Manufacturing Process of Automobile Airbags

WHAT IS AN AIRBAG

An Airbag is a type of vehicle safety device and is an occupant restraint system. The airbag module is designed to inflate extremely rapidly then quickly deflate during collision or impact with a surface or rapid sudden deceleration. It consists of the airbag cushion, a flexible Nylon66 fabric bag, inflation module, and impact sensor.

The purpose of the airbag is to provide the occupants a soft cushioning and restraint during a crash event to prevent any fatal injuries.

Airbag production involves three different separate assemblies that combine to form the finished end product, the airbag module. The propellant must be manufactured, the inflator components must be assembled, and the airbag must be cut and sewn.

Manufacturing Process of Airbags

Airbag production involves three different separate assemblies namely,

• PROPELLENT
• INFLATOR
• AIRBAG FABRIC

How airbags work

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1. When a car hits something, it starts to decelerate (lose speed) very rapidly.
2. An accelerometer (the electronic chip that measures acceleration or force) detects the change of speed.
3. If the deceleration is great enough, the accelerometer triggers the airbag circuit. Normal braking doesn’t generate enough force to do this.
4. The airbag circuit passes an electric current through a heating element (a bit like one of the wires in a toaster).
5. The heating element ignites a chemical explosive. Older airbags used sodium azide as their explosive; newer ones use different chemicals.
6. As the explosive burns, it generates a massive amount of harmless gas (typically either nitrogen or argon) that floods into a nylon bag packed behind the steering wheel.
7. As the bag expands, it blows the plastic cover off the steering wheel and inflates in front of the driver. The bag is coated with a chalky substance such as talcum powder to help it unwrap smoothly.
8. The driver (moving forward because of the impact) pushes against the bag. This makes the bag deflate as the gas it contains escapes through small holes around its edges. By the time the car stops, the bag should have completely deflated.

Different types of airbags

There are four main types of airbags: frontal, side, knee, and rear curtain. With continued research regarding safety, new types of airbags have been introduced recently, such as inflatable safety belts and pedestrian airbags. Front airbags are designed to protect the driver and front-seat passenger.

The driver’s airbag is installed in the steering wheel while the airbag for the passenger is mounted in the dashboard. When inflated, they keep a person’s chest and head from coming into contact with hard surfaces in the vehicle. For front airbags to provide maximum protection, the driver and passenger should be sitting properly and wearing seat belts. Modern airbags have a built-in sensor that detects whether occupants are wearing seat belts or not.

Many newer vehicles also have a front-seat weight sensor that prevents the airbag from deploying if the person sitting there is young or small. Airbags come in various shapes and sizes, but their core purpose is the same – to keep you from experiencing severe injuries during a vehicular accident. Although it’s best if you will never have to use them, the different types of airbags are there to protect you if ever you need them.

• Side torso bags are normally mounted in the seat. Their main purpose is to protect the person from smashing his body on the door and reduce injuries in the abdomen and pelvic areas.
• Side curtain airbags are installed behind the roof trim directly above the doors. They’re designed to lessen the risk of head injuries. When side curtain airbags inflate, they normally cover the front and rear windows to protect passengers in both seats. Some more advanced curtain airbags have sensors that can detect if a vehicle starts to roll following a collision.
• Head side airbags protect a person’s head from hitting the object which the car has collided with, such as a pole, tree, or another vehicle

• Knee airbag. In high-speed crashes, people in the vehicle are likely to sustain knee injuries. Studies have shown that a person’s kneecaps can shatter during impact. Knee airbags distribute the force of impact to prevent not only knee injuries but also breaking a leg bone or bruising. They also help keep people in their position during a head-on collision, which reduces the force on a person’s chest and abdomen.
• Rear airbags. During rear-end collisions, rear airbags protect passengers in the backseat. The rear-window airbag prevents them from crashing into the back window while the rear-center airbag keeps passengers from bumping into one another
• Seat belt airbags. The purpose of this type of airbag is to spread the force of impact over a larger part of the person’s body, at present, not a lot of makes and models make use of seatbelt airbags yet.
• Pedestrian airbags. Introduced in 2012 by Volvo, this exterior airbag inflates if the driver hits someone. Pedestrian airbags are supposed to reduce the number and severity of injuries that a pedestrian may suffer from.

