Tài liệu Design and optimisation of conformal cooling channels in injection muolding tools docx

An optimum shape of a 3D CAD model of a typical component suitable for injection moulding was designed and the core and cavity tooling required to mould the part then generated.. Analysi

Design and optimisation of conformal cooling channels in

injection moulding tools D.E Dimlaa,∗, M Camilottob, F Mianib

aSchool of Design, Engineering and Computing, Bournemouth University, 12 Christchurch Road, Bournemouth, Dorset BH13NA, UK

bDIEGM, Universit`a Degli Studi di Udine, via delle Scienze 208, 33100 Udine, Italy

Abstract

With increasingly short life span on consumer electronic products such as mobile phones becoming more fashionable, injection moulding remains the most popular method for producing the associated plastic parts The process requires a molten polymer being injected into a cavity inside a mould, which is cooed and the part ejected The main phases in an injection moulding process therefore involve filling, cooling and ejection The cost-efficiency of the process is dependent on the time spent in the moulding cycle Correspondingly, the cooling phase is the most significant step amongst the three, it determines the rate at which the parts are produced The main objective of this study was to determine an optimum and efficient design for conformal cooling/heating channels in the configuration of an injection moulding tool using FEA and thermal heat transfer analysis An optimum shape of a 3D CAD model of a typical component suitable for injection moulding was designed and the core and cavity tooling required to mould the part then generated These halves were used in the FEA and thermal analyses, first determining the best location for the gate and later the cooling channels These two factors contribute the most in the cycle time and

if there is to be a significant reduction in the cycle time, then these factors have to be optimised and minimised Analysis of virtual models showed that those with conformal cooling channels predicted a significantly reduced cycle time as well as marked improvement in the general quality of the surface finish when compared to a conventionally cooled mould

© 2005 Elsevier B.V All rights reserved

Keywords: Tool design optimisation; Injection moulding

1 Introduction

Injection moulding is one of the most exploited industrial

processes in the production of plastic parts Its success

re-lies on the high capability to produce 3D shapes at higher

rates than, for example, blow moulding The basic principle

of injection moulding is that a solid polymer is molten and

injected into a cavity inside a mould; which is then cooled

and the part ejected from the machine The main phases in an

injection moulding process therefore involve filling, cooling

and ejection The cost-efficiency of the process is dependent

on the time spent in the moulding cycle Correspondingly,

the cooling phase is the most significant step amongst the

three, it determines the rate at which the parts are produced

As in most modern industries, time and costs are strongly

∗Corresponding author.

E-mail address: dimla@bournemouth.ac.uk (D.E Dimla).

linked The longer is the time to produce parts the more are the costs A reduction in the time spent on cooling the part before its is ejected would drastically increase the production rate, hence reduce costs It is therefore important to under-stand and thereby optimise the heat transfer processes within

a typical moulding process efficiently Historically, this has been achieved by creating several straight holes inside the mould (core and cavity) and forcing a cooler liquid to circu-late and conduct the excess heat away so the part can be easily ejected The methods used for producing these holes rely on the conventional machining process such as drilling However this simple technology can only create straight holes and so the main problem is the incapability of producing compli-cated contour-like channels or anything vaguely in 3D space

An alternative method that provides a cooling system that

‘conforms’ to the shape of the part in the core, cavity or both has been proposed This method utilises a contour-like channel, constructed as close as possible to the surface of the 0924-0136/$ – see front matter © 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.jmatprotec.2005.02.162

Specific software was used to optimise the design and

con-struction of the mould, with attention on refining the tool

de-sign through application of finite element and thermal flow

analyses Successively, a study on the effectiveness of the

conformal cooling channel based on virtual models was

per-formed using I-DEASTMsoftware for prototyping and

simu-lation The study is on going and hopefully would culminate

in the suggestion of the level of proficiency required using

virtual models in deciding moulding specifications for

pro-duction parts

2 Brief overview of the injection moulding process

The injection moulding industry, like all industries, at

present needs to reduce costs to remain competitive This

need has been addressed using various technologies

rang-ing from design software to computer numerical control

ma-chinery After these technologies are in place and moulding

begins the cost is usually based on cycle time Adjustments

can be made to the moulding machine to help reduce the

time to mould but in the final analysis the time is dictated

by the ability of the mould to carry the heat away from the

molten polymer Liquid is passed through cooling channels

in the mould at the required temperature This must allow the

molten polymer to flow into all sections of the cavity while at

the same time remove the heat as quickly as possible Up to

now these channels have been produced by drilling which can

only produce straight lines If the channels carrying the water

could be conformed to the shape of the part and their

cross-section changed to increase the heat conducting area then a

more efficient means of heat removal could be realised This

may also help to reduce warpage when the part is ejected, as

the plastic would be cooled more uniformly

2.1 Temperature control

Temperatures such as those for the molten polymer, the

mould, the surround temperature and the clamping system

temperature need to be controlled (Fig 1) When molten

plas-tic is injected in the mould it must be solidified to form the

object The mould temperature is regulated by circulation of

a liquid cooler, usually water or oil that flows inside channels

inside the mould parts When the part is sufficiently cooled it

can be ejected Most (95%) of the shrinkage happens in the

mould and it is compensated by the incoming material; the

remainder of the shrinkage takes place sometime following

the production of the part[1]

Fig 1 Temperature history during injection moulding [2].

