Blocked kitchen sinks and bathroom sinks

What are the most effective ways to clear a blocked kitchen sink? I have used a plastic rod (made specially for this purpose) with its numerous "sharp edges/wing type ends" along the whole length of the rod but it always breaks during the exercise.

For blocked bathroom sinks - it is very difficult to clear it because the metallic circular cover is permanently sealed onto the base of the discharge hole at the bottom of the sink. Why do the manufacturers seal it?. What is the reason for one to seal the cap. There is no way that I could clear the bathroom sink if I am unable to open the protective metallic crossed cap where the water drains away. How do I do it? The metallic cap has many crosses which compounds the problem for me. I can't use a suction pump? I can't use a product called One Second Plumber.

Is there something that I could pour into the bathroom sink to dissolve the blocked article (whatever it is)? Is there a rod that is made for bathroom sinks?

I will need your help.

Thanks