Purpose and working of Airbags

An airbag is an inflatable cushion designed to protect automobile occupants from serious injury in the case of a collision. The airbag is part of an inflatable restraint system, also known as an air cushion restraint system (ACRS) or an airbag supplemental restraint system (SRS) because the airbag is designed to supplement the protection offered by the seat belts. Seat belts are still needed to hold the occupant securely in place, especially in side impacts, rear impacts, and rollovers. Upon detecting a collision, airbags inflate instantly to cushion the exposed occupant with a big gas-filled pillow.

The most important filament nylon 66 fabric Characteristics/requirements of an airbag fabric needed to meet main objectives are listed below

• High Tensile strength
• High Tear strength
• Low Air permeability
• Good Heat capacity
• Good Folding behavior
• Better Energy absorption
• Good Coating adhesion
• Functionality at extreme hot and cold conditions
• Package ability
• Reduced skin abrasion (softness)
• Good heat stability
• Free of knots, splices, spots, and broken ends.
• Low specific fabric weight
• High tenacity both in warp and weft directions
• High tenacity for further tearing
• High elongation
• Good resistance to aging.
• Heat resistance up to 190⁰
• Good resistance to UV light.
• Low and very even air permeability.
• Precisely controlled gas permeability.
• Excellent seam integrity.
• Reduced value or burn-through resistance.
• Improved pliability and pack height
• Reduced cost.

A typical airbag system consists of an airbag module (containing an inflator or gas generator and an airbag), crash sensors, a diagnostic monitoring unit, a steering wheel connecting coil, and an indicator lamp. These components are all interconnected by a wiring harness and powered by the vehicle’s battery. Airbag systems hold a reserve charge after the ignition has been turned off or after the battery has been disconnected. Depending on the model, the backup power supply lasts between one second and ten minutes. Since components vital to the system’s operation might sit dormant for years, the airbag circuitry performs an internal “self-test” during each startup, usually indicated by a light on the instrument panel that glows briefly at each startup.

The crash sensors are designed to prevent the airbag from inflating when the car goes over a bump or a pothole, or in the case of a minor collision. The inflator fits into a module consisting of a woven nylon bag and a break-away plastic horn pad cover. The module, in turn, fits into the steering wheel for driver’s-side applications and above the glove compartment for front passenger applications.

In a frontal collision equivalent to hitting a solid barrier at nine miles per hour (14.48 kilometers per hour), the crash sensors located in the front of the car detect the sudden deceleration and send an electrical signal activating an initiator (sometimes called an igniter or squib). Like a light bulb, an initiator contains a thin wire that heats up and penetrates the propellant chamber. This causes the solid chemical propellant, principally sodium azide, sealed inside the inflator to undergo a rapid chemical reaction (commonly referred to as a pyrotechnic chain). This controlled reaction produces harmless nitrogen gas that fills the airbag. During deployment, the expanding nitrogen gas undergoes a process that reduces the temperature and removes most of the combustion residue or ash.

The expanding nitrogen gas inflates the nylon bag in less than one-twentieth (1/20) of a second, splitting open its plastic module cover and inflating in front of the occupant. As the occupant contacts the bag, the nitrogen gas is vented through openings in the back of the bag. The bag is fully inflated for only one-tenth (1/10) of a second and is nearly deflated by three-tenths (3/10) of a second after impact. Talcum powder or corn starch is used to line the inside of the airbag and is released from the airbag as it is opened.

Objectives of Airbag

The main objective of an airbag is to lower the number of injuries by reducing the force exerted by the steering wheel and the dashboard or any point on the body. This is accomplished in two ways:

1. By increasing the interval over the force being applied
2. By spreading the fore over a large area of the body

The airbag system has been engineered to work with the pressure between the passenger and steering wheel, in a fraction of a second. The airbag unit must also stay intact at low-velocity collisions. The crash sensor, which detects the collisions and triggers the bag, to inflate must take all those constraints into account. The operation of deflation happens when N₂ generation stops, gas molecules escape the bag through vents. The pressure inside the bag decreases and the bag deflates slightly to create a soft cushion. By 2 seconds after the initial impact, the pressure inside the bag has reached atmospheric pressure.

In the modern epoch, airbags are almost present in every car. In order to mitigate the injuries due to car accidents, a variety of airbags are used. By making use of various types of airbags, road car accidents tend to decrease. An airbag is employed to prevent the major injuries that are caused by automobile collisions. The airbag will automatically inflate in less than a second in the event of a collision. Most of the airbags can be used only once. The main objective of the paper is to discuss numerous types of airbags with their functions and applications.

