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A Conceptual Design and Numerical Analysis for a Small-Scale
and Low-Cost Plastic Recycling Machine
Anh T. M. Le1, Hoang D. Doan1, Loc P. Ngo1, Ly T. Huynh1, Tuan N. Huynh1, Huy T. Phan1, and Thanh T. Tran2
1Graduate Student, Department of the Global Production Engineering and Management, Vietnamese-German University, 2 Le Lai, Thu
Dau Mot City, Binh Duong, Vietnam
2Lecturer, Department of the Global Production Engineering and Management, Vietnamese-German University, 2 Le Lai, Thu Dau Mot
City, Binh Duong, Vietnam
Abstract. A new conceptual design for a small-scale and low-cost plastic recycling machine is generated
by combining melting part and compression process. Starting with one of the outstanding requirements is in
terms of an affordable-priced machine that can perform two processes with high accuracy and capacity,
some issues related to balancing among quality, capacity and cost of machine occurred during a discussion.
After implementing various designing methods such as Quality Function Deployment, Reverse Engineering,
Morphological Matrix and Pugh Method, an idea of final concept about using an electric oven and hydraulic
system to melt down and compress plastic tile which has a dimension of 300x300x9 mm was created. The
design of concept is divided into two parts which are mechanical and electrical systems. In a mechanical
section, the technical drawing and simulation are made to see how machine performs under operation.
Besides, we examined the forces that applied in the moulds to evaluate the strength of the system. In heating
and electricity section, we chose electrical components, designed oven parameters and conducted the
heating simulation on the mould. In addition, the heating and cooling time was calculated based on the
principles of thermodynamics and heat transfer. Furthermore, the manufacturing plan is created to estimate
the essential resources producing a certain number of heat-forming machines. In general, the machine needs
to be prototyped for controlling its main function and finding practical issues. After that, some
improvements could be made to enhance efficiency and increase capacity by designing an optimal mould to
more heat absorb and reduce post process, calculate and design more efficient oven, create faster lock
mechanism and other improvements for an automatizing machine.
1 Introduction
Increasement of plastic waste has been raised an urgent
problem to the whole society. The reduction of plastic
and invention of machines must be implemented to ease
the problem causing by plastic waste [1]. The amount of
plastic and their variation need to be handled by
developing plastic recycling system. Large-scale
recycling plastic plants are built in developed countries
because a huge amount of plastic waste is collected and
classified. While plastic waste is being burned and buried
in many developing/emerging countries. To reduce the
proportion of plastic to be incinerated, this conceptual
design contributes an improvement for plastic recycling
machine in Vietnam. Sorting, cleaning, drying, shredding,
heating and forming are main steps in the plastic
recycling process. Climate change and plastic pollution
have impacted governments’ awareness of countries.
There are many social media campaigns were conducted
to increase citizens’ awareness about household sorting
of waste at source. However, these campaigns were
implemented asynchronously and almost citizens did not
recognize benefits of classifying garbage. The change in
habit has happened slowly and plastic recycling rate was
low or no data. This is the reason why investment in
plastic recycling plants is quite risky. The cost of
recycling plastic machines is range from $20000 to
$100000 depending on the capacity and resin type [2].
Enterprises also pay for workforce, machine maintenance,
and energy consumption. Profit problems make it hard to
attract investment from large-scale enterprises.
Recycling plastics to make base products is the potential
direction which is getting attention from small-scale
enterprises, start-up company, universities and non-
government organizations. Recycling plastic tile is a type
of base product which has promised future in furniture
and construction field. Because of small size, recycling
plastic tiles have more advantages to produce in every
household. The Precious Plastic project by Hakkens et al
[3] suggested a compression machine which is made from
recycling components. Although the cost of this machine
is low, there are many factors having to be considered.
The components are recycled from discarded machines
which are very hard to mass production. The productivity
is low because the machine just has one simple mould
inside. The heat loss is high because of lack of thermal
E3S Web of Conferences 93, 02007 (2019) https://doi.org/10.1051/e3sconf/20199302007
CGEEE 2018
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution
License 4.0 (http://creativecommons.org/licenses/by/4.0/).
isolation layer. The ReForm project from Evergreen Labs
et al [4] aims to create a household recycling process
which includes shredding, heating, forming, pressing
machine. However, this project still cannot complete
because of many defects appear in experiments. The
shrinkage during the cooling process is significant and
lead to deforming during the cooling process and air
enclosures. The arrangement of heating elements made a
distribution of heating temperature is uneven. The heating
temperature is also hard to control and lead to burning
plastics. There is a problem related to quantification in
production such as maximum dimensions of tile, weight
of material inside cavities, colour pattern.
