DOI: 10.3303/CET23107042 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 28 June 2023; Revised: 17 July 2023; Accepted: 19 October 2023 
Please cite this article as: Jakab S.K., Dogossy G., 2023, Effect of Injection Molding Parameters on the Chemical Foaming of Acrylonitrile 
Butadiene Styrene (ABS) Using Breathing Mold Technology, Chemical Engineering Transactions, 107, 247-252  DOI:10.3303/CET23107042 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 107, 2023 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Petar S. Varbanov, Bohong Wang, Petro Kapustenko 

Copyright © 2023, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-07-6; ISSN 2283-9216 

Effect of Injection Molding Parameters on the Chemical 

Foaming of Acrylonitrile Butadiene Styrene (ABS) Using 

Breathing Mold Technology 

Sándor K. Jakab*, Gábor Dogossy 

Department of Materials Science and Engineering, Széchenyi István University, Győr, Hungary 

jakab.sandor.kalman@sze.hu 

Manufacturers have to adapt very quickly to the accelerating world and use specialized machines for production 

to meet the demands. This also means that those machines are not used as often as their mass-producing 

counterparts and sometimes become obsolete. In the case of foaming with injection molding, a complicated 

system has to be built around the machine and has to be structured in a specific way, which is not always cost-

efficient. In this paper, two methods of chemical foaming of Acrylonitrile Butadiene Styrene (ABS) were 

performed on a regular injection molding machine. Chemical Blowing Agent (CBA) and Expandable Microsphere 

(EMS) were used with breathing mold technology. Injection molding parameters were modified, and specimens 

were produced for mechanical and morphological examinations. Properly selected parameters for foaming play 

an important role in retaining the mechanical strength of the product. With low switch-over volume and using 

the core-back method, it is possible to foam ABS using a Chemical Blowing Agent. 

1. Introduction 

Polymer foaming is a widely used technology, mainly because it reduces weight and, most of all cost of the 

manufactured product. The basic premise of this process is to introduce a blowing agent into the plasticized 

polymer that generates gases inside the material or is itself a gas that expands within (Standau and Altstädt, 

2019). The goal is to create a hollow, homogenous cell-like structure inside the produced good, maintaining the 

strength and/or functionality of its solid counterpart. This technology can be used on extrusion and injection 

molding product lines (Jin et al., 2019). 

Physical and chemical foaming are the most used methods for mass production. Mixing, heating, and pressuring 

of materials is carried out in the by means of a screw in the extrusion/injection equipment. Therefore, separate 

and sometimes complicated systems are used by the manufacturers to guarantee the proper introduction of the 

foaming material into the melted plastic (Altan, 2017). Most of the time, the foaming happens at the end of the 

extrusion part, so the whole system requires unique parts assembled in a way that cannot be easily tampered 

with (Rohleder and Jakob, 2015). 

To combat this issue, the foaming system has to be simplified, and the current easiest solution is chemical 

foaming, pre-mixing the polymer with a Chemical Blowing Agent (CBA), usually in the form of pellets. Although 

this technique is simple, finding proper parameters is quite challenging since improper temperatures, speed, 

and mold structure lead to imperfect or no foaming at all (Roch et al., 2014). 

The main takeaway from most of the literature is that the process has to be fast and continuous. The Chemical 

Blowing Agent (CBA) can easily dissolve if left for a long time in the heating zone, and gases can escape or 

merge with the plastic. Selecting the proper amount of blowing agent is also important since the required 

percentage differs from plastic to plastic. Generally, a low amount can be used and later increased with a trial-

and-error method until the desired foaming is achieved. However, the injected amount of polymer has to be 

adjusted parallel to the changing of blowing agent content so the blowing agent can truly expand inside the die. 

The whole clamping and pressuring system of the mold have to be lower so the hollow parts are not crushed 

inside. 

247



Most of the articles about foaming with chemical blowing agents discuss new technologies, such as microcellular 

foaming and core-back technologies. The usage of supercritical fluid allows foaming with smaller cellular sizes 

but much denser, while the mechanical properties barely change (Llewelyn et al., 2020). Although this method 

is quite efficient, it still requires a separate system to dispense the blowing agent. The other method is the core-

back or breathing mold method. During the injection molding, while the melted plastic cools inside the die, its 

two parts open, making a small gap for cooling airflow to pass through the molds and allowing space for the 

foaming agent to take effect, and the product can expand freely along the shape of the die. This method can be 

used on conventional injection molding machines (Wu et al., 2019). 

