[Amitaabh]

Lets say a plastic block is heated, melts and changes shape then allowed to cool. Are the properties of the plastic any different than before the process of heating etc... In particular, is the plastic more prone to leaching?

[Arkcon]

That really does depend on the type of plastic, and how hot it got above the plastic's melting point.  You'll have to describe your application better.

[Corribus]

Most solids don't have "memory".  Which means that if molecules are arranged in a material in a certain way through a certain process (orientation, say), then the material is melted, then cooled back to a solid, the molecules don't "remember" the way they were oriented before.  In polymers, one property that can be greatly changed by melting and subsequent cooling is crystallinity, and the degree of crystallinity can greatly impact the ability of substances (from gasses like oxygen to bigger organics like flavors or odors) to diffuse into, through, and out of polymeric materials. 

The same thing happens, by the way, with chocolate.  One of the most desirable properties in good quality chocolate is its "snap" - if you break it, it should be loud enough to hear.  Good snap is due in large part to the amount of fat crystals in the chocolate, which in turn is formed by cooling the chocolate to a very specific temperature, heating to a specific temperature, and cooling again.  This process is called tempering, and it's very precise - as little as a degree or two can result in poor snap.  If you take tempered chocolate and melt it to completely liquid state, then cool it again, you'll get a dull chocolate (no shine) with poor snap.  It'll come apart like clay.  This is because the crystallinity of the chocolate is no longer the same.  Identical principle.  You can also take this melted chocolate and retemper it by the same process.  But any time you melt chocolate, it has no memory of its previous microscopic/nanoscopic state.

Same thing goes with polymers.  So yes, physical properties of a solid polymer can change quite a bit by melting it and cooling it back down.

[Schrödinger]

Look up Thermosetting and thermoplastic materials. You may be able to find your answer in these