2.2 Pressure control

Both the injection unit and the clamping system require pressure with the latter developed to resist the former (Fig 2) Three different pressures can be distinguished in the injec-tion unit: initial, hold and back All these are obtained by the action of a screw In the clamping unit the oil pump of the hydraulic system controls the pressure needed to move the mould Holding pressure is required to finish the filling operation and maintained during solidification to supply the shrinkage

2.3 Time control

Time is the most significant parameter in the entire opera-tion Cost and machine efficiency can be estimated from the cycle time The principle temporal aspects to be controlled include: gate-to-gate time, injection time and cooling time A simple schematic illustration of a typical cycle time is shown

inFig 3

2.4 Thermal proprieties

Despite their large diffusion, for all plastic materials tem-perature range is a limit to their purpose Both high and low temperature can create damage to plastic components It is important to study thermal proprieties to understand and pre-dict this behaviour Therefore cooling times in moulding

ma-Fig 2 Pressure history during injection moulding [2].

Fig 3 Cycle time in injection moulding [2].

chines must be set carefully to permit, first, plasticization of

the thickness and secondly dissipation of melting heat Unlike

metals, the thermal capacity of plastics is high with crystalline

plastics having a higher capacity than non-crystalline

Plas-tics have a large coefficient of thermal expansion if compared,

for example, with metals A way to modify these values is to

use mineral fillers such as fibre glass

2.5 Cooling channels

As with most manufacturing fields, production time and

costs (lead and lag) are strongly correlated The longer it takes

to produce parts the more are the costs, and with injection

moulding production industries cooling time is often taken

as the indicator of cycle time Improving cooling systems will

reduce production costs A simple way to control

tempera-ture and heat interchange is to create several channels inside

the mould where a cooler liquid is forced to circulate

Con-ventional machining like CNC drilling can be used to make

straight channels Herein, the main problem is the

impossi-bility of producing complicated channels in three-dimension,

especially close to the wall of the mould This produces an

inefficient cooling system because the heat cannot be taken

away uniformly from the mould and the different shrinkage

causes warpage and cooling time increase (Fig 4) On the

other hand, if the cooling channels can be made to conform

to the shape of the part as much as possible (Fig 5), then the

cooling system the cycle time can be significantly reduced

with cooling taking place uniformly in all zones

Further-Fig 4 Cavity (A) and core (B) of a drilled channels mould [3].

Fig 5 Same mould as Fig 4 with conformal channels [3].

more, if the part is ejected with the same temperature in every point the subsequent shrinkage outside the mould is also uni-form and this avoids post-injection warpage of parts Another advantage is that a mould equipped with conformal channels reaches the operation temperature quicker than a normal one equipped with standard (or drilled) cooling channels[3,4]

In this way one can reduce the time required when the moulding machine is started When the polymer is injected,

it solidifies immediately touching the wall of the mould If the volume of the part is sufficiently big and its thickness is too small, polymer solidified can obstruct the flow and hinder

a complete filling of the cavity In this case the mould must be heated to a particular temperature in order to permit the poly-mer to flow Despite all these advantages it may be noticed that new technologies involved in the production of moulding tools with conformal channels can increase initial costs for the additional complexity of the construction process

3 Conformal channels—an overview

Results from an investigation of the effectiveness of con-formal channels by Ring et al.[5]through the construction

of three different moulds with and without conformal cool-ing, showed that the latter technique led to significant im-provements and a general reduction of the cycle time while ameliorating heat transfer

