Air Trap

In injection molding it may happen, that the air can’t escape or that it moves only very slowly out of the cavity.
An air trap will occur if the air is surrounded by the melt completely or if it is at the end of the flowpath (missing or insufficient venting). Therefore, in this situation burn marks may occur.
The air trap can be located in the inside or at the surface of the part.
The later causes an incomplete filling.
With a good venting system and a mainly uniform wall thickness, air traps can be avoided.

Air Trap in Plastic Industry

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.

Flash Formation

The pressure of the melt pushes the mold to open due to a too low clamp force.
On the parting line, the mold opens a little and the outcoming melt freezes and a flash remains.
The risk of the flash formation can be reduced by the use of a machine with a higher clamp force or the reduction of the specific pressure in the cavity.
Another reason for flash formation can be the wrong positioning of the touching surfaces at the parting line.

Flash Formation in Injection Molding

Injection molding defect: At the touching surfaces on the parting line it comes to flash formation.

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.

Hesitation

In injection molding, the hesitation is one of the situations where molten plastic flows inside cavity with different velocities at the front as a result of different resistance to flow. At a region with higher resistance, the melt front tends to stagnate and slows down. The causes can be high variations in wall thickness and locally low mold wall temperature. The hesitation could lead to molding defects such as short shot, burn mark, air trap and poor part surface.
In perspectives of part and mold designs, the hesitation can be avoided by uniform wall thickness and cooling layout. On the other hand, higher melt temperature, higher mold temperature, and higher injection speed could ease the hesitation during the production.

Hesitation in Plastic Industry
Hesitation in Plastic Industry

Injection Molding Defect: Mold halves are not centered to each other leading to non-uniform wall thickness and hesitation effect subsequently.
Injection Molding Defect: At narrow lines of the filling contour hesitations occur.

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.

Jetting

When the material enters a larger cavity with a high flow rate jetting might occur.
While the melt doesn’t hit a wall, a snake-like cord is build until a wall is reached.
The cooling down of the cord surface prevents a good merging between the cord and the new entering melt.
This leads to a reduction of the stiffness and a visible mark.
Slow injection or a bulging wall directly behind the gate will prevent the jetting effect.

Jetting in Plastic Industry

Injection Molding Error: Jetting visible due to polarized light.

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.