How do time-release pills work?

[Evert]

So for example you have Adderall XR which contain gel-covered things inside, right?
How does the gel exactly cause it to be time released? Does it dissolve at a constant (extremely slow) speed or what?

And how accurate can one make these? Could one make a pill which, for example, will start working 6 hours after ingestion (plus or minus 10 minutes)?

 

[nuke]

See: the patent

Also: The cited literature at the end of the patent

 

[Amu]

Adderall XR uses bimodal release, which releases half the amount immediately and the other half a few hours later, the latter part is covered so it takes longer to metabolize. It is not true time-release actually, and I find it too unstable, especially if one has unusually low urine pH values