An introduction to Silicone Dispensed Foam Gaskets

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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

silicone dispensed foam gaskets

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

silicone dispensed foam gaskets

Potential issues with high aspect ratios:

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

silicone dispensed foam gaskets

High aspect ratios can contribute to bad knit lines

 

CIPG/Non-slump Silicone Dispensed Foam Gaskets Designs

silicone dispensed foam gaskets

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

silicone dispensed foam gaskets

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

silicone dispensed foam gaskets

silicone dispensed foam gaskets

 

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

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