# Scientific Molding Viscosity Curve Worksheet

> Source: https://netray.co/tools/scientific-molding-viscosity-curve-worksheet
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Log a rheology (viscosity curve) study at several injection speeds and see relative viscosity and shear rate index calculated per run.

## Category

mes-shop-floor

## Industry

Plastics and Injection Molding

## Audience

- Process engineer
- Scientific molding technician
- Molding engineer

## Tool

**Kind:** worksheet

**Mode:** editable

**Columns:**

#### Run

**Key:** run

**Type:** text

#### Injection speed setting (mm/s)

**Key:** speed

**Type:** number

#### Fill time (s)

**Key:** fill

**Type:** number

**Unit:** s

#### Peak injection pressure (bar)

**Key:** pressure

**Type:** number

**Unit:** bar

#### Relative viscosity (bar x s)

**Key:** visc

**Type:** number

**Expr:** fill * pressure

**Format:** number

#### Shear rate index (1/s)

**Key:** shear

**Type:** number

**Expr:** 1 / fill

**Format:** decimal

#### Part weight (g)

**Key:** weight

**Type:** number

#### Observations

**Key:** notes

**Type:** text

**Rows:**

**Run:** 1

**Speed:** 20

**Fill:** 4

**Pressure:** 600

**Weight:** 24.1

**Notes:** Short shot ruled out, slow fill

**Run:** 2

**Speed:** 30

**Fill:** 3

**Pressure:** 640

**Weight:** 24.6

**Run:** 3

**Speed:** 45

**Fill:** 2

**Pressure:** 710

**Weight:** 24.8

**Run:** 4

**Speed:** 60

**Fill:** 1.5

**Pressure:** 770

**Weight:** 24.9

**Run:** 5

**Speed:** 90

**Fill:** 1

**Pressure:** 880

**Weight:** 24.9

**Notes:** Good fill

**Run:** 6

**Speed:** 120

**Fill:** 0.8

**Pressure:** 950

**Weight:** 25

**Notes:** Slight flash starting at parting line

**Run:** 7

**Speed:** 150

**Fill:** 0.6

**Pressure:** 1050

**Weight:** 25

**Notes:** Flash at parting line

**Totals:**

#### Lowest relative viscosity

**Key:** visc

**Agg:** min

**Format:** number

#### Highest relative viscosity

**Key:** visc

**Agg:** max

**Format:** number

#### Highest peak pressure (bar)

**Key:** pressure

**Agg:** max

**Format:** number

**Summary:**

#### Viscosity change across the study

**Key:** drop

**Expr:** if(gt(max_visc, 0), (max_visc - min_visc) / max_visc * 100, 0)

**Format:** percent

**Primary:** true

**Description:** How much relative viscosity fell between slowest and fastest fill

**Notes:** 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.

## Content

**Intro:** 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.

**Sections:**

#### How to run the study

**Body:** 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.

**Bullets:**

- 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

**Body:** 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

**Body:** 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

**Body:** 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.

**Cta:** 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.

**Faq:**

#### What is relative viscosity in scientific molding?

**Answer:** 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?

**Answer:** 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?

**Answer:** 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?

**Answer:** 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.

## Search terms

scientific molding viscosity curve, rheology study injection molding, relative viscosity fill time pressure, injection molding rheology curve excel, scientific molding worksheet, injection speed study template, decoupled molding viscosity, shear rate injection molding

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