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Design Guidelines: Liquid Silicone Rubber

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

We offer a number of secondary moulds and tooling services to support your project.

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Size

Typically, Protolabs can maintain a machining tolerance of +/- 0.08mm with a linear tolerance of +/- 0.025mm/mm.

Design Feedback
For additional questions, please contact an applications engineer at customerservice@protolabs.co.uk or +44 (0) 1952 683047

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Materials

VIEW ALL MATERIALS >

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Surface Finishes

FINISH	DESCRIPTION
PM-F0	non-cosmetic, default
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

 

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Draft

VERTICAL FACES	0.5°
MOST SITUATIONS	2°
MINIMUM FOR SHUT OFF	3°
MINIMUM FOR LIGHT TEXTURE (PM-T1)	3°
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 mould construction permits.

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Wall Thickness

Recommended wall thickness considerations for LSR moulding:

• Walls as thin as 0.25mm 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 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.

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Undercuts

LSR moulding 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.

Radii

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