A contribution to understanding the importance of confor-mal channels and the employment of new high-conductivity materials is given by Jacobs[6] This research showed that using nickel/copper moulds with conformal channels (cop-per layered) led to productivity improvements of about 70% when compared to a similar mould made with conventional steel with drilled cooling channels A comparison between conformal channels and drilled cooling channels has also been conducted by Sachs et al.[3] They based their investiga-tion on modelling the core and cavity coupled with software using both techniques and proceeded to construct the moulds

to compare theory and experimental data Subsequent anal-ysis shows that the conformal channel mould reaches opera-tional temperature faster than the convenopera-tional one, attaining

a more uniform temperature distribution with efficient heat transfer capacity

A method of controlling the moulding temperature sug-gested by Bayer[7]presents a means of finding the right

posi-gradient on the surface could be predicted illustrating that

de-sign sensitive analysis is a natural way to obtain an optimum

mould design, especially on sizing and positioning of

cool-ing channels With the same instrument, optimal processcool-ing

conditions in the cooling operation was found, minimising

functions linked to process quality and productivity

The problem of cooling channels disposition in not only to

find a way to construct them, but also to look for a method to

apply this technology in all kinds of cavity shapes A solution

to overcome this issue is proposed by Xu et al.[9] The initial

object is divided in small zones easy to be analysed and for

each of these a cooling channel system is constructed Then,

all the information is used to build the final tool A

simi-lar approach for solving cooling problem is proposed by Li

[10], who suggests a feature-based method where complex

moulds are divided into simple shapes through a recognition

algorithm Then for each shape, a specific cooling system is

constructed and at the end all of these are assembled The

algorithm is based on the “superquadrics”, a family of

para-metrical shapes capable of modelling features, such as those

used in computer graphic The main problem in this method

is selecting the best superquadric in order to approximate the

whole part Once this is done, the cooling system becomes

easy to be modelled This approach is very useful when there

are complex parts to create

4 Mould part adviser (MPA) analysis

The basic idea was to construct a virtual model using

Model Master in I-DEASTMand then use its Moldflow

anal-ysis option to find the best position for the runner Then a

cooling system was designed for the part Successively the

model was ready for further analysis such as finite element

analysis to refine the design, etc MPA is a tool used

ex-clusively on the virtual solid model of the object, to help the

designer to determine the manufacturability of the parts of the

mould The only requirement of the software is the choice of

the material from which the object is intended to be made

from

4.1 The model

The geometry of the model used in this exercise was

cho-sen according to specifications and characteristics required

in the object such as the inclusion of a draft angle to permit

the part to be ejected easily once cooled To create this model

(Fig 6) a rectangular surface was created and than extruded,

Fig 6 3D solid view of the model.

with a draft angle of 8◦ A rib was joined to the part in the

in-ternal cavity to increase the mechanical resistance and avoid the possibility of deformation

4.2 Gate location

The best position of the gate is found by trial and error and different positions for the gate are suggested and visualise via inspection of the model part in terms of quality, weld line presence, air traps and sink (Fig 7shows a typical scenario with weld lines) With the gate placed in the centre of the bottom surface of either of these two solutions are possible: internal position and external position From flow analysis it emerges that the two times for the total cycle are of the same order, but with the gate positioned on the external surface the weld lines are lower than in the internal position, especially

on the external surface The criteria that can be adopted for the choice of the position can opt for the quality surface or the production time The gate in the external position involves more time spent in cutting the eventual track of polymer, which forms on the gate part area when this is ejected, and this can be unacceptable for production purposes Placing the runner inside the cavity can lead to problems in creating the cooling system, as there is not much space so it can be difficult to place complex shape channels and runners

An increase in the number of gates not only does not lead to

an improvement in both an improvement in cooling time and quality, but obliges the creation of a complex shape runner

So the solution of one gate runner is preferred

Fig 7 Weld lines on the model surface.

Fig 8 Reduced weld lines on the external surface.

Fig 9 Single gate position: quality prediction.

The following observations were made from the optimised

solution of the part through computer prediction:

• Significantly reduced weld lines on both the external and

internal surfaces (Fig 8) compared toFig 7

• Improved quality prediction (Fig 9)

• Further check on quality, cooling and sink marks was

con-ducted Results indicated that the position of the gate was

optimum as no visible marks were evidenced and the

in-jection time reasonable at about 1 s (Fig 10)

Fig 10 Results form from Moldflow window analysis.

Fig 11 Surface temperature.

Fig 12 Freezing time.

• Similarly, good prediction rates were achieved for both the surface temperature and freezing time (Figs 11 and 12) Fig 10essentially shows that the condition chosen from the optimisation lead to a better product as the process falls within the zone that indicates a good possibility of creating the object without problems (green area) In the same form the injection time is estimated to be 1.18 s

5 Cooling channel positioning

5.1 General considerations

The final aspect of the object and the precision of its shape are determined not only by the process condition, but also by the temperature of the wall of the cavity[11] An accurate positioning of the cooling channel system is thus needed to satisfy quality standards of the production, as the tempera-ture of the mould must be kept high in order to permit the crystallisation of the material The problem on positioning channels is to assure a uniform and equal temperature in both core and cavity If there is a strong gradient in the cavity

be-Fig 13 Effect of the temperature gradient of the wall surface on the

non-reinforced plastics [11].

