For the last 15 years, Wes Arbo looked to Portugal as a source for imported plastic molds. Today he sees it as a potential export market.

Mr. Arbo, president of Diversified Services In Plastics Co. of Leominster, Mass., said Wednesday that because of the country's rapidly expanding economy, he's thinking of selling plastic materials in Portugal for the first time and perhaps manufacturing there.Mr. Arbo is not alone.

Other U.S. businessmen say that although the idea of a consumer market in Portugal seemed far-fetched a few years ago, it is quickly becoming reality.

Ford Motor Co., which for years has produced pickup trucks in Portugal for export, will also begin selling the trucks in Portugal next month, said Botelho Tome, managing director of Ford's operations there.

Such changes in business attitudes toward Portugal are a measure of the country's rapid progress since 1986 in economic output, living standards, road building and a host of other areas, according to a high-level delegation of Portuguese trade officials who have been touring the United States and promoting further investment with the help of a glitzy TV ad campaign.

Like many other European countries, Portugal is portraying itself as the ''gateway" to 1992.

"We are no longer a market of 10 million people. We are a market of 320 million people and even more," said Antonio N. Silva, Portugal's secretary of state for foreign trade. Of course, such a claim might be made by any of the 12 nations in the European Community.

But in Portugal's case, there are some significant differences. Because of the emerging nature of its economy, for example, Portugal has been granted full access to the single market while it continues to enjoy protection from single-market rules and penetration for an additional three years.

Portugal also has the fastest-growing economy in Europe, according to both its ads and its finance minister, Luis M. Beleza, who spoke here Wednesday. The country's gross domestic product grew at a 5.4 percent real annual rate last year, marking the fourth straight year of growth at 4 percent or more.

But Portugal is starting from a far smaller base than the rest of the EC. The country's GDP was $45.2 billion last year compared with just $29.6 billion in 1986 when it entered the EC.

Much of the credit for the recent growth spurt has been claimed by the Social Democrats who gained full control of the government and its policies in 1987. The party, which Mr. Beleza terms "the extreme center," has embarked on a program of gradual privatization and liberalization of the economy.

But with growth have come problems, which the country's officials are quick to admit. Inflation is the worst. While per capita income grew 8.2 percent last year, inflation rose 12.6 percent, more than twice the average in the EC.

"I have to say that this is not a great result," Mr. Beleza conceded. Inflation in some sectors may actually be higher. Mr. Silva said that some real estate prices in Portugal have doubled in the past year.

The government has tried to balance encouragement of economic growth with a tight monetary policy. The commercial bank prime rate averaged 20.4 percent last year compared with 18.2 percent the year before, according to government figures.

The government hopes its policies will cool inflation to around 10.5 percent by the end of the year without dampening investment. "We must fight inflation without penalizing the private sector," Mr. Beleza said.