set clamping force over several cycles and registers the change in the peak value of mold breathing. Using a stored algorithm, the required clamping force is calculated automatically from the obtained data.

Dr.-Ing. Harald Sambale
sambale <AT> hanser.de

additional links

Article: Breathing Exercises for Process Optimization, Kunststoffe International 2015/10

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