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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If you have a problem with industrial adhesives, sealants, lubricants or electronic protection products and require advice, you can contact our technical support service. Our team can help you to optimize your processes and applications.

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