Design Guidelines: Liquid Silicone Rubber Molding

Our basic guidelines for liquid silicone rubber molding include important design considerations to help improve part moldability, enhance cosmetic appearance, and reduce overall production time.

Size

Maximum Dimensions

SIZE 12 in. x 8 in. x 4 in.
VOLUME 13.3 cu. in.
DEPTH No greater than 2 in. from any parting line; deeper parts are limited to a smaller outline
PROJECTED MOLD AREA 48 sq. in.
SIZE 304.88mm x 203.2mm x 101.6mm
VOLUME 217948 cu. mm
DEPTH No greater than 50.8mm from any parting line; deeper parts are limited to a smaller outline
PROJECTED MOLD AREA
30968 sq. mm

Typically, Proto Labs can maintain a machining tolerance of +/- 0.003 in. (0.08mm) with a linear tolerance of +/- 0.025 in./in. (0.025mm/mm).


Materials

  • Standard silicone (30, 40, 50, 60, and 70 durometers)
  • Medical-grade silicone
  • Optical-grade silicone
  • Fluorosilicone (fuel and oil resistant)

VIEW MATERIALS >

press operator examines liquid silicone rubber part

Surface Finishes

FINISH DESCRIPTION
PM-F0 non-cosmetic

PM-F1

low-cosmetic, most toolmarks removed

SPI-C1

600 grit stone, 10-12 Ra
PM-T1 SPI-C1 + light bead blast
PM-T2 SPI-C1 + medium bead blast
SPI-A2 grade #2 diamond buff, 1-2 Ra
optical liquid silicone rubber part on a mold

Draft

VERTICAL FACES 0.5°
MOST SITUATIONS
MINIMUM FOR SHUT OFF
MINIMUM FOR LIGHT TEXTURE (PM-T1)
MINIMUM FOR LIGHT TEXTURE (PM-T2) 5°+

 

While 1 degree of draft is common in LSR parts, zero draft can occasionally be tolerated on shallow parts. The nature of LSR allows the rules of draft to be stretched more so than thermoplastics, if mold construction permits.

injection molding draft ilustration

Adding draft angles to design, even on LSR parts, helps facilitate ejection of a part from a mold, and improves overall moldability.


Wall Thickness

Recommended wall thickness considerations for LSR molding:

  • Walls as thin as 0.010 in. (0.254mm) are possible, depending on the size of the wall and the location of adjacent thicker sections.
  • Rib thickness should be from 0.5 to 1.0 (127mm to 25.4mm) times the adjoining wall thickness.
  • The radii of inside fillets should typically be approximately equal to the wall thickness. Radii that are much larger or smaller than this may cause porosity.
injection molding wall thickness illustration

Proper wall thickness will reduce the risk of cosmetic defects in plastic parts.


Undercuts

LSR molding has the ability to produce parts with undercuts. Many simple undercuts can be easily removed by the press operator without mechanical assistance. Each part will be reviewed on a case by case basis for feasibility.

Undercuts, such as those shown, can complicate and, in some cases, prevent part ejection, so eliminate them if possible.

Undercuts, such as those shown, can complicate and, in some cases, prevent part ejection, so eliminate them if possible.

Radii

Some part corners will have a radius rather than a sharp edge since we use an automated CNC milling process to make the mold for your parts. This typically does not require a change to your model, but resulting radii are identified before the mold is milled.

molding a radiused corner

Proper placement of corner radii can improve the moldability of parts.


operator examines part

Free moldability analysis within hours

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