Cured-In-Place Compressible Gasket (CIPCG)

The cured-in-place compressible gasket – CIPCG for short – is a variant of the seal applied and cured directly onto the component, in which a compressible, foamed sealing compound is used. The decisive difference from the CIPG: with the CIPG, the bead consists of solid silicone rubber, which is displaced sideways during joining. With the CIPCG, by contrast, a foaming sealing compound is applied that can be compressed after vulcanisation – the sealing effect is created by compressing the foam, not by material displacement.

This makes possible a considerably lower joining and compression force while at the same time reliably sealing joint faces that are uneven or subject to tolerances.

Material and process

Foamed RTV silicone systems are used as the sealing compound – typically two-component RTV-2 types in which a blowing agent is integrated into the mixture.

As it foams, a closed outer skin forms on the surface of the sealing bead – the so-called integral skin. It is this skin that produces the actual barrier effect against water and dust, not the foam inside. The core remains open-celled or mixed-celled and, without the closed outer skin, would be capillary-active.

After application, the compound foams up in a controlled manner and vulcanises in this structure. The result is a foam structure with a defined density and compressibility – with a closed outer skin (integral skin) that forms on the surface during foaming. This skin produces the actual barrier effect against water and dust; the open-celled core beneath it would be capillary-active without it.

The application process corresponds to that of the CIPG: the sealing compound is applied as a bead onto the carrier part via a mixing and metering unit, guided fully automatically by robot or coordinate table, and subsequently cured in a drying oven or IR channel. The finished seal adheres captively to the component and passes into further processing as a pre-assembled unit.

The most important process parameter is the consistency of the foam density: even slight variations in the mixing ratio of the RTV-2 system or in the dynamics of the mixing head alter the pore size and density – and thus directly the mechanical characteristic curve of the seal and the required closing force of the housing. In series production, the foam density is monitored via the shot weight.

Design and sealing effect

Since foamed silicone rubber is genuinely compressible – unlike solid silicone – different design parameters apply to the CIPCG than to the CIPG:

• The compression is typically 40–60 % of the original foam cross-section
• The joint geometry must permit lower closing forces, which simplifies the design of the seat
• Unevenness and tolerances of the joint faces are compensated better by the compressibility of the foam than with solid silicone
• The sealing-bead cross-section must take account of the foaming behaviour – the bead continues to grow to its final volume after application

The compression set of foam structures behaves differently from that of compact material. Under sustained load, cell walls can collapse – the material loses its recovery force and the seal leaks without this being visible from the outside. The compression set of CIPCG systems depends heavily on the cell structure and the density set, and must be qualified and tested for long-term applications.

Strengths and limits

Compared with conventional sealing concepts and with the CIPG, the CIPCG process offers specific advantages:

• Very low closing and holding forces during joining – relevant for lightweight housings, plastic components or applications with limited bolting force
• Good compensation of tolerances and unevenness – suitable for joint faces with larger manufacturing deviations
• Captive adhesion to the carrier part, component that can be fully pre-assembled
• Sound and vibration decoupling by means of the foam structure

The limits of the process lie first of all in the sensitivity of the integral skin: burrs on the housing, sharp edges or excessive assembly pressure can tear it open locally – at such points, the foam core absorbs moisture and conducts it into the interior of the housing. Clean joint faces and burr-free housing contours are therefore a prerequisite for reliable IP protection.

Beyond this, foamed seals are limited to static low-pressure applications. For high internal pressures, aggressive media or dynamically stressed sealing points, solid-silicone systems, O-rings or profile seals are more suitable. As with the CIPG, in the event of a repair the seal must be renewed completely; selective replacement is not possible.

Typical applications of cured-in-place compressible gaskets

CIPCG seals are used wherever low joining and closing forces are required and the sealing function is sufficient at low to medium pressure:

• Housing lids and covers in the electrical and electronics industry
• Control-unit housings and sensor housings in vehicle construction
• Lightweight plastic housings in mechanical and plant engineering
• Applications with elevated requirements for sound and vibration decoupling