Plastic guns aren't new. Congress first voted to outlaw them in 1988 and twice has renewed that ban.

The spread of inexpensive technology to produce the weapons adds urgency to the need to renew the Undetectable Firearms Act before it expires Dec. 9 and, for once, there appears to be a basic agreement in Congress to do so.

In May, Texas law student Cody Wilson posted online a blueprint for constructing a handgun, which he called the Liberator, on an inexpensive 3-D printer. The Bureau of Alcohol, Tobacco, Firearms and Explosives constructed two such weapons and tested them. One exploded; the other fired eight rounds before ATF halted the test.

Because plastic weapons contain no metal, they don't show up on standard detection equipment such as metal detectors and X-rays.

The House passed a renewal Tuesday that extends the current ban for another 10 years.

The Senate is in recess and will be in one session for only a day before the current ban will expire. Senate Democrats want to close a loophole in the existing law and in the extension passed by the House.

Plastic guns are exempt from the law if they include metal parts that show up in detection scans. But some plastic weapons include removable metal parts that could enable someone to slip a plastic weapon through a detection point at an airport, government building or other venue.

Senate Democrats want to require that plastic weapons include permanent metal parts, but there might not be time to adopt the amendment and send it back to the House before the existing law expires. The Senate should pass the renewal now and attend later to closing the loophole.