We are using Graphical LCD in IP65 plastic enclosure. We cut the IP65 box front panel for GLCD's display space. How to maintain the IP65 rating after cutting the space on the boxes for GLCD Display??? What's your solutions/methods for this job?

 

What's a IP65 rating?

 

Glass or perspex bonded with a good quality adhesive sealer such as Sikaflex https://www.sika.com/ or Tec7 http://www.tec7.com.
They both have a wide range of products that will cover your needs.
Available in black, white and clear.

 

What's a IP65 rating?

IP = Ingress Protection , see http://www.dsmt.com/resources/ip-rating-chart/
IP rating indicates how well protected, an enclosure is against dust and moisture.

 

if your LCD is IP65 rated and you followed the cutout/mounting instructions then your enclosure is still IP65 rated. If your LCD is not IP rated or you did not follow the instructions, then your enclosure is no longer IP-anything rated and there's nothing you can do about it. You could slather a puddle of caulk over the whole thing and make it so it doesn't leak, but you can't keep calling it IP65. Once you violate the IP rating by installing a non-IP-rated fixture, it's null.