By Design: Part design 103 — Sink marks in nominal walls

Past articles in this series have established the importance of an injection molded part's nominal wall. Selecting the optimum wall thickness and maintaining that thickness throughout the part have also been reviewed. This article will be devoted to sink marks in the nominal wall. Sink marks are an inherent part of the injection molding process. They are the topic of endless debates among marketing, quality assurance, product designers, and injection molding suppliers. And the trade press, resin suppliers, and textbook authors have devoted reams of paper to describing the causes and cures for sink marks. As a result, sink marks have developed a bad reputation.

What Is a Sink Mark?
Whittington's Dictionary of Plastics defines a sink mark as a "shallow depression or dimple on the surface of an injection molded article, caused by local internal shrinkage after the gate seals, or by a short shot."

The interpretation of this definition depends on to whom you are talking. To the end user, a sink mark is a curiosity. To upper management, it is "a waste of our valuable time"; to marketing, it is "unacceptable"; to the designer, it is "a surprise"; to quality control, it is "not on the drawing"; to the molder, it is "to be expected"; to the moldmaker, it is "not my responsibility"; and to the material supplier, it is the detail "described on page 24 of our design manual."

In spite of these different interpretations, everyone seems to agree that sink marks are unwanted depressions on the surface of a molded part. Sink marks are considered to be undesirable because they detract from the appearance of an otherwise attractive part.

What Causes Sink Marks?
A sink mark is usually caused by a localized increase in shrinkage that results from an increase in wall thickness, or the lack of adequate cavity packing pressure. These marks may also appear near sharp outside corners, or at abrupt changes in wall thickness. They occur with annoying frequency on appearance surfaces opposite bosses, stiffening ribs, and standoffs. Sometimes, sink marks show up in unexpected locations with no apparent justification.

The root cause of sink marks is that all solid materials expand when heated and contract when cooled. This phenomenon is important to the injection molding process, as thermoplastic materials exhibit a relatively high coefficient of thermal expansion. During injection molding the plastic material is first heated and then cooled. As the material is heated and cooled, it expands and contracts.

The amount of expansion and contraction is determined by a lot of interrelated factors. The characteristics of the specific plastic material, the maximum/minimum temperature range, and cavity packing pressure are the most significant factors. The size and shape of the part being molded and the rate and uniformity of cooling are other important considerations.

The amount of expansion and contraction that takes place during the molding of a plastic material is related to the coefficient of thermal expansion of the material being molded. However, other variable factors combine to produce differences in the amount of expansion and contraction that will take place in a specific molding situation. The molding industry lumps all of these variations together and refers to the coefficient of thermal expansion during the molding process as "mold shrinkage."

Figure 1 depicts a cross-sectional view of an injection molded part and a portion of the mold cavity. The cavity has been filled, and there is no additional flow of melt into the cavity. At this point in time, the plastic is pressed tightly against the cool surfaces of the cavity. As the molded part cools, it gives up its heat to the cool surfaces of the cavity. That heat is conveyed away from the cavity by the round cooling channels, which are shown as four small, out-of-scale circles in this view.

As the material cools, the coefficient of thermal expansion comes into play. The part cools, contracting or shrinking, and becomes smaller. This shrinkage pulls the part away from the walls of the cavity, as shown by the broken line just inside of the walls of the cavity.

To digress for a moment, it is important to note that as the part shrinks to a size smaller than the cavity, it loses its intimate contact with the cooling surfaces of the cavity. From that point on, there is a reduction in the efficiency of the cooling.

As the part continues to cool, it will continue to shrink by an amount determined by the combination of factors mentioned previously.

Theoretically, the hot plastic material gives up its heat uniformly along all four sides of this .125-inch-thick square part. However, further consideration of this situation indicates that the plastic material forming the four corners of the part becomes progressively thinner the nearer it comes to the sharp corner. These sharp corners contain a smaller mass of material than any other location on the part. These sharp corners are also being cooled from two sides. As a result, the corners on the part cool rapidly and regain their strength before the other surfaces on the part. This condition is particularly prevalent while molding crystalline plastic materials, which have a definite solidification temperature.

