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CONFORMAL COOLING FOR PLASTICS INJECTION MOULDING Ing. Branislav DULEBA, prof. Ing. Frantiek GREKOVI, CSc.

Department of Technologies and Materials, Faculty of Mechanical Engineering, Technical University of Koice, Letn 9, 042 00 Koice, tel. 055/602 4220, e-mail: branislav.duleba@tuke.sk

ABSTRACT This paper deals about the possibilities of cooling of injection moulded parts and its latest trend conformal cooling

manufactured by technology of direct metal laser sintering. The cooling phase starts immediately after the injection of material and includes both the injection and holding pressure. However, the cooling time must be extended beyond the holding pressure phase, as the moulding normally has not yet cooled down sufficiently and is not stable enough for demanding. Extension of cooling phase can improve the quality of manufactured part, but recede the economy of production. Conformal cooling shortens cycle times, improves plastic part quality, and above all cost reductions.

Keywords: mold cooling, conformal cooling, direct tool, Direct Metal Laser Sintering, DMLS

1. INTRODUCTION

Injection moulding is a broadly used manufacturing process in the fabrication of plastic parts. The main 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 is ejected from the machine. And so the main phases in an injection moulding process contain filling, cooling and ejection. The cost-effectiveness of the process is mainly reliant on the period spent on the moulding cycle in which the cooling stage is the most significant phase. Period spent on cooling cycle limits the rate at which parts are produced.

2. COOLING OF INJECTION MOULDED PARTS

Since, in most modern industries, time and costs are strongly connected, the longer is the time to produce parts the more are the costs. A reduction in the time spent on cooling the part would radically increase the production rate as well as reduce costs. So it is important to understand and improve the heat transfer process in a typical moulding process. The rate of the heat exchange between the injected plastic and the mould is a key factor in the economic performance of an injection mould. Heat has to be taken away from the plastic material until a stable state has been reached, which permits demolding. The time needed to accomplish this is called cooling time or freezing time of the part. Proper design of cooling system is necessary for optimum heat transfer process between the melted plastic material and the mould.

Optimal properties of engineering plastics can be achieved only when the right mold temperature is set and maintained during processing. The mold temperature has a substantial effect on:

Mechanical properties, Shrinkage behaviour, Warpage, Surface quality, Cycle time, Flow length in thin walled parts.

In particular semi-crystalline thermoplastics need to cool down at optimal crystallization rate. Parts with widely varying wall thicknesses are likely to deform because of local differences in the degree of

crystallization. Additionally the required cooling time increases rapidly with wall thickness. Theoretically, cooling time is proportional to the square of the heaviest part wall thickness or the power of 1.6 for the largest runner diameter. In other words, doubling the wall thickness quadruples the cooling time.

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where the Thermal diffusivity of polymer melt is defined as:

*,-.//0123 =(*,-4.0423)5.6

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Traditionally, cooling process has been realized by creating several straight holes inside the mould core and cavity and then forcing a cooling fluid (i.e.water) to circulate and conduct the excess heat away from the molten plastic. The methods used for producing these holes rely on the conventional machining process such as straight drilling, which is incapable of producing complicated contour-like channels or anything vaguely in 3D space. The distance from cavity to cooling channel differs as only straight line drilling channels are possible, shown on Fig. 1 and Fig. 2 and as a consequence the heat dissipation cannot take place uniformly in the material. This results in:

Uneven temperature levels on the cavity surface. Uneven cooling-down processes resulting in

internal stresses and thus negative impact on part quality (warpage).

Actively influencing cooling-down processes inside the melt can often not be achieved.

On top clogging of dead drilling ends creates areas with zero flow velocity thus facilitating dirt agglomeration. The drilling procedure itself is not without certain risks: in case of deep drilling there is always a danger to hit ejector holes (wandering drill), or the drill can even break. As a consequence, the whole mould insert could get unusable.

