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Molding Defects

Weld Lines

  • Describe the narrow V-shaped line that occurs at the point where two different flow fronts meet. To the untrained eye, a weld line may seem to be a crack.
  • This phenomenon is guaranteed to occur whenever using inserts, lattices, or multi-point gates.
Plastic conditions
Die conditions
Molding machine
  • Holes (i.e., lattices, round holes, square holes)
  • Differences in material thickness (or uneven thickness)
  • Fluidity (or viscosity)
  • Die temperature
  • Gates (number and location)
  • Gas vents
    (method and location)
  • Cooling ducts
    (and die temperature distribution)
  • Performance (precision and response)
  • Injection and plasticization

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.

Shaping conditions used to minimize weld lines

  • Plastic temperature a High
  • Die temperature a High
  • Injection speed a Low
    1. * 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
    1. * Marks are made less noticeable using the compression effect of the weld's V-notch.

Verification : Die temperature which is most effective in minimizing weld lines

写真:延性破壊 写真:延性破壊 破壊面

Normal molding
Die temperature: 40deg.C
Weld section (central)
Width: Approximately 10 mm
Depth: Approximately 120 mm

High-speed heat and cool molding
Die temperature: 100deg.C
Weld section (central)
Not visible with the naked eye

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molded part cracking
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