This research aims to create a conceptual design and
numerical analysis for machine elements of plastic
recycling machine which can solve problems from
previous projects. The technical parameters of this
machine are converted from a voice of customer
(Evergreen Labs) by Quality Function Deployment (QFD)
method. The heating elements are rearranged and
archived uniform temperature distribution by thermal
simulation. The heating time and cooling time of mould
set on the principles of thermodynamics and heat transfer.
The design of mould is based on dimensions of products
and try to solve not only quality problems but also the
productivity of the machine. The final design consists of
main components of this machine and their blueprints
which can be used for mass production directly.
2 CONCEPTUAL DESIGN
2.1 Convert Requirements to Technical
Parameters
Based on Evergreen Labs requirements’ list, the Quality
Function Deployment tool makes a system engineering
process in which product development process is linked
to quality of product as defined by Evergreen Labs [5].
From that point, the engineering parameters of products
may be posed to satisfy those expectations. Through
result of QFD report, readers can pay their attention
effectively on key criteria rather than overviewing
without priority values. By this way, machine builders
can resolve each engineering specification in an orderly
manner from highest importance to lowest one. The most
three customer demands after QFD consideration include
product dimension, output quality (no bent, no air
bubbles), and high productivity. To resolve them, five
engineering specifications shall be observed, consisting
of the output dimension, bending angle, pressing force,
air-filled density, and the maximum diameter of air-
bubbles. All of them are necessarily reduce as much as
possible. Turning on the creation of ideas of product
design, by using technical specifications derived from
QFD, Reverse Engineering method has been applied to
detect function process and its needed function parts [6]
(shown in Fig. 1). Morphological Matrix which concerns
the analysis and permutations of possible solutions of
machine elements is used for construction elements
development process [7]. There are three options for the
machine consisting of an injection-molding machine,
injection-machine with rollers, and compression machine
with multi-mold. Nevertheless, Pugh method eliminates
two options with lower rating than compression machine
with multi-mold.
For further evaluation, Computer Aided Design/
Computer Aided Engineering (CAD/CAE) application
and drawing built help to demonstrate Pugh’s method
result of compression molding machine with multi-mold
Fig. 1 Functional process of Healing-forming plastic
machine
2.2 Heating technology
The heating process in the oven up to now, there have
been many different technologies which can provide a
very high efficiency and short heating time such as
microwave, heating element, inductor and infrared.
However, every technology has specific disadvantages
which affect the heating process of recycling plastic. The
microwave cannot be able to melt down plastic because it
can be used to heat up the material which contains the
water molecule inside only [8]. For the inductor oven, it
requires a kind of mold material called ferromagnetic that
causes more difficulties in the mold manufacturing [9].
The infrared oven is not suitable as well because of high
cost and power consumption in the current scale.
Therefore, the electric oven using heating element is the
most enough and feasible choice for heating technology
in this research.
2.3 Mould Design
Conceptual design and optimizing design
To approach the desired capacity with the compression
forming method, two design concepts have been
evaluated. In the first concept, plastic is formed inside a
rectangular box by using a single mold whose dimension
is 300x300x50 mm. The main advantage of this concept
is the height of the product is flexible and can be changed
easily by changing the dimension of the fixture used in
slicing process. However, this concept requires an
additional slicing machine and a fixture for slicing
process, which could increase the cost for equipment and
the final weight of the whole system.
For the second concept, a set of molds is used to make
separated tiles with the dimension of 300x300x9 mm.
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This concept has disadvantages such as lack of ability to
change dimensions of product and the additional cost for
manufacturing five molds. However, additional slicing
machine is not required. Moreover, cycle time and quality
of the final product are controlled. With the good result
from the practical experiments of Evergreen Labs with
the similar mold dimension for only one mold, the second
concept is chosen. In this study, the method of injection
molding in which the plastic material will be melted
down inside the barrel and then it is injected into the
closed mold for forming the final product is used [10].
