Both the shrink mark and shrinkage porosity are defects of injection products due to the molding shrinkage at thick walls where no sufficient follow-up compensation is made.

During the injection molding process, the surface layer of products often condenses first, followed by the center layer, which will cause sinking on the surface of parts with thick walls, thereby producing obvious inner concave on surface of products, wherein the small concave is called shrink mark and the large concave is called depression; conversely, when parts with thick walls have condensed and when the surface intensity and rigidity are large enough to resist the shrinkage stress resulted from the subsequent shrink of the center part, a large shrinkage porosity or a series of minute shrinkage holes will form inside the parts with thick walls. Usually when such shrinkage porosities or holes appear, depression will more or less occur at thesurface of products. Although shrink mark has no impact on the structural intensity of products, it will cause rather obvious visual defect on the surface whereof; depression influences both surface quality and structural property of products; shrinkage porosity can bring bad effect on the structural property of products. The three defects usually occur at parts with relatively thick walls or at hot spot of products such as the back of lug boss, stiffening rib as well as straight gate of sprue. During actual production, optimization of structure and material of product, the injection technics as well as mold structure are usually adopted to avoid shrinkage porosity and meanwhile to control the surface shrink mark of products at the minimum degree that can be accepted by users. The specific solutions are as follows:

Now a sink mark injection molding is simply depression on the surface of a molded object or part. When it comes to what causes sink marks in injection molded parts, there can be various factors depending on the type or material of the part, i.e., whether it is plastic, metal, etc. Specifically, in injection molded plastic parts, whenever a substance having thick characteristics like bosses or ribs is compressed during the injection molding process, more than the substance present in the abutting wall, sink marks tend to appear.

It is mainly because these characteristics of thick plastic parts lead to a thicker zone that takes more time to cool down than its surrounding dimensions/areas. These distinctive cooling rates create a sink mark in plastic molded objects, which is seen as a dip on the neighboring surface of the object. Such defects, known as sink mark injection molding, are especially problematic when developing and molding thin-walled plastic items like TV bezels and screen monitors. Yet, these marks may not be equally visible on all consumer products made with injection molded plastic. For instance, customer electrical parts may not have a visible sink mark, but plastic toys may have such marks on their exterior.

Although the depressions that often occur in the form of sink marks are small, products with these depressions reflect light from different angles. Hence, due to the reflection of light from diverse angles, the sink marks are easy to spot and very obvious in the molded parts. In addition, a sink mark is evident depending on the color and texture of the molded object. Thus depth is only one issue to consider. Even though sink marks do not influence part longevity or performance, they are considered severe quality flaws.

The mold surface across the rib has a greater temperature than the area close to it. In general, the mold temperature remains high because of the local concentration of molten rubber and the immense heat load. Hence, for the condensation layer to form more quickly, with more stiffness and less likelihood of depression, the mold surface opposite the rib must be cooled down to lower the mold temperature.

· Usage of Very Small Sprue, Runner, and Gate for the Production of Injection Molded Parts:
The flow resistance increases if the sprue, runner, or gate is too tiny. In contrast, if the injection pressure is insufficient to produce injection molded parts, the cavity may not be filled, leading to a low melting density and a significant likelihood of denting in the molded parts. This is another reason that can cause a sink mark to appear.

· Improper Location or Amount of Gates Used During the Production of Injection Molded Objects:
Regardless of whether the number or placement of gates is incorrect, the flow length and resistance will be excessive. However, if the injection pressure is insufficient, the cavity cannot be filled, leading to a lower density of molten rubber is low. This lower density leads to a high possibility of denting, which further causes sink mark injection molding.

Sometimes, no matter how the machine is adjusted, the shrinkage of the outer surface of the injection molded item at the cylindrical (cylinder) location is challenging to address owing to a problem in mold manufacture. In such scenarios, even if the issue is resolved, the injection molded component is already a full-body batch front, making shrinkage non-apparent on these parts.

· Limited Cooling Time:
As mentioned earlier, due to insufficient cooling time, the plastic condensation layer is not thick enough to withstand the internal melt glue curing shrinkage caused by stress, resulting in the formation of a dent. This issue can often be prevented with the help of material suppliers’ suggestions on cooling times for various plastics and product thicknesses.

· Extremely High Temperature of Material Tube Leading to a Shrink Mark:
When the material tube’s temperature is too high, the density of the melt glue is low, and as it cools, the melt glue near the cavity’s surface first hardens into a condensation layer (Frozen Layer). As a result, this causes the volume of the plastic to shrink. Now, if you wait until the central melt glue is also gradually hardened, the cavity will be hollowed out, and the cavity wall will be thick with tensile stress. If the condensation layer’s stiffness is insufficient, it will collapse inward and create a depression. In this scenario, lowering the material’s temperature will increase the molten glue’s density and decrease the likelihood that it will sink.

