Tài liệu A scaffolding architecture for conformal cooling design in rapid plastic injection moulding doc

ORIGINAL ARTICLE
A scaffolding architecture for conformal cooling design
in rapid plastic injection moulding
K. M. Au & K. M. Yu
Received: 4 August 2005 /Accepted: 25 March 2006 / Published online: 8 June 2006
#
Springer-Verlag London Limited 2006
Abstract Cooling design of plastic injection mould is
important because it not only affects part quality but also
the injection moulding cycle time. Traditional injection
mould cooling layout is based on a conventional machining
process. As the conventional drilling method limits the
geometric complexity of the cooling layout, the mobi lity of
cooling fluid within the injection mould is confined.
Advanced rapid tool ing technologies based on solid free-
form fabrications have been exploited to provide a time-
effective solution for low-volume production. In addition,
research has made attempts to incorporate conformal
cooling channel in different rapid tooling technologies.
However, the cooling performance does not meet the mould
engineer’s expectations. This paper proposes a novel
scaffold cooling for the design of a more conformal and
hence more uniform cooling channel. CAD model for
constructing the scaffolding str ucture is examined and
cooling performances are validated by computer-aided
engineering (CAE) and computer fluid dynamics (CFD)
analysis.
Keywords Conformal cooling
.
Scaffolding
.

Rapid tooling
.
Plastic injection moulding
1 Background on cooling channel design in plastic
injection mould
In recent years, rapid prototyping and tooling [1] pro-
cesses have found widespread use in speeding up tooling
production. These processes greatly reduce the manufac-
turing cost and the lead time required for tool produc-
tion. Figure 1 illustrates the difference between traditional
tooling production and contempo rary rapid tooling
fabrication.
1.1 Conventional cooling channel in plastic injection mould
The use of conventional cooling channel [2] allo ws coolant
or water to circulate within the injection mould, removing
the heat by dissipation. It is the most common method of
controlling mould temperature. The channel is formed by
hole-drilling in various sizes as close as possible to the
actual moulding area of the cavity sets. Figures 2 and 3
illustrate the conventional cooling channel in the injection
mould. According to the part dimensional accuracy re-
quired, the drilled holes are always machined using boring
tool or drilling machine. The side wall of the mould is
plugged and coolant is directed into cross bores and
changed in direction. The freeform geometric cavity is
surrounded by a straight-line cooling pattern. This will
cause uneven cooling in the mould part. The uneven
cooling will result in a tendency of several mould defects
occurrence and increase the cooling time. A more accept-
able cooling method is performed by the coolant flows in a

pattern that closely matches the geometry of the part being
moulded.
2 Conformal cooling channel in rapid soft tooling
formed by copper duct
Conformal cooling [4] is defined as the cooling channels
that conform to the surface of the mould cavity (or core) for
effectively transferring the heat from the mould cavity to
Int J Adv Manuf Technol (2007) 34:496–515
DOI 10.1007/s00170-006-0628-x
K. M. Au
:
K. M. Yu (*)
Department of Industrial and Systems Engineering,
The Hong Kong Polytechnic University,
Hung Hom, Hong Kong, People’s Republic of China
e-mail:
the coolant channel. The term conformal means that the
geometry of the cooling channel follows the mould surface
geometry. The aim is to maintain a steady and uniform
cooling performance for the moulding part. Figures 4 and 5
illustrate the geome tries of the different conformal cooling
channels.
From experimental results by several researchers, the
injection mould cooling performance after utilizing confor-
mal cooling channels can offer nearer uniform temperature
distribution within the mould than the traditional cooling
method. Heat can be evenly transferred or dissipated
through the conformal cooling channel. Figures 6 and 7
illustrate the conformal cooling channel of direct AIM
prototype tooling, designed by 3D Systems in 1997 [5].

However, the geometry of the copper duct can only
partially follow the shape of the moulding part. It cannot
provide a true uniform temperature distribution in the
injection mould. The bending of the copper duct is limited
by its diameter, mechanical strength and the size of the
moulding part. Further bending of the copper duct will
damage the cooling channel. It is worth to focus on the
relationship between the geometry of the moulding surface
and the cooling channel. The technique shown in Figs. 6
and 7 is proposed to realize the conformal cooling channel
with better cooling performance.
Besides, properties like thermal conductivity and coeffi-
cient of thermal expansion are important in the rapid
tooling process. Thermal conductivity is the quantity of
heat transmitted through a distance in a direction normal to
a surface with a certain area due to a temperature
difference. An increase in thermal conduct ivity of the
mould shortens the time required to cool down the
moulding part. As epoxy is the material having low thermal
conductivity, aluminium filler is added or mixed with
epoxy. On the contrary, the coefficient of thermal expansion
is the fractional change in dimension (or length) of a
material for a unit change in temperature. The value
decreases when aluminium filled compounds are added.
Aluminium filled epoxy have a better dimensional stability
than unfilled epoxy for injection moulding in RT.
Table 1 indicates the coefficient of linear thermal
expansion and thermal conductivity of various metal filled
epoxies.
Fig. 1 Difference in time between traditional and concurrent rapid tooling fabrications

