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The term "weld line" is used to describe the narrow
V-shaped line that occurs at the point where two different flow
fronts meet.
This phenomenon is guaranteed to occur whenever using inserts, lattices,
or multi-point gates, and there is no theoretical means for its
elimination; accordingly, its effect must be minimized or the resulting
marks must be moved from a decorative face to a side face. To the
untrained eye, a weld line may seem to be a crack. In terms of product
specifications, the presence of weld lines in areas of stress concentrations
may lead to strength problems, and therefore, countermeasures should
be implemented in advance.
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Weld lines disappear when the angle of meeting is 120deg. or more.
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| Cause : |
Weld line factors |
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Shape of molded component
- Holes (i.e., lattices, round holes, square holes)
- Differences in material thickness (or uneven thickness) |
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Plastic
- Fluidity (or viscosity) |
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Die
- Die temperature
- Gates (number and location)
- Gas vents (method and location)
- Cooling ducts (and die temperature distribution) |
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Molding machine
- Performance (precision and response)
- Injection and plasticization |
| Countermeasures: |
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Shaping conditions used to minimize weld lines |
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| -Plastic temperature a High
-Die temperature a High
-Injection speed a Low
*Although weld lines will become less obvious when the speed is
increased
(i.e., the depth is minimized), there is a tendency
for their length to increase.
-Holding pressure a High
* Marks are made less noticeable using the compression effect of
the weld's V-notch.
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| Verification: |
Die temperature which is most effective in minimizing
weld lines |
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Normal molding
Die temperature: 40deg.C
Weld section (central)
Width: Approximately 10 mm
Depth: Approximately 120 mm
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High-speed heat and cool molding
Die temperature: 100deg.C
Weld section (central)
Not visible with the naked eye
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