Trouble Shooting

Sink marks or shrink marks are hollows or indentations that occur on the outer surfaces of molded components.

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)

Warping and Twisting Checkpoints

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	Factor	Cause	Countermeasures
Product	Material thickness	Thick or uneven (thin)	Removal of material, standardizing thickness, thickening [Reference] Product design guide (thick)
	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

Sink marks on entire molded component

Warping and Twisting Checkpoints

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	Factor	Cause	Countermeasures
Product	Material thickness	Thick or uneven (thin)	Use suitable thickness [Reference] Product design guide (thick)
	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/add/change
	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

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.