MES & Shop FloorFree worksheetPDF, Excel, Word exportPlastics and Injection Molding

Scientific Molding Viscosity Curve Worksheet

A viscosity curve shows how easily the resin flows at different injection speeds, and it guides which fill speed makes a stable process. Record each run here, compute relative viscosity and a shear rate index, and export the data to plot your curve in Excel.

Download:

Run	Injection speed setting (mm/s)	Fill time (s) (s)	Peak injection pressure (bar) (bar)	Relative viscosity (bar x s)	Shear rate index (1/s)	Part weight (g)	Observations
				2,400	0.25
				1,920	0.3333
				1,420	0.5
				1,155	0.6667
				880	1
				760	1.25
				630	1.6667

Lowest relative viscosity

630

Highest relative viscosity

2,400

Highest peak pressure (bar)

1,050

Viscosity change across the study

73.8%

How much relative viscosity fell between slowest and fastest fill

Relative viscosity here is fill time multiplied by peak injection pressure, a common scientific molding index. Plot viscosity (y) against shear rate index (x) and choose a fill speed on the flat part of the curve, where viscosity is least sensitive to speed. Keep melt temperature, transfer position and tool the same across all runs, and fill 95 to 99 percent in velocity control, then complete the fill with pack.

Store process studies with tool and lot data

Optional: we can outline how to keep process study results linked to tool and resin lot records.

Optional. The tool and every export work without this. No spam.

How to run the study

Fill the cavity to about 95 to 99 percent with velocity control only, with pack and hold off, at a series of injection speeds from slow to fast. Record the fill time and peak injection pressure for each run. Keep melt temperature, transfer position and back pressure constant. Let each setting stabilize for several shots before recording. Enter the numbers in the table, and the worksheet calculates relative viscosity as fill time multiplied by peak pressure and a shear rate index as one divided by fill time.

Stop pack and hold so only the fill phase is measured
Use the same tool, resin lot and melt temperature
Record part weight to confirm the fill is consistent

Reading the curve

Plot relative viscosity against the shear rate index. Most thermoplastics are shear thinning, so viscosity falls as speed rises, then flattens. Choose a fill speed on the flat part of the curve, where small speed changes barely change viscosity, which makes the process robust against variation in melt temperature and material. Avoid the steep slow-speed region, where viscosity changes quickly with speed, and avoid speeds so high that flash, burning or shear damage begin.

Using the result in your process

Once you pick the speed, document it in the process sheet along with the viscosity curve and the date. Repeat the study when you change resin grade, lot, tool or machine, and compare curves over time to detect material variation. A shift in the curve between lots may mean a melt flow difference, which you can correlate with resin lot data in your quality or ERP system.

Common mistakes

Frequent errors are mixing pack effects into the fill measurement, changing melt temperature between runs, using the set speed instead of measured fill time, and choosing the fastest speed because pressure looks lower. Pressure behavior differs from viscosity, so decide using the viscosity curve. Remember the index is relative, not an absolute material property, so compare only runs from the same tool and machine.

Frequently Asked Questions

What is relative viscosity in scientific molding?

It is an index calculated as fill time multiplied by peak injection pressure. It is not an absolute viscosity, but it shows how resistance to flow changes with injection speed on the same tool and machine.

Which fill speed should I choose?

Choose a speed on the flat part of the viscosity curve, where viscosity changes little with speed. That gives a robust process. Also confirm that the part has no flash, burning or visible shear problems at that speed.

Why must I turn off pack and hold?

Pack and hold add pressure after fill and distort the pressure and time readings. A study of the fill phase alone, at about 95 to 99 percent fill, shows how the melt flows, which is what the curve is meant to capture.

Can I compare curves from different machines?

Only with care. The index depends on pressure measurement, screw geometry and tool, so compare curves from the same tool and press. Use curves from different machines as a guide only.

If you want process studies stored with the tool and resin lot in your ERP or MES, Netray can help you structure and query them on your own network.

Talk to an Expert Explore More Free Tools

Related Tools

Industry Templates

More MES & Shop Floor tools

See all

Back to All Tools