The dents and hollows on the surface of a plastic product are referred to as “sink marks”. In addition to the appearance of the product, sink marks also affect the quality and strength of the final product. The reasons for sink marks are related to molding processing, mold design and the choice of the plastic material.
Raw Material
The shrinkage rates of different plastic materials are different. Usually, the raw materials that are prone to sink marks are crystalline, such as nylon. During the injection molding process, the crystalline plastic is heated to a fluid state, with the molecules randomly arranged; when filled into the cold cavity, the plastic molecules are slowly arranged neatly to crystallize. As a result, the volume is shrunk to be smaller than the specified dimensional range, which is referred to as the “sink mark”.
The shrinkage rates of various plastic materials(mold shrinkage rate) are shown below:
| Name of the polymer | Explicit name of the polymer | Min Value(%) | Max Value(%) |
| ABS | Acrylonitrile-Butadiene Styrene | 0.7 | 1.6 |
| ABS FR | Acrylonitrile-Butadiene Styrene flame retardant | 0.3 | 0.8 |
| ABS High Heat | Acrylonitrile-Butadiene Styrene High Heat | 0.4 | 0.9 |
| ABS High Impact | Acrylonitrile-Butadiene Styrene High Impact | 0.4 | 0.9 |
| ABS/PC | Acrylonitrile-Butadiene Styrene/Polycarbonate | 0.5 | 0.7 |
| ABS/PC 20% GF | Acrylonitrile-Butadiene Styrene/Polycarbonate 20% glass fiber | 0.2 | 0.3 |
| ABS/PC FR | Acrylonitrile-Butadiene Styrene/Polycarbonate flame retardant | 0.3 | 0.6 |
| ASA | Acrylonitrile Styrene Acrylate | 0.4 | 0.7 |
| ASA/PC | Acrylonitrile Styrene Acrylate/Polycarbonate | 0.3 | 0.7 |
| ASA/PC FR | Acrylonitrile Styrene Acrylate/Polycarbonate flame retardant | 0.4 | 0.8 |
| ASA/PVC | Acrylonitrile Styrene Acrylate/Polyvinyl Chloride | 0.3 | 0.7 |
| CA – Cellulose Acetate | Cellulose Acetate | 0.3 | 1 |
| CPVC – Chlorinated Polyvinyl Chloride | CPVC – Chlorinated Polyvinyl Chloride | 0.3 | 0.7 |
| EVA | Ethylene Vinyl Acetate | 0.4 | 3.5 |
| HDPE – High Density Polyethylene | HDPE – High Density Polyethylene | 1.5 | 4 |
| HIPS – High Impact Polystyrene | HIPS – High Impact Polystyrene | 0.2 | 0.8 |
| HIPS FR V0 | High Impact Polystyrene flame retardant V0 | 0.3 | 0.6 |
| LDPE – Low Density Polyethylene | LDPE – Low Density Polyethylene | 2 | 4 |
| LLDPE – Linear Low Density Polyethylene | LLDPE – Linear Low Density Polyethylene | 2 | 2.5 |
| PA 11 30% Glass fiber reinforced | Polyamide 11 30% Glass fiber reinforced | 0.5 | 0.5 |
| PA 11 conductive | Polyamide 11 conductive | 0.7 | 2 |
| PA 11 flexible | Polyamide 11 flexible | 1.4 | 1.8 |
| PA 11 rigid | Polyamide 11 rigid | 0.7 | 2 |
| PA 12 conductive | Polyamide 12 conductive | 0.7 | 2 |
| PA 12 fiber reinforced | Polyamide 12 fiber reinforced | 0.7 | 2 |
| PA 12 flexible | Polyamide 12 flexible | 0.7 | 2 |
| PA 12 glass filled | Polyamide 12 glass filled | 0.7 | 2 |
| PA 12 rigid | Polyamide 12 rigid | 0.7 | 2 |
| PA 46 | Polyamide 46 | 1.5 | 2 |
| PA 46 30% GF | Polyamide 46 30% glass fiber | 0.3 | 1.3 |
| PA 6 | Polyamide 6 | 0.5 | 1.5 |
| PA 6-10 | Polyamide 6-10 | 1 | 1.3 |
| PA 66 | Polyamide 6-6 | 0.7 | 3 |
| PA 66 30% GF | Polyamide 6-6 30% glass fiber | 0.5 | 0.5 |
| PA 66 30% mineral filled | Polyamide 6-6 30% mineral filled | 0.6 | 1 |
| PA 66 IM 15-30% GF | Polyamide 6-6 impact modified 15-30% glass fiber | 0.2 | 0.6 |
| PA 66 impact modified | Polyamide 6-6 impact modified | 1.2 | 3 |
| PBT | Polybutylene Terephthalate | 0.5 | 2.2 |
| PBT 30% GF | Polybutylene Terephthalate 30% glass fiber | 0.2 | 1 |
| PC 20-40% GF | Polycarbonate 20-40% glass fiber | 0.1 | 0.5 |
| PC 20-40% GF FR | Polycarbonate 20-40% glass fiber flame retardant | 0.1 | 0.5 |
| PC high heat | Polycarbonate high heat | 0.7 | 1 |
| PC/PBT | Polycarbonate/Polybutylene Terephthalate blend | 0.6 | 1.1 |
| PCTFE | Polymonochlorotrifluoroethylene | 0.5 | 1.5 |
| PE 30% GF | Polyethylene 30% glass fiber | 0.2 | 0.6 |