tween the two halves the part may warp and distort its shape

(Fig 13)

So the targets that a correct cooling system has to

fol-low are the uniformity of the wall temperature and a gradual

reduction of the polymer temperature, in order to find a

com-promise between the necessity of reducing cycle time and

allowing for the crystallisation

5.2 Temperature behaviour

During the production cycle the temperature of the mould

follows a periodic fluctuation (Fig 14), due to several

fac-tors such as the properties of the material of the mould and

the polymer The cooling system is not able to control the

amplitude of these fluctuation, but what is important is the

maximum pick of temperature, reached when the flow of hot

polymer arrives and touches the inside of the cavity[11] To

keep the temperature uniform some physical effects must be

Fig 14 Fluctuation of temperature of the wall inside the cavity during the

moulding cycle [11].

ter of the cross-section (or the cross-section area if not circu-lar), the distance between channels and the distance between channel and wall of the mould The main problems that arise when choosing these dimensions concerns the pressure losses derived from the choice of the diameter and the design of the channel A heating/cooling relationship reported in Zollner [11]gives a guideline on the channels positioning This states that the value resulting from the solution of the relationship should stay between 2.5 and 5% for semi-crystalline thermo-plastics and between 5 and 10% for amorphous thermoplas-tics

5.3 Cavity channels positioning

Different solutions for the core and the cavity cooling sys-tem were suggested for this analysis, consisting of a confor-mal cooling system (Fig 15) and a straight drilled cooling system (Fig 16) for comparison Because these parts had to

be analysed after with a FEM package, only a quarter of each insert was analysed A system of four channels was created following the surface of the object, with three channels placed

to cool the lateral surface and one to cool the bottom one

5.4 Core channels positioning

The conformal channels system for the core consisted

of two channels that followed the geometry of the shape (Fig 17) with one channel cooling the upper and the short side surfaces and the second one cooling the big side surface All corners of the channels were filleted to decrease fluid-dynamic losses of the liquid cooler In the straight channel solution one cooling line was created (Fig 18) requiring three

Fig 15 Conformal channel proposition for the cavity.

Fig 16 Straight channel solution for the cavity.

Fig 17 Conformal cooling channel solution for the core.

drilling operations In this case, it was impossible to fillet the

corners, so losses of the liquid cooler are greater than in the

previous case The next step is the finite element analysis to

check if parts can resist against injection pressure

Fig 18 Straight channels solution for the core.

This work is currently ongoing and the analysis will con-centrate on the mechanical resistance of the bottom surface, where there is the maximum amount of pressure during the injection operation

6 Conclusion

A design and optimisation of conformal channels in cool-ing an injection-moulded component has been conducted us-ing virtual prototypes The method pursued involved con-structing a 3D CAD model of the object, from which the core and cavity of the tool was created The ensuing simulations showed that it was possible to optimise and predict the best location for such channels to reduce the cooling times when compared to straight-drilled channels The study is on-going and hopefully would culminate in the suggestion of a level of proficiency required using virtual models in deciding mould-ing specifications for production parts

Further work is required to test core and cavity samples using FEA to check the mechanical resistance to the injection pressure and eventually modify the thickness of the mould Some meshing of the object, reducing it to a surface model to use planar elements is required and this should lead to a bet-ter understanding of the cooling times between conformally cooled tools and conventional ones

References

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[2] Anon., Intelligent Systems Laboratory, Michigan State Univer-sity, 1999 [accessed October 30, 2003] http://islnotes.cps.msu.edu/ trp/inj/inj time.html.

[3] E Sachs, et al., Production of injection molding with conformal cool-ing channels uscool-ing the three dimensional printcool-ing process, Polym Eng Sci 40 (5) (2000) 1232–1247.

[4] K.W Delgarno, Layer manufactured production tooling incorporat-ing conformal heatincorporat-ing channels for transfer mouldincorporat-ing of elastomer compounds, Plastic Rubber Compos 30 (8) (2001) 384–388 [5] M Ring, et al., An investigation of effectiveness of conformal cool-ing channels and selective laser sintercool-ing material in injection mould-ing tools, RPD (2002) 1–5.

[6] F Jacobs, High-conductivity Materials and Conformal Cooling Channels, Warwick Manufacturing Group, Warwick University, UK [accessed September 29, 2003] http://www.nasatech.com/NEWS/ rpd399.xpress.html.

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[9] X Xu, et al., The design of conformal cooling channels in injection molding tooling, Polym Eng Sci 41 (7) (2001) 1265–1279 [10] C.L Li, A feature-based approach to injection mould cooling system design, Comput.-Aided Des 33 (2000) 1073–1090.

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