Obtaining a green card via consular processing is used for immigration purposes only -- to live in the USA permanently. Do not use the green card as a visitor visa, as you've stated that your parents intend to visit for just over six months. Using the green card instead of a visitor visa for a temporary stay is abusing the green card.
Please see the link below for Consular Processing:
5. Go to Your Appointment
Once a visa is available or a beneficiary’s priority date is current (earlier than the cut-off date listed in the monthly Visa Bulletin),the consular office will schedule the applicant for an interview. The consular office will complete processing of the applicant’s case and decide if the beneficiary is eligible for an immigrant visa.
6. Notify the National Visa Center of Any Changes
You do not need to contact the National Visa Center about your petition, they will contact you for the information they need. You should, however, contact the NVC if there is a change in your personal situation or if you change your address. For NVC contact information, see the “NVC Contact Information” link to the right. It is important to notify the NVC if you reach the age of 21 for a child or have a change in your marital status, as this may affect your eligibility or visa availability.
7. After Your Visa is Granted
If you are granted an immigrant visa, the consular officer will give you a packet of information. This packet is known as a “Visa Packet.” You should not open this packet.
Upon your arrival to the United States, you should give your Visa Packet to the Customs and Border Protection officer at the port of entry. You will be inspected by a Customs and Border Protection officer and if found admissible, will be admitted as a permanent resident of the United States, which gives you the authority to live and work in the United States permanently.
8. Receive Your Green Card
You will be mailed your green card. If you do not receive your green card within 30 days of your arrival, please call the USCIS National Customer Service Center at 1-800-375-5283 or visit your local office by making an InfoPass appointment. See the “Make an Appointment” link to the right.
http://www.uscis.gov/portal/site/uscis/menuitem.eb1d4c2a3e5b9ac89243c6a7543f6d1a/?vgnextoid=62280a5659083210VgnVCM100000082ca60aRCRD&vgnextchannel=62280a5659083210VgnVCM100000082ca60aRCRD