Fig. 2 Configuration of an injection mould with conventional cooling
channel (side view)
Fig. 3 Configuration of conventional cooling channel with coolant
circulation [3]
Int J Adv Manuf Technol (2007) 34:496–515 497
2.1 Related works in injectio n mould cooling channel
design via RT techniques
The advancement of SFF gives rise to the production of
injection mould with intricate cooling channel geometry.
Rapid tooling based on SFF technology includes RapidTool,
SL, SLS or rapid casting, [8] etc. They provide significant
advantages to plastic injection mould manufacturing. Much
research has focused on improving the geometric design of
the cooling channel via RT technologies.
In 2001, Xu [9] studied injection mould with complex
cooling channels based on SFF processes. He described the
conformal cooling layout that can be realized with
substantial improvements in part quality and productivity.
He presented a modular and systematic technique for the
design of cooling layouts by using 3DP. He suggested the
decomposition of the injection moulded surface into
definite controllable parts, called cooling zones. Then the
cooling zones with the system of cooling layouts are further
divided into definite cooling cells for analysis with the
assistance of six design rules or constraints. He demon-
strated his methodology via application to complex core
and cavity for injection moulding. Figure 8 shows the green
part of an injection mould with conformal cooling system
design made by MIT’s 3D printing [ 9 ].
Li [10] studied a new design synthesis approach with the

use of a feature-recognition algorithm to optimize the
cooling system of a complex shape plastic part at the initial
design stage. The plastic part model is divided from integral
domain into simpler shape features. Then the individual
shape feature is matched with its corresponding cooling
design layout to form the mould cavity. This design
synthesis technique can offer uniformity in mould temper-
ature distribution. The ineffective computation time and
complexity in domain part subdivision may give rise to
some technical problems during the mould design process.
Figure 9 illustrates the proposed conformal cooling design
based on feature recognition algorithm.
In 1999, Jacobs [11] described the use of conformal
cooling channels in an injection mould insert. The channels
are built by electroformed nickel shells. From finite element
simulation, the conformal cooling channel formed by
copper duct bending can increase the uniformity of mould
temperature distribution. It can also decrease the cycle time
and part distortion. As common injection moulding
materials, such as steel, have not been included in his
research, the application is only restricted to copper or
nickel duct bending.
Schmidt [12] investigated and generated a series of
design of experiments in an attempt to evaluate and
measure the benefits of conformal cooling for injection
moulding. He presented an overview of the mould design
methodology, cooling channel simulation and analysis, and
tool product ion through MIT’s 3D Printing process. The
simulation results show that conformal cooling can reduce
both cycle and cooling times, and in part shrinkage.

However, the mechanical strength, thermal stress of mould
material and other mould defects are not taken into
consideration in this work . Figur e 10 illustrates the
Fig. 4 Conformal cooling channel in cavity side
Fig. 5 Location of conformal cooling channel [6]
Fig. 6 Conformal cooling channel formed by copper duct [5 ]
Fig. 7 Bending of cooling duct evenly around the cavity wall
(surrounding the ejector pin)
498 Int J Adv Manuf Technol (2007) 34:496–515
comparison between conventional and conformal cooling
design for cooling simulation.
Ferreira [ 13] attempted to use r apid soft tooling
technology for plastic injection moulding. His work
integrates rapi d tooli ng with a composi te materia l of
aluminium-filled epoxy. The mould is cooled by conformal
cooling channels. With the assistance of a decision matrix
algorithm, a proper choice of materials and processes can
be selected. The cooling layouts of the soft tooling are
inserted with a bending copper duct before the epoxy filling
process. However, in reality, the geometries of the cooling
layouts are not fully conformed to the model. The cooling
and moulding performance are affected directly with the
rough metal mould surface finish. Mould defects such as
flash, weld line, sink marks and low back pressure appeared
and cannot be avoided. Figure 11 shows the soft RT mould
with conformal cooling channel.
From the above review, much research has attempted to
apply SFF technologies to the design of conformal cooling
channel. However, the increase in complexity of part
geometries hinders the realization of conformal cooling

layout fabrication in some RT processes. It is worthwhile to
investigate further a more effective approach in order to
obtain better cooling performances.
3 RapidTool fabrication with conformal cooling design
RT, such as RapidTool process [14] by 3D systems, has
successfully applied to the production of prototype in recent
years. Figures 12 and 13 indicate the workflow of the
RapidTool process for tooling fabrication. The application
of RT for injection mould fabrication can assess to complex
metal-type prototype more rapidly than other contemporary
rapid proto typing technologies. As mould cooling is one of
the limiting factors in the injection-moulding cycle, cooling
channel design in RT is important for controlling the
production time and quality.
3.1 Laminated steel tooling (LST)
Laminated steel tooling (LST) [15] is a process that is
employed to produce a laminated tool made of sheets of
steel from laser-based cutting technology. The process is
based on sequentially combining sheets of steel layer by
layer with high-strength brazed joints for the laminated
injection-mould fabrication. The advantage of LST is the
production of tools that have dimensional accuracy com-
parable to injection moulding. The technology can give rise
to produce complex geometric configuration within the
injection mould. However, LST moulds are used only for
low melting thermoplastics and are not appropriate for the
injection-moulding process with thermosetting plastics or
high-temperature glass fibre. The layered manufacturing
feature of LST is capable of fabricating injection moulds
insertion of conformal cooling channels into any shape or

