Final answer:
When the melt-index value of plastic increases, its viscosity decreases, indicating that the plastic is more fluid and flows more easily under pressure.
Step-by-step explanation:
As the melt-index value of a plastic goes up, the viscosity goes down. This is because the melt index is a measure of the flow of molten plastic or resin and indicates how easily the plastic can flow under pressure. A higher melt-index value suggests that the plastic has lower viscosity and will flow more readily. For instance, a plastic with a high melt index can be used in processes that require it to be poured or extruded into fine details, whereas a plastic with low melt index is more viscous and might be better suited for applications needing more structural integrity.