Airbag Production Process

Airbag production involves three different separate assemblies that combine to form the finished end product, the airbag module. The propellant must be manufactured, the inflator components must be assembled, and the airbag must be cut and sewn. The powdery substance released from the airbag, by the way, is regular cornstarch or talcum powder, which is used by the airbag manufacturers to keep the bags pliable and lubricated while they’re in storage.

Airbag control module

An airbag control module is an important part of your vehicle. In addition to storing crash data and sending cut-off signals to your engine following a crash, it also deploys your airbags.

Also known as the airbag sensor, diagnostic unit, and computer module, the airbag module is responsible for receiving information from the collision sensor. Found in all vehicles, the airbag module is a computer that stores information. Thus, during a collision, the impact data is stored in the airbag module.

Do we need to replace the airbag module after an accident?

If you have been in an accident where the airbags were set off, you will need to replace the airbags and the SRS airbag control module in order to make the car safe to drive again. It is always best to send your original module for a reset after an accident.

PROPELLENT

Airbag propellant chemicals are used to bring about rapid inflation of an airbag as soon as a sensor of automobile senses the crash force above a predetermined value in case of a collision. Chemicals used for the airbags should react or decompose spontaneously so as to produce enough amount Airbag propellant chemicals are used to bring about rapid inflation of an airbag as soon as a sensor of the automobile senses the crash force above a predetermined value in case of a collision.

Chemicals used for the airbags should react or decompose spontaneously so as to produce enough amount of required gas for the instantaneous inflation and deflation of the airbags. In case of a collision, the airbag’s electrical circuit passes current towards the heating element which in turn causes the chemical explosion through which a certain amount of gas is generated so as to inflate the airbags.

Some of the chemicals that are commonly used to affect this controlled explosive reaction include sodium azides, ammonium nitrate, and Potassium 5-amino tetrazole among others. Along with propellant chemicals, certain additives are used in order to neutralize the effect of the chemical before exposure to the occupants. The demand for airbags that adheres to stringent government regulations has resulted in efforts towards the development and adoption of new propellant chemicals over the recent past.

INFLATOR

An airbag inflator (consisting of a casing containing an igniter, a booster material, a gas generant, and, in some cases, a pressure receptacle (cylinder)) is a gas generator used to inflate an airbag in a supplemental restraint system in a motor vehicle. An airbag module is the airbag inflator plus an inflatable bag assembly. What is the function of the airbag inflator?

The airbag inflator is part of the airbag module, which also includes the airbag as well. The job of the airbag inflator should be self-explanatory. It is to quickly inflate the airbag in the airbag module so that it comes out and shields you from flying out of the car. inflator into the airbag, which is folded up inside the airbag unit. In a crash, those gases inflate the airbag before the person in front of it ever gets close to the dashboard or steering wheel. Airbag Inflator. The airbag inflator is part of the airbag module, which also includes the

1. Impact Sensors. The airbag system depends on impact or crash sensors so that the airbag opens up
2. SRS Airbag Module. Every time that you start your vehicle, the diagnostic monitoring unit will function
3. Indicator Lamp. The indicator lamp is basically the warning light for the airbag system.

The airbag fabric is typically made of nylon. Either nitrogen or argon gas is used to inflate an airbag. Both of these gases are non-toxic. Immediately after deployment, “smoke-like” residue will be present in the air. Most of this residue is talcum powder that is used as a lubricant to help the airbag deploy smoothly. However, a minute amount of sodium hydroxide, an irritant, may also be present. Be sure to wash off any residue as soon as possible after deployment.

AIRBAG FORMATION 

When airbag material has been finished, it is cut into panels by laser. This technique is fast and accurate, it fuses the edges of the fabric to prevent fraying and reduces cost by eliminating cutting dies. The normal design of the driver-side bag is two circular pieces of fabric sewn together. The passenger bag is tear-drop shaped, made from two vertical sections and a main horizontal panel. Airbags are sewn with nylon 6,6, polyester and Kevlar, aramid yarns, the sewing patterns and stitch densities being chosen carefully to maximize performance.

