Screening Life Cycle Assessment comparing One-step and Two-step

Injection Molding Compounding using Conservative and Optimistic

Scenarios

ULRIKE KIRSCHNICK1, ZAHRA SHAHROODI2, NINA KREMPL2, RALF SCHLEDJEWSKI1

1Department Polymer Engineering and Science – Processing of Composites Group

Montanuniversitaet Leoben,

Franz Josef-Strasse 18, 8700 Leoben,

AUSTRIA

2Department Polymer Engineering and Science – Institute of Polymer Processing,

Montanuniversitaet Leoben,

Otto Gloeckel Strasse 2, 8700 Leoben,

AUSTRIA

This article is dedicated to Univ-Prof. Dr.-Ing. Ralf Schledjewski

Abstract: - One-step injection molding compounding (IMC) is an innovative process to manufacture short-

fiber-reinforced polymer composites. The aim of combining compounding and injection molding into one

process is to enhance component quality and minimize environmental impacts. In this study, a screening Life

Cycle Assessment (LCA) is conducted to evaluate and compare the environmental impacts of the IMC process

with standard two-step manufacturing. Two scenarios for the IMC are considered, each differing in terms of

machinery requirements, energy consumption, and material usage. Mechanically recycled polypropylene and

glass fiber are used, and considered in the LCA employing a simple cut-off approach without awarding credits

for substituting (primary) materials. The functional unit is the composite produced via the respective process,

assuming equal functionality. Inventory data are obtained from initial experiments, literature, and the ecoinvent

database. The impact assessment method selected is ReCiPe2016. Results indicate that the environmental

performance improvement achieved by the IMC compared to the reference process is minimal in the

conservative scenario where energy and material usage can be reduced but machinery usage is increased.

However, in an optimistic scenario, the IMC can reduce the impacts of composite manufacturing by 34 %. The

contributions at the midpoint level vary, and metal usage and energy consumption are the main contributors in

all scenarios. A variation of the energy source for manufacturing shows the dependency of environmental

impacts of components produced in both processes on the geographical location of production and its electricity

supply. Methodological choices, such as the definition of the functional unit and modeling of recycled

materials, have a large influence on LCA results, and alternative options are discussed.

Key-Words: - Injection molding compounding, Life Cycle Assessment, recycled fiber-reinforced polymers,

recycled glass fibers, recycled polypropylene, scenario analysis.

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1 Introduction

Fiber-reinforced polymer (FRP) composites are

high-performance materials that can help to reduce

negative environmental impacts, e.g. through

application in wind power and transport sectors,

where they can reduce fuel consumption due to their

lightweight potential, [1]. On the other hand, FRP

composites are also facing challenges under the

sustainability paradigm, such as their reliance on

fossil resources for production and processing,

limited recyclability at End-of-Life (EoL), and

limited availability of functional manufacturing

processes for recycled and novel materials, [2], [3].

The development and improvement of a combined

injection molding compounding (IMC) process aims

to contribute to advancing technological solutions to

foster the Circular Economy of FRP composites, as

this process is suitable for the usage of recycled

glass fibers (rGF) and thermoplastics, such as

Polypropylene (rPP). The goal of the improved

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Ulrike Kirschnick, Zahra Shahroodi,

Nina Krempl, Ralf Schledjewski

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processing is to decrease material degradation

during processing, while simultaneously improving

economic and environmental performance of

recycling and remanufacturing processes.

The IMC is an innovative process technology

for manufacturing different types of fibers, fillers

and additives in a thermoplastic matrix by directly

connecting a continuously conveying extruder to a

discontinuously operating injection molding (IM)

machine through a melt pipe and pot, [4]. This

connection allows for processing in a single

plasticizing process, which can potentially increase

composite quality due to reduced degradation of the

polymeric matrix and maintained fiber length during

processing. Potential economic and environmental

advantages are the reduction in processing time,

production cost, and machine wear as well as energy

savings, [5].

Nevertheless, there remain challenges of the

IMC process that need to be addressed to exploit the

full potential of the concept, [6], [7]:

 Improvement of the connection between the

continuous compounding and discontinuous

IM processes,

 Optimised configuration of processing

parameters, such as shear energy input and

residence time,

 Identification of suitable material

combinations and compositions,

 Usage of additives to improve processing of

recyclates and component quality.

