 Glasses and certain types of plastics are commonly welded materials . Unlike metals , which have a specific melting point , glasses and plastics have a melting range , called the glass transition . When heating the solid material into this range , it will generally become softer and more pliable . When it crosses through the glass transition , it will become a very thick , sluggish , viscous liquid . Typically , this viscous liquid will have very little surface tension , becoming a sticky , honey @-@ like consistency , so welding can usually take place by simply pressing two melted surfaces together . The two liquids will generally mix and join at first contact . Upon cooling through the glass transition , the welded piece will solidify as one solid piece of amorphous material . 