Most of the polymers can be foamed, but each of them has their own requirements. Polystyrene can be easily 

foamed because of its amorphous crystallinity (Chung et al., 2021). Polyamide (Jiang et al., 2021) and 

polypropylene also can be foamed, but there is not much research about Acrylonitrile Butadiene Styrene (ABS) 

foaming, as the previous works available in literature report failures of foaming of ABS (Aycicek et al., 2018). 

The reason for the failure of foaming can be traced back to the parameters used for manufacturing. Therefore, 

it is reasonable to research the effects of parameters. Too much blowing agent causes the foamed cell structure 

to collapse, while too little amount does not form enough. The mold temperature also influences foaming by 

allowing it if it is low enough (Ronkay et al., 2017). The mass volume injected in the mold also plays an important 

role, as the too-small amount doesn’t fill out the mold properly and allows the escape of gases from the part. 

Injection speed plays an important role in the surface quality of the product, while the holding pressure ensures 

the proper filling of the mold. The main parameter that can greatly affect of foaming in the case of core-back 

technology is a core-back rate, in other words, how much and how fast the mold opens. The increase in mold 

opening allows better foaming, but if it happens too fast, the melt polymer doesn’t have time to solidify, which 

leads to improper captivity fill (Wu et al., 2019). 

The reason for failure, specifically in the case of ABS, is their poor heat conducting property (Lendvai and Rigotti, 

2023), and it needs a high processing temperature, which can hinder the foaming process, as opposed to the 

low-temperature requirement for successful foaming to occur. However, the lack of articles and research into 

the topic of ABS foaming with CBA gives enough reason to conduct an investigation into this topic. 

In this paper, foamed ABS injection molding was researched using a conventional injection molding machine 

and chemical blowing agents with the core-back method. The research aims to find the proper parameters for 

this method (for example, mold opening, switch-over volume, and foaming agent used) and make mechanical 

and morphological measurements on the produced specimens to obtain data about the rate of foaming possible 

with this technique. With this contribution, the unexplored parameters of ABS foaming were expected to be 

clarified and expand knowledge about this operation type. 

2. Materials and methods 

For this research, foamed ABS specimens with injection molding were produced. The used ABS was Terluran 

GP35 type polymer from BASF Chemical Company. This was mixed with a chemical blowing agent (CBA) called 

PRISMA AD PE Expansor 91205 from ICC-CHEMOL Ltd., which was mixed with the ABS at a 2 % rate. We 

also produced a few specimens with the same ABS mixed with an Expandable Microsphere (EMS) type blowing 

agent called Expancel 980 MB 100 from the Nouryon Gmbh. The used machine for injection molding was an 

Arburg Allrounder 420C Golden Edition injection molding machine, equipped with a mold capable of 

manufacturing specimens corresponding to EN ISO 527-2 international standards and having a mold cavity of 

42 cm3. The main settings of the machine for manufacturing can be seen in Table 1. 

Table 1: Main settings for injection molding (with highlighted variable data) 

Settings name Unit Value Settings name Unit Value 

Mold opening mm 0.4/0.6 Injection pressure bar 1,400 

Drying temperature °C 80 Switch-over cm3 2 

Drying time h 9 Holding volume rate cm3/s 10 

Clamp force kN 700 Holding pressure (p1) bar 500 

Mold temperature °C 35 Holding pressure (p2) bar 400 

Nozzle temperature °C 220 Holding time (t2) s 1 

Switch over cm3 34/36/38/40 Holding Pressure (p3) bar 150 

Screw rotation m/min 20 Holding time (t3) s 1 

Back pressure bar 20 Residual cooling s 20 

Injection speed cm3/s 35    

 

248



As can be seen in Table 1, we tried different mold openings and switchovers to determine how these parameters 

affect foaming. The summary of the main difference between the made specimens can be seen in Table 2. 