Because of the thin thickness of the final product, the
main force, in this case is to 30,000 N which is equal to
the force used in experiments of Evergreen Labs. By
using Solidworks Modeling software, a concept for a set
of molds has been designed as Fig. 2a in which five
layers of mold will be compressed together at the same
time. The dimension of the product is determinate by the
shape of cavities inside the mold and the bottom face of
the upper mold pressing on lower mold. One of the most
critical points of this concept is the guiding system for six
layers have to be pressed at the correct position to avoid
the exceeding material and create the desired shape of the
product. There are four guide pins are located at the four
corners of the base mold for guiding the whole system.
Additionally, the guide pins are used as the fixture which
keeps the springs inside the set of molds. Springs are
used to create the forces to separate each layer of molds
after releasing the clamp. There are three kinds of mold
in each set of molds (as in Fig. 2b) which are layer mold
(LM), base mold (BM) and top cover plate (TC). BM and
LM have the same inner dimension for forming the shape
of the product. Because the whole heat-forming system
will be operated by local civilians and the amount of
input plastics is controlled by its weight, the exceeding
material cannot avoid. Therefore, the solution is to reduce
the length of each bottom surfaces of LM and TC to
298mm for keeping these materials. Theoretically, BM
and TC are developed based on the design of LM (as Fig.
3a) with some major modifications. The ears of BM
thickness have been increased 10mm for carrying the
guide pins and the rest of molds have ears’ thickness is 5
mm. Consequently, the thickness of the BM is also
increased leading to the weight of BM is increased. To
solve this problem, four slots have been made in the
bottom surface of the BM for reducing weight.
Fig. 2. (a) Main components;
(b) three kinds of mold in set of molds
Fig. 3. (a) Detail design of layer mold ; (b) Shape of base
mold
Another modification of BM is two thick ears in two
sides of the mold for the location the clamping system.
The clamping system will be fixed at the BM and freely
slide to the holding ears of TC. The clamping of TC will
be fixed after the set of molds is fully compressed. Beside
of two additional ears for clamping, TC has been
increased the thickness of the bottom layer to 12mm
because of the pushing head has the smaller touching
surface which lead to the increase of pressing force to the
TC and may lead to the fracture of TC.
- Nominating material and structure analyses
As mentioned, this set of molds has the same working
method as injection molding machine where the pressing
force is applied to press and close the mold, and the result
is an impact between each surface of the mold. This is the
reason why medium carbon steel is not used for making
injection molds. However, in this machine, the
compressing force is much lower than the compressing
force of normal injection machine and medium carbon
steel is a decent option. For reducing cost and widely
approach in local markets, AISI 1045 steel, which
contains 0.43% to 0.5% Carbon, is referred [11].
Moreover, the hardness of AISI 1045 steel can be
increased by surface heat treatment to reach the hardness
approximate 55HRC [12] and the Yield Strength of AISI
1045 steel is 2.42e+08MPa. To confirm the suitability of
AISI 1045 steel in the concept design, Solidworks
Simulation is used for checking the internal stress of the
mold and the deformation of molds while being
compressing. Regarding the design of LM, the TC is the
most critical mold in which the force of springs below
focusses on the ears of LM.
Fig. 4 describes the force distribution on the TC mold in
which the red arrows representative for the pressing force
from TC and equal to the compressing forces (30,000N).
Blue arrows representative for the counter force of the
plastic of below mold to the bottom surface of top cover
mold. Due to lacking practical experiment, this force is
assumed to equal to 15,000 (50% of compressing force)
as green area represents the touching surface of this mold
to the edge of next mold.
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Fig. 4. Force distribution of top cover mild
Fig. 5. Force distribution of top cover mold
and it is fixed. Brown and purple arrows represent for the
force caused by the springs. By using the medium spring
from the Mitsumi with the length of 25mm without
compression and 12.5mm when it is fully compressed,
each spring generates nearly 390N. Therefore, the
magnitude of brown arrows and purple arrows are 390N
and 1,560N respectively. The analysis of BM is ignored
because BM is similar with LM and the bottom surface of
BM is even thicker than LM. For analysing TC, the same
counterforce is applied. However, the pressing area is
narrowed down to the circle (which is the profile of
pressing head) with the diameter is 240mm. The upper
spring force is removed, and the bottom spring force is
increased to 1,950N caused by 5 springs linear
connection (as Fig. 5).
