Spiral flow data are obtained by using a standard spiral mould and measuring the distance the material has flowed under the test conditions.  They are a way of describing the flow behaviour of a polymer.  Machine, mould and process conditions all influence the results of spiral flow tests.

A polymer's melt flow rate (MFR) is an indicator of its flow capabilities under given conditions.  A higher MFR indicates an easier flowing resin.

Of course, processing at increased melt temperature, or with higher injection pressure, will increase the length of the polymer's flow, and varying either of these conditions is one method for achieving flexibility with any given grade of thermoplastic.

Spiral flow and MFR data only provide a very rough guide to the processability of a resin.  To accurately compare the flow characteristics of two resins, a complete curve of viscosity versus shear rate is required for each resin.

Mould Shrinkage Value

The mould shrinkage value reflects, for the plastic from which a part is made, the amount of contraction from the actual mould dimensions that a finished part exhibits after removal from the mould and cooling to room temperature  (23 degree Celcius ) for 48 hours.

It is possible to obtain less shrinkage by close control of all processing conditions.  However, because of the slower cycles involved in such control, economics are usually adversely affected.  The addition of fillers and/or reinforcements can also decrease mould shrinkage.  In all practicality, however, part and mould design must take the stated amount of shrinkage into account.

Table 1 below gives mould shrinkage values of some representative thermoplastics.