position required. Figure 14 shows the hot platening
process for LST production.
3.2 RapidTool
RapidTool is a proprietary process from 3D Systems
(formerly from DTM) based on selective laser sintering of
LaserForm powder (thermoplastic coated steel powder) and
subsequent bronze infiltration. Conformal cooling channels
can be incorporated into the moulds, which last for
hundreds of thousands of shots of common plastic.
Table 1 Mechanical properties of various metal-filled epoxies [7]
Epoxy for casting resins
and compounds
Unfilled Silica-
filled
Aluminium-
filled
Coefficient of linear thermal
expansion, (10
−6
/°C)
45–65 20–40 5.5
Coefficient of thermal
conductivity, (W/(m•K))
4.5 10–20 15–25
Fig. 8 Green parts of an injection mould with conformal cooling
channel design made by MIT’s 3D Printing [9]
Fig. 9 Conformal cooling design based on a feature-recognition
algorithm [10]
Int J Adv Manuf Technol (2007) 34:496–515 499
3.3 Copper polyamide

Like RapidTool, the Copper Polyamide process is now
available from 3D Systems and uses a mixture of bronze
and polyamide powders and conformal cooling channels
can be incorporated into the moulds.
3.4 Direct metal laser sint ering (DMLS)
EOS’s DMLS process utilizes specially developed
machines and multi-component metal powders (mixture of
bronze or steel with nickel). The SLS process is used for
sintering, but no bronze infiltration is needed. Figure 15
shows the core and cavity of inserts with conformal cooling
channel designed by EOS.
3.5 Direct AIM (accurate, clear, epoxy solid-injection
mould)
The advancement in rapid prototyping provides the capa-
bility for the development of rapid tooling for injection
moulding via 3D Sy stems’ stereolithography (SL). In the
SL process, a photo-curable epoxy formed resin is
solidified by exposing to a UV laser beam. In order to
further improve thermal conductivity, copper channels or
aluminium shots can be added to the low-melt alloy mix.
The proposed design of cooling channel limits the
consistency of the mould surface for heat transfer. Figure 16
shows the cross section of an injection mould assembly by
the SL technique.
3.6 ProMetal
ProMetal is an application of MIT’s Three Dimensional
Printing Process to the fabrication of injection moulds. The
ProMetal system creates metal parts by selectively binding
metal powders layer by layer. It uses a wide area inkjet
head to deposit a liquid binder onto the metal powders. The

final metal mould is obtained by sintering and bronze
infiltration simil ar to RapidTool of 3D Systems. Figure 17
shows the design of the cooling channel that can be located
on any position within the mould.
4 Proposed model of porous scaffold architecture
for an injection mould
Scaffold technique [16–20] has been widely used in the
medical, bio-technological and architectural disciplines. It
can offer a desirable three-dimensional interconnectivity
with tough mechanical strength. The dimension can be
accurately controlled by the highly repeatable solid free-
form fabrication processes. The design and fabrication of
various complex geometries with a porous network can be
performed by various RP&T processes. Figure 18 shows a
porous structure formed by the assembly of scaffold
elements. A mechanical and chemical feasible three-
dimensional porous scaffold architecture can be fabricated.
The maturity and high resolution of various RP and RT
techniques allow scaffold architectural model to be devel-
oped in various applications.
4.1 Possible methods for the design of a cooling
passageway
The use of rapid tooling technologi es offers a compact
fabrication of a complex 3D model. With the purpose of
enabling the production of a cooling passageway con-
formally, this section outlines the surface offsetting method
for the approximation of autom atic design of cooling
Fig. 10 Comparison between conventional and conformal cooling design for cooling simulation [12]
Fig. 11 Soft RT mould with conformal cooling channel [13]
500 Int J Adv Manuf Technol (2007) 34:496–515

passageway with the scaffolding technique. Firstly, spatial
occupancy enumeration is used to approximate the array of
the whole conformal cooling passageway with scaffolding
elements. Figure 19 shows the flowchart of scaffold cooling
surface approximation.
a) Formulation and numerical solution of conformal cool-
ing passageway formed by mould surface offsetting.
Offsetting method is widely applied in various applica-
tions. In theory, surface offsetting [21] is defined as the
locus of points that are at constant distance d along the
normal from the original surface. The offset surface
r
0
(u) of a parametric surface r(u) can be expressed by
Eq. (1)
r
0
uðÞ¼r uðÞþdn uðÞ ð1Þ
Here, the surface of the mould cavity is under surface
offset. The intention is to define the geometric
approximation of cooling passageway with a specific
offset distance d. The new offset surface will identify
the location of the cooling passageway of the mould
cavity. The new offset surface is then offset again with
a specific distance to form the layout and size of
cooling channel. Figure 20 illustrates the location of
offset surface with a particular offset distance d.
b) Spatial enumeration of the conformal cooling channel
by scaffolding element approximation. Spatial enumer-
ation is one scheme to represent the geometry of three-

