The press controls the speed using valves, servos, etc. Thinner flow fronts flow easier, however injection speed is established through scientific procedure to be at the mean of slow to fast.
Viscosity has little effect on fill time. Again, it is important to measure melt temperature to verify that heats are within the melt window.
Lower temperatures provide faster cycle times, but increase wear on plastics equipment if the temperature becomes too low.
Reducing heat thickens the flow and slows down the fill rate. Melt temperature should be measured to assure that barrel temperature are within the tolerances of the melt window provided by the machine manufacturer. It is important to note here that higher temperatures add to cycle time, and that there is a point when temperature becomes detrimental by producing more gas and causing degradation. Adding heat will lower the viscosity of a material, thus making the flow thinner and faster. Temperature plays an important role in adjusting viscosity. Materials that are a lower viscosity than styrene are recorded in MSDS data as negative values. As such, materials that are at a higher viscosity than styrene are recorded as positive. Styrene falls in the middle of the material scale, and is considered to be the mean. For instance, nylon flows thinner and faster than styrene, thus nylon has a lower viscosity than styrene. Materials that are low viscosity flow thin and quick, while high viscosity materials flow thick and slower. The same terminology applies in different plastic injection materials. Molasses would be considered to be high viscosity, and water would be low viscosity. If you were to pour water and molasses at the same time, water would flow much easier than the molasses. A good comparison would be the difference between molasses and water. Viscosity as it relates to plastic injection is the measurement of how thick or thin a material flows. This article will first explain viscosity, and then delve into different situations where viscosity may help or be hurting a processors goal of zero scrap and high output. It is also one of the most important factors in a molder’s arsenal of making process changes. 
Material viscosity strongly effects standardized molding processes.
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