Homopolymer is used for hot fill applications. However, Homopolymer has lesser impact strength. Random Co-polymers are used where better
gloss and clarity are desired. Air tight containers are made with tight fitting lids moulded out of LDPE/LLDPE. high Density Polyethylene is also used to mould houseware items like kitchen containers, mugs and buckets. Buckets made out of HDPE have better impact strength than those made out of Random Copolymer.
Homopolymer has excellent integral hinge properties. Impact Co-polymers are used where
enhanced impact properties are required. Flip top lids are made out of Polypropylene because of
its hinge properties. Grades used will depend upon the number of impressions in the mould.
Multi-cavity moulds need grades of higher flow. Controlled rheology grades of PP are effective in giving warpage free mouldings. When stress- is applied, like flexing at the hinge, the stressed area, i.e. hinge becomes white, when impact Co-polymer is used. This phenomenon is known as stress whitening. LDPE, LLDPE & HDPE are also used to mold closures. PP closures give good dimensional stability. LDPE/LLDPE is used if flexibility is required in lids.