Why is my Casio DR-220T calculator heating up for a short time?

I know that when a machine heats up, it means that it's working. My calculator which wasn't used up after 10 years heats up about 3-5 minutes after it was turned on. What causes that? It all works (including it's printing mechanism), the only problem is that the keys does not even respond easily and the heating problem. How do I take the keys off and clean the dust inside it?