The thick mass of plastic material near the center of the part is the farthest from the cool surfaces of the cavity. This is the last portion of the part to give up its heat. As this mass of material near the center of the part cools, it continues to shrink or contract long after the corners have regained their strength.

The four flat surfaces halfway between the sharp corners are being cooled from only one side and they will not have regained as much strength as the material at the corners.

When this combination of circumstances occurs, all of the necessary criteria are present for the creation of a sink mark, of the type shown by the broken lines in Figure 2. In this illustration, the relatively weak surfaces between the partially cooled and stronger corners have been pulled inward by the cooling and shrinking of the plastic in the center of the part. It is this shrinkage in the center of thick sections that creates sink marks on the surface of injection molded parts.

This phenomenon of sharp outside corners cooling quickly and shrinking less also explains why shadowy little sink marks so often appear near the edge of flat panels with square edges.

Sink Marks Indicate Stress
There are many instances in which sink marks are unacceptable. In other cases, such as industrial parts where appearance is less critical, the sink marks are tolerated. Still, sink marks should never be ignored, as these surface depressions are indicators of a more serious problem. The presence of a sink mark proves that there is a higher rate of mold shrinkage in that location than there is in the surrounding area on the part. If a molded part is shrinking more in one area than in another, there will be stresses set up in the molded part. These molded-in stresses are one of the reasons why injection molded parts warp.

Molded-in residual stress also reduces a part's impact strength and temperature resistance. In some instances, a more costly higher performance plastic material is substituted in order to compensate for these problems related to molded-in stress. This is unfortunate, as in many cases an acceptable appearance and adequate temperature and impact resistance could have been achieved with the lower cost material if the part were properly designed and molded.

Minimizing Sink Marks
Molding considerations. In some cases, sink marks can be avoided by adjustments in the molding conditions. For instance, during the packing part of the cycle, additional plastic material is pushed into the cavity to compensate for the mold shrinkage. Some large, complex parts simply do not allow the whole part to be properly packed out. In most cases, the gate will be much thinner than the rest of the part. These small gates solidify while the molded part is still hot and continuing to shrink. Packing will have no effect on the part in the cavity, once the gate has solidified. If this is the cause of sink marks, then a hotter mold and/or a larger gate may be helpful.

Material considerations. Taking into account the origin of sink marks, it is apparent that the semicrystalline plastic materials, with their higher mold shrinkage factors, will exaggerate this problem. Amorphous materials, which have a lower shrinkage factor, will minimize the sink mark problem. Filled and fiber-reinforced plastics have an even lower shrinkage factor and they have less of a tendency to create sink marks.

Design considerations. Thicker sections take longer to cool, resulting in additional shrinkage in those locations. These thick sections are the root cause of sink marks and are the result of how the engineer designed the part. Wherever possible, localized thick sections should be eliminated. Where this cannot be done, thick sections should be cored out. If this is not possible, then the thicker sections should be smoothly blended into the nominal wall thickness. The undesirable sink marks that are created near sharp corners can be minimized or eliminated by replacing square corners with generous radiuses.

The part shown in Figure 2 has radiuses on the two corners on the right-hand side of the molded part. Referring to the figure, it can be seen that the mass of plastic material at these two radiuses does not thin out as much as the material that forms the two sharp corners shown on the left side of the same part. These rounded off corners do not cool as quickly as the sharp corners. As a result, the plastic material in these corners and the material halfway between these radiused corners cools more uniformly. This allows all of the outside surfaces to be more uniformly pulled toward the center of the part by shrinkage in that area.

If the square shape of this part were changed to be round, the rate of cooling and the mold shrinkage would be uniform in all locations on the part. Design engineers should also consider the fact that highly polished surfaces reflect light, and that exaggerates the appearance of even a minor sink mark. Matte finishes and textured surfaces tend to disguise sink marks.

If the size of the .125-inch square part shown in Figure 2 was increased to .250 inch, the sink marks and molded-in stress would also increase. Shrinkage in a molded part of that thickness could also create internal voids, which are the topic for the next article in this series.