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Fig. 1 Conventional heating/cooling channels

Fig. 2 Conventional tooling mould temperature control

3. CONFORMAL COOLING

An alternate method of cooling system that conforms or fits to the contour of the cavity and core of the mould can provide better heat transfer in injection process, is called conformal coolingconcept behind conformal cooling is that all heating/cooling lines in the tool follow the contour of the part, maintaining an optimal distance from the part surface (shown on Fig. 4) in order to maintain a constant temperature by channels of different cross

Fig. 3 Conformal heating/cooling channels with DMLS

Conformal cooling is defined as the ability to make cooling/heating configurations within a tool that essentially follows the contour of the tool surface or deviates from that contour as thin/thick sections of the part

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ng/cooling channels

Conventional tooling mould temperature control

An alternate method of cooling system that conforms or fits to the contour of the cavity and core of the mould can provide better heat transfer in injection moulding process, is called conformal cooling shown on Fig.3. The concept behind conformal cooling is that all heating/cooling lines in the tool follow the contour of the part, maintaining an optimal distance from the part surface

r to maintain a constant els of different cross-section.

Conformal heating/cooling channels with DMLS

Conformal cooling is defined as the ability to make cooling/heating configurations within a tool that essentially follows the contour of the tool surface or deviates from that contour as thin/thick sections of the part

may dictate for optimal thermal managobjective typically is to cool or heat the part uniformly. This is especially useful in tight areas on a part, such as in ribs or clips.

Fig. 4 Conformal cooling with DMLS

Design recommendations for the layout of heating/cooling channels with DMLS are the same as the ones given for conventional designed channels: they are both based on the plastic recrystallization and heat conductivity theories, with the necessary adaptations to conformal mould temperature control systems on the one hand and the advantages of DMLS on the other hand (for example the possibility to change the cross section along the channel path). The channel diameter should be chosen depending on the distance between the heating/cooling channel and the cavity. Optimal desigand distances between holes/cavity is shown on Fig.

Fig. 5 Optimal design of a three dimensional channel system

Tab.1Dimensions of o

Wall thickness of molded part

(in mm)

Hole diameter (in

mm)

b

0 - 2 4 8 2 4 8 - 12 4 - 6 12 - 14

2

may dictate for optimal thermal management. The objective typically is to cool or heat the part uniformly. This is especially useful in tight areas on a part, such as in

Conformal cooling with DMLS

Design recommendations for the layout of with DMLS are the same as the

ones given for conventional designed channels: they are both based on the plastic recrystallization and heat conductivity theories, with the necessary adaptations to conformal mould temperature control systems on the one

and the advantages of DMLS on the other hand (for example the possibility to change the cross section along the channel path). The channel diameter should be chosen depending on the distance between the heating/cooling channel and the cavity. Optimal design of hole diameter and distances between holes/cavity is shown on Fig.5.

Optimal design of a three dimensional channel system

Dimensions of optimal channel system

diameter (in Centerline distance

between holes

Distance between center of holes and

cavity

a c

2 - 3 x b 1.5 - 2 x b 2 - 3 x b 1.5 - 2 x b 2 - 3 x b 1.5 - 2 x b

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4. BENEFITS OF COMFORMAL COOLING

In terms of tool geometry Direction-finding options for cooling passages are

almost infinite. This makes it possible to create an ideal cooling channel in a well-defined distance to the cavity. A conventional drilled cooling mechanism cannot achieve this (Fig. 1).

Cooling channel cross sections can take almost any shape (e. g. oval vs. round) - (Figure 6). Turbulence of the coolant (the desired high Reynolds number) within the system can thus be controlled by actively choosing different cross sections and by switching between different cross sections. As a consequence, turbulence inside of the coolant is generated, close to cavity along the whole path of the channels. A bended path should improve in most of the cases this effect.

Fig. 6 Shape of cooling channel cross sections

Changing cross sections or forking the cooling channel can easily be done without splitting up the form. This allows for additional heat/cooling advantages in areas that cannot be reached by conventional methods.

Quality in the process of injection moulding A more effective mould temperature control system

saves time and costs in the process of injection moulding.

