Factory orders roll economy forward; Inventory fall comes to a halt . . .

The ''great inventory liquidation'' is ending. The corporate closet cleaning that resulted in cut-rate prices and low production rates all over the country is finally coming to an end.

As the economy begins to rebound, manufacturers are starting to increase production to cope with a rising order rate - and raising their inventories again. An indication of this came Wednesday afternoon, when the government reported that new factory orders rose by 2.1 percent at the same time as inventories gained 0.1 percent.

''This (the government's report) is the first sign the inventory liquidation is over,'' says Robert Scott, an economist with Chase Econometrics. Gordon Richards, an economist with the National Association of Manufacturers (NAM) in Washington, says, ''Looking at the April industrial production, I assume inventories are being rebuilt.'' In April the Federal Reserve Board reported that industrial production rose by 2.1 percent, following a 1.2 percent gain in March.

The gain in inventories, Mr. Richards notes, is a healthy sign for the economy. ''With demand rising and inventories low, we will see much stronger growth,'' he says. He is now predicting the economy, based on the gross national product, will grow at a 6 percent rate this quarter and at a 5 percent rate for the third and fourth quarters. ''Merely a slowdown in the decumulation (selling) of inventory pushes the GNP up,'' Mr. Richards comments.

This swing in inventories, says Charles Lieberman, an economist with Morgan Stanley & Co., an investment banking firm, ''implies gains for employment and production.''

One of the biggest shifts, the Morgan Stanley economist says, is taking place in the durable-goods area, particularly among producers of steel and automobiles. The steel manufacturers slowed production dramatically last year and sold off their inventories. Thus, when demand for steel picked up in Detroit this spring, steel companies filled the orders by increasing production. Operating rates went from the low point of 30 percent to 50 percent.

Smaller companies, too, are noticing a change. Sol Halpern, the president of H&S Tool & Mold Corporation in Brooklyn, N.Y., says in the last two to three months his injection-molding business has perked up. He has added a third shift to fill orders coming in from electronics companies, manufacturers of exercise bicycles and other housewares.

''Our sales have tripled in the last three months,'' he says. ''Inventories are low in all kinds of industries.''

When inadequate, low inventories can harm businesses. One Florida cement manufacturer has suddenly seen demand spurt much faster than his production. Since he has low inventories, he plans to import cement in order to fill his orders. This will take some of the profit out of his pocket.

Aqua Meter, a manufacturer of precision marine compasses, saw its distributors this winter order too few of some of its higher-priced equipment in an attempt to keep inventories down. Thus, when sailboat sales picked up this spring, many customers could not get the compasses they wanted.

Comments William O'Brien, director of sales and marketing, ''Everyone down the line is trying to keep inventories as low as possible, and now that business has picked up, inventories are slack.

Although many businessmen are more confident about the economy, they are not expected to increase inventories in a major way. In a May survey of purchasing managers to be released next week, the National Association of Purchasing Management will report that purchasing managers found new orders up slightly and inventories down slightly. However, in a semiannual survey, notes Paul Hassen, editor of the NAPM survey, 70 percent of the members felt the second half of the year would be stronger than the first half.

In addition, factory utilization remains low. Thus, for many companies, filling increased orders merely means increasing production rates. In some cases this can be done by adding overtime. And, as in the case of H&S Mold & Tool, by adding another shift.

Businessmen will also keep inventories down, says Mr. Richards, the NAM economist, because interest rates remain high by historical standards. ''This raises the cost of carrying inventory,'' he notes, adding, ''While the recovery is strong in 1983, it might be tenuous in 1984, depending on the federal deficit problems.

Because of this conservative posture of businessmen, Mr. Richards is estimating inventories will rise by only $8 billion this year. ''This would imply businessmen are by and large placing orders in response to existing demand ,'' he adds.

This is definitely the case at F&L Packing Company, a small canning, recreational packaging, and importing firm. Because of slack business last year, the firm was slow to replace inventory this year, says Burton Hurvich, company president. And, he was particularly cautious about placing orders for European and imported camping goods, placing orders only in response to definite interest at the 18 trade shows he attends each year. Businesses may adopt a wait-and-see approach, he says, to determine whether or not demand will be sustained.

This attitude carries through to larger companies, as well. Duncan Muir, a spokesman, says J.C. Penney Company will not increase inventories until it sees a sustained sales pickup.