Table 2: Summary of manufactured specimens 

Materials/Mold opening/Switch over Designation 

ABS/0/38 ABS 

ABS+CBA/0.4/34 ABS_434 

ABS+CBA/0.4/36 ABS_436 

ABS+CBA/0.4/38 ABS_438 

ABS+CBA/0.4/40 ABS_440 

ABS+CBA/0.6/34 ABS_634 

ABS+CBA/0.6/36 ABS_636 

ABS+CBA/0.6/38 ABS_638 

ABS+CBA/0.6/40 ABS_640 

ABS+EMS/0.6/40 ABS_EMS 

The produced specimens were examined via tensile, bending, and impact testing, and their density was 

measured. The tensile test was done under the guidance of the ISO 527, while the bending was under the ISO 

178 standard. 5-5 specimens were collected from each sample for the test. Both measurements were done with 

an Instron 5582 universal testing machine, with 10 mm/s constant crosshead speed, but while the gauge length 

for tensile was 100 mm, the span between the supports for the bending test was 64 mm. For the impact test, 

the ISO 179 standard was followed, and a Ceast 6545 impact testing machine was used with a 15 J hammer 

and a support span of 62 mm. The required 80 mm length unnotched rectangular specimens were made from 

the injection molded specimens. 5 measurements were performed from the produced specimens of each 

sample. The standard deviation of measurements was calculated and served as a comparison of the change in 

properties, assuming normal distribution with 1 standard deviation (68 % confidence). 

Densities of raw material and foamed samples were determined by mass measurement with Ohaus Explorer 

analytical scale, calculated by Eq(1) according to Archimedes Principle: 

𝜌𝑠 =
𝜌𝑒𝑚𝑎

𝑚𝑎 −𝑚𝑒
 (1) 

where ρs (g/cm3) is the density of the sample, ρe (g/cm3) is the density of ethanol, ma (g) is the measured mass 

of the sample in air, me (g) is the measured mass of the sample in ethanol. 

3. Results and discussion 

From the manufacturing and early inspection of the created specimens, visible increases in thickness and small 

foam structures can be observed in the case of ABS+CBA specimens. Unfortunately, the ABS+EMS blend did 

not show any change in structure, and it was determined that no foaming occurred during the production of 

these specimens, even with varying parameters like temperature, mold opening, and switchover. 

3.1 Density measurement 

The density measurements showed a decrease in the properties of all foamed ABS mixtures when compared 

to the pure ones. Table 3 shows the measurements result for all the evaluated materials, with the designation 

at the first and third column and the corresponding density right to the second and fourth column. 

Table 3: The density of the materials 

Designation Density [g/cm3] Designation Density [g/cm3] 

ABS 1.002±0.0093 ABS 1.002±0.0093 

ABS_434 0.919±0.0021 ABS_634 0.906±0.0061 

ABS_436 0.920±0.0019 ABS_636 0.915±0.0049 

ABS_438 0.950±0.0055 ABS_638 0.949±0.0045 

ABS_440 0.958±0.0006 ABS_640 0.966±0.0030 

ABS_EMS 0.975±0.0014 ABS_EMS 0.975±0.0014 

 

249



The biggest decrease is shown at the lowest mass volume injected into the mold in the case of ABS_434 and 

ABS_634, with their 0.919 and 0.906 g/cm3. Although the ABS_EMS specimens didn’t foam, they show a 

decrease in density compared to the 1.002 g/cm3 of the ABS with their 0.975 g/cm3. The reason for that is in the 

carrier of the blowing agent. The carrier of the microspheres is made of Ethylene-Vinyl Acetate (EVA), which 

has a lower density than ABS, and mixing with it decreases this value. 

3.2 Tensile test 

The results of the tensile test can be seen in Table 4. The specimens made with expandable microsphere 

foaming achieved one of the largest property improvements. However, when compared with the density values, 

it can be concluded that no foaming was achieved, the improvement being due to the EMS substrate (EVA). Of 

the foamed samples, the best results for both mold openings were obtained with the 36 cm3 switchover. 