dimensional model. A three-dimensional solid model
can be represented in a computer by decomposing its
volume into smaller primitive cells, such as cuboids,
which are mutually contiguous and non-intersecting.
Generally, the divided cubical cells can be set at a
specific resolution and models are modeled by listing
the cells that they take up. Here, the cubical cells are
substituted by equal-sized, porous cells or scaffold
volume elements. The integer coordinate system that it
induces and offers on a shape can be used for Boolean
operations and volum e computations. The representa-
tion of continuous variation in space can be imple-
mented easily and efficiently with scaffolding models.
Figure 21a shows the modeling of mould cavity surface
and Fig. 21b the cavity mould half with scaffolding
elements inserted for uniform cooling.
c) Unionization of scaffolding elements. After the co oling
passageway subdivision, Boolean unionization of con-
secutive scaffolding elements will be applied to
generate the whole conformal cooling passageway.
The scaffold elements are combined to form the whole
porous structure.
Fig. 12 Workflow of DTM RapidTooL Process [27]
Int J Adv Manuf Technol (2007) 34:496–515 501
Fig. 13 Workflow of common RT mould development and fabrication
Fig. 14 The hot platening process for LST production
Fig. 15 Core and cavity of inserts with conformal cooling channel
[28]
Fig. 16 Cross-sectional view of an injection mould assembly by SL
technique [29]

Fig. 17 CAD design and prototype of a rapid mould by ProMetal [30]
Fig. 18 Numerous scaffold elements with porous structure arrangement
502 Int J Adv Manuf Technol (2007) 34:496–515
4.2 Discrete scaffolding elements formation by solid offset
The positive and negative solid offset [22] of the solid
primitives can be easily computed by changing the size.
The c ooling passageway can be formed by unionization of the
equal-sized scaffolding e lements from a negative offset. The
subdivided curve is re placed b y union ization of scaffold-
ing elements. Here, a cube is applied as a scaffold to
speed up the processing time and smoothness of the
approximated model. Connected scaffold elements can be
produced to form the cooling channel which is confor-
mal to the surface of the mould cavity. Th e dimen sion of
the scaffoldin g elem ent is set as L a nd the edge of th e
scaffolding element being used is 8 mm (based on the
theoretical data of mould engineering). Figures 22 and
23 show the d imension of the scaffolding elements and
their assemblies.
Scaffolding element formation is shown as follows:
SE ¼ S À
Ã
S
À
ðÞ ð2Þ
where SE is the scaffolding element; S is the original solid
box; S
−
is the negative offset solid from S;
The unionization of the discrete scaffolding elements

generates the whole cooling channel conformally. In this
approach, set Z denotes the set of integers, Z
3
becomes the
set of points whose coordinates are all integers in the three-
dimensional Euclidean space E
3
: and a set of discrete
volume data is given as a finite subset of Z
3
: A primitive of
scaffolding element in Z
3
are defined. The union of the
scaffold elements is based on the connectivity of the chain
structure. The chain structure is obtained by the vertex,
edge and face connection to generate the whole cooling
channel.
We can define and locate the solid volume with the
union of the scaffolding primitives. Let C be a Euclidean
cube within the subset Z
3
: Then, we define the scaffolding
elements of C as follows:
– Vertices (V
a
), a = 1, 2, 3 8 are labeled with (i, j, k)
where i, j, k are the three plane indices, and those
planes have at least one point in common;
– An edge (E

b
), b = 1, 2, 3 12 is drawn between two
vertices if the vertices’ labels have two planes in
common.
The Euclidean cube primitives within the subset Z
3
develop the shape of the cooling system. The whole
structure is defined by the corresponding attributes of
vertices and edges of the primitives. The position of the
cooling channel is tracked from the previous section of
scaffolding curve approximation process. The connectivity
of the scaffolding element primitives is based on the
Boolean operation. Figure 24 shows the union of two
consecutive scaffolding elements. The interior surfaces of
the scaffolding elements will form the cooling surface for
the proposed model.
4.3 Coolant flow through the scaffolding architecture
The scaffold cooling syst em is desig ned with a complete
coolant circulation which has an inlet, an outlet, and a
pumping system. The coolant inlet and outlet are connected
directly to the mould halves. Heat transfer during the
injection-moulding cycle includes heat exchange originated
from polymeric melt to the mould material by conduction.
Fig. 19 Flowchart of scaffold
cooling surface approximation
Fig. 20 Surface offsetting of mouse model (mould cavity)
Int J Adv Manuf Technol (2007) 34:496–515 503
Fig. 21 Schematic diagrams of
the injection mould half;
a Modeling of mould cavity

surface; b Cavity mould half
inserted with scaffolding ele-
ments inserted for uniform
cooling
Fig. 22 Graphical representa-
tion of a solid scaffolding
element
Fig. 23 Assembly of scaffolding elements
Fig. 24 Union of two consecu-
tive scaffolding elements;
a Before merging; b After
merging
504 Int J Adv Manuf Technol (2007) 34:496–515
The heat is then conducted from the mould material to the
coolant in the cooling passageway via the scaffold cooling
passageway. For the direction of coolant flow, a single
scaffolding element has six faces that provide one face as
the inlet and five faces as the outlet pathways for the
coolant flow. Figure 21b is an example of a cavity mould
half that is integrated with a scaffold cooling architecture.
When the coolant inlet and outlet are connected with a
high-pressure water pump and connector, a complete
coolant circuit is formed. The assembly of numerous
scaffolding element forms the conformal cooling surface
which generates a multiple orientation passageway. The
coolant flows from the inlet with high fluid pressure and
run into the scaffolding architecture passageway in the
cavity mould half. The coolant then brings the heat from
the polymer and flows away via the outlet. As the
scaffolding architecture follows the shape of the mould