2.4 Heating Time Calculation
There are the parameters for calculating the heating time
of heat-forming machine: m is the mass of materials (kg);
C is heat capacity of materials (J/kgK); T is temperature
(K); Q is thermal energy transfer (J); t is time (s); ρ is
density of material (kg/m3); V is volume of material (m3);
P is pressure (atm); n is mol number (dm3). The
dimension of:
The oven is 500 x 500 x 500 mm
A plastic plate is 300 x 300 x 9 mm
A layer mold is 300 x 300 x 50 mm
The volume of pressing plate is 1.8685359 x 106 mm3
The total volume of chamber and guide plate is
1.8005518 x 107 mm3
The volume of base mold is 1.0834136 x 106 mm3
Qplastic = mplastic x Cplastic x Δt = 840 000 (J (1)
Qmold = mmold x Ccarbon-steel x Δt = 3323481 (J)
(2)
Qair = mair x Cair x Δt = 17574 (J) (3)
Sum of thermal energy transfer
Q = Qplastic + Qmold + Qair = 4181 x 103 (J) (4)
Thermal energy supply is calculated based on Joule Lenz
law:
Qs = Pt = 3600t (5)
In this case, we assume the efficiency of energy
conversion is 100 %.
Thermal equilibrium equation:
Q = Qs = 4181 × 103 = 3600t (6)
Then the heating time t is 1161 (s). In other words, the
needed time for heating plastic from 30 oC to 180 oC is
19.34 minutes.
3 Numerical Simulation and Analysis
3.1 Heating Element Arrangement
- Oven troubleshooting
The heating element in the electric oven is normally
arranged at the top and bottom of the oven where the heat
is distributed unevenly.
As Fig. 6a, the middle layer of plastic (layer 3) has not
absorbed enough heat. It leads to the middle layers cannot
be molten down and a failure result in consequently. For
all that reason, the simple act of rotating the oven an
angle of 90 degrees to change the position of the heating
element and also the thermal flow. The modification is
described in the Fig. 6b, the position of the heating
element makes the heat distribute evenly for the entire
mold from both sides. On the other hand, the thin layer of
plastic is separated by a mold layer which can conduct
enough heat for the heating process because of the high
thermal conductivity of metals. To verify the result of this
modification, the thermal simulation is performed on the
next section to observe the heat flow and the temperature
in the mold to ensure the good condition and result of a
heating process.
- Thermal simulation
In this section, the thermal simulation is performed on
one mold to track down the direction of the thermal flow
inside the mold and to examine whether it is transferred
enough heat. First, several constraints are put into the
model to archive the accurate and expected result (Fig.
7a). The blue line indicates the direction of convection to
the environment. Convection is a phenomenon where the
air transfers from the high pressure to the low-pressure
zone namely from high temperature to low-temperature
zone. The red line is the position of the heat load put on
the mold. The yellow line is the flow direction of heat
transfer from oven to the mold.
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Fig. 6. (a) Heat is distributed unevenly by top-down
arrangement ; (b) Heat is distributed evenly left-right
arrangement
Fig. 7. (a) Thermal contraints ;
(b) thermal distribution in simulation
The thermal constraints and thermal distribution are
described in the mold in the Fig. 7b. The hottest zone
(white zone) approximately 185 Celsius degrees is the
two edges near the oven wall. The coldest zone (red zone)
is the rest two edges which has temperature of nearly
175oC. Because the melting point of polyethylene
plastics spreads over the range from 120oC to 180oC, so
with 175oC compared to 180oC, this result is good
enough and the model will work sufficiently. In
conclusion, the thermal simulation result proves that the
modification of heating element position is feasible and
can adapt the heating requirement for the good outputs.
