Knit Line Affect On Material Strength

[JMulligan]

I am looking for a technical explanation of knit line affect on material strength in PC/ABS injection molded parts. Any help would be greatly appreciated.


Thank You
Joe

 

[MikeHalloran]

The material is as strong as ever, it's just not _joined_ at a knit line.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Do you mean you want an explanation as to why it is not as strong, or you want an explanation as to what strength you can expect, and maybe parameters that will cause variations.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JMulligan]

patprimmer,

I was looking for the strength I can expect, and maybe parameters that will cause variations. In the application I am working on the knit line area is still very strong. My customer was asking for a "scientific explanation" that compares the base material to the knit line areas.

It seems to me that it would be a very complex explanation in order to deal with the multitude processing variables.

Thank you for taking time to reply, and you too Mike Halloran.

 

[patprimmer]

Some weld lines are as strong as the base material, some have zero strength.

In a reasonable design a weld line will typically be 80% or better.

Some variables are:-

Temperature of the melt at the flow fronts where the two fronts meet.

Pressure of the melt at the flow fronts where the two fronts meet.

Effects of trapped gas and volatiles at the flow fronts where the two fronts meet.

Effect of fillers and reinforcement materials at the flow fronts where the two fronts meet.

Effect of additives such as waxes and stabilisers at the flow fronts where the two fronts meet.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[orbit03]

Material plays a role in the strength too. Another variable is how the flow fronts meet (what angle).

You mentioned ABC/PC blend. I do know that straight PC has fairly poor weld line strength. Especially if it is a butt weld (head on flow fronts).



------------
Jason Williams
K Development, Inc.

 

[moldflowk]

Hi,

I would like to post a doc from Moldflow HELP.


The Weld lines result is generated at the end of a filling analysis. This analysis shows where weld lines are likely to occur on your model along with the angle at which the flow fronts converge to form the weld line. Weld lines occur where two or more flow fronts converge. The presence of weld lines may indicate a structural weakness and/or a surface blemish. For a full list of flow analysis results



Weld lines can cause structural problems, and they can also make the part visually unacceptable. However, some weld lines are unavoidable, so you need to look at the processing conditions and the weld line position to decide if the weld lines will be of a high quality.

Weld line strength is influenced by the temperature at which the weld line is formed and the pressure exerted on the weld while the material is within it's recommended processing temperature range.



Structural problems - The part may be more likely to fracture or deform at a weld line, especially if the weld line is of a low quality. This weakness will be more of a problem in areas of the part which will be subject to stress.
Visual defects - A weld line can cause a line, notch or color change on the surface of the part. If the weld line is positioned on a non-critical part surface (for example, the bottom of the part), this may not be a problem.


Remedy


To move weld lines, change the fill pattern to make the flow fronts meet at a different place:

Increase the wall thickness.
Decrease the part thickness ratio.
Adjust the gate position and dimension.
To improve the quality of weld lines:

Increase the diameters of gates and runners.
Move injection locations to make weld lines form closer to the gates.
Move injection locations to make flow front meet more obliquely.
Increase the melt temperature, injection speed, or injection pressure or packing pressure.
Place a vent in the area of the weld line. This will remove air traps, which should further weaken the weld line.


Regards,

 

[KeithFawcett]

This may not be exactly relevent to your problem, but I have had cosmetic problems caused by knit lines due to sub-gating at 120° around the perimeter of a circular cover. By changing to two gates 30° apart inline with apertures in the covers the length and visibility of the knit lines was reduced considerably. Shorter knit lines must equate to a stronger part.

 

[lorelei5607]

Weld Lines occurs due to temperature difreences between two material fronts.
The material (melt) flows into cavity in a FOUNTAIN FLOW.
If an obstacle (any geometry futures in the cavity which obstructs the flow) will create two material flows (these two material flows will have a diferent temperature): when the two material fronts meet they form a WELD LINE.
The weld line has a lower strength than material which was molded at same temperature (you may experience microscopic cracks at weld line which will develop in full cracks during part application or assembly).