Preformed vs. bulk cavity fillers for noise reduction

  • 15-Jul-2013 03:16 EDT
Figure 1.jpg

Thermoplastic baffles before and after bake (left) and rubber-based baffles before and after bake.

The use of acoustic cavity fillers to prevent the propagation of air- and structure-borne noise, water, and dust into the interior spaces of vehicle structures has been in practice for many years. During the 1990s, two technologies emerged to allow more robust and efficient cavity sealing: preformed heat-reactive expandable materials and chemically reactive expandable polyurethane (PU) foams.

Preformed parts include thermoplastic-based systems that incorporate a heat-reactive thermoplastic sealer applied to a nylon or steel carrier for attachment to the body structure and heat-reactive rubber-based sealer systems that incorporate a carrier, push pin, or pressure-sensitive adhesive layer for attachment. Bulk systems refer to chemically reactive, two-component polyurethane or expandable foam systems, which include polyol and methylene diphenyl diisocyanate (MDI) components.

Thermoplastic baffle designs range in complexity from simple extrusions of heat-reactive sealer applied to die-cut carriers to highly engineered two-shot injection-molded parts, where the nylon for the carrier is shot into the tool and the sealer over-molded onto the carrier in a fully automated process. Rubber-based designs typically include a co-extrusion of heat-reactive sealer with a pressure-sensitive adhesive or an extrusion of heat-reactive sealer with a push pin applied for attachment to the body-in-white (BIW).

The parts are applied during assembly of the sheet metal panels that form the body structure of a vehicle. Sealer and primer coatings are then applied in the sealer deck and paint shop operations. When the body structure is removed from the primer coat bath, it proceeds to a bake oven to cure the primer coat. The sealer material expands with exposure to heat that occurs in the bake ovens in the paint shop. Expansion rates can vary from several hundred percent to 2000% or more. These materials are designed to form a complete seal of the cavity cross-sections in which they are applied.

With the bulk systems, the two components are kept in storage tanks in the assembly plant and are pumped to an application station as needed. The foam systems are applied after paint and bake cycles. There, a line operator uses a dispensing gun that mixes the two components together in the gun head and dispenses the PU material as a liquid into specific areas of the body structure. An exothermic reaction occurs as the material is applied to each cavity section, which causes the material to begin to gel and expand within about 4 s. This exothermic reaction is the curing mechanism for the two-component polyurethane foam cavity filler systems.

With “low MDI” formulations, for which the mix ratio is one part MDI for multiple parts of polyol (i.e., 24:1), ventilation booths are not necessary in assembly plants. These low-MDI systems are now the norm in most assembly plants in North America.

On the horizon, development activity centered on safer “green” soy-based bulk foam formulations may reflect the future direction of chemically reactive PU foam cavity filler materials.

Material performance comparison

Material requirements vary among the OEMs, but they typically identify similar attributes that the technologies must possess to be considered for material approval and use, including expansion, adhesion, sealing, stability, water absorption, volatile organic compounds (VOCs), flammability, and acoustical performance. There is a wide range of required expansion rates for these materials based on the type of technology selected for the application. Bulk foams provide a higher expansion rate, but it is necessary to control the flow of the product into the body structure to avoid over-filling the cavity section. The priority is to ensure complete sealing of the cavity with a minimum amount of baffle or foam material applied.

Adhesion requirements typically stipulate 80-100% cohesive failure of the cavity-filler material to electrodeposition coatings. In addition, the materials should have little to no shrinkage during or after the cure process. The preformed and bulk technologies exhibit similar performance in this area.

Water absorption is a key metric for cavity-filling technologies, particularly when applied to lower elevations of the body structure. Long-term exposure to and absorption of water can lead to corrosion issues. Thermoplastic baffles exhibit the lowest water absorption values, followed by rubber-based baffles, and finally the bulk foam systems.

Many methods exist to evaluate the noise-blocking performance of cavity-filler materials. Component-level noise reduction tests can be performed using cavity sections cut from a BIW or generic cavity sections that represent typical pillars, posts, and sills of vehicle bodies—i.e., the SAE J2846 test method. Two different cross-sectional channel dimensions are cited in the SAE J2846 specification: 75×75×250 mm and 150×150×250 mm.

According to the J2846 specification, data below 800 Hz is influenced by channel resonances and geometry and should not be relied upon for data analysis. Tests indicated that the bulk foam formulation performs better than the preformed baffles that were tested from 1000 to 1600 Hz and then drops off significantly at higher frequencies, while preformed baffles block more noise in mid- and high-frequency bands. It is likely that the greater mass and sample thickness of the foam, due to the volume of material required to seal a specific section, provides higher performance from 1000 to 1600 Hz, while the foam’s greater stiffness has a negative impact on acoustic performance at higher frequencies, when compared to the preformed baffles that were tested.

Cavity-filler materials can be tested when applied to full vehicles under operating conditions. Full-vehicle results correlate to previously published SAE J2846 Insertion Loss data comparing preformed baffles to bulk foam. The heavier and thicker foam application, based on the volume of material that is required to seal the section, demonstrates higher acoustic performance at low frequencies, while the less-rigid post-bake expanded baffle sealers perform better at mid and high frequencies when tested using this method.

How the technologies are used

In the heavy truck market, both baffle and foam technologies are used today. In many cases, body structure design is changing to better accommodate available cavity-sealing technologies during the design process. At some heavy truck OEMs, bulk foam systems are used in plants where the paint ovens do not reach temperatures that will activate the sealer that is used with heat-reactive thermoplastic or rubber-based expandable cavity-filler systems. However, lower-bake formulations of preformed baffles are being developed currently to address this requirement.

In the lower elevations of the body structure, cavity sections can sometimes be very large and/or complex in construction; therefore, bulk foam may be preferred by the OEM as a less costly solution than a highly engineered, preformed baffle part. In some plants, the manufacturing process includes multiple oven bakes that the body structure must go through during sealing and paint operations. Some thermoplastic baffles are not well suited to multiple bake cycles, which can cause the sealer to shrink back from the sheet metal, exposing leak paths through the baffle. Bulk foams are applied after these bake processes and will not be affected by this phenomenon.

In the quest to improve fuel efficiency across the automobile and truck markets, weight becomes a key factor in selecting appropriate cavity-filler solutions. Preformed baffles are typically 40-50% lighter than foam systems when comparing specific applications. A preformed part is designed to expand enough to seal a cross-section, without adding an excessive amount of sealer material. The foam systems require a greater volume of material to be applied, as it will spread over a greater volume within the cavity during application, before it gels and completely seals across the diameter of the section.

With bulk foam systems, the OEM is responsible for purchasing the infrastructure necessary to apply the material to the body structure. The manufacturing plant is also responsible for the maintenance of the entire system. In the case of preformed parts, much of the responsibility and effort falls upon the supplier.

These are just a few of the considerations that an OEM will review to determine which sealing technology to use for a specific application.

This article is based on SAE International technical paper 2013-01-1946 written by Michael Fasse of Sika Corp.

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