Terry B,

You're probably using what's called RTV silicone which is room temperature
vulcanizing - meaning it sets at room temperature (usually after a few
hours). The lower ratios (e.g. 5:1) will set faster than the higher
(20:1). Heating makes it set quicker. Most silicones will "cure" at room
temperature eventually, but that can take days for some ratios/mixtures.

You can get the two halves to adhere in a number of ways:

1 - don't cure the two completely
2 - Cure one completely and the other partially
3 - use uncured silicone as a glue
    (if your features are really large you can paint it on)
4 - O2 plasma and UV/ozone cleaning

In any case, you probably want to cure the joined halves overnight at
elevated temps - say 70C.

I never had much luck with the varying ratios method. Best for me was
curing to about 80% set, then bonding, then overnight heat. If you have a
UV/ozone cleaner handy, a 1 to 2 minute clean of completely cured silicone
and a glass "cap" gives good bonding between the two.

btw.. Acrylic enamel spray paint worked well for me in keeping the
silicone from adhering too much to my photoresist molds (if you run into
that problem). Dunking in ice water helps in peeling off thin layers
without distorting or tearing the silicone also.

Best of luck,

Mike