Achieving the Optimal Residence Time in Injection Moulding and Extrusion incl. New Online Calculator

Achieving the Optimal Residence Time in Injection Moulding incl. New Online Calulator

Hello and welcome to this blog post on how to achieve the optimal residence time in injection moulding. 

Let us start with a brief recap on melt residence time.

How to calculate the residence time? 

The total time the resin is in molten state, from the time it is completely melted, leaving the barrel, entering the mould runner system into the cavity is defined as melt residence time. Figure 1 shows how it can be calculated and this formula is used later in my calculator tools too. A detailed post with all calculations can be found here .

Try out my tool right here below or in this section of my blog

What are recommended residence times (at melt temperature) of engineering and high performance polymers?

In the following, the recommend residence times of selected polymers are shown which can serve you as a guideline: 

• Polyamide 6 (PA 6): 10 minutes
• Polyamide 6 with glass fibers (PA 6-GF): 10 minutes
• Polyamide 6.6 (PA 6.6): 15 minutes
• Polyamide 6.6 impact modified (PA 6.6): 10 minutes
• Polyamide 6.6 with 30 wt% glass fibers (PA 6.6-GF30): 15 minutes
• Polyamide 6.6 with 25 wt% glass fibers  and flame retardant (PA 6.6-GF25 FR): 10  minutes
• Polyphthalamide (PPA): 4 minutes
• Polyketone (PK): 10 minutes
• Polyethylene terephthalate with 30 wt% glass fibers (PET-GF30): 8 minutes
• Polybutylene terephthalate (PBT): 8 minutes
• Polybutylene terephthalate with 30 wt% glass fibers (PBT-GF30): 8 minutes
• Polycarbonate (PC): 6 minutes
• Polycarbonate (PC) / Polybutylene terephthalate (PBT) blend: 6 minutes
• Polycarbonate (PC) / Acrylonitrile butadiene styrene (ABS) blend: 6 minutes
• Polysulfones (PSU): 5 minutes
• Polyethersulfone (PESU): 5 minutes
• Polyphenylsulfone (PPSU): 5 minutes
• Polyphenylene sulfide (PPS): 5 minutes 
• Polyetheretherketone (PEEK): 5 minutes
• Liquid crystal polymer (LCP): 1.5 minutes (max. 4 minutes)

How to achieve the optimal residence time in injection moulding?

An extensive melt residence time will result in degradation of the molecular weight of the and as a consequence your plastic part will not have the desired chemical resistance and thermal stability. It is crucial to keep the molecular weight as close to the virgin material as possible. Degradation is not always visible at first sight, however it can backfire once the part is in use. 

Optimal residence time is achieved by having a good mould design, together with optimal part design (minimum wall thickness), and an efficient running moulding process. 

Let us dig deeper into the running of the moulding process. Apart from the temperature, and cycle time, the biggest impact on the residence time is the size of your plasticizing unit and its optimal selection. If you have a high metering stroke, residence time is low. If you have only a small metering stroke, residence time will be up and you have to check if you are not above the maximum residence time of the polymer. It can encounter this by using an increasing temperature profile on your plasticizing unit.

How about extrusion residence times?

Yes, also there I have you covered. The extrusion residence time can be approximately be calculated by building the quotient of the volume of the extruder filled with melt and the volume flow of the melt. This is the background of the online calculation tool shown below. 

Thanks for reading and #findoutaboutplastics

Greetings,

Herwig 

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://www.tritanmoldit.com/blog/mold-design-critical-factors-1

[2] https://www.ptonline.com/articles/minimizing-melt-residence-time

[3] https://www.findoutaboutplastics.com/2023/03/rule-of-thumb-residence-time-and.html

Residence Time Calculation for Extrusion - Polymer Processing

Hello and welcome to a new blog post. In a previous post, I presented the residence time calculation in injection moulding. Today we will focus on the extrusion process.

Residence time in extrusion is referring to the time, which a polymer pellet, and as consequence, polymer melt needs to travel from the melting zone until the end of the metering zone.

The online calculation

For an approximate calculation of the residence time, building the quotient of the volume of the extruder filled with melt and the volume flow of the melt is possible (underlying basic laws of physics):

Equation 1 - extrusion residence time (average residence time): t=V/Q

V = volume of the extruder filled with melt

Q = flow rate of the melt

t = extrusion residence time

This is the background of the online calculation tool shown below:

Check out the new tool here

In literature, extruder output (flow rate; kg/h) can be calculated by the following equation [3]:

The leakage flow is calculated by addition of mass flow rate (drag) and pressure flow rate minus the extruder output.

