AIM Processing Small Plastic Parts Blog

How to avoid them and produce high-quality injection molded parts

Injection molding creates complex plastic parts with high precision and repeatability. However, like any manufacturing process, it comes with its challenges, including defects in the molded part. One of the most common defects is sink marks. These small surface depressions can compromise not only the aesthetic appeal of a product but also its functionality. This blog will describe sink marks, explain what causes them to occur and offer methods to prevent these defects.

Sink marks defined

Sink marks are shallow depressions or indentations that appear on the surface of injection-molded, plastic parts. They typically occur at thick sections of a part or near structural features like ribs (thin, wall-like structures perpendicular to a surface that add support and rigidity) and bosses (typically cylindrical shapes used for fastening a part to another part). While they might seem like a minor issue, sink marks can detract from a product’s appearance and may indicate underlying problems in the molding process.

Causes of sink marks

Sink marks are primarily caused by uneven thermal contraction. As molten plastic cools and solidifies in the mold, it contracts. When the outer layers of a part cool faster than the interior, the shrinking inner material pulls the surface inward, forming a sink mark. This can occur near a thicker section of the mold which takes longer to cool. The outer portion of the part which is in contact with the mold surface cools faster than the internal portion. As molecules in the thicker sections begin to contract, they pull on contracted molecules on the outer surface of the part causing sink marks to appear.

Other factors that contribute to sink marks include:

• Inadequate melting temperature – If the plastic resin is heated to a level near the lower threshold of its melting temperature, uneven cooling is more likely. However, the mold temperature cannot be too high; if it is, the gates may not seal properly.
• Low packing pressure and hold time - Insufficient pressure during the molding process fails to push enough material into the mold to compensate for shrinkage. Furthermore, the total packing and holding time are not long enough to allow the gates to solidify after filling the mold.
• Insufficient cooling time – A short cooling time can prevent the resin from fully solidifying in the mold.
• Poor Mold Design – Uneven wall thickness, improper gate locations, inadequate gate size or inadequate cooling channels contribute to non-uniform cooling.
• Inadequate materials - Some materials, especially those with high shrinkage rates like polypropylene, are more susceptible to having sink marks.

Recommendation for preventing sink marks

Careful attention to design, materials and process parameters can minimize and potentially eliminate sink marks. The following paragraphs present recommendations in all phases of plastic injection molding design and manufacturing to avoid creation of sink marks.

Optimize the design of the part

A proper design can ensure a uniform cooling rate. Maintaining consistent wall thickness throughout the part helps obtain homogeneous cooling and shrinkage rates. Use ribs to strengthen a wall instead of increasing the wall thickness (See the sidebar on ribs). This can save material and reduce the potential for uneven cooling at a thick-wall-to-thin-section interface. Using fillets (curved transitions) or tapered transitions between thick and thin sections avoids stress concentrations and reduces localized shrinkage.

Locate and design bosses to avoid sink marks. Placing a boss adjacent to or into a wall creates a high concentration of material that will require longer to cool than surrounding surfaces (See the sidebar on bosses). Thinner walls of bosses and spacing from walls contribute to more uniform cooling and can avoid sink creation near a boss.

Optimize process parameters

Tuning the injection molding machine parameters is essential for making a high-quality part which will not be plagued with sink marks. Verify that the machine applies sufficiently high packing pressure to supply enough material into the mold. This adjustment compensates for material shrinkage during cooling. Verify that the packing time is long enough to prevent creation of voids in material and for the gates to solidify. Also, allow sufficient cooling time so that the entire part solidifies at approximately the same rate.

Use an appropriate material

Select materials with low shrinkage rates. Alternatively, use additives that reduce shrinkage rates. Engineering plastics such as ABS or polycarbonate have low shrinkage rates and are superior to commodity plastics.

Employ good mold design techniques

The mold design determines how evenly the part cools. Position gates near locations which require a greater amount of material so that these locations experience adequate material flow and packing pressure. Design cooling channels so that uniform cooling occurs evenly throughout the mold. Consider the use of conformal cooling channels if the part design is a complex shape. Also, include vents to release trapped air in cooling channels. The trapped air negatively impacts heat transfer from the part to the cooling channel.

Perform an additional mold design or manufacturing step

Another option is covering up sink marks. Applying a textured finish to the mold surface can hide small sink marks. In addition, performing a secondary process step such as painting or filling can remove the sink mark.

Avoiding Deep Sink Marks Near Ribs

Ribs are thin, vertical walls integrated into a plastic injection molded part to strengthen its structure without increasing wall thickness. Ribs help to prevent deformation or warping. Excessively thick ribs, greater than two-thirds of the wall thickness, can lead to the creation of sink marks. Keeping ribs as thin as possible, without compromising the strength of the part, can eliminate or minimize sink marks. Also, designing sufficient space between ribs allows more uniform cooling and helps to prevent sink marks.

Avoiding Sink Marks Near Bosses

Bosses are fasteners that enable one part to attach to another part or an assembly. Bosses are typically in the shape of a cylinder. They can be spacers or housings for threaded fasteners. Since bosses can be a thicker amount of material than surrounding surfaces, sink marks can occur nearby. The large amount of material cools at a slower rate than the outer wall, resulting in sink marks.

Conclusion: Take advantage of expertise from AIM Processing

Sink marks are a common problem in injection molding. By understanding their causes and applying the recommended design techniques, optimizing process parameters and selecting appropriate materials, manufacturers can produce high-quality parts absent of sink mark issues.

AIM Processing will not let sink marks sink your parts. The company has over 25 years of experience in manufacturing small plastic parts. Our skilled engineers have decades of experience and develop creative solutions for design and manufacturing challenges. Our engineers will relieve your parts of sink mark issues and keep your parts afloat. Contact us at Contact AIM Processing to get assistance with your plastic injection molded part challenges.