Meaning
Mass measurements per unit of volume establish the density of a polymer above its melting point. The value for melt density is always lower than the solid density because the molecular chains move further apart as they heat up. Moulders use this figure to convert the volume of the injection cylinder into a weight of plastic.
Accurate data prevents overfilling or underfilling the mould cavities.
Polymers Expansion
Chains expand at different rates depending on their chemical structure.
Shot Calculation
The volume of the mould must match the volume of material delivered by the screw. By using the melt density of the resin, the processor can calculate the exact stroke length required for the machine. This step is vital for precision parts where the weight must be held to a fraction of a gram.
Incorrect calculations lead to flashing at the parting line or sink marks on the surface.
Buying Efficiency
Purchasing resin by weight but using it by volume creates a financial discrepancy. A resin with a lower melt density will produce more parts per tonne than a denser alternative. Procurement teams use this information to compare the true cost of different materials.
This calculation helps in selecting the most economical polymer for high volume production runs.
