Mold balancing part 2/2

>> Saturday, 20 September 2008

Mold balancing is design and mold manufacturing problem, machine, set up factors have lower impact. We can not skip factors result from viscosity changing, which are filling stage set up and polymer rheology. Factors which have impact on polymer flow parameters (temperature, pressure, viscosity). It is important to secure symmetry distribution in velocity, shear rate, temperature, viscosity, for mold balancing. Title and body refer to mold, injection mold, but in the same way can be analyze any tool in polymer processing (e.g. extrusion die). Interesting problem is flow balancing in extrusion blow molding and parison centering, measured by wall thickness (e.g. botlle), the problem will be described soon.

How mold balance impact parts?

Each cavity is filled and packed in different stage for imbalanced mold. One cavity can be full filled during injection (over pack) and other partly filled during holding, when we fill shrinkage losses. Effect we receive parts with different stresses distribution, shrinkage, and dimensions.

How find reasons in mold imbalance ?

Imbalance for molding cycle to cycle can be machine and viscosity difference. Factors influence imbalance for the same cycle, can be divided for:

geometrical (geometrical differences between cavities, nozzle gate dimensions and shape),
thermal (design and heating and cooling system function),
mechanical (damage, blocked cavities)

How check mold balance?

Procedure you will find in part 1/2.

When we do the mold balance test?

The best time is to do it when you validate your mold during commissioning. I establish that all your molds are balanced otherwise we can not talk about process stability and capability.

How to deal with imbalanced mold?

Identify the reason. In most cases setting up: mold, hot runner or nozzle temperature is sufficient. If not follow the rule always chose mold adjustment not molding parameters. Mold adjustment you will always do once, parameters you have to always monitor, any deviation and you will receive bad part.

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