Rib is a kind of mechanical profile which is very common to be seen in plastic part design. Most of the time, the purpose of rib is to increase the strength of plastic part. See illustration of rib in Pic.-1 and 2.

Pic.-1 illustration of crate
Pic.-2 illustration of phone case

While rib is only necessary under the scenario when the area need more strength. it is important to follow these principle while adding rib:

  • Too thick rib will lead to shrink mark on part surface finish
  • Too thin rib will lead to stress mark on part surface finish

Therefore, enough but minimized rib thickness shall always being concerned in order to provide enough plastic part strength and keep part surface finish at acceptable level.

Different rib thickness requirement could be raised within the entire product life cycle including simulation of part design, testing during product development, customer complain after product introduced into market.

If the rib thickness found not enough (more strength is required at certain area of the plastic part), we can increase the rib thickness.
If the rib thickness found too much (found sink mark on plastic part surface finish), we can reduce the rib thickness.

Usually, the required/enough rib thickness on an individual part leads to sink mark. Other then changing the rib thickness, we can change plastic part design according to following direction:

  • Add grooves at opposite side of rib
  • Add beads at opposite side of rib
  • Add surface texture

These plastic part design changes help to reduce the shrink mark on the surface finish, see illustration in Pic.-3

Pic.-3 illustration of different part surface finish design

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