# Barrel Residence Time Calculator for Injection Molding

> Source: https://netray.co/tools/barrel-residence-time-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Estimate how long resin sits in the barrel and how much of the machine capacity a shot uses, to spot degradation risk from oversized presses.

## Category

manufacturing

## Industry

Plastics

## Audience

- Process engineer
- Molding technician
- Quality engineer

## Tool

**Kind:** calculator

**Currency:** $

**Inputs:**

#### Machine maximum shot (polystyrene)

**Key:** cap_ps

**Type:** number

**Default:** 450

**Min:** 1

**Step:** 10

**Unit:** g

**Help:** From the machine spec sheet, usually quoted in grams of general purpose polystyrene.

#### Resin melt-equivalent density

**Key:** density

**Type:** number

**Default:** 0.91

**Min:** 0.5

**Step:** 0.01

**Unit:** g/cm3

**Help:** Solid density of the resin on the datasheet, used to scale the polystyrene capacity.

#### Shot weight (parts plus runner)

**Key:** shot_g

**Type:** slider

**Default:** 90

**Min:** 5

**Max:** 450

**Step:** 5

**Unit:** g

#### Cycle time

**Key:** cycle_s

**Type:** number

**Default:** 30

**Min:** 1

**Step:** 1

**Unit:** s

#### Hold-up factor

**Key:** holdup

**Type:** number

**Default:** 2

**Min:** 1

**Step:** 0.1

**Unit:** x

**Help:** Melt in the barrel, screw flights and nozzle compared with the maximum shot. Two is a commonly used planning figure.

#### Resin residence limit

**Key:** max_res

**Type:** number

**Default:** 8

**Min:** 1

**Step:** 0.5

**Unit:** min

**Help:** Maximum safe time at melt temperature, from the resin supplier. Heat sensitive resins are far lower than polyolefins.

**Outputs:**

#### Machine maximum shot in this resin

**Key:** cap_res

**Expr:** cap_ps * density / 1.05

**Format:** decimal

**Description:** Polystyrene capacity scaled by density ratio (1.05 g/cm3 for GPPS).

#### Shot size as percent of capacity

**Key:** shot_pct

**Expr:** shot_g / cap_res * 100

**Format:** percent

**Primary:** true

**Description:** A common guide is 20 to 80 percent.

#### Estimated residence time

**Key:** residence

**Expr:** cap_res / shot_g * cycle_s / 60 * holdup

**Format:** minutes

**Primary:** true

#### Margin to residence limit

**Key:** margin

**Expr:** max_res - residence

**Format:** minutes

**Description:** Negative means a degradation risk.

#### Over the limit (1 = yes)

**Key:** status

**Expr:** if(gt(residence, max_res), 1, 0)

**Format:** integer

**Chart:**

#### Residence time (min) vs shot weight (g)

**Type:** line

**X:** shot_g

**From:** 20

**To:** 400

**Steps:** 39

**Series:**

- residence

**Disclaimer:** A planning estimate using a common rule of thumb. Actual hold-up depends on screw design, back pressure and barrel length. Confirm limits with the resin supplier.

## Content

**Intro:** A small shot on a large press leaves resin cooking in the barrel for too long. This calculator estimates residence time and shot utilization so you can decide whether a job belongs on a smaller machine.

**Sections:**

#### How to use it

**Body:** Enter the machine maximum shot from its spec sheet, normally in grams of polystyrene, and the density of your resin so the capacity can be scaled to what the machine really holds in your material. Add the shot weight including runner, the cycle time and a resin residence limit from the supplier. The result shows percent of capacity used and an estimated time at melt temperature.

#### The formula

**Body:** Capacity in your resin is the polystyrene capacity times the resin density divided by 1.05. Residence time in minutes is capacity divided by shot weight, times cycle time in seconds divided by 60, times a hold-up factor to account for melt in the screw flights and nozzle beyond one maximum shot. The factor of two is a widely used planning number, not a physical constant.

**Bullets:**

- Shots below roughly 20 percent of capacity raise degradation risk
- Shots above roughly 80 percent leave little room for recovery and consistent plasticizing
- Heat sensitive resins such as PVC, POM and some polycarbonate blends need short residence

#### Reading the result

**Body:** With the defaults, a 90 g shot on a 450 g polystyrene-rated press in polypropylene-density resin runs about 17 percent of capacity and an estimated residence near 9.5 minutes, which is over an 8 minute limit. The fix is a smaller press, a lower hold-up through a smaller barrel, or a shorter cycle. The chart shows how steeply residence time rises as shot weight falls.

#### Common mistakes

**Body:** Using the shot weight of one part rather than the whole shot with runner, ignoring density, and forgetting that downtime multiplies residence: a press stopped with a full barrel degrades resin quickly, so purge procedures and temperature setback matter. Also check the real melt temperature, not the setpoint.

**Cta:** Netray can match job shot weights from your ERP routings against the presses that exist on your floor and flag oversized machine assignments before they cause rejects.

**Faq:**

#### What shot size percentage is recommended?

**Answer:** A common guideline is to keep the shot between about 20 and 80 percent of machine capacity. Below that, residence time grows and consistency suffers. Above it, recovery time and shot control can become limiting.

#### Why scale capacity by density?

**Answer:** Machine ratings are given for polystyrene. A lighter resin such as polypropylene fills the same screw volume with less mass, so the maximum shot in grams is lower for that resin.

#### How do I know the residence limit?

**Answer:** Check the resin supplier datasheet or technical service. Polyolefins tolerate many minutes at melt temperature, while heat sensitive resins may allow only a few. Use their figure for your melt temperature.

#### Is the hold-up factor of two accurate?

**Answer:** It is a rule of thumb. Screw design and barrel length change it. If you have measured purge or color change data for the machine, use that to calibrate the factor.

## Search terms

barrel residence time calculator, injection molding residence time, melt residence time formula, shot size percent of barrel capacity, resin degradation in barrel, right size injection molding machine, polystyrene equivalent shot capacity

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