Research and development aim to address these

issues through improvements in machinery

conception and process development (especially

concerning the connection of the compounder and

IM machine), in-line monitoring, formulation of

rGF, rPP, and additives, and the analysis of cause-

effect relationships.

Environmental advantages associated with the

improved processing and the usage of recycled

materials are the main motivation for the process

development. It is important to verify and quantify

these potential environmental sustainability benefits

using a suitable methodology such as Life Cycle

Assessment (LCA), [8], [9].

To develop a better understanding of the

anticipated environmental benefits of the combined

process and identify hotspots, this study conducts a

screening LCA. The one-step IMC process is

compared in two different scenarios to a

conventional two-step compounding plus injection

molding (IM+C) process to produce a component

using rPP reinforced with rGF. The goal of this

paper is the depiction of the status quo in IM+C

processing and the development of two scenarios

(conservative and optimistic) to estimate the

environmental impacts of the IMC process.

Elaboration at an early stage of the project helps to

identify hotspots for environmental performance in

the process functioning. Furthermore,

methodological choices in LCA, such as the choice

of the functional unit and approaches to model

recycled materials, are critically discussed.

2 Methodology

The goal and scope of the LCA are to determine the

potential environmental advantages of the IMC

process in comparison to a conventional two-step

IM+C process using different scenarios. The

scenarios vary according to the key areas expected

to be different, namely machinery requirements,

energy consumption, and generation of waste. The

geographical scope of the LCA for manufacturing is

Austria, whereas additional materials are supplied

from the European market. While the research

activities are performed in Austria with Austrian

and German machinery manufacturers, the IMC

technology can be used worldwide to produce FRP

products.

2.1 Functional Unit

The functional unit (FU) constitutes the object

investigated and represents the quantified reference

unit for inventory data and environmental impacts,

[10]. To ensure comparability of different products,

the FU needs to include quantitative and qualitative

aspects of the object’s function(s) concerning: the

service provided (what?), extent of the service (how

much?), level of quality (how well?), and duration

or lifetime of the product (for how long?), [11].

In this screening, LCA, the FU is the injection-

molded rGF/rPP component produced in the

respective process. The components produced in the

two processes have uniform geometrical dimensions

and shape (convex hull) and are composed of the

same materials. They are expected to provide an

equivalent level of functionality.

Nevertheless, there are potential differences in

the level of quality provided by the two processes.

Previous research showed that Young’s modulus of

PP nanocomposites manufactured in an IM+C two-

step is up to 7 % higher than in the counterpart

manufactured using a one-step IMC process, [6].

This is explained by differences in shear energy

introduced and by the long residence time of the

melt in the non-optimized melt conveying system.

Research and process developers aim to solve these

shortcomings of the IMC process, and it is expected

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that in the future, the IMC component will exhibit

better mechanical properties than the IM+C

counterpart due to the decrease in material

degradation during manufacturing.

The adaptation of the FU is one option to

account for these differences in LCA. Before

undertaking any modification of the FU in a process

comparison LCA, it is important to reflect on the

cause-effect relationship leading to differences in

component quality. It should be analyzed to what

extent these differences are a result of processing

differences or material-induced variability.

Recycled materials are often subject to inherent

heterogeneity as a consequence of polymer

degradation, cross-contamination with other

substances, and variable input streams into recycling

from the first life, [12], [13], [14].

Afterward, the FU can be modified by reflecting

on the consequences of quality differences during

the life cycle of the component in its field of

application. Possibilities for quantification in the FU

are the inclusion of differences in component

lifetime and effects on emissions during the use

phase. To make products from the two processes

comparable, a modification of component geometry

and composition to reach equal mechanical

properties is another possibility to redefine the FU.

Such a change can be considered in LCA by

modeling components at equal strength or stiffness

as expressed by Ashby indices, [15], [16]. At the

same time, this change does not only require the

adaptation of inventory data concerning material

inputs but also concerning mold design and energy

consumption. The influence of mold and cavity

design on energy consumption in IM has been

illustrated by [17].

