An introduction to Silicone Dispensed Foam Gaskets

Silicone Dispensed Foam Gaskets : Sealing and Gasketing

With the information below you will be able to introduce yourself into the Silicone Dispensed Foam Gaskets world.

40+ Years of Silicone-based Solutions

 

Agenda

Dispensed silicone discussion – gaskets and seals terminology

• Unique properties of silicone
• What are silicone dispensed foam gaskets (DFG)?
• Common applications
• Design considerations
• Equipment requirements for DFG
• Questions/discussion

 

Unique Properties of Silicones

• Performs in temperatures from -70 to 200˚C
• Withstands weathering, ozone, corona, radiation, moisture, chemicals, weak acids and bases, oils and fuels
• Operates in automotive, aerospace and general industrial applications
• Chemically inert
• Physical properties can provide high tensile, tear, modulus, elongation, fatigue life, resilience, damping and compression set
• Provides low flammability and smoke
• Provides extended low temperature performance and low swell in fuels (fluorosilicones)
• Heat curing (HTV) and room temperature curing (RTV) materials allow for flexibility in design

 

Dispensed Foam Gasketing (DFG)

What is Dispensed Foam Gasketing?

• Two-part foam dispensed directly on a component and cured
• Platinum-catalyzed, room temperature or heat cure, 1:1 mix ratio
• Self-foaming – does not require an added blowing agent o Provides a low modulus sealing option.
• Typically used where the part needs to be serviceable

When to use Dispensed Foam Gasketing

• Seal in/out air, dust, dirt, water
• Low modulus needed for plastic parts
• Low compression set needed o Squeak and rattle elimination (NVH)

 

Chemistry

Hydrosilation (Addition Cure)

• Accelerated by heat
• No by-products
• Catalyst easily poisoned
• Variable pot life

Curing of Silicone Foams from Dow

When the A and B components are mixed, the following reactions occur:

1. Hydrogen formation
2. Formation of gas cells and cell growth (foam expansion)
3. Curing (formation of the elastomeric network)

All reactions are temperature-dependent but at different rate constants

To ensure a homogeneous cell size distribution, a proper mixing of the components and proper dispense tip selection are essential

Foaming mechanism:

• DFG products from Dow create the foaming gas during the crosslinking reaction (hydrogen as by-product)
• Typical amount of H2 evolved: 0.5-1.3 g/kg product (based on “worst case” consideration using low density 8257 foam)
• Typical expansion ratio: 1:2.5 to 1:4
• To ensure optimal foam structure a good mixing of the components is required
• In case of dynamic mixing air nucleation is typically recommended

 

DFG Applications

Typical Automotive Applications:

• ABS brake modules
• Power distribution/ECU Modules
• Filter housings
• Beauty covers
• Transportation inspection covers
• Thermostat housing
• Timing chain and timing belt covers
• Wheel cover
• Exterior lighting
• NVH/acoustic

 

CIPG/DFG: Primary Design Considerations

• Bead design
• Aspect ratios
• Flange groove/void volume design

 

Gasket Bead Design

Generally limited to rounded, hump shape

 

Aspect Ratio

Potential issues with high aspect ratios:

• Leaves small area for adhesion
• Could have roll-over tendency

High aspect ratios can contribute to bad knit lines

 

CIPG/Non-slump Silicone Dispensed Foam Gaskets Designs

DFG Special Situations

Stacked beads may be possible if using SILASTIC™ 3-8186 Thixotropic Foam due to low slump nature of the material.

Processing

CIPG and DFG typically require:

• Fixturing
• Meter/mixing o Robotic dispensing o Oven curing

Can be manual or highly automated for mass production

 

Typical Layout of a Production Line: DFG/CIPG

Processing

Fixturing:

• Good fixturing is critical for repeatable/accurate dispensing
• Corrects for part-to-part variation
• Maintains part alignment
• Poor fixturing leads to irregular gasket beads

 

Typical meter/mix dispense systems contain:

• Material feed pumps
• Teflon-lined stainless steel flex hoses
• Material filters
• Pressure regulators
• DC Servo motor rod-displacement metering (preferred)
• Air operated dispense gun, and static mixer elements

 

Robotic Dispensing:

Multiple dispensing axis depending on part complexity; robot provides an even, accurate and repeatable gasket bead with high throughput

 

Dispense Options:

• Articulated arm
• Cylindrical (SCARA)
• XYZ gantry
• Oven curing is typically used to cure the seal materials*
• • Basic oven types:
    • Infrared
    • Gas fired
    • Electric heated
• Any method that heats the seal material is acceptable, heat is the important factor

*Except for DOWSIL™ 3-82XX series dispensed foams

 

Typical CIPG/DFG Application Cell

 

Summary: Main Considerations for Sealing and Gasketing

Does this part need to be serviceable?

• • Yes: CIPG, DFG
  • No: FIPG

Does this gasket need to seal in fluids (oil, coolant, etc.)?

• • Yes: CIPG, FIPG
  • No: DFG

Does the gasket need to seal in pressure?

• • Yes: CIPG, FIPG
  • No: DFG

Is there capital for equipment? Y/N?

How many parts*?

• • 100,000 or less usually FIPG
  • 100,000 or more, CIPG, DFG

*Ball-park numbers – simply used for reference