The spiral fl ow test is a form of viscosity measurement used to measure the fl ow of resins in a mold. Using a special mold, as shown in Figure 7.9 with a specifi ed section thickness and circular fl ow path length, materials are evaluated under typical Injection Molding processing conditions.

The length of fl ow in the mold is a function of a resin’s viscosity versus injection pressure, fi ll rate, melt temperature, and setup. All these factors affect the resin’s fl ow length.

The lower the viscosity, the longer the fl ow. This test is less sophisticated than GC, but under standard molding parameters of pressure and temperature, the resin’s fl ow length can be measured. Variations in fl ow (viscosity) will give an indication of the material’s fl ow properties, crystallinity and freeze – off time for each lot of material. In hard – to – fi ll tools, it can indicate whether the resin has the fl ow length necessary to fi ll the tool under standard molding conditions. Any major fl ow variances in lot – to – lot testing would justify the use of more sophisticated viscosity measurements and a discussion with the material supplier.

As an example, a standard grade of ABS was evaluated in a spiral flow mold with varying melt temperatures and fi ll rates or injection speeds. Another variable was mold temperature, which also has an effect on fl ow length but is not as signifi cant for ABS. The fl ow length changes are given below.

An example of generic resin fl ow qualities is the viscosity of fl ow length of nylon 6 versus 6/6. Cable ties vary in the length and the number of individual cavities in a mold. Nylon 6/6 is the preferred material based on fl ow to fi ll qualities. Nylon 6 has higher elongation but lower fl ow qualities versus 6/6. Each can perform equally well but 6/6 is the prime material based on fl ow and economics.

One must also remember that the mold cavity gate size has a defi nite effect on resin melt temperature (shear heat). This affects fl ow as well as the physical properties of the ABS and other shear sensitive polymers, such as PVC. If a resin picks up too much shear heat when pushed through a small gate, the physical properties of the resin and molded part will be lowered dramatically.

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