2.2 System Boundaries

The cradle-to-gate system boundaries and flow chart

for the recycling and two manufacturing schemes

are depicted in Fig. 1. The life cycle of the

components starts with the collection of the post-

consumer polymer waste and post-industrial GFRP

waste. The input material is the same in both

processes, an rPP from the mechanical recycling of

the Austrian post-consumer packaging waste. After

packaging waste collection, the size is reduced by

shredding and the PP fraction is sorted from the

mixed waste using Near-Infrared (NIR) sensors.

During the washing step residues are removed

before the flakes are extruded and pelletized. The

rGF is provided by the size reduction and sieving of

post-industrial GF/PP tapes.

For the consideration of the recycled materials,

a simple cut-off approach is chosen. The recycled

materials come burden-free from their first life and

no credits for the replacement of (virgin) materials

are awarded to emphasise on manufacturing

processes instead of the materials being recycled.

Therefore, the environmental impacts incorporated

by the recycled materials are solely related to

necessary recycling steps from waste treatment

(sorting, shredding, washing, and extrusion) to

generate the input material for the IMC and IM+C

processes.

Fig. 1: Flow chart of the IM+C and IMC processing

with system boundaries of the LCA (red box)

Using the recyclates, the component

manufacturing takes place according to the two

described processes for comparison: In the separate

IM+C process, the first step is the production of an

rGF/rPP granulates using a compounder with a

granulation unit. The granulate is dried before

further usage to minimize humidity. In the second

step, the granulate is used in an IM machine, where

the polymeric matrix is melted again and

mechanically injected into the mold to obtain the

desired component shape. In the alternative

combined IMC process, the melt of the compounder

is directly conveyed to the IM machine to obtain the

final product. The use phase and treatment of the

component at EoL are neglected in this study as the

focus lies on the manufacturing stage.

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2.3 Inventory Data

Inventory data describe the type and amount of

resources consumed as well as process outputs

(products, wastes, and emissions). They can be

categorized in three main groups for this study:

machinery, energy, and material requirements. Data

for modeling the two-step IM+C process are

retrieved from preliminary experiments, the

ecoinvent v3.8 cut-off database, [18], and literature.

Data for the mechanical recycling of the PP from

Austrian post-consumer, mixed packaging waste

were retrieved from [19], [20], whereas data for

GF/PP tape shredding were extracted from [21]. The

inputs required for sorting the mixed packaging

waste were allocated based on the mass of the

different waste fractions according to the waste

composition described by [22].

In addition to experimental data for

compounding and IM processing, the energy

consumption of the IM machine (in the two-step

IM+C manufacturing scenario) was modeled

according to [23]. Transport between the recycler

and manufacturer has been neglected as it is

expected to be the same in both manufacturing

processes.

For the two scenarios of the IMC process, the

inputs are varied about the IM+C process as visible

in Table 1. The conservative scenario (IMC-CON)

expects an increase in machinery and a moderate

decrease in energy and material consumption,

whereas the optimistic scenario (IMC-OPT) expects

a significant reduction in all three regards.

Table 1. Variation of process input and output of the

IMC process scenarios about the IM+C reference

process

IMC-CON

IMC-OPT

Machinery usage

110 %

50 %

Electricity

90 %

67 %

Heat (natural gas)

90 %

67 %

Heat (other)

90 %

67 %

Water

10 %

2 %

Processing waste

80 %

50 %

2.3.1 Machinery Usage

The usage of machinery in the IMC process can

potentially be reduced as the granulation and drying

units after compounding become redundant. An

additional machinery effort is needed to connect the

compounder to the IM machine through the (heated)

melt pipe and melt pump. The design of the IM

machine remains the same in the one-step and two-

step processes as the plasticizing unit and screw are

still used to convey and inject the melt into the

cavity. In the optimistic scenario, overall

requirements are still reduced in the IMC whereas

the conservative scenario assumes the overall

demand for machinery is increased compared to the

IM+C process.

2.3.2 Energy Consumption

Energy usage can be divided into thermal energy

and electricity consumption for the main drive

(motors etc.), electrical heating, and auxiliary

equipment. Similar to machinery requirements, the

energy consumption of the granulation and drying

unit and partially from the IM machine can be

reduced in the IMC scenario. Even when taking into

consideration the additional consumption for the

heating of the melt pipe and melt pump, there is still

a net decrease in energy consumption expected in

the conservative scenario.

