Every car body is full of hollow steel sections: pillars, rocker panels, roof rails and floor cross-members. Left open, these cavities act like drums and ducts. They carry road noise, wind noise and structure-borne vibration into the cabin. Sealing them during body assembly is one of the most cost-effective ways to improve interior comfort.
NVH stands for Noise, Vibration and Harshness. NVH expandable sealant is a heat-activated material that stays solid at room temperature, then expands in the paint oven to fill the cavity, bond to the walls and form a sealed foam barrier. It adds no extra assembly step, because it rides through the existing paint shop cycle.
That raises a practical question many buyers ask: why do sealants with different expansion ratios give different filling and sealing results, and how do you choose the right one? This article answers it step by step.
The expansion ratio is the volume of the sealant after baking divided by its volume before baking. An 8× grade grows to about eight times its starting volume; an 18× grade grows to about eighteen times. The exact figure depends on the test method, so always confirm how the supplier defines and measures it.
The sealant is a thermoplastic or rubber-based matrix blended with a chemical blowing agent. When the part temperature reaches the agent's decomposition threshold, it releases gas evenly through the softened matrix, and the material swells into a closed-cell foam. On Joysun's product page, grades activate at 150°C or 170°C. Joysun also develops chemical foaming agents, which is why blowing-agent selection can be tuned in-house.
A small starting strip can fill a large cavity, but only if the ratio, the bake temperature and the available space match. Expansion is not unlimited: the cavity walls stop the foam, and the foam then presses against them to form contact and adhesion.
The ratio decides how much space one strip can fill, how dense and hard the foam will be, and how much material you must buy per vehicle. It is the first number on most data sheets, but, as the next sections show, it is never the only one.
Body cavities have flanges, weld nuts, drain holes and bends. A foam with enough expansion can flow around these features and reach corners a rigid insert cannot. Expansion is what lets a simple strip handle a complex shape.
Ovens do not heat every point equally. Thick sections lag behind thin ones. Uniform expansion across that temperature gradient keeps foam density consistent, which is why Joysun's R&D team tests expansion uniformity across realistic oven profiles, not only at one set point.
An unfilled gap is a path for air, water and sound. Complete contact around the full cross-section is what turns an open tube into a sealed volume.
Low ratios (around 8–10×) give a denser, firmer foam and suit small or medium sections. High ratios (15–18×) give more volume per gram and suit large cavities, but the foam is lighter and, if the formulation is not tuned, softer.
Coverage per strip rises with the ratio. A high-expansion grade can mean a smaller starting piece, less weight on the line, and fewer parts to place.
| Expansion ratio | Typical use (guideline) | Main consideration |
|---|---|---|
| 8× | Small, tight sections; secondary sealing | Dense foam, low material volume change |
| 10× | Medium cavities; sill or underbody sections | Balance of fill and hardness |
| 15× | Large pillar and rocker cavities | Needs enough free space and uniform heating |
| 18× | Very large voids; wide gaps | Highest fill; check for over-expansion risk |
A high ratio wastes performance if the cavity is small, and it can lower foam hardness. Sealing quality depends on contact, uniformity and adhesion, not on the biggest number.
It is important to separate three mechanisms. Sealing blocks airborne noise paths. Sound absorption converts sound energy into heat inside porous material. Vibration damping reduces panel resonance. Expansion ratio does not equal sound insulation; it affects NVH only through how well the cavity is filled and sealed.
A filled cross-section interrupts the air column inside a pillar, so noise cannot travel freely along it.
Gaps at flanges and holes let outside noise leak into the cabin. Continuous foam contact closes them.
Foam bonded to the walls adds damping and can shift thin-panel resonances. Higher-hardness grades add some local stiffness as well.
One weak spot can be the dominant leak. Consistent expansion from vehicle to vehicle gives consistent NVH results. This matters even more in electric vehicles, where there is no engine noise to mask road and wind noise.
Start from the free cross-section and the planned starting volume, then pick a ratio that fills it with a small margin.
Pillars are long, with changing sections. Larger sections usually call for high-expansion grades; narrow lower sections may need a medium ratio.
Rockers are large and often torsion-loaded, so a medium ratio with higher hardness can help. Floor pan cavities and hem flanges mainly need reliable sealing against low-frequency structure-borne noise and airborne leaks.
Consider vertical or horizontal installation (foam sags and flows differently), the carrier type, and the real part temperature in the oven, which can be far below air temperature. Also check substrate: steel, coated steel or nylon.
Selection should weigh cavity size and structure, installation position, bake temperature and time, substrate adhesion, cured hardness and sealing demand, and cost per vehicle. Customised grades are possible through Joysun R&D.
Zhejiang Joysun Advanced Material Co., Ltd. was founded in 2005 and offers four NVH grades. Full details are on the NVH expandable adhesive page.
| Product grade | Activation temperature | Expansion ratio |
|---|---|---|
| NVH182Q-3 | 150°C | 15× at 150°C; 18× at 180°C |
| NVH185Q-1B | 170°C | 10× at 150°C; 12× at 180°C |
| NVH192Q | 170°C | 8× at 150°C; 10× at 180°C |
| NVH445Q-3 | 170°C | 8× at 150°C; 10× at 180°C |
Data are taken from the Joysun product page. Ratios are measured at different temperatures, so read each value on its own test condition and do not treat the table as a ranking under identical conditions.
When you contact an NVH expandable sealant manufacturer, request the TDS, expansion test data, adhesion reports and expansion tables at several temperatures, along with samples. Use the Joysun contact page to reach the technical team.
Expansion ratio strongly affects cavity filling, leak-path closure and, indirectly, NVH performance. But the best choice is the ratio that fits the cavity, bake schedule, substrate and hardness requirement, not simply the highest one. Joysun supplies 8×–18× grades for automotive body cavities and high-speed rail structures; browse the full product range, which also includes high-performance TPEE, or read more company news.
Need help selecting a grade? Contact Zhejiang Joysun Advanced Material Co., Ltd. at lh@joysunsh.com or visit our contact page for data sheets and samples.