3.2 Mold Simulation
By using Solidworks simulation, the maximum stress of
top cover mold focuses on the edge of the mold where the
pressing force has the impact on both bottom surface and
side surfaces. The maximum value of internal stress of
top cover mold is 1.78e+08 MPa and it less than the
maximum yield strength of AISI 1045 steel which is
2.42e+08 MPa. For deformation analysis, Solidworks
Simulation has carried out the result as Fig. 8a in which
the red zone is the high deformation zone of this mold
with the value approximate 0.158mm and the
deformation increase gradually from edges of the mold
into the center of the mold. Therefore, the center of top
cover mold is the center of distribution of compressing
force. Regarding to the analysis results of TC, the results
are quite familiar with results of top cover mold in which
the value of internal maximum stress is 1.25e+08 MPa
and it is slightly smaller than the result of top cover mold
and the reason may come from the distribution of the
force on TC is only focus on the bottom surface and the
increase of bottom thickness to 12mm. Without the
pressure of plastic to the side surface, the shear stress on
the edge of the mold is reduced. The same phenomena for
deformation result in which the maximum deformation is
0.14 mm at the centre of TC (as Fig. 8b).
Fig. 8. (a) Top view deformation result of top cover mold; (b)
Bottom view deformation result of top cover mold
arrangement
3.3 Final concept design
The final concept (shown in Fig. 9) is developed from
compression technology which is one of the molding
technologies that applies force to the preheated material
to manufacture metal and plastic object. The raw
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materials for a heating-forming machine in this paper is
shredded plastic particles. The main concept of plastic
molding is putting a molten plastic into the mold cavity
so that shape of plastic can be achieved with the
adjustment of calculated temperature and pressure. The
mold is then closed, and pressure is applied to force the
molten plastic to fill the cavity. The pressure and heat are
kept until the plastic is formed into the desired shape.
After melting and compressing, set of molds is locked by
clamping mechanism and taken out for cooling down.
With the set of 5 molds, the cycle time of the machine is
reduced because the cooling process would be taken
outside of oven instead of putting molds insides. For
achieving the realistic design, components are designed
for easy manufacturing instead of making hard and
unique shapes with local suppliers so that the
manufacturing cost could be reduced. In addition, some
components which currently exist on the market are also
used for designing based on 3D software of Misumi
Company.
Fig. 9. Final Prototype for the machine
4 Conclusion and Discussion
Although this conceptual design is referenced from
compression machine of Precious Plastic project, there
are many improvements were defined and tested by
simulation software. The problems related temperature
(uneven melting, burning, Granular patterns) are solved
by a new arrangement of heating elements. The heating
elements change from top-down arrangement to left-right
arrangement and make thermal flow distributed evenly.
The hottest zone is approximately 185oC and the coolest
zone is about 175oC. This temperature range is suitable
to melt polyethylene (PE) and polypropylene (PP) plastic.
The length of each bottom surfaces of LM and TC is
298mm to avoid excess material. Every mold has 4 ears
and connects with 4 pins to guide force of the hydraulic
system. The deformation of products in cooling process
prevents the clamping which is fixed by tightening bolts.
Every set of molds include 5 molds to increase the
productivity of machine, the cycle time is 19.34 minutes
per product based on the result of heating and cooling
time calculation. By simulation force distribution of mold,
the center of top cover mold is the center of distribution
of compressing force but still in the limit yield strength of
AISI 1045 steel. The set of mold achieves durability to
use in a long time. Beside improvements, this research is
still in the conceptual phase in which further experiment
on the prototype of components should have been
conducted. The final dimension of mold, arrangements of
the heater is tested by simulation software and some
assumptions. The heating time and cooling time are
calculated totally based on theories of thermodynamics,
then they need to do experiment in laboratories later. The
weight of material inside cavities of molds must be
measured carefully based on resin type. Generally, the
compression machine using multi-mold of five plates can
meet Evergreen Labs requirements, especially in terms of
output quality, product dimension, productivity, and
durability. The final option of a plastic heating-forming
machine has already been compared to other heating
methods as well as plastic treatment ways; hence, it may
pose not only advantages but also disadvantages of
solution one-to-one and result in the most proper option
for PP and PE plastic recycling. In view of economic, the
equipment may be quite proper to a small-scale operation
such as households, scrap merchant bases, and even
recycling factory. Furthermore, to enhance its
performance, buyers can customize and improve it by
expanding suitable applications such as adding
ventilation to gather fume exhausted generated during
processing, two-hand trigger to ensure safety
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