cavity surface, it increases the contact area of heat transfer
from the poly meric melt and a near uniform cooling
performance can be achieved.
5 Results of scaffold cooling performance
The advent of computer-aided engineering (CAE) technol-
ogy for plastic injection moulding provides a large support
to injection mould design. Injection mould design simula-
tion modules allow precise determination of the effective-
ness of the mould cooling system at the desired mould
temperature, avoiding some mould defects, and finding the
desired injection moul ding cycle time. A variety of CAE
simulations are performed for the proposed scaffold cooling
system. Section 5.1 deals with the cooling performance
analysis. Section 5.2 considers the mecha nical properties of
the scaffold cooling design method for loadings during the
injection-moulding cycle. Section 5.3 discusses the thermal
management of the proposed method. Section 5.4 tests the
effect of dimensional stability from shrinkage analysis. The
CAE results illustrate the feasibility of the proposed
scaffold cooling approach as for rapid plastic injection
mould.
5.1 Cooling performance investigation of cooling channel
by meltflow analysis
Cooling performance analysis will find the temperature
distribution in a plastic injection mould during the
moulding process. Heat transfer will be analyzed between
the plastic, the mould material and the coolant within the
cooling system. An optimal cooling performance for
designing the cooling system can be identified. Moreover,
shrinkage and thermal stress analysis are conducted. In this

research, Moldflow Plastic Insight 3.1 [23] is used to
investigate the thermal effects of cooling channel design on
the injection mould. The set of analysis sequence in this
study is cool and flow. The parameters included injection
mould pressure, maximum temperature of part, thermal
stress, cooling time and volumetric shrinkage. The aims are
to create uniform cooling along the circu lar cooling channel
above and below the injection moulded part. Figure 25a
shows the mouse model for the meltflow analysis. It
consists of a thin shell with three buttons. Figure 25b
illustrates the modeling of the mould cavity and core mould
halves with scaffolding architecture. Figure 25c shows the
opening and closing of the mould.
Figure 26 shows the meltflow analysis workflow by
Moldflow Plastics Insight 3.1. The procedures can be
grouped into: the pre-processing step, the solver and the
post-processing step. Tables 2 and 3 tabulate the specifica-
tions and cooling parameters for meltflow analysis. Figure 27
compares geometric design of traditional and scaffold cool-
Fig. 25 CAD models for CAE analysis;, a Mouse model; b Cross
section of mould cavity and core, and c mould closing and opening
Int J Adv Manuf Technol (2007) 34:496–515 505
ing channels. The geometry of the scaffold cooling method
follows closer to the mouse model surface.
Many injection mould parts have complex three-dimen-
sional geometries. Figure 28a reveals that the moul d
temperature distribution of scaffold cooling method is more
uniform than the traditional cooling method (Fig. 28b). The
reason is that the scaffold cooling configuration closely
matches the shape of the part being moulded. Heat can

transfer more evenly from the mould surface. Figures 29
and 30 indicate that residual stress and volumetric shrink-
age accumulate near the corner of the mouse model under
scaffold cooling. Also, mould defect occurrence in the
traditional cooling method is higher than the s caffold
cooling method. In both cases, the mould defects are built
up on the surface and corner of the model. This is because
heat cannot be effectively transferred without any cooling
channel insertion. Uniform mould temperature distribution
cannot be obtained in the case of restricted cooling in
traditional cooling method.
5.2 Mechanical properties investigation
The mechanical performance of an injection mould is
important as it directly affects the durability of the injection
moulded part production. The mechanical strength of the
mould material has to withstand any force or load produced
from damaging or dislocation during the mould opening
and closing stages.
In injection mould production, material selection
depends on the experience of the mould engineers. It is
necessary to select a suitable mould material to prevent any
chemical deterioration during the moulding process and to
withstand the mechanical impact during the locking
process. Table 4 compares the mechanical properties of
some common mould steels in the industrial market.
During mould opening and closing in injection moulding,
the mould plates are loaded by the clamping force. Figure 31
indicates the stress distribution of a typical injection mould.
The pressure inside the cavity is considerably less than the
Fig. 26 Workflow of meltflow

analysis by MPI 3.1
Table 2 Specifications for meltflow analysis
Specification(s) Units
Materials (mould steels) P20 H13 A6
Mould temperature (°C) 55
Injection pressure (Nm
−2
) 190
Tolerance +/− of accuracy (mm) 0.01
Table 3 Cooling channel parameters
Cooling parameter(s) Descriptions
Cooling channel
diameter (mm)
8
Cooling pitch (mm) 16
Mould materials Tool steel (P20) Tool steel (H13)
Tool steel (A6)
506 Int J Adv Manuf Technol (2007) 34:496–515
Fig. 27 Geometries of two dif-
ferent cooling channel design;
a traditional cooling and b scaf-
fold cooling
Fig. 28 Performance of maxi-
mum mould temperature be-
tween a traditional cooling and
b scaffold cooling
Fig. 29 Performance of in-
cavity residual stress between
a traditional cooling and
b scaffold cooling