| PEEK | Polyetheretherketone | 1.2 | 1.5 |
| PEEK 30% CF | Polyetheretherketone 30% carbon fiber | 0 | 0.5 |
| PEEK 30% GF | Polyetheretherketone 30% glass fiber | 0.4 | 0.8 |
| PEI | Polyetherimide | 0.7 | 0.8 |
| PEI 30% GF | Polyetherimide 30% glass fiber | 0.2 | 0.4 |
| PEI mineral filled | Polyetherimide mineral filled | 0.5 | 0.7 |
| PEEK– Low cristallinity grade | Polyetherketoneketone– Low cristallinity grade | 0.004 | 0.005 |
| PESU | Polyethersulfone | 0.6 | 0.7 |
| PESU 10-30% GF | Polyethersulfone 10-30% glass fiber | 0.2 | 0.3 |
| PET | Polyethylene Terephtalate | 0.2 | 3 |
| PET 30% GF | Polyethylene Terephtalate 30% glass fiber | 0.2 | 1 |
| PET 30/35% GF Impact modified | Polyethylene Terephtalate 30/35% glass fiber impact modified | 0.2 | 0.9 |
| PET G | Polyethylene Terephtalate Glycol | 0.2 | 1 |
| PE-UHMW | Polyethylene -Ultra High Molecular Weight | 4 | 4 |
| PMMA | Polymethylmethacrylate (Acrylic) | 0.2 | 0.8 |
| PMMA high heat | Polymethylmethacrylate (Acrylic) high heat | 0.2 | 0.8 |
| PMMA Impact modified | Polymethylmethacrylate (Acrylic) impact modified | 0.2 | 0.8 |
| Polyamide 66 (Nylon 66)/Carbon Fiber, Long, 30 % Filler by Weight | Polyamide 66 (Nylon 66)/Carbon Fiber, Long, 30 % Filler by Weight | 0.3 | 0.3 |
| POM | Polyoxymethylene (acetal) | 1.8 | 2.5 |
| POM impact modified | Polyoxymethylene (acetal) impact modified | 1 | 2.5 |
| POM low friction | Polyoxymethylene (acetal) low friction | 1.8 | 3 |
| POM mineral filled | Polyoxymethylene (acetal) mineral filled | 1.5 | 2 |
| PP 10-20% GF | Polypropylene 10-20% glass fiber | 0.3 | 1 |
| PP 10-40% mineral filled | Polypropylene 10-40% mineral filled | 0.6 | 1.4 |
| PP 10-40% TALC | Polypropylene 10-40% talc | 0.9 | 1.4 |
| PP 30-40% GF | Polypropylene 30-40% glass fiber | 0.1 | 1 |
| PP copo | Polypropylene copolymer | 2 | 3 |
| PP homo | Polypropylene homopolymer | 1 | 3 |
| PP impact modified | Polypropylene impact modified | 2 | 3 |
| PPA | Polyphthalamide | 1.5 | 2.2 |
| PPA – 30% mineral | Polyphthalamide– 30% mineral | 1 | 1.2 |
| PPA – 33% glass fiber | Polyphthalamide – 33% glass fiber | 0.5 | 0.7 |
| PPA – 33% glass fiber – high flow | Polyphthalamide– 33% glass fiber – high flow | 0.74 | 0.76 |
| PPA – 45% glass fiber | Polyphthalamide– 45% glass fiber | 0.1 | 0.3 |
| PPE | Polyphenylene Ether | 0.5 | 0.8 |
| PPE 30% GF | Polyphenylene Ether 30% glass fiber | 0.1 | 0.4 |
| PPE FR | Polyphenylene Ether flame retardant | 0.6 | 1 |
| PPE impact modified | Polyphenylene Ether impact modified | 0.6 | 1 |
| PPE mineral filled | Polyphenylene Ether mineral filled | 0.3 | 0.7 |
| PPS | Polyphenylene Sulfide | 0.6 | 1.4 |
| PPS 20-30% GF | Polyphenylene Sulfide 20-30% glass fiber | 0.2 | 0.5 |
| PPS 40% GF | Polyphenylene Sulfide 40% glass fiber | 0.2 | 0.5 |
| PPS conductive | Polyphenylene Sulfide conductive | 0.3 | 1 |
| PPS GF & mineral | Polyphenylene Sulfide glass fiber & mineral | 0.3 | 0.7 |
| PS 30 % GF | Polystyrene 30% glass fiber | 0.2 | 0.2 |
| PS crystal | Polystyrene crystal | 0.1 | 0.7 |
| PS high heat | Polystyrene high heat | 0.2 | 0.7 |
| PSU | Polysulfone | 0.7 | 0.7 |
| PSU 30% GF | Polysulfone 30% glass fiber | 0.1 | 0.6 |
| PSU mineral filled | Polysulfone mineral filled | 0.4 | 0.5 |
| PVC 20% GF | Polyvinyl Chloride 20% glass fiber | 0.1 | 0.2 |
| PVC plasticized | Polyvinyl Chloride plasticized | 0.2 | 4 |
| PVC plasticized filled | Polyvinyl Chloride plasticized filled | 0.8 | 5 |
| PVC rigid | Polyvinyl Chloride rigid | 0.1 | 0.6 |
| SAN | Styrene Acrylonitrile | 0.3 | 0.7 |
| SAN 20% GF | Styrene Acrylonitrile 20% glass fiber | 0.1 | 0.3 |
Injection processing:
Regarding control of the injection technology, the causes of sink marks include: insufficient pressure, excessively slow injection speed, small gate or long runner. Therefore, when using the injection molding machine, it is necessary to pay attention to the molding conditions and whether the holding pressure is enough to prevent the appearance of sink marks.