Quality of injection moulded parts is improved by better control of the injection moulding process. Warping and sink marks are minimized by evenly cooling out the plastic melt thus minimizing internal stress. Scrap rates are reduced or eliminated. Avoiding internal stresses helps to produce better parts with the same amount of required material. Certain geometries are only possible to manufacture at required quality standards with conformal cooling. The Fig. 7 describes the cooling process of two different approaches of cooling- conformal and traditional. After 10 cycles by 15 seconds the difference between these hot spots is more than 17oC. Difference of hot spot temperatures of mold at same conditions is shown on Fig. 8 and is almost 23oC.

Even combined systems with separated cooling and heating channels are possible or the split between main systems (for the control of the global temperature) and specific systems (for the handling of close to cavity critical temperatures) can be performed with DMLS.

Fig. 7 Simulation of cooling of plastic part, cycles no. 10 @ t=15s

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Fig. 8 Simulation of cooling of mould, cycles no. 10 @ t=15s

Part of the mould with integrated cooling system is shown on Fig. 9 and the conformal cooling system of the same part is shown on Fig. 10.

Fig. 9 Part of the mould with integrated cooling system

Fig. 10 Shown conformal cooling channels in the mould

5. MANUFACTURING PROCESS OF DIRECT TOOL

Direct Metal Laser Sintering (DMLS) is well known as a leading technology for tool making, an application known as Direct Tool. Direct Tool is best known for plastic injection moulding. However, the technology is also used for other tooling types including blow molding, extrusion, die casting, sheet metal forming. Tool inserts are built overnight or even in just a few hours. Also the freedom of design can be used to optimize tool performance by integrating conformal cooling channels into the tool.

Direct Metal Laser Sintering (DMLS) was developed jointly by Rapid Product Innovations (RPI) and EOS GmbH, starting in 1994, as the first commercial rapid prototyping method to produce metal parts in a single process. In this process shown on Fig.11, powdered metal free of binder or fluxing agent is sintered by the scanning of a high power laser beam at 20 or 40 micron layers, which are traced in the X and Y-axes before the build tray lowers. The recoater arm then sweeps over a new layer of powder, allowing a new layer to be sintered on the already built layer. Though layers are sintered, support structures are required, an element also found in the SLA process. Parts may require a variety of post processes, including heat treating, support removal, shot peening and more.

Fig.11 Schematic picture of the instrument used for laser sintering

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There are many metal powders available for Direct Metal Laser Sintering and new powders are in development. A high grade EOS Maraging Steel MS1 (1.2709) is best-suited for production tooling. The material is melted in the machine to produce fully dense parts with a hardness of 36-39 HRC. Parts are easily post-hardened to 53-55 HRC and produce an ultimate strenght of more than 1 900 MPa. Components can be later machined, eroded and polished same way as a conventional tool steels.

6. CONCLUSIONS

The aim of this paper was to describe capabilities and performance of conformal cooling and its manufacturing by direct metal laser sintering. Injection moulding is the most common method of part manufacturing. It is ideal for producing high volumes of the same object. The standard method of cooling is passing a coolant (usually water) through a series of holes drilled through the mould plates and connected by hoses to form a continuous pathway. The coolant absorbs heat from the mould (which has absorbed heat from the hot plastic) and keeps the mould at a proper temperature to solidify the plastic at the most efficient rate. Cooling phase of injection moulding is one of the most important phases for quality and preciseness of moulded part. Its often also the one of the most time consuming phase. So improvement of cooling process will raise the quality of moulded part and also improves the economy of production.

Conformal cooling has many advantages. In general, low-volume manufacturers get their greatest savings from the reduced tooling costs of DMLS. Midrange volumessay, up to a million partsincur savings from shortened cycle times that increase productivity. High-end producers of plastic parts (in the millions) also reap the rewards of faster cooling times, of course, but at these quantities, the elimination of scrap caused by uneven temperature distribution becomes an important source of savings as well.

ACKNOWLEDGMENTS

This paper is the result of the project implementation: Technological and design aspects of extrusion and injection moulding of thermoplastic polymer composites and nanocomposites (PIRSES-GA-2010-269177) supported by The international project realized in range of Seventh Frame Programme of European Union (FP7), Marie Curie Actions, PEOPLE, International Research Staff Exchange Scheme (IRSES).

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