ribs of plastic mold

Enhancing Stiffness and Durability with Ribs

Ribs offer a method to economically increase stiffness and durability in casted components without increasing overall wall size. Different uses for ribs include:

  • Locating and Captivating Parts: Ribs help secure parts in relation to an assembly.
  • Providing Positioning: They assist in aligning mating components.
  • Serving as Stops: Ribs can function as stops or guides for mechanisms.

Rib Design Guidelines

Appropriate rib design consists of four principal considerations: height, size, quantity, and moldability. Consider these factors carefully while designing ribs.

Determining Rib Size

Several aspects influence the suitable rib size:

  • Thickness: Thick ribs often lead to sink and aesthetic issues on the opposite side of the wall.
  • Material Properties: The polymer type, rib size, surface texture, color, proximity to an entrance, and various running conditions affect the severity of sink.
  • Surface Quality: Extremely glossy, critical surfaces may require thinner ribs.
  • Thin-Walled Components: Components with walls less than 1.5 mm can often accommodate thicker ribs than those suggested by typical guidelines.
  • Wall Thickness Considerations: For components with wall thicknesses of 1.0 mm or less, the rib size should match the wall thickness.

Moldability and Filling Challenges

Rib size directly impacts moldability. Very thin ribs are typically difficult to fill. Due to flow hesitation, thin ribs near the entrance can be harder to fill compared to those further away. The flow entering the thicker ribs hesitates and solidifies as the thicker wall sections fill. Ribs usually project from the main wall in the mold-opening path and are formed in blind holes within the custom mold metal.

Draft Requirements

To facilitate component ejection from the mold, ribs generally require at least a half-degree of draft per part. More than one degree of draft can lead to excessive rib size reduction and filling difficulties in tall ribs.

Impact of Rib Thickness

Thick ribs create thickened flow channels where they meet the base wall. These channels can enhance flow within the rib path and alter the filling pattern.

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