Washington — Cardboard boxes keep rolling out of Container Corporation of America's Philadelphia plant, even though the factory's unionized work force went out on strike Aug. 9. Salaried employees are tending the production equipment.
A growing number of companies are using computers and highly automated machinery to keep churning out products during labor disputes. The result is a reduction in the effectiveness of one of labor's preeminent weapons - the strike.
''The Philadelphia plant is one of our newest and most automated,'' says Container Corporation spokesman Robert Best. So it is ''relatively easier'' to operate without union members.
Charles Perry, an associate management professor at the Wharton School, University of Pennsylvania, notes, ''Increasing automation and also competitive pressure have made (some companies) more able and willing to attempt to operate in the face of a strike.'' At companies using this tactic, ''the balance of power (in labor relations) has shifted in their favor.''
Picket lines are also under electronic attack. As companies decentralize their computer operations and upgrade communications capabilities, ''if a picket line is around the home office, work can be processed 100 miles away or 1,000 miles away,'' says Harley Shaiken, a professor at the Massachusetts Institute of Technology.
High-tech strikebreaking is just one way labor unions are being challenged by increasing automation and computerization. Other technological hurdles include:
* A loss of jobs, and union members, to robots and other sophisticated manufacturing equipment. An AFL-CIO study predicts 100,000 robots will join the work force by 1990. Over a much longer period, the machines are expected to replace 3 million workers in the metalworking industries alone.
* A hostile climate toward union organizing in fast-growing high- tech companies. When the American Electronics Association surveyed its 1,900 members last year, less than 5 percent reported having any unionized workers. By contrast, 35 percent of all manufacturing workers are organized.
''Many of the (new electronics firms) imitate . . . the labor relations policies of IBM, the model of a union-free environment,'' admits Steve Early, an organizer for the Communications Workers of America (CWA). IBM offers generous wages and benefits to keep unions out.
Experts warn against overemphasizing technology's impact on labor unions. For example, while some new technology replaces workers, other machines work alongside existing employees or actually create new jobs, notes David Lewis, a professor at the Columbia University Business School. ''I believe every major (technological) change in the long run creates jobs, although that is little comfort if you are displaced,'' he says.
And despite appearances, analysts note that the United States is not being engulfed in a high-tech wave. ''People vastly exaggerate the extent to which high-tech is all around the economy,'' says Richard B. Freeman, director of labor studies at the National Bureau of Economic Research.
For example, even though computer-related occupations will increase by more than 45 percent during the 1980s, this is an increase of only 600,000 jobs, and these occupations will make up only an estimated 1.5 percent of the estimated 1990 labor force, according to AFL-CIO estimates.
Finally, although management's power to withstand strikes is increasing, ''it isn't happening as fast as people think,'' says a spokesman for the General Electric Company, a leader in factory automation.
Of course some companies - including utilities, oil refiners, and chemical producers - have always tried to keep plants operating during strikes. But Professor Perry and others say the trend is spreading - for instance, to paper producers, building product manufacturers, and jet engine makers, among others, although there are no statistics to confirm this. Nevertheless, Mr. Perry's research shows that capital-intensive companies now can get up to 90 percent of normal production during a strike, while less capital-intensive ones can manage 50 to 80 percent.
The degree to which a plant can operate plays a role in the outcome of labor negotiations. Companies that can maintain a 90 percent production level with a union on strike usually ''get what they want'' at the bargaining table more than those with less ability to produce, Mr. Perry says.
Union members resent this, and vandalism of machinery often results. For example, at the highly automated AT&T there were more than 3,000 incidents of vandalism during its recent three-week strike, spokeswoman Paula Horii says.
Forming alliances with other unions is one way for labor to cope with management's growing ability to take strikes, says Mr. Shaiken at MIT. If more than one union at a company walks out, pressure on managers is increased.
Added efforts to sell the union's case to the public are also needed, says CWA spokesman Duayne Trecker. ''Unions are beginning to understand the need for making known labor's contribution to the productivity of any company,'' he says.
Education also plays a key role in union efforts to organize high-technology companies. The CWA, along with three other unions, is trying to organize high-tech companies around Boston's Route 128. To do this, the unions are trying to build a network of what Mr. Early calls ''union-minded high-tech workers.'' These employees engage in organizing on the plant floor, try to win some small victories over management, and lay the base for later formation of a union. Organizers admit this is a time-consuming strategy.
In the battle for high-tech workers, some argue that time is on the unions' side. As start-up firms get larger, workers are less likely to duplicate the large salary gains they received for joining the company. And as companies expand, more workers hold jobs that are traditionally organized, thus boosting a union's prospects, Mr. Trecker asserts.
Despite all the obvious challenges in labor-management relations, the nation still has an unparalleled opportunity to make full use of worker ideas in improving the way industry operates, some experts contend. ''We have a better shot at this than any other country,'' says Ben Fischer, director of Carnegie-Mellon University's Center for Labor Studies, because of ''our own culture and democratic tradition and lack of a class tradition.''