Apart from this, various other scenarios can be linked with different aspects of the injection molding process, like the equipment, injection molding conditions, materials, and the temperature that may also lead to a sink mark.

Besides exploring what causes sink marks in injection molded parts, it’s also essential to be familiar with the appropriate solutions to deal with them in injection molding parts after they are created. Here are a few practical solutions that can help better handle a sink mark in molded products:

The cooling time should be kept to a minimum to prevent the deformation of the injection-molded pieces. It is because a higher temperature results in prematurely expelling the injection-molded components.
Usually, the rising temperature of the mold surface makes cooling insufficient, which leads to sink and concavation issues. Therefore, the injection molded object is adequately chilled to preserve a specific hardness on its surface after prematurely releasing it so that surface sink marks cannot quickly form. However, cooling won’t eliminate the sink markings if the issue with the sink is severe. Hence, to deal with a more severe sink mark, the surface of the molded object must immediately harden to avoid shrinking without excluding the inner shrinkage holes from the product. You can accomplish this by freezing the water.
Sometimes the sink issue may not be fully resolved by the aforementioned techniques. In such situations, a sufficient amount of an anti-shrinkage chemical should only be used as a last option if surface sink marks absolutely must be eliminated. Transparent components, however, cannot be used in this manner.
Preventing Sink Marks:
Lastly, there is no denying the statement that prevention is better than cure. So before you even end up facing a sink mark issue in your injection molded products or objects, you can use effective injection molding procedures and work in ideal environments to proceed with the molding process without worrying about sinks.

Specifically, emphasizing the manipulation of procedure conditions or designing a component or part to quickly produce more of a particular part without any sink marks – are both ideal approaches to prevent a sink mark in the first place.

Dent marks are caused by localized internal shrinkage after gate sealing or out of stock injection. Dimples or dents on the surface of injection molded parts are an old problem in injection molding process.
Dent is generally caused by local increase in shrinkage rate of plastic product caused by increase in wall thickness of plastic product. It may appear near outer sharp corners or at sudden change of wall thickness, such as protrusions, ribs or behind support, and sometimes in unusual locations. Root cause of dents is thermal expansion and contraction of material, as thermoplastics have a fairly high coefficient of thermal expansion.
Extent of expansion and contraction depends on many factors, of which properties of plastic, maximum and minimum temperature ranges, and cavity holding pressure are the most important factors. There are also size and shape of injection molded parts, as well as cooling rate and uniformity.
Amount of expansion and contraction during molding process of plastic materials is related to thermal expansion coefficient of plastic being processed, and thermal expansion coefficient during molding process is called “molding shrinkage”. As molded part cools and shrinks, molded part loses close contact with cooling surface of cavity. At this time, cooling efficiency decreases. After molded part continues to cool, molded part continues to shrink. Amount of shrinkage depends on combined effect of various factors.
Sharp corners on molded part cool the fastest and harden earlier than other parts. Thick part near center of molded part is the farthest from cooling surface of cavity and becomes last part of molded part to release heat. After solidified material is solidified, as melt near center of part cools, molded part will continue to shrink, plane between sharp corners can only be cooled on one side, and its strength is not as high as that of material at sharp corners.
Cooling shrinkage of plastic material at center of part pulls inward relatively weak surface between partially cooled and more cooled sharp corners. In this way, dents are created on the surface of injection molded part. Presence of dents indicates that molding shrinkage here is higher than shrinkage of surrounding areas. If molded part shrinks more in one place than the other, that is cause of molded part warping.
In-mold residual stress reduces impact strength and temperature resistance of molded part. In some cases, adjusting process conditions can avoid sink marks. For example, during holding process of molded part, mold cavity is additionally injected with plastic material to compensate for molding shrinkage. In most cases, gate is much thinner than rest of part. While molded part is still hot and continues to shrink, small gate has cured. After curing, holding pressure has no effect on molded part in cavity.
Semi-crystalline plastic materials have high molding shrinkage, which exacerbates dent problem; non-crystalline materials have lower molding shrinkage, which minimizes dents; fill and maintain reinforcement materials, which have shrinkage rates lower and less likely to dent.
Thick injection molded parts have a long cooling time and will produce larger shrinkage. Therefore, large thickness is root cause of dents. Attention should be paid to design. Thick-walled parts should be avoided as much as possible. If thick wall cannot be avoided, it should be designed to be hollow, and thick part will be smoothed to nominal wall thickness. Using large arcs instead of sharp corners can eliminate or minimize dents generated near sharp corners.