Recommended Tolerances

Understanding achievable tolerances early in the design phase is critical. In RIM, tolerances are influenced by part size, geometry, and complexity. Features such as ribs, bosses, holes, draft angles, and wall thickness variation all affect dimensional control. 

General tolerance guidelines include: 

• Parts under 12 inches in length typically achieve linear tolerances of ±0.020 in, depending on geometry. 

• Parts between 12 and 60 inches in length generally fall within 0.0015–0.002 in per inch. 

• Holes up to 3 inches in diameter can typically be held to ±0.010 in. 

Texture

RIM materials replicate mold details exceptionally well, making them ideal for textured and patterned surfaces. Aluminum tooling can be photo-etched to create thousands of available textures, ranging from subtle matte finishes to aggressive patterns. Parts may incorporate a single texture, multiple textures, or a combination of textured and polished surfaces. 

Because textured surfaces require additional draft for proper release, texture selection should be considered early in the design process. 

Draft guidelines for textured surfaces: 

• For every 0.001 in of texture depth, add 1–1.5 degrees of draft. 

• Typical texture depths range from 0.002–0.0045 in, resulting in draft requirements of approximately 2–6.75 degrees on vertical walls. 

• Draft requirements may be reduced depending on material selection and part geometry. 

Color

Color can be incorporated into RIM parts using several methods: 

• Pigmented raw materials (color throughout the part) 

• Post-mold painting 

• In-mold coating (IMC)  

The most common approaches are IMC and post painting. Both provide durable color and UV resistance; however, IMC delivers these benefits in a single, cost-effective process with superior adhesion and no additional surface preparation. IMC is often the preferred solution for high-quality cosmetic finishes. 

Nominal Wall Thickness

RIM offers significantly more design freedom than traditional injection molding, particularly when using foamed systems that are less prone to sink. 

Guidelines include: 

• Localized wall thicknesses may range from 1/8 in to 1 in. 

• Typical nominal wall thicknesses fall between 3/16 in and 0.5 in. 

• Changes in thickness should generally remain below a 2:1 ratio to avoid flow issues and trapped air. 

While thicker sections are possible, they increase cure time and production cost. Large thickness variations may also lead to uneven shrinkage, read-through, warpage, or thermal splitting. When additional stiffness is required, over-molding structural inserts is often more effective than increasing wall thickness. 

Parting, Lines, Venting & Gating

The parting line is a critical design element that influences gating, venting, draft strategy, and part aesthetics. 

Best practices include: 

• Position the parting line as high as possible to promote effective venting. 

• When this is not feasible, a stripper plate may be required to create a secondary vented parting line. 

• Maintain a sharp parting edge—radiused edges can restrict venting and cause pinhole voids. 

• Textured or IMC-coated parts require special consideration, as coating both core and cavity surfaces can reduce venting efficiency. 

Most RIM parts are filled using edge gates: 

• Typical gate lengths range from ~2 inches for small parts to 8 inches or more for larger components. 

• Straight gate edges are preferred for uniform filling and clean de-gating, though curved gates may be used when necessary. 

Ribs

Ribs primarily add stiffness and improve venting by providing airflow paths during filling. 

Design recommendations: 

• Add fillets at rib roots to reduce stress and improve flow (1/32–1/8 in typical). 

• Radius rib tops to aid tool machining and maintenance. 

• Apply 1–2 degrees of draft per side. 

• For cosmetic surfaces, rib width (including radii) should not exceed nominal wall thickness to minimize read-through. 

Draft

All molded features require draft to enable part release. 

General guidelines: 

• Smooth surfaces typically require 1–2 degrees per side, depending on depth. 

• Textured surfaces often require 3–6.5 degrees, increasing with feature height. 

• Interior draft often exceeds exterior draft to encourage part retention in the cavity. 

• Ribs and bosses typically require 1–3 degrees per side. 

• Cored holes require 2–3 degrees, or 1.5 degrees when used for threaded inserts. 

Radii & Fillets

Radii improve part strength, material flow, and tool manufacturability. 

Typical guidelines: 

• External radii: 0.125–0.25 in for 0.25 in wall thickness 

• Internal fillets: 1/32–1/8 in 

• Rib and boss fillets should maintain a ≤1:1 ratio between root width and nominal wall thickness to minimize sink. 

Threaded Inserts

Threaded inserts are typically post-installed for cost efficiency and flexibility. Molded-in holes provide higher pull-out strength due to the dense surface skin formed during molding. 

Fi supports both metric and standard inserts from suppliers such as Stafast and Yardley. 

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