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.

Cracks formed on the surface of molded products are called cracks.

 

Fig. 1 Schematic diagram of cracks

1. Residual stress is too high. In terms of mold design and production, direct gates with minimum pressure loss and higher injection pressure can be used, and forward gates can be changed to multiple pin point gates or side gates, and gates can be reduced diameter. When designing a side gate, a tab gate can be used that can remove cracked part after molding.
In terms of process operation, reducing residual stress by reducing injection pressure is the easiest method, because injection pressure is proportional to residual stress. Temperature of barrel and mold should be appropriately increased, temperature of melt and mold should be reduced, cooling time and speed of parison in mold should be controlled, so that oriented molecules have a longer recovery time.

 

Fig. 2 Schematic diagram of residual stress
2. Residual stress concentration caused by external force. Under normal circumstances, this kind of failure always occurs around ejector rod. After such a failure occurs, ejector device should be carefully checked and adjusted, ejector rod should be set at part with the largest demoulding resistance, such as bosses, reinforcing ribs, etc. If number of ejector pins set cannot be expanded due to limitation of ejection area, method of multiple ejector pins in a small area can be used. If demolding slope of mold cavity is not enough, surface of plastic part will also be scratched to form a wrinkle pattern.
3. There is a difference in thermal expansion coefficient of molding material and metal insert. Metal inserts should be preheated, especially when cracks on the surface of plastic parts occur at beginning of machine, most of which are caused by low temperature of inserts.
In addition, in selection of insert materials, materials with linear expansion coefficients close to resin characteristics should be used as much as possible. When selecting molding raw materials, high molecular weight resins should also be used as much as possible. If low molecular weight molding materials must be used, thickness of plastic around insert should be designed to be thicker.
4. Raw materials are improperly selected or impure. Practice has shown that low-viscosity loose resins are not prone to cracks. Therefore, in production process, appropriate molding raw materials should be selected according to specific situation. In process of operation, special attention should be paid not to mix polyethylene, polypropylene and other resins together, which will easily cause cracks. In molding process, release agent is also a foreign matter for molten material. If amount is not appropriate, it will also cause cracks, so amount of release agent should be minimized.
5. Poor structural design of plastic parts. Stress concentration is most likely to occur at sharp corners and gaps in structure of plastic part, resulting in cracks and ruptures on the surface of plastic part. Therefore, outer and inner corners of plastic body structure should be made as arcs with the largest radius as far as possible. Tests show that optimal transition arc radius is ratio of arc radius to wall thickness at corner of 1:1.7.

 

Figure 3 Unreasonable product design leads to deformation
6. Cracks on mold reappear on the surface of plastic part. During injection molding process, due to repeated injection pressure of mold, fatigue cracks will occur at edges with acute angles in cavity, especially near cooling holes.
When cracks on the surface of mold cavity are reflected on the surface of plastic part, cracks on the surface of plastic part always appear continuously in same part in same shape. When such a crack occurs, surface of cavity corresponding to crack should be checked immediately for same crack. If crack is caused by re-image, mold should be repaired by machining.

Uneven wall thickness of product causes uneven surface shrinkage and thus sink marks.

 

Figure 1 Schematic diagram of sink marks

1. Improper control of molding conditions. Appropriately increase injection pressure and injection speed, increase compression density of melt, prolong injection and pressure holding time, compensate for shrinkage of melt, and increase injection buffer amount. But holding pressure should not be too high, otherwise it will cause convex marks.
If dents and sink marks occur near gate, it can be solved by extending pressure holding time; when plastic parts are dented at wall thickness, cooling time of plastic parts in mold should be appropriately extended; If there are depressions and sink marks around insert due to local shrinkage of melt, this is mainly caused by temperature of insert being too low, try to increase temperature of insert; If surface of plastic part is depressed due to insufficient material supply, amount of material supplied should be increased. In addition, cooling of plastic part in mold must be sufficient.
2. Mold defects. According to specific situation, gate and runner section should be appropriately expanded, gate position should be set as symmetrical as possible, and feed port should be set at thick-walled part of plastic part. If dents and sink marks occur far from gate, it is generally due to poor flow of melt in a certain part of mold structure, preventing pressure transfer.
In this regard, structural size of mold gating system should be appropriately expanded, and it is best to extend runner to part where depression occurs. For thick-walled plastic parts, wing gates should be preferred.
3. Raw materials do not meet molding requirements. For plastic parts with high surface requirements, resins with low shrinkage rate should be used as much as possible, and an appropriate amount of lubricant can also be added to raw materials.
4. Structural design of plastic parts is unreasonable. When designing structure of plastic parts, wall thickness should be as consistent as possible. If wall thickness of plastic parts varies greatly, it can be solved by adjusting structural parameters of gating system or changing wall thickness distribution.

 

Figure 2 Change wall thickness to reduce sink marks