densityplot png in a for loop

Hi all,

I am trying to plot a list of densityplots as png files, but when I do
it in a for loop, I get empty png files as a result.

If I manually run the instructions inside the loop, it works... any hints?

library(lattice)
names_list = c("alfa","beta","gamma")
for (i in 1:length(names_list)) {
 pngfile = paste("example_",names_list[i],".png", sep="")
 png(file=pngfile, width=1280,height=1024)
 input = rnorm(1000,i)
 densityplot(~input,
             plot.points=FALSE,
             auto.key=TRUE
             )
 dev.off()
}

Thanks,

   Albert.