Fig. 30 The performance of
volumetric shrinkage between
traditional cooling (a) and
b scaffold cooling
Int J Adv Manuf Technol (2007) 34:496–515 507
injection pressure at the injection nozzle. The stress is
investigated by the following equations.
The maximum stress S
max
under load W is:
S
max
¼À
WL
4Z
ð3Þ
where Z is the section modulus in mm
3
.
Z ¼
1 Â d
2
6
ð4Þ
where the unit width is 1. S
max
must be equal to or less than
the critical fatigue stress developed by the steel mould
plate.
As the porous structure of the scaffolding element

provides less regular support than a solid volume, the
mechanical strength of the injection mould have to be high
enough to withstand the force and stress from moul d
opening, closing and locking. Here, mechanical CAE
software provides the insight to the non-linear dynamical
analysis. Thermal stress, temperature distribution and
mechanical strength are investigated to determine the
mould’s mechanical performance. FEA package of COM-
SOS/Works [24] is used as it integrates tightly with the
SolidWorks CAD software. Figure 32 shows the scaffold-
ing assembly to be tested.
Table 5 shows the results of CAE simulation to evaluate
the mechanical properties of different cooling methods. The
injection pressure in the simulation is set at 1 Nm
−2
. The
analysis results indicate that scaffolding architecture has a
higher residual stress value (2.216 Nm
−2
) than the solid
assembly structure (1.407 Nm
−2
). The injection pressure is
accumulated near the vertical columns or bone-like config-
uration as shown in Fig. 33 for the porous structure. The
protection of the mould cavity against injection pressure
highly depends on the mechanical strength of the mould
material and the arrangement of the bone-like configura-
tion. Scaffolding assembly can provide the extensive
mechanical properties for rapid plastic injection mould

with proper mould material selection.
Table 4 Material properties of typical mould materials
Material(s) Hardness
Rockwell (C)
Tensile strength
(N/mm
2
)
Thermal
conductivity
(W/m.K)
Thermal
expansion
(10
−6
/K)
Wear
resistance*
Compressive
stress*
Dimensional stability in
heat treatment*
Mould steel
(P20)
30–36 640 29 12.7 2 4 7
Mould steel
(H13)
50–52 1,170–1,950 24.6 12–13 6 7 8
Mould steel
(A2)

56–60 745 62.3 14.9 9 9 9
*Properties rankings on scale of 1 to 10 (10 = best)
Fig. 31 Stress distribution of the injection mould Fig. 32 Import of scaffolding assembly
508 Int J Adv Manuf Technol (2007) 34:496–515
5.3 Thermal management and heat transfer in an injection
mould
Within the duration of the injection mould cooling process,
a three-dimensional, cyclic, transient heat conduction and
convection problem on the cooling channel and mould
surfaces is involved. Figure 34 shows how coolant flows
through the cavity mould half with scaffold cooli ng system
configuration.
A cooling fluid or coolant such as water is pumped with
coolant flow rate V
fvia
the scaffold cooling configuration,
entering at tempe rature T
o
and leaving at temperature T
e
.
Assume that the coolant flow rate V
f
is maintained as a high
pumping pressure so as to facilitate high local heat transfer
from the solid surface into the coolant. The efficiency of
heat removal depends on the offset distance between the
scaffold cooling system and mould cavity surface. The heat
transfer to the coolant increases as either the offset distance
decreases or the surface to volume ratio increases by

increasing the amount of scaffolding elements. Figure 35
indicates the heat transfer within the mould and the cooling
channel.
The local heat-transfer coefficient at the surfa ce of the
scaffold element is h (Wm
−2
K
−1
). During the injection
moulding process, heat is removed from the mould surface
and through the injection mould plate with heat conductiv-
ity λ (Wm
−1
K
−1
). Then the heat is removed by the coolant
via conduction and forced convection.
The scaffold cooling configuration provides a network-
ing system that transfers heat from the hot surface into the
coolant. The performance of thermal management is based
on the heat transfer coefficient, h
eff
(Wm
−2
K
−1
), which is
related to the heat flux per unit area, q (Wm
−2
), from the

hot surface,
q ¼ h
eff
ΔT ð5Þ
where ΔT is the temperature change between the mould
surface and the coolant. The heat transfer coefficient, h
eff
,
for metal can be obtained from theoretical derivation and
experimental results.
The rate of convective heat transfer is the diffusion of
energy according to random molecular motion with energy
transfer due to bulk motion. Once the heat transfer
coefficient for a given geometry and the flow conditions
are known, the rate of heat transfer can be expressed by the
Newton’s law of cooling [25],
q
c
¼ h
c
AT
s
À T
f
ÀÁ
ð6Þ
where q
c
(W) is the rate of heat transferred from a surface at
uniform temperature T

s
, (K) to a fluid with temperature
T
f
, (K). A is the surface area (m), h
c
is the mean coefficient
of heat transfer (Wm
−2
K
−1
).
dq
c
¼ h
x
dA T
s
À T
f
ÀÁ
ð7Þ
Table 5 Comparison of mechanical properties between solid assembly and scaffolding assembly
Model Material Temperature
(K)
Pressure
(N/m
2
)
Restraints