The electricity consumed is provided by the

average market mix of electricity in Austria at a low

voltage level. To depict alternative energy scenarios,

supply from the Swedish grid serves as an

exemplary low-emission supply with an emission

factor for electricity consumption of 0.033 t

CO2e/MWh, and Poland gives exemplary results for

manufacturing in a country relying on fossil fuels to

generate electricity with 0.796 t CO2e/ MWh, [24].

The shares of different energy sources to produce

electricity in the three countries, [25], are visible in

Fig. 2.

(a)

(b)

(c)

Fig. 2: Electricity mixes of Austria (a), Sweden (b)

and Poland (c), [25]

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Fig. 3: Overview of ReCiPe2016 methodology for LCIA from inventory results to midpoint impact categories

and endpoint areas (Based on [26])

2.3.3 Material Requirements

The material origin and blend ratio are the same in

all analyzed processes. In the optimistic and

conservative scenario, it is expected that processing

wastes (purging and lumps) in the IMC are reduced

in comparison to the IM+C process. Material input

quantities are also expected to be reduced as a

consequence of the reduced waste occurrence.

2.4 Impact Assessment Method

The Life Cycle Impact Assessment (LCIA)

determines the influence of the mass and energy

flows described in the inventory on the

environment. ReCiPe2016, [26], (as implemented in

the ecoinvent version 3.8) has been chosen due to its

significance in LCA research, [27], [28]. At the

endpoint level, the aggregated single score allows

for easy comparison of results while the

differentiation into 18 midpoint impact categories

enables a more detailed analysis.

The functioning of the ReCiPe2016 method is

depicted in Figure 3. The midpoint level uses

characterization factors that represent the

environmental flow (e.g. greenhouse gases emitted

to air). At endpoint, flows are translated into effects

on the life of the earth using endpoint

characterization factors, which provide more

relevant and comprehensive information on damage

caused by environmental flows to human health,

ecosystems, and resource availability. At the

midpoint level, the LCIA results in “a score list with

different environmental effects”, [26]. These effects

are independent of each other concerning their

impact pathways and affected areas of protection.

They contribute to the endpoint score through a set

of normalized rules.

Furthermore, ReCiPe2016 offers the possibility

to choose among three perspectives that represent

value choices regarding the parameters of the

assessment, such as time horizon, included effects,

and uncertainty. The hierarchist perspective has

been selected as it provides a balance of the three

proposed perspectives. The LCA was conducted

using OpenLCA v2.0 software, [29].

3 Results

The results of the impact assessment using

ReCiPe2016 endpoint level are depicted in Fig. 4.

Fig. 4: ReCiPe2016 endpoint results of the IM+C

reference process and the two IMC scenarios

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The IMC process modeled employing

conservative assumptions leads to a marginal

improvement of approximately 2 % in

environmental performance compared to the

reference two-step process. On the other hand,

environmental impacts from manufacturing via IMC

can potentially be reduced by around 34 % using

optimistic assumptions. Nevertheless, it is important

to disaggregate these results to have a more

complete and differentiated understanding of the

advantages and drawbacks of the IMC process in

both possible scenarios.

Next to the absolute amount of impacts at the

endpoint level, a change in environmental impacts at

the midpoint level and consequently, a change in the

type of environmental endpoint area can be

observed. As depicted in Fig. 4, the IMC-CON

process exhibits a larger absolute amount in the

endpoint area “damage to resources availability”

than the IM+C process (0.25 and 0.26 Pt

respectively). The reason for this shift in

environmental impacts is visible in Fig. 5, which

shows the normalized midpoint impacts for the 18

ReCiPe2016 midpoint categories. The elevated

usage of machinery in the IMC-CON scenario

requires additional materials (mainly metals) for

constructing the connecting parts between the

compounder and IM machine, which leads to a

comparatively higher metal depletion. On the other

hand, even in the conservative scenario, more than

5 % of environmental impacts can be reduced

compared to the reference two-step process

concerning freshwater and marine eutrophication,

ionizing radiation, water depletion, and climate

change.

The global warming potentials over 100 years

(GWP100) of IM+C, IMC-CON, and IMC-OPT are

3.86, 3.56, and 2.64 kg CO2e per piece

manufactured in the respective processes. The main

contributors to climate change in all three scenarios

are emissions from direct energy consumption

(electricity and heat) responsible for over 50 % of

the GWP100, and incineration of the processing

waste which accounts for up to 16 % of the overall

GWP100.

