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Traditionally, a "meeting angle" of 135º is used to differentiate
between weld lines and meld lines, as illustrated in Figure 6. For
most polymers the weld line surface mark tends to reduce once the
meeting angle passes through 120º to 150º. However, there is always
an exception to the rule and some materials by nature tend to
exhibit more visible weld lines irrespective of the angle. In
addition many fillers and additives also highlight weld & meld
lines. Glass fibres and metallic pigments are good examples.
Typical
fountain flow results in a hot core of polymer flowing through the
centre to the flow front and a frozen layer at the mould wall.
Incomplete joining of the frozen layer influences weld line
visibility, therefore mould temperature and melt temperature are key
factors influencing weld line visual quality and strength.
The
exact strength of the weld line depends on the ability of the flow
fronts to weld (or knit) to each other. The strength of the
weld-line area can range from 10 to 90 percent as strong as the pure
material. Weld lines are generally considered to be of lower quality
than meld lines, since less molecular diffusion occurs across a weld
line after it is formed.
Conditions that are favourable for better weld-line strength
include: High injection pressure and speed; High melt and mold-wall
temperature; Formation of the weld lines closer to the gate; A
temperature difference of less that 10ºC between the two emerging
melt fronts.
If a
weld line forms before the filling phase is complete and is
immediately subject to additional packing pressure, the weld line
will typically be less visible and stronger.
The most effective way to improve
the strength of weld and meld lines is by increasing the local
temperature and pressure. |
