When producing a part by injection molding, it can happen that the contours of the molded part are not fully formed, which is the result of a not completely filled cavity. So-called undersupply, or short-shots, then develops. They happen quite often during the first tests. Read our material and learn how they arise and how to eliminate them quickly by changing the right injection parameters.

Underfilling is encountered when the outer contours of the molding are not completely formed. They usually appear at some distance from the filler, in long sections of flow or in the area where thin-walled fragments (e.g., ribs) are present. However, if the tool is inadequately vented, underflows can also appear in guilty places.

Causes of underdevelopment

Incomplete filling of parts can have several physical causes:

– Too little plastic in a single injection,

– Venting problems have impeded the flow of mass,

– Injection pressure too low,

– premature solidification of the channel section (e.g., insufficient injection speed or improper mold temperature).

In the case of under-airing related to inadequate venting, it can be difficult to identify the cause of this defect, as generally this type of problem leads to a diesel effect.

If the molding is too small, it may not be completely filled. This can be the result of too small a portion of plastic injected at one time, venting problems that impede the flow of the compound, too little injection pressure or too much melt flow resistance in the mold.

We can counteract this by changing the relevant processing parameters such as.

– Increasing the injection stroke of the screw,

– Increasing the injection speed,

– Checking the patency of vent channels, improving venting,

– Reducing the resistance of mass flow through the nest, adding baffles to facilitate flow.


It can be helpful to eliminate this surface defect in moldings by examining the posted step-by-step diagram.


The article was prepared on the basis of materials from the Dopak Research and Development Center and the KraussMaffei company


If you are interested in details about over-blowing and other defects of moldings and ways to eliminate them, we invite you to our
training courses at the Research and Development Center in Wroclaw.


Also read another article about defects in moldings: Where do overspills come from and how to eliminate them?