Liquid Silicone Rubber Molding

Liquid silicone rubber (LSR) molding is a thermoset process that mixes a two-component compound together, which is then heat cured in the mold with a platinum catalyst to produce a final part. LSR parts are formed through an injection-molding process similar to that of conventional plastic injection molding with one major difference — the material delivery system is cooled and the mold is heated.

Final LSR parts are strong and elastic with excellent thermal, chemical and electrical resistance. They maintain their physical properties at extreme temperatures and can withstand sterilization. LSR parts are also biocompatible, so they work very well for products that have skin contact. Those benefits lend themselves well to automotive, medical and food appliance industries, typically in the form of seals, gaskets, valves and cables.
25 to 5,000+ parts
Shipped in 1 to 15 days
Molds start at $2,000
PROCESS
DESIGN GUIDELINES

Design Guidelines

Liquid silicone rubber is a free-flowing, shear thinning thermoset material that injects easily into a mold and is less prone to many defects like sink as compared to typical thermoplastic materials. However, LSR flashes extremely easily and into gaps as small as 0.0002 in. When designing a part for LSR, you should keep in mind best practices for tooling of standard plastic molds. Though the materials are different, the molds are constructed in a similar fashion.

Size

Maximum part size limits at this time are approximately:
  • Size: 12 in. (304mm) by 8 in. (203mm) by 4 in. (100mm)
  • Depth: No greater than 2 in. (50mm) from any parting line; also note that deeper parts are limited to a smaller outline
  • Surface area: 48.4 sq. in. (312 sq. cm.)
  • Volume: 13.3 cu. in. (217 cc)


Recommended wall and rib thicknesses

Walls as thin as 0.010 in. (0.25mm) are possible, depending on the size of the wall and the location of adjacent thicker sections. Liquid silicone rubber typically fills thin wall sections with minimal challenges. The constraint is if the thin detail can be milled into the mold or removed from the mold without damage. Rib thickness should be from 0.5 to 1.0 times the adjoining wall thickness. The radius of inside fillets should typically be approximately equal to the wall thickness. Radii that are much larger or smaller than this may cause porosity.

Mass reduction and uniform wall thickness

The rules of general plastic part design apply for liquid silicone rubber. Although, LSR is much more accommodating to variations in wall section, and sink is almost nonexistent.

Parting lines

LSR flows into the mold as a liquid and will fill into gaps as small as 0.0002 in. creating flash. Considerations to simplify and minimize parting lines on parts for the LSR process will help to get cleaner parts as quickly as possible.

Undercuts

One of the fortes of LSR molding is an 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.

Part ejection

Unlike thermoplastic molds, ejector pins are not used as part of a standard practice in LSR molding. The part should be designed in a fashion that the entire part is retained on one half of the mold when it is opened at the end of the molding cycle. Ideally, part features will also rise above the parting line surface of the mold so that the operator has something to begin part extraction with.

Draft

Generally, liquid silicone rubber parts need draft similar to what is used in plastic injection molding for ease of manufacturing. A good rule of thumb is 1 degree of draft per inch (25.4mm) of depth. While 1 degree of draft is common, 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.

Gating and venting

Liquid silicone rubber parts require relatively small gates compared to injection-molded parts due to the shear thinning nature of the material. Generally, a gate should feed into the thickest or broadest cross-section of the part, like thermoplastics, although this is not an absolute rule in LSR. A vast majority of LSR gates are some form of edge gate. Gates will leave a vestige or blemish, so they should be placed on a surface that is not dimensionally or cosmetically critical, or a recess should be provided for gating.

In some cases, Proto Labs will need to add vents, overflows or tear strips to accommodate processing requirements of the part. These features are used to improve the overall part quality, but may leave a small vestige on the finished part. A venting pin or insert may be added, in certain cases, to allow otherwise trapped air to escape, leaving a blemish that looks like an ejector pin witness line. Proto Labs will propose gate and vent locations (that require your approval) before manufacturing your tool.

Tool and part finishes

Proto Labs offers the following finishes (to Protomold discretion) on LSR molding tools:
  • PM-F0
  • PM-F1
  • SPI-C1
  • PM-T1
  • PM-T2
  • SPI-A2

More finishes will be added in the near future, and special requests will be given consideration.

Expected tolerances

Typical achieved tolerances for a well-designed part are linear tolerances of +/-.003 in. (0.08mm) with an included resin tolerance of no less than +/-.01 in./in. (.25mm/mm). The LSR process is prone to significant flash, so please contact your Proto Labs Account Manager or Customer Service Engineer for more details on flash allowance.

Material selection

Proto Labs offers a number of engineering-grade liquid silicone rubber materials in various colors (black, purple, red, white, blue, clear, optical) and durometers (30, 50, 70). Please consult the available data sheets for information on material properties. At this time, we do not process customer-supplied material. Our current LSR selection includes:
Material Features Durometer Tensile Strength Elongation at Break
ASTM D412
Tear Strength, Die B
ASTM D624
Data Sheet

Dow Corning MS-1002

  • Optically clear
  • Primary or Secondary lens, light pipes or guides
72 shore A 1625 psi (11.2 Mpa) 80% PDF

Dow Corning QP1-250

  • Water clear
  • USP class IV
51 shore A 1204 psi (8.3 Mpa) 499% 47.4-52.4 kN/m
270-299 ppi
PDF

Elastosil 3003/30 A/B

  • General use
  • Good processability and appearance
  • Transparent
31 shore A 7.5 N/mm² 620% 23 N/mm PDF

Elastosil 3003/40 A/B

  • General use
  • Good processability and appearance
  • Transparent
41 10.00 N/mm² 610% 33 N/mm PDF

Elastosil 3003/50 A/B

  • General use
  • Good processability and appearance
  • Transparent
50 shore A 9.9 N/mm² 480% 29 N/mm PDF

Elastosil 3003/60 A/B

  • General use
  • Good processability and appearance
  • Transparent
60 shore A 9.40 N/mm² 340% 27 N/mm PDF

Elastosil 3003/70 A/B

  • General use
  • Good processability and appearance
  • Transparent
70 shore A 8.6 N/mm² 290% 21 N/mm PDF

Silastic FL 60-9201 (Fluorosilicone)

  • Fuel and oil resistant
  • Seals, gaskets or extrusions
60 shore A 6.5 Mpa 220% 14 N/mm PDF