Ncaj phab ntsa ob-txheej khob

Txhawm rau nthuav dav daim ntawv thov ntau ntawm cov khob ntawv, xyoo 1940, ncaj-walled ob-txheej ntawv khob tau qhia rau lub khw. Lub khob ntawv no tsis yog yooj yim nqa xwb, tab sis kuj siv tau los tuav cov dej kub. Tom qab ntawd, cov chaw tsim khoom siv cov latex rau cov khob no los npog cov "paperboard tsw" ntawm daim ntawv thiab ntxiv dag zog rau cov ntaub ntawv pov thawj ntawm cov khob ntawv. Ib txheej wax-coated khob kho nrog latex txheej yog dav siv nyob rau hauv tus kheej-service vending machines los tuav kub kas fes.