(N/m
2
)
Residual stress
(N/m
2
)
Displacement
(m)e-014
Strain e-012
Solid assembly Steel H13 373 1 1 1.407 1.366 4.919
Scaffolding assembly Steel H13 373 1 1 2.216 9.969 2.912
Fig. 33 Comparison of thermal
stress with static finite element
analysis of mould material
(H13); a Scaffolding assembly,
and b solid assembly
Int J Adv Manuf Technol (2007) 34:496–515 509
Fig. 34 Cross-sectional views
of the cavity mould half with
scaffold cooling system config-
uration; a XZ-plane cutting, and
b YZ-plane cutting
Fig. 35 Direction of heat transfer from mould surface via scaffolding
architecture of cooling channel
Fig. 36 CAD model of scaffold cooling architecture for COSMOS/
FloWorks analysis
Fig. 37 3D-profile of coolant flow analyzed by COSMOS/FloWorks
Fig. 38 Flow trajectories of coolant analyzed by COSMOS/FloWorks
510 Int J Adv Manuf Technol (2007) 34:496–515

Fig. 40 a Strain and b defor-
mation of tool steel H13
Fig. 39 a Strain and b defor-
mation of tool steel P20
Fig. 41 a Strain and b defor-
mation of tool steel A6
Fig. 42 a Strain and b defor-
mation of tool steel P20 at solid
structure
Int J Adv Manuf Technol (2007) 34:496–515 511
where dq
c
is the rate of heat trans ferred from a differential
surface area dA, and h
x
represents the local coefficient of
heat transfer. While the mean and local coefficient of heat
transfer are related by Eq. (8).
h ¼
1
A
Z
A
h
x
dA
s
ð8Þ
5.4 The pressure distribution of coolant flow
within the scaffolding architecture

The scaffolding architecture provides a more uniform
cooling surface over the mould cavity surface, as every
location of the mould cavity surface and scaffold cooling
surface can experience an even rate of heat transfer.
However, the volum e of coolant flow increases within the
scaffolding structure as well as the pressure drop. In order
to investigate the effect of pressure drop of coolant within
the scaffold cooling configuration, computational fluid
dynamics (CFD) simulation software of COSMOS /Flo-
Works is used [26]. A CAD model of the cavity mould half
with internal scaffolding architecture, coolant inlet and
outlet is designed and analyzed by internal fluid flow
analysis. Figure 36 illustrates the CAD model of scaffold
cooling architecture for COSMOS/FloWorks analysis. By
setting the boundary conditions of inlet mass flow rate
80 kg/h, the results of pressure distribution can be obtained
and illustrated graphically. Figures 37 and 38 show the
pressure drop distribution along the scaffolding architec-
ture. From the results of the coolant flow from the inlet to
the outlet within the cavity mould half, the pressure reduces
constantly from 6.294e
12
Pa to 1.801e
12
Pa. It indicates that
the large contact area of the coolant flow will lead to a large
pressure drop of the coolant flow. The turbulent flow
cannot be maintained within the cavity mould half. To
maintain the cooling performance, it is necessary to modify
other cooling parameters in order to compensate effect of

the pressure drop.
5.5 Dimension stability of scaffold cooling channel design
Dimensional stability reflects the change in length of an
unrestrained film sample subjected to a specific elevated
temperature. It depends on the properties of plastic material
and tool steel. Units are reported as percentage change from
the original dimension. During the injection moulding
process, the increase in mould temperature from normal to
critical will cause physical change in the tool material. The
dimensional accuracy will decrease due to thermal expan-
sion of the tool materials. Figures 39, 40, 41, 42, 43 and 44
show the difference in strain and deformation of common
tool steels between solid and scaffold structures at injection
mould temperature from normal 328K to 483K.
Fig. 43 a Strain and b defor-
mation of tool steel H13 at solid
structure
Fig. 44 a Strain and b defor-
mation of tool steel A6 at solid
structure
512 Int J Adv Manuf Technol (2007) 34:496–515
From CAE results by COSMOS/Works 7.0 in Tables 6
and 7, thermal stress and thermal strain at different moul d
temperatures are close to zero in the tool materials (P20,
H13 and A6) being tested. The results indicate that the
variation in mould temperat ure from 328 to 483 K have no
significance in thermal stress and thermal strain. The
mechanical strength of these tool materials can withstand
the vast change in mould temperat ure during the plastic
injection moulding process. The scaffolding architecture of

