All polymers show classic Non-Newtonian behaviour during flow due to their very long but thin chain structure. This means that the melt viscosity they exhibit will fall as the shear rate being applied increases. It is preferable to select an injection speed that will result in the minimum melt viscosity value so as to ensure that the material chains have sufficient time to relax, cool uniformly and minimise any post moulded in stress levels. These situations will also result in higher weld line strengths and shorter cycle times.

Tests should be conducted on the correct size of machine for which the mould will be used in production. Once the correct injection speed has been established then its use will result in a reduction to the potential batch to batch variations which would otherwise be created.

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