The sink marks on a hard-plastic product are usually caused when the molten plastic shrinks due to cooling, but at the same time sufficient melt is not provided through the gate to fill up the space created by concentrated shrinkage. Therefore, factors that are not conducive to shrinkage compensation will affect our solution for the sink mark issue.
Most people know that it is easy to cause sink marks when the mold temperature is too high. Therefore, they usually prefer to solve the problem by lowering the mold temperature. However, sometimes if the mold temperature is too low, it will not be helpful for solving the problem of sink marks.
When the mold temperature is too low, the melt is cooled very fast. For the thick part that is far away from the gate, sufficient shrinkage compensation will not be possible, since the passage in the middle section is blocked due too fast cooling, thus making it harder to solve the sink mark issue. Sink marks on larger and thicker molded products are particularly serious. Therefore, when dealing with tough sink mark issues, it is helpful to check the mold temperature. Each material has its proper mold temperature.
It is not conducive to solving the sink mark issue when the melt temperature is too low
Similarly, most of us know that a plastic injection molded product is prone to the sink mark issue when the melt temperature is too high. If the temperature can be properly lowered by 10 to 20°C, the sink mark issue will be improved.However, when the injection molded part has a sink mark in a relatively thick part, then setting the melt temperature to a too low level, e.g., close to the lower limit of the melt temperature, will not be helpful for solving the sink mark issue, or even make worse. The thicker the injection molded part, the more obvious it is. PC material is a raw material that solidifies quite quickly, so its sink mark issue can be said to be a big problem in plastic injection molding.Also, if the melt temperature is too low, it will not be conducive to increasing the overall shrinkage amount, resulting in an increase in concentrated shrinkage, thereby exacerbating the sink mark issue.
To solve the sink mark issue, the first thing that comes to mind is to raise the injection pressure and extend the injection time. However, if the injection speed is already very fast, it will not be conducive to solving the sink mark issue. Therefore, when it is difficult to eliminate the sink mark, it should be solved by lowering the injection speed. When the injection speed is lowered, a large temperature difference can be created between the melt front and the gate, which is helpful for melt solidification and shrinkage compensation from the far end to the near, while also allowing the sink mark far away from the gate to get a higher pressure for shrinkage compensation, thus helpful for the problem solving. Due to the lowering of the injection speed, the temperature of the melt front is relatively lower, while the speed has been slowed down, so it is not easy for the molded part to flash. The injection pressure and time can be further increased, to better solve the serious sink mark issues.
Mold & Product Design
The mold runner and cooling designs greatly influence the final product. Due to the low heat transfer capacity of the plastic material, it solidifies and cools slowly. There should be enough plastic to fill the cavities, so that the plastic does not flow back to cause pressure drop when the screw of the injection molding machine is performing injection or pressure holding.
On the other hand, the gate cannot solidify too fast, so that the semi-solid plastic will not block the runner and cause pressure drop, and subsequently sink marks on the product. Different mold flow processes lead to different shrinkage rates. The properly controlled barrel temperature is able to prevent the overheating of the plastic part; extending the cycle is able to allow sufficient time for the product to cool down.