Why do car-seats have an expiration date?

Question

My wife and I are expecting our first child, and therefore are expecting to purchase a car seats. My wife and I have discovered that car seats have "expiration dates": a car seat, no matter how lightly used, is deemed unsuitable for use after a particular date. We wonder why.

A car seat which has been in an accident could be compromised in ways that simple visual inspection could not confirm. I am not asking about these car seats. I am asking about the car seats which have experienced the repeated loading/unloading which comes from placing a small human child in and out of the seat and driving them about in a driving style that could only be described as "not aggressive."

Attempting to look up answers for this question have not yielded any satisfactory results! So I ask you, engineering stack exchange, why do infant and child car seats have expiration dates?

I have a few possibilities, but I have issues with all of them:

1. The car seat is designed to withstand a number of cyclic loads, and critical car-seat parts have a fatigue life which the manufacturers base the expiration date on. This seems the most likely, but babies are really small and the materials are meant to protect them against high-speed crashes for the entire product life!
2. The car-seat manufacturers are greedy, using social pressures and the label of "unsafe" to force parents (as a group) to buy car seats at regular intervals and deny the second-hand market any 'viable' material.
3. Something in the car seats really does degrade with use, heat, or sunlight and therefore actually expires. (This seems unlikely, but terrifying!)
4. Safety standards are expected to update after some amount of time. This particular explanation seems very odd to me: safety standards are set be governments and industry groups, and I doubt car-seat needs have evolved so much that a regular update every few years is entirely needed, or that those updated industry safety standards will always render old products unsafe.

Or maybe it's something else! Sources are welcome!

Answer 1

It's a combination of 1, 3, 4, the product development cycle, and the nature of the product.

1 - Damage in materials is a cumulative process, the product of hundreds or thousands of little events (e.g. the tightening and loosening of the belts) or one big event (a crash). So, the product has to be designed to always have a certain damage capacity throughout its intended lifetime.

3 - Plastics break down and their mechanical properties degrade over time. Specifically they lose their impact resistance and become more brittle. Both of which are bad things for a car seat intended to protect a child in the event of a crash.

4 - Standards do change slowly, but safety technology changes at a much faster pace. The safety standards in many cases are simply a minimum requirement and not an optimal one.

Validating the performance of a product like a car seat requires testing...a lot of testing and testing to a very high standard since the consequences of a defect in the seat can be lethal. Testing is expensive. Making the high standards higher to extend the product life makes the testing even more expensive. The further out you go, it becomes exponentially more expensive.

Since the plastic materials decline over years, this means the manufacturer has to artificially age these materials and there are limits to how good these procedures actually are.

When it comes to product lifetimes and guarantees, for the manufacturer, it's a lot like looking into a crystal ball that gets fuzzier and more costly the further into the future you look.

The expiration dates are NOT a legal requirement in that they are not legislated, but they are there, in part for legal reasons. The expiration date is a way of communicating to the consumer that the company is only willing to guarantee a standard of performance for a certain period of time due to those factors. It's also a means of limiting the company's liability in the event an older seat is involved in an accident where injury occurs.

You can bet that these companies have looked long and hard at what the average consumer's usage cycle is for car seats and compared that with what consumers are willing to payfor a car seat when they are developing these products. Could manufacturers make a car seat that they would have an expiration date that's further out in the future? Sure. How much more are you willing to pay for a car seat that can still be used when your infant is a teenager even though it will be outdated (and they won't fit in it anyway)?

Source: 20yr of product design experience and the answers I got when I researched the same question after buying a car seat for my first child 16yr ago

Answer 2

I would add #5, dumbing down parental advice so even very stupid parents can get the message clearly without confusion.

The classic example is "put your baby in the back seat". That advice is applicable to just a few model years of cars from the mid-90s where they had passenger side airbags, but did not have the seat-occupied detector or the "baby seat locked in" detector which would shut off the airbag in those cases. Nor to cars without passenge air bags. So, inapplicable to most cars.

Yet this advice was hammered into the skull of every parent, and made law in many places. A parent would be stopped and confronted by parents, officials, and police. Explaining was hopeless. The lower ranks say "don't wanna hear that technobabble", and the upper ranks would say "You know that, I know that, but you're also smart enough to know we have to send a consistent, simple message that everyone can understand, and enforce fairly and uniformly. As a smart one, you have a social responsibility to set a good example."

That last bit is what it's all about.

So yes, you as an engineer can carefully inspect a car seat, look for the telltale crazing of UV damage or whiting of imapact damage... Evaluate the provenance of friends and family's seats... But the officials are worried that others cannot, and would recklessly use a seat with obvious damage.

And I'm not so sure that advice is right. Even a damaged car seat is better than no car seat. And also better than a family who can't afford a new seat foregoing food or vaccines to get the new seat. So in that sense, I think the advice is self-serving to both the NHTSA and the manufacturers, with little consideration to other consequences.** And so I consider #2 to be also valid.

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** such as the epidemic of infants being forgotten in the back seats of cars, dying to the tune of 30-40 a year, a higher rate than babies ever were hurt by passenger side air bags even when those cars were common. Those cars are mostly off the road, but the forgotten-baby fatalities continue apace. What a legacy!