PLA is truly a semi-crystalline resin and by choice of whether or not to include a nucleating agent and choice of mold temperatures, one can dictate whether the final part will have amorphous or crystalline morphology.
If application will not be exposed to temperatures above 120°F (50°C), it is acceptable to mold PLA in the amorphous morphology. Amorphous morphology is achieved by not adding any nucleating agents and using a mold surface temperature that is controlled to a temperature of less than 75°F (24°C). Mold temperatures higher than this will cause parts to stick in the mold and be very soft and flexible upon ejection.
If application will be exposed to temperatures above the Tg of 130°F (55°C) regardless of pressure, or will be exposed to above 120°F (50°C) while under moderate pressure, ie 50 psi (340 KPa), RTP Company recommends that the PLA be in the crystalline state. Crystalline morphology is achieved by adding a nucleating agent (RTP Company offers either a standard cycle or a fast cycle nucleating agent). The mold surface temperature must be controlled at a temperature above 180°F (82°C), preferably about 220°F (105°C), and mold closed time must typically exceed 60 seconds for standard cycle or 40 seconds for fast cycle nucleating agent. This is typical for 1/8 inch (0.32 cm) thick parts. Thinner parts may be ejected at reduced cycle times depending on sprue & runner thickness and ejector design. Mold temperatures of less than 180°F (82°C) or shorter cycle times will cause parts to stick in the mold and be very flexible upon ejection. It may sound counter-intuitive, but for a nucleated PLA you want to raise your mold temperature to above 200°F (93°C) to make the part stiffer upon ejection.