Although recession has affected its customers in the food service and food packaging industry, Maryland Cup Corp. said yesterday it had record sales and earnings for the second quarter and first half of 1982.

Maryland Cup, the nation's largest manufacturer of paper and plastic food service products, had profits of $4.5 million (67 cents a share) in the three months ended March 31, compared with $4.4 million (66 cents) for the same period of 1981.

For the half, the Owings Mills firm posted earnings of 48.5 million ($1.25) compared with $8.1 million ($1.21) last year. Second-quarter sales were $150.3 million, compared with $143.4 milllion last year, and six-month sales climbed to $286.2 million from $274.5 million the year before.

Company officials said the first and second quarters traditionally are the smallest for Maryland Cup, which does about 60 percent of its business in the second half of its fiscal year--during the summer months. Maryland Cup's main trademarks are Sweetheart and Eat-It-All.

"We are encouraged by the company's performance, in view of the state of the economy," said Samuel Shapiro, president. "Many of our customers in food service and food packaging are being adversely affected by the recession."

Working on the highest backlog in its history, Radiation Systems Inc. of Virginia, a designer and manufacturer of antennas, reported nine-month profits of $1.7 million (87 cents a share) compared with $689,000 (54 cents) for the similar period of 1981. Earnings-per-share figures for the nine-month period reflect an additional 657,000 shares outstanding.

For the latest quarter ended March 31, Radiation Systems had earnings of $652,000 (33 cents), compared with $310,000 (22 cents) last year.

First-quarter receipts for new orders and the backlog of contracts grew at Isomet Corp., a manufacturing and engineering company involved in scientific instruments and electronic components, and revenues increased to $1.1 million from $1.03 million. But compared with 1981 profits, which included $149,000 of tax-loss carry forwards, earnings for the first quarter of 1982 declined.

The Springfield firm had profits of $163,000 (11 cents a share) in the first quarter compared with $334,000 (22 cents) last year.

The Washington Corp., a Chevy Chase real estate firm, posted a loss of $179,198 in the first three months this year compared with profits of $150,000 (6 cents a share) for the first quarter of 1981, which included extraordinary gains, company officials said.

Gross income was $281,000 compared with $497,000 for the same period last year. A company representative said 1982 results were affected by slower sales of residential housing units. The firm expects two deals involving land with hotels to be concluded by the end of the summer that would give the company a gain of about $1.9 million.

New life insurance sales at United Services Life Companies climbed during the first quarter to $427.7 million from $373.6 million in 1981.

The D.C.-based company, which sells life insurance to military personnel, civil service employes and civilians, had net earnings of $3.3 million (58 cents), compared with $3.1 million (56 cents) in the comparable period last year.