Parts that are too thick will result in warping and sink marks. By shelling out thick areas a more uniform part can be achieved while also reducing material costs. Rigidity can still be maintained in a shelled part by placing ribs inside the shelled out part. Wall thickness details are below:

Sink marks and warpage best avoided by designing your part with uniform wall thicknesses. Even further different polymers shrink at different rates. To yield the best results, Eck Plastics recommends using the wall thicknesses listed below:

The entire industry accepts that tolerances specified on a print for injection molded parts should vary with respect with the polymer used, and the overall size of the part.  This is because industry wide tolerances greatly rely on the molder’s ability to predict how the polymer will shrink as it changes from a liquid to solid both inside the mold, and after the part has been ejected.  Below is a chart that specifies generally accepted tolerances for the injection molding industry.

The entire industry accepts that tolerances specified on a print for injection molded parts should vary with respect with the polymer used, and the overall size of the part.  This is because industry wide tolerances greatly rely on the molder’s ability to predict how the polymer will shrink as it changes from a liquid to solid both inside the mold, and after the part has been ejected.  Below is a chart that specifies generally accepted tolerances for the injection molding industry.

By rounding or tapering transitions in part thickness will minimize read-through and possible surface blemishes. Additionally, the practice of material blending may reduce molded-in stresses and stress concentration associated with abrupt changes in thickness.

Providing ribs within a design reinforces strength and stiffness in molded parts without increasing overall wall thickness. Proper rib design involves five main criteria: thickness, height, location, quantity, and mold-ability.

Other uses for ribs are as follows:

• They act as stops or guides for mechanisms
• Ribs locate and captivate components within an assembly
• They provide alignment in mating parts

There are many factors that determine the appropriate rib thickness for a part. Often if ribs are too thick they can cause sink, a cosmetic problems on the opposite wall surface. The material, rib thickness, surface texture, color, proximity to a gate, and a variety of processing conditions determine the severity of the sink.

Rib Location & Quantity:

Proper thought should be used when deciding on the location and quantity of ribs within a part. For example, ribs added to increase part strength and prevent breakage may reduce the ability of the part to absorb impacts without failure.

• Furthermore, a grid of ribs added to ensure part flatness may lead to mold-cooling difficulties and warpage.
• Typically much easier to add than remove, ribs should be applied sparingly in the original design and added as needed to fine tune performance.

Bosses are projections on a part designed to add strength, to facilitate alignment during assembly or allow for fastening additional parts. The most common variety consists of cylindrical projections with holes designed to receive screws, threaded inserts, or other types of fastening hardware.

Normally, the boss hole should extend to the base-wall level, even if the full depth is not needed for assembly. Shallower holes can leave thick sections, resulting in sink. Deeper holes reduce the base wall thickness, leading to filling problems, knit lines, or surface blemishes. Because of the required draft, tall bosses (those greater than five times their outside diameter) can create a filling problem at their top or a thick section at their base. Additionally, the cores in tall bosses can be difficult to cool and support. Think about coring a tall boss from two sides or extending tall gussets to the standoff height instead of than the whole boss.