Melt Front Time

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Melt Front Time

Melt Front Time result shows the position of melt front with respect to time during the filling stage.


In general, an optimized Melt Front Time result should show balanced flow contribution of each gate and all flow path should reach the cavity wall at the same time. It is the most useful result in injection molding simulation since you can extract lots of information solely from Melt Front Time.


Possible issues that could be interpreted from Melt Front Time result are listed below:



Hesitation is the condition where the flow significantly slows down along a particular path. If the plastic flows too slowly and eventually stops before it completely fills the cavity, it is called short shot.


You can check the occurrence of hesitation by looking for slowly moving melt front in this result.




To solve hesitation problem, you can:

Increase injection flow rate.

Increase mold temperature or melt temperature.

Change gate location.

Increase wall thickness where the hesitation occurred.

Use a different plastic material with higher MFI (Melt Flow Index).



Short Shot

Short shot is incomplete filling of the mold cavity. When short shot occurs, extending the filling time will not solve the issue.



You can check the occurrence of short shot by looking for incomplete filling from Melt Front Time result.


To solve short shot problem, you can:

Ensure sufficient injection amount.

Increase injection flow rate.

Increase mold temperature or melt temperature.

Change gate location.

Use a different plastic material with higher MFI (Melt Flow Index).

Modify the thickness of cavity or the diameter of runner.

Improve venting.



Weld line

Weld line is the line formed by two different melt fronts joining together during the filling stage. It will decrease the strength of the final product and produce cosmetic defect.


You can identify possible weld line positions by looking for places that two melt fronts join together in Melt Front Time Animation result.

In most cases, it is very difficult to completely remove weld lines. One alternative solution is to move the weld line to area that does not require good strength and smooth surface. Another solution is to diminish the appearance of the weld line.


To move the weld lines, you can:

Change gate location.

Modify part thickness


To diminish the appearance of the weld line, you can:

Increase mold temperature or melt temperature.

Modify runner system design.

Reduce runner diameter to take advantage of frictional heating.



Air Traps

An air trap is formed by converging melt fronts trapping a small bubble of air. It may occur at multiple locations inside the cavity.

You can conclude that an air trap will occur at the location where melt front comes in all directions.




During filling, injected plastic melt flows in several different paths depending on the cavity geometry and the gate location. Usually each flow path does not have the same length or same cavity thickness, so some flow paths will be filled completely before others will. These filled flow paths will keep being filled with extra plastic melt until the entire cavity is filled. The condition of some flow paths being over-filled is called overpacking.


Overpacking could cause warpage due to non-uniform density distribution.

You can identify possible overpacking areas by looking for flow paths that have shorter fill time in Melt Front Time result.


To solve overpacking problem, you should balance the flow paths by following methods:

Move the gate to a position that defines flow paths with similar length.

Balance the flow resistance in each flow path by thickening or thinning the cavity.

Add flow leader or deflector.



Racetrack effect

As described in Overpacking, plastic melt in each flow path travels at different speeds. Because thicker wall region has lower flow resistance, the plastic melt tend to flow faster than thinner wall region. If the flow path is long enough, then backfill into thinner region may occur and further causes air trap and weld line.


You can identify possible racetrack effect by checking if there is any two adjacent flows travel at very different speeds from Melt Front Time result.