2nd way to calculate the average residence time

average residence time t = V/Q= (2 * z) / (v_bz *(1-a)) = 

z =dz = unwound channel length = L / sin φ

φ = outer slope angle f.e. 30.72°

L = screw length f.e. 14*D

D = screw diameter (mm)

n = RPM (1/min)

h = groove depth

b = π*D*sin φ-e

e =  bridge width

dp = pressure drop f.e. 20 bar = 20*10^5 Pa

v_bz = circumferential speed in Z direction = n*π*D*cos φ 

a = throttle coefficient = ((b*h^3)/(12*η))*(dp/dz)* (2)/(v_bz*b*h)

Example with following extrusion screw and process data

D=45 mm

L=14*D

h=3 mm

e=4.7mm

n=60 1/min

φ=30.72°

η = 800 PAs

1. calculate b = 67.52 mm

2. calculate v_bz = 121.53 mm/s

3. calculate  z = 1233.26 mm

4. calculate a = 0.25

5. calculate t = 2*1233,26 / 121,53 (1-0,25) = 27,07 s

Also as a general rule, the higher the temperature, the lower should be the melt residence time. 

Thanks for reading and #findoutaboutplastics

Greetings

Herwig Juster

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature:

[1] https://www.findoutaboutplastics.com/2017/10/how-to-calculate-residence-time-in.html

[2] https://www.extrusion-training.de/extruder-dimensionierung/

[3] Rao, Natti S. - Design Formulas for Plastics Engineers

[3] N. Rao: Design formulas for plastics engineers

How to Calculate the Residence Time in Plastics Injection Moulding [incl. online calculation tool]

The term residence time in injection moulding operations refers to the time that a plastic pellet takes from entering the injection moulding barrel until entering the injection mould. It relates to the amount of polymer material present in the cylinder of the injection unit, the shot weight and the total cycle time. Often, residence time is also referred to as Hold-Up Time (HUT).

Melting of plastics for processing is usually attained by bringing the plastics over a certain temperature, i.e., glass transition temperature for amorphous thermoplastic polymers and glass transition temperature as well as crystalline melting temperature for semi-crystalline thermoplastic polymers. For both types of thermoplastics longer than necessary heat exposure, especially in the presence of oxygen (air), may induce chemical degradation. Therefore, the residence time in injection moulding at polymer-sensitive melt temperatures needs to be optimal. In this context, residence time is especially important for polymers such as, for example, PVC, POM, ABS, PBT and PET.

Melt temperatures have to be chosen in a way that the material’s thermal stability during processing is ensured [1, 2]. Guidance about optimal residence time and residence time for different polymers is given by material manufacturers in processing and design guides.  In practice, tools for accurately calculating the melt residence time depending on the utilized machine and processing conditions are usually not available. This prevents processing engineers from making quick process assessments. For this reason, I have created an online tool to calculate the residence time of your injection moulding operation. This can be used online or downloaded. The calculation is based on the formula below [3].

Formula for calculating the residence time in injection moulding

Here, number 8 represents the volume of the molten polymer in the barrel. This is the ratio between flight height and screw length, which for most injection moulding machines is approximately 8. Part A gives the number of shots in the barrel and Part B represents the cycle time to produce the part.

Finally, keeping the residence time at an optimum level will help you keeping materials’ degradation to a minimum and, consequently, the mechanical properties of your final moulded part to a maximum.



Successful residence time injection molding calculation  and thanks for reading!

Till next time!

Greetings, 

Herwig Juster

Interested to talk with me about your polymer material selection, sustainability, and part design needs - here you can contact me 

Interested in my monthly blog posts – then subscribe here and receive my high performance polymers knowledge matrix.

New to my Find Out About Plastics Blog – check out the start here section

My YouTube channel

My personal website


Literature

[1] http://www.solvay.com/en/binaries/Sulfones-Quick-Molding-Guide_EN-227546.pdf

[2] GE Plastics - Injection Moulding Guide

[3] Christoph Jaroschek - Spritzgießen für Praktiker
  [4] https://www.wittmann-group.com/sites/default/files/2021-05/wiba_prnews_plasticizing-screws-article-series-part2_04-2020_en.pdf