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Painting Defects

As the easiest plastic to paint, ABS is widely used for painted components.
 Nevertheless, painting defects can occur as a result of factors such as grade selection, molding conditions,paint and thinner characteristics, and painting method. The following photographs are provided in order to allow painting defects to be better understood, thus contributing to their resolution.



Pinholes

picture:Pinholes picture:Pinholes enlarged view
Characteristics
  • Often seen in urethane painting, this phenomenon is characterized by the appearance of small holes on the painted surface, and it is also known as "cratering.
    (picture:Urethane paint)
Cause 1
  • Inclusion of air

Pinholes can be caused when air bubbles trapped in the paint film expand during drying and break through the surface.
Furthermore, this phenomenon is not dependent on the substrate,
and it occurs often in high-temperature, high-humidity environments;
with thick application of paint; when there is insufficient setting time;
and when the speed of thinner evaporation during drying is high.

Countermeasures
  • Modification of the painting environment
  • Lengthening of the setting time
  • Reduction of the paint's viscosity
  • Slowing of the thinner evaporation speed
Cause 2
  • Residual strain

When the plastic substrate contains residual stress, the material is attacked
by the paint's solvent and fine cracking occurs. The solvent penetrates
into these cracks, and when this then evaporates during the drying process,
the resulting vapor bursts through the surface of the painted film.

Countermeasures
  • Selection of a less aggressive solvent
  • Modification of the paint and solvent mixture ratios
  • Annealing of the unpainted component (at between 60deg.C and 70deg.C for 20 to 60 minutes)
  • Raising of the cylinder temperature and die temperature
  • Lowering of the injection pressure and raising of the injection speed

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Cracks

picture:crack

Characteristics
    As with pinholes, these fine cracks on the painted surface are caused by solvent attack, and this phenomenon is often seen in acrylic painting. These cracks generally occur near gates and at edges in particular."
    (picture: Acrylic paint)
Cause Solvent in the paint attacks sections of the substrate material
where levels of residual strain are high, thus causing chemical stress.
Countermeasures
  • Selection of a less aggressive solvent
  • Modification of the paint and solvent mixture ratios
  • Annealing (at between 60deg.C and 70deg.C for 20 to 60 minutes)
  • Raising of the cylinder temperature and die temperature.
  • Lowering of the injection and dwell pressures,
    and raising of the injection speed

* Generally speaking, the addition of retarder is not a good means of
treating chemical cracks.

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Sinking

picture:Sinking picture:Sinking enlarged view

Characteristics From the paint penetrates the component near the gates, at areas of
varying thickness, or near welds; consequently, luster becomes unevenand fine cracks develop.
(picture: Metallic acrylic paint)
Cause This problem often occurs at areas close to gates with molding defects such as gate flash, flow marks, or jetting; furthermore, these factors are difficult to eliminate even by annealing.
Countermeasures
  • Selection of a less aggressive thinner
  • Slight increase of the paint's viscosity
  • Raising of the cylinder temperature and die temperature.
  • Lowering of the injection pressure and raising of the injection speed
  • Modification of the gate type (i.e., switching to the use of tab gates)

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Crawling

picture:crawling picture:crawling enlarged view

Characteristics With indentations of 1 mm or more in diameter occurring on the painted surface, this phenomenon results in the substrate becoming visible.
(picture:Urethane paint)
Cause

This problem occurs when a contaminant with lower surface tension (i.e., lower wetting) than the paint adheres to the surface of the molded component.

  • Mold release agent (i.e., silicon oil)
  • Machinery grease
  • Dust or other contaminants attracted by static electricity
Countermeasures
  • Regulation of the painting environment to eliminate the causative agents
  • Degreasing of the molded component's surfaces (using n-hexane, IPA, etc.)

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Sealing defects

picture:Sealing defects

Characteristics One of a number of phenomena where paint peels from the surface of the component, poor adhesion is generally identified during adhesion testing.
(picture:Poor adhesion as identified in a cross cut test)
Cause Although this phenomenon readily occurs when contaminants
such as grease or mold release agent are present on the surface of
the molded component, it is also affected by matching of the
substrate and paint (i.e., solubility and wetting, etc.) and by the
condition of the paint itself (i.e., thinner, viscosity, film thickness).
Countermeasures
  • Degreasing of the molded component's surfaces (using n-hexane, IPA, etc.)
  • Switching to paint with lower cohesive power
    (cohesive power < adhesive power a no peeling)
  • Thickening of the paint film
  • Increasing of the injection speed ( increase orientation and raise thinner attack )

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Others

Lifting

Wrinkling occurs when multiple layers of paint are applied (i.e., repainting).

Orange peel

Rather than being flat and smooth, the surface takes on the appearance of orange peel,with numerous small bumps and indentations.

Whitening

The surface of the painted component becomes cloudy and white.(Often occurs with acrylic lacquers.)

Welds

Welds on the surface of molded components become more obvious after painting.

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!

 UMG ABS Ltd. accepts no responsibility for the quality or safety of any customer products which use our materials or which have made use of any type of data provided by this company.
 Customers are requested to independently determine the suitability of our materials for their products. We also request that sufficient attention also be paid to laws, regulations, and industrial rights.


molded part cracking
molding defects
weathering
painting defects