While the IMC process in the optimal scenario

leads to a reduction of environmental impacts in all

midpoint categories, there are differences in the

magnitude of the change: Impact categories where

IMC-OPT has a very large potential to reduce

environmental impacts compared to the IM+C

reference are metal depletion (reduction of 51 %)

and terrestrial ecotoxicity (reduction potential of

42 %). The latter is mainly a result of the reduction

in processing waste for incineration.

Next to differences in the environmental

performance of the two compared processes as a

result of the chosen LCIA method (and level of

aggregation), potential benefits associated with the

usage of recycled materials and location of

production are discussed as they have a large

influence on LCA results.

Fig. 5: Normalised results of the ReCiPe2016 impact categories at the midpoint level for the three

manufacturing process models

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3.1 Substitution of Primary Materials

The consideration of substitution effects includes

avoided burdens for amounts and types of materials

being replaced by the recycled ones in the LCA.

Credits for substitution offer also another option to

integrate component differences derived from the

manufacturing processes into LCA (as discussed in

Chapter 2.1): A component with superior material

properties can potentially replace a more advanced

material type at a higher material quality ratio

(quality of the ingoing secondary material compared

to quality of the material being substituted).

There exist several methods to model recycling

in LCA, [30], [31], [32]. The majority of methods

differ in how they consider and allocate impacts

among multiple material life cycles (and

applications) and how they answer the following

question: Do the recycled materials replace another

material and if yes, which material and at what

quality? The answer is case-specific and depends on

the suitability of the different modeling approaches

for the field of application. Various parameters,

such as type and processability of the materials,

material functionality and homogeneity, economic

performance, market availability, and environmental

impacts play an important role in determining type

and quality ratio for materials substituted. Fig. 6

provides some examples of different types of

materials and levels of functionality that can

theoretically substitute each other. For example,

recycled and virgin FRP composites can be used to

replace parts manufactured using aluminum and

steel in the automotive sector, [33], [34]. In many

cases, recycled thermoplastics, such as PP, are

expected to replace a virgin thermoplastic but with a

decrease in functionality, [14], [35], [36].

Fig. 6: Examples of materials being substituted with

recycled ones considering material type and

functionality

In this LCA, no credits for substitution have

been awarded because the focus is on comparing the

manufacturing processes and not the materials used.

Nevertheless, rPP can potentially replace its virgin

counterpart (with a decrease in mechanical

properties and adaptation of processing parameters)

and rGF can replace short virgin GF or another type

of filler.

3.2 Location of Production

As visible in Fig. 7, the energy provision at the

place of production plays an important role when

assessing the overall environmental performance of

the IM+C and IMC processes. In comparison to

production in Austria, production in a country with

an electricity mix provided in large parts by

renewable energy (such as Sweden) can decrease

ReCiPe2016 endpoint scores by up to 16 %. On the

other hand, production using an electricity mix

relying on fossil fuels (such as in Poland) leads to an

increase of up to 41 % compared to the respective

production process in Austria.

Fig. 7: ReCiPe2016 endpoint results of the IM+C

reference process and the two IMC scenarios for

production in Sweden (SE) and Poland (PL)

including change in endpoint impacts compared to

production in Austria

Given the reduced contribution of energy

provision to the overall score for production in

Sweden (20 to 30 % of total impacts), the benefits

of the combined process in the conservative IMC

scenario become less pronounced: With 0.429 Pt,

the impacts of the IMC-CON in Sweden are

marginally lower than the IM+C reference

production with 0.433 Pt. Besides energy provision,

the metals used for construction and waste treatment

(incineration of processing waste) are the main

levers of improvement and emphasize the need for a

circular economy of waste metals and (post-

industrial) plastics.

Due to the differences in electricity provision,

there is also a change of impacts at the midpoint

level. For example, ionizing radiation contributes

0.2% to the total score for production in Sweden,

while the contributions in Poland (0.02 %) and

Austria (0.03%) are much lower. Nuclear energy

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provision in the Swedish electricity mix is the main

reason for this difference. Similarly, the GWP100

per process varies significantly as visible in Table 2.

