Injection Molding Plastic Part Design Guidelines

To ensure your parts are moldable, please review the basic guidelines laid out on this page and follow these basic steps when designing your injection molding plastic parts. A PDF image of these guidelines is available by clicking the link below.
Basic Plastic Injection Molding Design Guidelines

Wall Thickness by Resin Material Guidelines

Selecting the proper material and observing uniform wall thickness in injection molded plastic parts helps avoid potential issues such as sink marks. We recommended using the plastic part design guidelines in the table to the right, as thicknesses vary by material.
Proper Materials List
Plastic Injection Molding Part Guidelines
Plastic Injection Molding Part Guidelines
Plastic Injection Molding Part Guidelines
Core or redesign thick areas to create a more uniform wall thickness to prevent sink or voids.
Making the outside radius one wall thickness larger than the inside radius will maintain constant wall thickness through the corners. R2 = R1 + t
Rounding or tapering thickness transitions will minimize read-through and possible blush or gloss differences. Additionally, blending reduces molded-in stresses and stress concentration associated with abrupt changes in thickness.
Plastic Injection Molding Part Guidelines
Rib Thickness
Plastic Injection Molding Part Guidelines


Ribs provide a means to augment strength and stiffness in plastic molded parts without increasing overall wall thickness. Other uses for ribs include:
• Acting as stops or guides for mechanisms
• Locating and captivating components of an assembly
• Providing alignment in mating parts
Proper rib design involves five factors: thickness, height, location, quantity, and moldability.

Rib Thickness

There are many factors that determine the appropriate rib thickness. Thick ribs often cause sink and cosmetic problems on the opposite surface of the wall to which they are attached. Factors including the material, rib thickness, surface texture, color, proximity to a gate and a variety of processing conditions determine the severity of sink.
The illustration above gives plastic part design guidelines for rib thickness for a variety of materials. These guidelines are based upon subjective observations under common conditions and pertain to the thickness at the base of the rib. Highly glossy, critical surfaces may require thinner ribs.

Rib Location and Quantity

The location and quantity of ribs is important to avoid worsening problems that the ribs were intended to correct. 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. Ribs are easier to add than remove. They should be applied sparingly in the original design and added as needed to fine-tune performance.
Plastic Injection Molding Part Guidelines
Proper Design of Bosses
Plastic Injection Molding Part Guidelines
Bosses find use in many plastic molding part designs as points for attachment and assembly. The most common type consists of cylindrical projections with holes designed to receive screws, threaded inserts or other kinds of fastening hardware. Generally, the outside diameter of bosses should remain within 2 to 2.4 times the outside diameter of the screw or insert
Avoid bosses that merge into sidewalls, because they can form thick sections that lead to sink. Proper bosses should be positioned away from the sidewall. If needed, use connecting ribs for support. Try using open boss designs for bosses near a standing wall.
Plastic Injection Molding Part Guidelines
Plastic Injection Molding Part Guidelines
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, causing 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.
Plastic Injection Molding Part Guidelines
Draft (Draft Angles)
Draft: Providing angles or tapers on product features such as walls, ribs, posts and bosses that lie parallel to the direction of release from the mold which eases part ejection. How a specific feature is formed in a plastic mold determines the type of draft needed.

Features formed by blind holes or pockets – such as most bosses, ribs, and posts – should taper thinner as they extend into the mold. Surfaces formed by slides may not need draft if the steel separates from the surface before ejection. Other guidelines for draft design include:
• Draft all surfaces parallel to the direction of mold separation.
• Angle walls and other attributes that are formed in both mold halves to assist ejection and retain uniform wall thickness.
• As a general rule, use the standard one degree of draft plus one additional degree of draft for every 0.001 inch of texture depth.
• Use a draft angle of at least one-half degree for most materials. If the design permits it, use one degree of draft for easy part ejection. SAN resins typically require one to two degrees of draft.
Plastic Injection Molding Part Guidelines