When this has been sewn it is folded inside its cover. Like a parachute, the fabric is folded with extreme care to ensure smooth development. A variety of folds are suitable including the accordion fold, reversed accordion fold, pleated accordion fold, and overlapped. Generally, the smaller airbags are preferred by the automotive industry which is the concept exemplified by a revolutionary airbag designed to fit inside a shirt pocket. It was conceived by accessing the fabric and seams of a normal airbag in a wind tunnel. The research found that the strain on a conventional airbag during deployment did not coincide with its strongest axis. Most of the strain was concentrated on its equator. The experimental results were fed into a computer and a new bag was designed to exert stress along the preferential axis. As a result, less stress was exerted on the seams, so less stitching was needed and the bag could be folded into a much smaller space.

Packing should also allow for tethers joined to the bags to control its protrusion into the car during deployment. Lastly, a cover can be fitted over the bag to protect it from abrasion. Dupont had made a jacket from Tyvek AC spun-bonded polyethylene for this purpose. It tears to release the airbag when it inflates.

The Chemistry Behind Airbags

Key Concepts:

• Chemical Reactions to Generate the Gas to Fill an Airbag
  • Decomposition of Sodium Azide (NaN₃)
  • Reactions to Remove Harmful Products
  • Reaction Stoichiometry
• Ideal-Gas Laws (Macroscopic Picture)
  • PV=nRT
  • Estimating the Pressure to Fill an Airbag
    • Acceleration
    • Force
    • Pressure
  • Deflation
• Kinetic Theory of Gases (Microscopic Picture)
  • Pressure as the Result of Molecular Collisions with Container Walls
  • Average and Root-Mean-Square Speed of Molecules
  • Maxwell-Boltzmann Distribution
• Protection in a Collision
  • Newton’s Laws
  • Airbags Decrease the Force on the Body
  • Airbags Spread the Force Over a Larger Area
• Undetonated-Airbag Disposal: Safety Considerations

Overview of How Airbags Work

• Timing is crucial in the airbag’s ability to save lives in a head-on collision. An airbag must be able to deploy in a matter of milliseconds from the initial collision impact. It must also be prevented from deploying when there is no collision. Hence, the first component of the airbag system is a sensor that can detect head-on collisions and immediately trigger the airbag’s deployment. One of the simplest designs employed for the crash sensor is a steel ball that slides inside a smooth bore. The ball is held in place by a permanent magnet or by a stiff spring, which inhibits the ball’s motion when the car drives over bumps or potholes. However, when the car decelerates very quickly, as in a head-on crash, the ball suddenly moves forward and turns on an electrical circuit, initiating the process of inflating the airbag.
• Once the electrical circuit has been turned on by the sensor, a pellet of sodium azide (NaN₃) is ignited. A rapid reaction occurs, generating nitrogen gas (N₂). This gas fills a nylon or polyamide bag at a velocity of 150 to 250 miles per hour. This process, from the initial impact of the crash to full inflation of the airbags, takes only about 40 milliseconds (Movie 1). Ideally, the body of the driver (or passenger) should not hit the airbag while it is still inflating. In order for the airbag to cushion the head and torso with air for maximum protection, the airbag must begin to deflate (e., decrease its internal pressure) by the time the body hits it. Otherwise, the high internal pressure of the airbag would create a surface as hard as stone– not the protective cushion you would want to crash into!

When a body hits the steering wheel directly, the force of this impact is distributed over a small area of the body, resulting in injuries to this area. The area that hits the steering wheel is shown in red.

When a body is restrained by an airbag, the force of the impact is distributed over a much larger area of the body, resulting in less severe injuries. The area that hits the airbag is shown in orange.

This bar graph shows that there is a significantly higher reduction in moderate to serious head injuries for people using airbags and seat belts together than for people using only seat belts.

Deaths among drivers using both airbags and seat belts are 26% lower than among drivers using seat belts alone.

Chemical Reactions Used to Generate the Gas

Inside the airbag is a gas generator containing a mixture of NaN₃, KNO₃, and SiO₂. When the car undergoes a head-on collision, a series of three chemical reactions inside the gas generator produce gas (N₂) to fill the airbag and convert NaN₃, which is highly toxic (The maximum concentration of NaN₃ allowed in the workplace is 0.2 mg/m³ air.), to harmless glass (Table 1). Sodium azide (NaN₃) can decompose at 300^oC to produce sodium metal (Na) and nitrogen gas (N₂).

The signal from the deceleration sensor ignites the gas-generator mixture by an electrical impulse, creating the high-temperature condition necessary for NaN₃ to decompose. The nitrogen gas that is generated then fills the airbag. The purpose of the KNO₃ and SiO₂ is to remove the sodium metal (which is highly reactive and potentially explosive, as you recall from the Periodic Properties Experiment) by converting it to a harmless material. First, the sodium reacts with potassium nitrate (KNO₃) to produce potassium oxide (K₂O), sodium oxide (Na₂O), and additional N₂ gas.