While this leads to different absolute Greenhouse

Gas emission abatement potentials, the relative

magnitude when replacing the IM+C process with

the IMC process is similar for all locations (up to

around 32 %).

Table 2. GWP100 for the IM+C, IMC-CON, and

IMC-OPT processes assuming usage of electricity

provided by the Austrian, Polish, and Swedish grids

Process

Country

GWP100

[kg CO2e]

IM+C

Austria

3.86

Poland

5.87

Sweden

3.04

IMC-CON

Austria

3.56

Poland

5.36

Sweden

2.81

IMC-OPT

Austria

2.64

Poland

3.98

Sweden

2.09

4 Conclusion

Reflecting on the initial research question, the IMC

process can lead to a potential improvement in

environmental performance compared to the two-

step IM+C process. This benefit depends mainly on

the additional effort required to connect the two

machines and energy consumption. Contributions to

midpoint categories vary, which results in effect-

specific hotspots. Generally, metal usage and energy

consumption are the main levers to improve the

environmental performance of the IMC processing.

The three-step procedure for the evaluation of

energy consumption demonstrated in this paper is

recommended to compare the processes and draw

meaningful conclusions regarding their

environmental performance: i) The separate analysis

of the amount of energy consumed in the inventory

stage compares processes’ energy efficiency and

potential optimization measures for further process

development. Future research should verify the

presented assumptions by collecting inventory data

for both, the two-step reference scenario as well as

in the one-step IMC manufacturing. ii) Investigating

the dependency on location of production and type

of energy provision, puts the contribution of energy

consumption to overall impacts into perspective. iii)

The consideration of multiple impact categories

allows for a holistic picture of environmental

damages associated with manufacturing. It also

helps to identify potential shifts from one impact

category to another when changing process

characteristics and also location of production.

The role of awarding credits in LCA for avoiding

burdens of (virgin) material use through substitution

with recycled ones has a potentially large influence

on LCA results. Whether and to what extent the

inclusion of these credits is appropriate and realistic

for the field of application of processes and

components should be subject to future research.

Furthermore, the options of adapting the FU (as well

as inventory data) should be investigated to

incorporate potential differences in component

quality as a result of the change in component

manufacturing.

Acknowledgement:

The authors would like to acknowledge the helpful

support of the following project partners: Engel

Austria GmbH, LIT Factory der Johannes Kepler

Universität Linz, Leistritz Extusionstechnik GmbH,

Gabriel Chemie GmbH and Institute of Material

Science and Testing of Polymers at

Montanuniversität Leoben.

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DOI: 10.37394/232015.2023.19.117

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Nina Krempl, Ralf Schledjewski

E-ISSN: 2224-3496

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DOI: 10.37394/232015.2023.19.117

Ulrike Kirschnick, Zahra Shahroodi,

Nina Krempl, Ralf Schledjewski

E-ISSN: 2224-3496

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Contribution of Individual Authors to the

Creation of a Scientific Article (Ghostwriting

Policy)

- Ulrike Kirschnick: Conceptualization,

Methodology, Formal analysis, Writing - Original

Draft

- Zahra Shahroodi: Resources, Data curation

- Nina Krempl: Project administration, Funding

acquisition

- Ralf Schledjewski: Supervision, Project

administration.

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

Report potential sources of funding if there are

any

The authors kindly acknowledge the financial

support through project LightCycle (project no.

864824) provided by the Austrian Ministry for

Climate Action, Environment, Energy, Mobility,

Innovation, and Technology within the frame of the

FTI initiative “Kreislaufwirtschaft 2021”, which is

administered by the Austria Research Promotion

Agency (FFG).

Conflict of Interest

The authors have no conflicts of interest to declare.

Creative Commons Attribution License 4.0

(Attribution 4.0 International, CC BY 4.0)

This article is published under the terms of the

Creative Commons Attribution License 4.0

https://creativecommons.org/licenses/by/4.0/deed.en

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WSEAS TRANSACTIONS on ENVIRONMENT and DEVELOPMENT

DOI: 10.37394/232015.2023.19.117

Ulrike Kirschnick, Zahra Shahroodi,

Nina Krempl, Ralf Schledjewski

E-ISSN: 2224-3496

1303

Volume 19, 2023