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Molding Defects: Sink Marks (or shrink marks)
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Sink marks or shrink marks are hollows or indentations
that occur on the outer surfaces of molded components.
Whether
or not sink marks are treated as a problem depends on the required
quality of appearance.
For example, this would not be acceptable for external molding
components which must be highly attractive in nature. It is often
the case that the decision on whether or not to treat this phenomenon
as a defect depends on product quality issues.
Sink mark behavior depends on the volumetric shrinkage
of the plastic (i.e., the isothermal PVT characteristic) and
the chronological history of all locations within the injection
molding process is important. In specific terms, this phenomenon
occurs during the transition from the molten condition upon
injection to the solid condition upon dwelling and cooling.
Molten plastic that has been injected into the
die begins to cool and solidify from the die surface. As the
plastic continues to cool and harden from the outside (i.e.,
during dwell and cooling), certain injection settings such as
the dwell pressure and time make it impossible to compensate
for changes in volume of the plastic (i.e., volumetric shrinkage)
resulting from the PVT characteristic. In these cases, the plastic
at the surface of the die can be drawn towards the inside of
the molding when volumetric shrinkage occurs in the molten plastic
still present at the interior, and this results in the cosmetic
defect referred to as sink marks.
Alternatively, when the outer layer of the molded
component has sufficient strength to resist the pull of volumetric
shrinkage, voids will be generated at the interior, and in certain
cases, this will not be manifested as an appearance-related
problem. Furthermore, if latent causes exist in the shape of
the product (i.e., bosses, ribs, thick sections, uneven thickness,
etc.) or in the construction of the die (i.e., cooling duct
design, die material, cooling agent, etc.), it will be impossible
to eliminate sink mark problems unless advance countermeasures
are implemented at the product design and die design stages.
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Localized sink marks (effect of rear surface shape and flow tips) |
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Factor
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Cause
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Countermeasure
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| Product |
Material thickness |
Thick or uneven (thin) |
Removal of material, standardizing thickness,
thickening |
| Gate position and method |
Inadequate (design restrictions) |
Addition of gates, modification of method |
| Molding conditions / Molding machine
performance |
Resin temperature |
High (or low) |
Reduce (or increase) |
| Die temperature |
High (or low) |
Reduce (or increase) |
| Injection speed |
High (or low) |
Reduce (or increase) |
| VP switching |
Fast |
Slower position |
| Dwelling pressure |
Low |
Increase |
| Dwelling time |
Short |
Lengthen |
| Back pressure |
Low (measurement disparity) |
Increase |
| Screw speed |
Temperature increase due to rotation |
Reduce |
| Amount of cushioning
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Insufficient, disparity |
Increase (excess is unacceptable)
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| Mechanical back-flow |
Replace with normal component |
| Measurement |
Insufficient |
Increase |
| Die clamping pressure |
Insufficient (presence of flash) |
Increase |
| Die |
Nozzle holes |
Small cross section |
Increase |
| Sprues |
Small cross section |
Increase |
| Runners |
Small cross section |
Increase |
| Gates |
Small cross section |
Increase |
| Cooling circuit |
Insufficient (temperature disparity) |
Increase, standardize |
| Unsuitable method (i.e., efficiency) |
Change method |
| Small flow volume / high pressure loss |
Improve efficiency |
| Die material |
Insufficient cooling ability |
Usage of material with good thermal
conductivity |
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Sink marks on entire molded component
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Factor
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Cause
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Countermeasure
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| Product |
Material thickness |
Thick or uneven (thin) |
Use suitable thickness |
| Gate position and method |
Inadequate (design restrictions) |
Addition of gates, modification of method |
| Molding conditions / Molding machine
performance |
Resin temperature |
High (or low) |
Reduce (or increase) |
| Die temperature |
High (or low) |
Reduce (or increase) |
| Injection speed |
High (or low) |
Reduce (or increase) |
| VP switching |
Fast |
Slower position |
| Dwelling pressure |
Low |
Increase |
| Dwelling time |
Short |
Lengthen |
| Back pressure |
Low (measurement disparity) |
Increase |
| Screw speed |
Temperature increase due to rotation |
Reduce |
| Amount of cushioning
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Insufficient, disparity |
Increase (excess is unacceptable)
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| Mechanical back-flow |
Replace with normal component |
| Measurement |
Insufficient |
Increase |
| Die clamping pressure |
Insufficient (presence of flash) |
Increase |
| Die |
Nozzle holes |
Small cross section |
Increase |
| Sprues |
Small cross section |
Increase |
| Runners |
Small cross section |
Increase |
| Gates |
Small cross section |
Increase |
| Cooling circuit |
Insufficient (temperature disparity) |
Increase, standardize |
| Unsuitable method (i.e., efficiency) |
Change method |
| Small flow volume / high pressure loss |
Improve efficiency |
| Die material |
Insufficient cooling ability |
Usage of material with good thermal
conductivity |
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