injection mould can maintain the mechanical stability to
withstand the injection mould pressure and locking pressure
during mould opening and closing. The mould designed
can perform with stable dimensions and hence the cooling
performance. The uniformity of the injection mould cooling
can then be maintained with a high efficiency and part
quality can be ensured.
6 Discussion
Straight-drilled cooling channel and conformal cooling
channel (CCC) systems provide a cooling passageway for
heat to be carried away from the injection mould.
Contemporary CCCs are characterized by offering near-
uniform cooling with consistent heat transfer. Better cool-
ing can be achieved with less residual stress initialized
defect formation during injection moulding. Compared to
the existing cooling channel systems design, the scaffold
cooling system provides ev en better cooling for injection
moulding. The cooling channel matches further to the
cooling surface. Extra uniform cooling can be provided and
extended to region that is always ignored due to restrictions
in traditional machining process. For the proposed scaffold
cooling system, the cooling surface provides a more
uniform heat exchange system as the cooling covers the
whole cavity surface. The surfa ce area of the cooling region
is greater than the conventional straight-drilled and copper-
duct bending cooling systems. The heat can be uniformly
transferred by the coolant via the coolant circulation
system. However, compe nsation for optimal heat transfer
by adjusting different parameter settings is necessary.
Table 8 illustrates the influence of various cooling

parameters towards the injection mould cooling process.
The increase in cooling circuit distance may cause more
pressure drop in the coolant flow. On the contrary, the level
of extra uniform cooling has increased to a greater extent.
Besides, the pressure drop within the scaffolding passage-
way may not sustain a turbulent flow. Ashby et al. proposed
that an incre ase in heat transfer coefficient will increase the
pressure drop when fluid flows through cellular metal. He
suggested that a trade-off between the pressure drop and the
heat dissipation is necessary. The limitation can be improved
by increasing the power of coolant pumping device via the
coolant circulation system. The faster and continuous heat
extraction can restrain the effect of the non-turbulent flow.
Lower coolant temperat ure can be sustained, and more heat
can be carried away by the coolant. The injection moulded
defect formation such as hot spot or residual stress can also
be prevented. From the proposed scaffold cooling system
design, better cooling performance can be achieved by
deciding the most favorable cooling parameters for the
proposed design. An equilibrium position for effective
cooling is based on the overall parameter settings rather
than considering a single parameter only.
Table 6 Comparison of thermal stresses due to expansion for different tool steels
Tool material(s) Thermal Stress(es) (N/m
2
) P20 H13 A6 P20 H13 A6
Structure Porous Solid
Mould temperature at 328 K 2958.088 2.90E+08 2.91E+08 3.38E+07 3.38E+07 3.34E+07
Mould temperature at 483 K 2964.531 2.91E+08 2.91E+08 3.38E+07 3.38E+07 3.34E+07
Variation in stress(es) (%)* 0.002 0.003 0 0 0 0

*Variation in thermal stress ¼
Thermal stressat 483KÀThermal stress at 328K
Thermal stress at 328K
x100%
Table 7 Comparison of thermal strains due to expansion for different tool steels
Tool material(s) Thermal strain(s) P20 H13 A6 P20 H13 A6
Structure Porous Solid
Mould temperature at 328K 8.91E-09 0.00083 0.000785 0.000113 0.000107 0.000101
Mould temperature at 483K 8.91E-09 0.00083 0.000785 0.000113 0.000107 0.000101
Variation in strain(s) (%)** 0 0 0 0 0 0
ÃÃ
Variation in thermal strain ¼
Thermal strain at 483KÀThermal strain at 328K
Thermal strain at 328K
x100%
Int J Adv Manuf Technol (2007) 34:496–515 513
7 Conclusions
In this paper, a novel approach using uniform-sized
scaffolding architecture is proposed for conformal cooling
design. This method is aimed at providing a more uniform
cooling surface for the injection moulded part. With proper
selection of mould material, the scaffolding structure can
offer additional mechanical strength so as to withstand the
force and stress experienced during the injection moulding
cycle. A genuine uniform cooli ng, pressure drop perfor-
mance and thermal distribution can be optimized by CAE
and CFD analysis. In the simulation results, the cooling
performance indicates that the scaffold cooling technique
can offer a more uniform thermal distribution with minor
in-cavity residual stress occurrence than the conventional

method. The mechanical strength of common mould
material can withstand any force or load produced during
mould opening and closing. The scaffolding architectural
design can be performed with acceptable dimensional
stability. Uniform cooling performance can be obtained
without severe mould shrinkage. Injection mould defects
such as thermal stress or warpage can be avoided. This
increases productivity and he nce reduces the time to
market.
Acknowledgement The work described in this paper was supported
by a grant from the Hong Kong Polytechnic University.
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−ve Indirect
Coolant flow rate Coolant flow rate determines the amount of heat energy being carried out within a period
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and laminar flow
+ve Indirect
Coolant temperature Coolant temperature at lower or higher degree can provide various heat capacities
for the heat transfer
−ve Direct
Contact area The larger the contact area of the cooling passageway for heat transfer, the greater
the region to achieve uniform cooling
+ve Direct
Thermal conductivity of
mould material
Thermal conductivity is the quantity of heat, Q, transmitted through a thickness L,
in a direction normal to a surface of area A, due to a temperature gradient
+ve Direct
Coolant selection Different coolants will have different thermal conductivities and specific heat capacities +ve Direct
Coolant circulation
within the injection mould
The longer the coolant circulation distance, the less effective the heat dissipation will be
resulted
−ve Indirect
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