Parts that are too thick can result in warping and sink marks due to inconsistent cooling. By shelling out thick areas, a more uniform part can be achieved while also reducing material costs. Part rigidity and functional strength can still be maintained through the use of internal ribs.

Sink marks and warpage are best avoided by designing parts with uniform wall thicknesses.

Maintaining consistent wall thickness is one of the most important factors in controlling shrink, warp, and cycle time. Abrupt transitions (transitions without fillets or chamfers), isolated thick sections, or unnecessary mass drive cosmetic defects and may increase cooling time. While ribs and gussets can restore stiffness, they must be sized and located carefully to avoid secondary sink or warp issues.

To achieve optimal results, Eck Plastics recommends using the wall thicknesses listed below:

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Industry-wide, tolerances for injection-molded parts vary depending on the polymer used and the overall size and geometry of the part. This is because tolerances rely heavily on a molder’s ability to predict polymer shrinkage as material transitions from liquid to solid—both in the mold and after ejection. This is a challenging endeavor at best.

Injection molding tolerances generally fall into two categories: commercially accepted tolerances and fine tolerances. Commercial tolerances are typically repeatable and economical. Fine tolerances may be achievable but are material- and geometry-dependent and often involve added cost and risk.

Tolerance stack-ups are frequently underestimated. Engaging with the molder early helps identify which dimensions are functionally critical and which can remain commercially accepted. Applying tight tolerances only where necessary can significantly reduce tooling complexity and piece price.

Tolerance also directly impacts pricing during the molding cycle. More restrictive tolerances require tighter process control, longer cycle times, and increased scrap risk. In many cases, molded parts may vary some from nominal dimensions yet still fit, function, and appear as intended.

Below is a chart outlining generally accepted tolerances for the injection molding industry.

Good corner design is critical in injection-molded parts for several reasons. Properly designed corners distribute stress more evenly, reducing weak points that can lead to warping or cracking in assembled parts. Sharp corners create stress concentrations and restrict material flow, increasing the risk of defects such as short shots.

Generous radii improve material flow, reduce molded-in stress, and enhance cosmetic consistency. From a tooling perspective, sharp internal corners are difficult to machine and accelerate tool wear, while radiused geometry improves manufacturability and tool longevity.

By rounding or tapering transitions in part thickness, read-through and similar surface blemishes can be minimized. Gradual transitions also reduce molded-in stress and stress concentration associated with abrupt thickness changes.

Abrupt transitions are a common source of sink, warp, and other cosmetic defects. Controlled transitions allow material to both flow and cool more uniformly, improving dimensional stability and part quality.

Ribs reinforce strength and stiffness in molded parts without increasing the overall nominal wall thickness. Proper rib design considers five main criteria: thickness, height, location, quantity, and moldability.

Additional uses for ribs include:

• Acting as stops or guides for mechanisms
• Locating and capturing components within an assembly
• Providing alignment in mating parts

While ribs are effective, overuse can create cooling challenges and lead to warpage. Ribs are typically easier to add than remove, so they should be applied conservatively in early designs and added incrementally as needed.

Rib thickness plays a major role in cosmetic and structural performance. Overly thick ribs can cause sink marks on the opposite wall surface. Factors relating to rib design that influence sink severity include: rib thickness, surface texture, color, gate proximity, and processing conditions.

Careful consideration should be given to rib placement and quantity. For example, ribs added to prevent breakage under a sustained load may reduce impact resistance. Similarly, dense rib grids intended to ensure flatness can introduce mold-cooling challenges and warpage.

Ribs are much easier to add than remove, so they should be applied sparingly in the initial design and refined during validation. Excessive ribbing can also increase cooling time, reducing the yield in parts per hour and impacting piece price.

Bosses are projections typically designed to facilitate alignment, or enable fastening. The most common type is a cylindrical projection with a hole for screws or threaded inserts.

The boss’ center hole should extend to the level of the part’s nominal wall, even if that full depth is not required for the chosen fastener. Shallow holes leave thick sections that increase the risk of sink, while overly deep holes can reduce nominal wall thickness, potentially causing filling issues, knit lines, or surface blemishes.

Because of draft requirements, tall bosses (greater than five times their outside diameter) may cause filling issues at the top or thick sections at the base. Tall cores can also be difficult to cool and support. Consider coring tall bosses from two sides (creating a hole through the part) or using gussets to achieve the required standoff height rather than extending the full boss.

Boss design benefits significantly from early review. Small geometry changes can minimize or eliminate sink, simplify tooling, and improve throughput. In many cases, alternative fastening strategies or gusseted designs achieve the same functional outcome with less material and shorter cycle times.