Theory
Viscosity
Cavity Balance
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Theory of Cavity Balance

As the plastic flows through the runners into the cavity, the melt has a given temperature, pressure and velocity. All these three variables are time dependent, which means that the value of each one of them will change within the short time the plastic flows till it reaches the end of fill. For example, melt temperature drops with time. If the melt temperature is T deg F at the start of injection, then after one second, the melt temperature is lower than T deg F. The final quality of the part being molded is a function of each of all these variables or in other words, the final dimension and quality of the part depends on the temperature, pressure and velocity of the plastic as it fills the cavity. 

Consider a one cavity mold. The melt temperature at the end of fill is 450 deg F, the plastic pressure is 8000 psi and the velocity at which the plastic entered the cavity was 4.5 inches per minute. This produced a part with a certain dimension and finish. Now, if the temperature drops to 400 deg F, the part will shrink less and will produce a part that is now larger than the previous shot. Similarly, if the end of fill pressure and velocity changes, there will be a change in the dimension and/or finish of the part. Now consider a two cavity mold with identical parts being molded in each cavity. If the two cavities do not fill under similar conditions, then based on the above discussion, we know that the two parts produced from each cavity will be different. That is why, to produce identical parts, it is imperative that the flow in the two cavities is balanced, demonstrating the fact that the plastic has reached the end of the fill under the same conditions and will result in identical parts. This is the importance of cavity balance.

 
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