The N₂ generated in this second reaction also fills the airbag, and the metal oxides react with silicon dioxide (SiO₂) in a final reaction to produce silicate glass, which is harmless and stable. (First-period metal oxides, such as Na₂O and K₂O, are highly reactive, so it would be unsafe to allow them to be the end product of the airbag detonation.)

 

Gas-Generator Reaction	Reactants	Products
Initial Reaction Triggered by Sensor.	NaN3	Na N2 (g)
Second Reaction.	Na KNO3	K2O Na2O N2 (g)
Final Reaction.	K2O Na2O SiO2	alkaline silicate (glass)
Table 1 This table summarizes the species involved in the chemical reactions in the gas generator of an airbag. Note: Stoichiometric quantities are not shown.

Estimating the Pressure Required to Fill the Airbag

 

An estimate for the pressure required to fill the airbag in milliseconds can be obtained by simple mechanical analysis. Assume the front face of the airbag begins at rest (i.e., initial velocity v_i = 0.00 m/s), is traveling at 2.00×10² miles per hour by the end of the inflation (i.e., final velocity v_f = 89.4 m/s), and has traveled 30.0 cm (the approximate thickness of a fully-inflated airbag).

The airbag’s acceleration (a) can be computed from the velocities and distance moved (d) by the following formula encountered in any basic physics text:

 

vf2 – vi2 = 2ad.

(1)

Substituting in the values above,

(89.4 m/s)2 – (0.00 m/s)2 = (2)(a)(0.300 m) a = 1.33×104 m/s2.

(2)

 

The force exerted on an object is equal to the mass of the object times its acceleration (F = ma) ; therefore, we can find the force with which the gas molecules push a 2.50-kg airbag forward to inflate it so rapidly.  2.5 kg is a fairly heavy bag, but if you consider how much force the bag has to withstand (see Figure 5), it becomes apparent that a lightweight-fabric bag would not be strong enough.  Note:  In the calculation below, we are assuming that the airbag is supported in the back (i.e., all the expansion is forward), and that the mass of the airbag is all contained in the front face of the airbag.

 

F = ma F = (2.50 kg)(1.33×104 m/s2) F = 3.33×104 kg·m/s2 = 3.33×104 N.

(3) (4)

 

• Pressure is defined as the force exerted by a gas per unit area (A) on the walls of the container (P = F/A), so the pressure (in Pascals) in the airbag immediately after inflation can easily be determined using the force calculated above and the area of the front face of the airbag (the part of the airbag that is pushed forward by this force).  Note:  The pressure calculated is gauge pressure.
• The amount of gas needed to fill the airbag at this pressure is then computed by the ideal-gas law (see Questions below).  Note:  the pressure used in the ideal gas equation is absolute pressure.  Gauge pressure + atmospheric pressure = absolute pressure.

·         Deflation of the Airbag

• When N₂generation stops, gas molecules escape the bag through vents. The pressure inside the bag decreases and the bag deflates slightly to create a soft cushion. By 2 seconds after the initial impact, the pressure inside the bag has reached atmospheric pressure.

Summary

Airbags have been shown to significantly reduce the number and severity of injuries, as well as the number of deaths, in head-on automobile collisions. Airbags protect us in collisions by providing a cushion to decrease the force on the body from hitting the steering wheel, and by distributing the force over a larger area. The cushion is generated by rapidly inflating the airbag with N₂ gas (from the explosive decomposition of NaN₃ triggered by a collision sensor), and then allowing the airbag to deflate.

Fundamental chemical and physical concepts underlie the design of airbags, as well as our understanding of how airbags work. The pressure in the airbag, and hence the amount of NaN₃ needed in order for the airbag to be filled quickly enough to protect us in a collision, can be determined using the ideal-gas laws, and the kinetic theory of gases allows us to understand, at the molecular level, how the gas is responsible for the pressure inside the airbag. Newton’s laws enable us to compute the force (and hence the pressure) required to move the front of the airbag forward during inflation, as well as how the airbag protects us by decreasing the force on the body.

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Ashok Hakoo. B.E(Textiles), MBA.
Technical and management consultant (Fabtechsolutions)
Acknowledgment: Technical and Technological Facts in this write-up have been selected from various research papers and reliable sources.