HOME    Trouble Shooting     Molded Part Cracking     Sink Mark

Molding Defects


Sink Mark


 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.


Localized sink marks (effect of rear surface shape and flow tips)


 
Factor
Cause
Countermeasure
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 Insufficient, disparity Increase (excess is unacceptable)
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

back to top

Sink marks on entire molded component

 
Factor
Cause
Countermeasure
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 Insufficient, disparity Increase (excess is unacceptable)
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

back to top


!

 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