There are 3 refs, but the last is BEST! It confirms JOIN LINES

MOULD LINES (if lines are caused by mould)

EXTRACT "Both are cast in relatively soft (very soft for many resin kits) two-part rubber moulds that can leave "mould lines" on bodyshells and small pieces, and possibly some thin "flash" (excess casting material that's been forced out between the two-part mould faces) in the cockpit and axle openings."

FROM: http://www.google.com/search?q=cache:www.grandprixmodels.co....

IF lines are ciaused by the joining, JOIN LINES (But extract doesn't refer spedifically to plastic)

EXTRACT "Compressed pieces of amber put in alcohol will show their join
lines. But amber is not affected by alcohol or benzole."

FROM: http://www.google.com/search?q=cache:www.florilegium.org/fil...

EXTRACT "The packaging industry uses vacuum forming extensively, biscuit trays, margarine containers, hardware blister packs to name a few. Refrigerator linings are commonly vacuum formed. Next time you open the door and gaze, focus on the egg shelf and you will notice the lack of any moulding marks or join lines."

FROM: http://www.google.com/search?q=cache:www.plasform.com.au/vac...

For more refs see Google ---> join+lines+plastic+moulds (2nd page)