Table 4: Summary of results of tensile test 

Designation Tensile strength [MPa] Tensile modulus [MPa] Elongation at tensile strength [mm] 

ABS 27.16 1,521.58 2.56 

ABS_434 27.72 1,527.29 2.50 

ABS_436 29.85 1,613.15 2.70 

ABS_438 29.12 1,531.39 2.85 

ABS_440 26.79 1,454.04 2.83 

ABS_634 27.83 1,505.08 2.83 

ABS_636 30.57 1,629.56 3.02 

ABS_638 29.71 1,575.97 2.75 

ABS_640 38.59 1,853.90 3.08 

ABS_EMS 30.61 1,836.07 2.85 

3.3 Bending test 

The results of the bending test can be seen in Figure 1 and Figure 2. Figure 1 shows the properties of the 

specimens with 0.4 mm mold opening, compared to pure ABS and ABS_EMS. It can be depicted that the 

changed switch-over value did not affect the bending properties for any of the samples as they remained 

between 50-60 MPa. The modulus values were also similar, except for the ABS_440, which decreased to 1,906 

MPa. The only noticeable change was in the case of ABS_438 labeled specimens with 57 MPa flexural strength 

and 2,008 MPa flexural modulus. The reason for ABS_EMS surpassing even the pure ABS is because the failed 

foaming within the specimens and the added EVA simply increased its flexural properties. 

 

Figure 1: Flexural test results for pure ABS, ABS foamed with CBA using 0.4 mm mold opening and distinct 

switchover volume (increasing from left to right), and ABS foamed with EMS 

0

10

20

30

40

50

60

70

ABS ABS_434 ABS_436 ABS_438 ABS_440 ABS_EMS

1500

1600

1700

1800

1900

2000

2100

2200

2300

Fl
ex

u
la

r 
St

re
n

gt
h

 [M
p

a]

Designation

Fl
ex

u
la

r 
M

o
d

u
lu

s 
[M

p
a]

Flexural Modulus [MPa] Flexural Strength [MPa]

250



In the case of the 0.6 mm opening, depicted in Figure 2, we do not see a change in tensile strength, as it remains 

between 50-60 MPa, with only a slight decrease in modulus, with the lowest amount presented by the ABS_634 

labeled specimen’s 1,872 MPa. 

 

Figure 2: Flexural test results for pure ABS, ABS foamed with CBA using 0.6 mm mold opening and different 

switchover volume (increasing from left to right), and ABS foamed with EMS 

3.4 Impact test 

The results of the impact test can be seen in Figure 3. The impact strength of pure ABS is around 121 kJ/m2, 

according to the test, towering above all the measurements. Therefore, it is not shown in the graph. The 

ABS_EMS has around the same impact strength (32 kJ/m2) as all the other foamed specimens. From the size 

of the standard deviation, it can be concluded that the parameters did not affect this property. 

 

Figure 3: Summary of results of impact test 

0

10

20

30

40

50

60

70

1500

1600

1700

1800

1900

2000

2100

2200

2300

ABS ABS_634 ABS_636 ABS_638 ABS_640 ABS_EMS

Fl
ex

u
ra

l S
tr

en
gt

h
 [M

P
a]

Fl
ex

u
ra

l M
o

d
u

lu
s 

[M
P

a]

Designation

Flexural Modulus [MPa] Flexural Strength [MPa]

25

27

29

31

33

35

37

39

41

43

45

34 36 38 40

Im
p

ac
t 

St
re

n
gt

h
 [k

J/
m

^2
]

Switch over [cm^3]

0.4 mm mold opening 0.6 mm mold opening

251



4. Conclusion 

Different properties of chemical foaming were investigated using conventional injection molding with breathing 

mold technique. Based on the results, it was found that foaming with expandable microspheres (EMS) is not 

feasible for the chosen material due to the too-large processing temperature difference. When using the 

Chemical Blowing Agent (CBA), the use of the smallest switchover point, i.e., the least amount of material 

injected into the mold cavity, results in the largest density reduction. Contrary to expectations, the mechanical 

properties did not always decrease in proportion to the density reduction. For both mold openings, the tensile 

properties associated with the 36 cm3 switchover showed an improvement compared to the original material. 

This may be a consequence of the homogeneous distribution and small cell structure. Further morphological 

studies are needed to confirm this. 

In summary: 

• Acrylonitrile Butadiene Styrene (ABS) foaming with a Chemical Blowing Agent (CBA) is possible using 

the core-back method. 

• ABS foaming with Expandable Microsphere (EMS) was unsuccessful because of the main difference 

between the processing temperature of the foaming agent and ABS. 

• The most favorable injection molding was with 0.4 mm mold opening and 36 cm3 switch-over rate, 

which is around 7 % volume reduction. 

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