Utilization of Direct Metal Laser Sintering in Injection Mold Design

JAN NAVRATIL, MICHAL STANEK, STEPAN SANDA, MIROSLAV MANAS,

DAVID MANAS, ALES MIZERA, MARTIN BEDNARIK

Tomas Bata University in Zlin

nam. T. G. Masaryka 5555, 760 01 Zlin

CZECH REPUBLIC

j1navratil@ft.utb.cz http://www.ft.utb.cz

Abstract: - The aim of this research paper is to design two variants of cooling systems (cavities) for an injection

mold. Both systems are designed to be used on the same injection mold for producing the same product. The

difference is in the manufacturing technology itself. The first variant was made by conventional methods of

machining and the second variant was made by combination of conventional methods and unconventional rapid

prototyping technology Direct Metal Laser Sintering (DMLS). It was dealt with design of the universal

injection mold frame and both cooling systems for an existing product in the first part of this research paper. In

the second part were both designs compared by mechanical analysis and their influence on final product by

flow analysis. Last part is focused on economical evaluation of both designs.

Key-Words: - Rapid Prototyping, Direct Metal Laser Sintering, Injection Molding, Analysis, Mold, Part

1 Introduction Rapid prototyping belongs among modern

manufacturing technologies, where the resulting

product is made by adding material layer-by-layer,

unlike subtracting technologies (milling and

drilling), where the resulting product is made by

removing material [1-3, 13-19].

Fig.1 Manufacturing technologies

There are several rapid prototyping technologies

based on the adding material, the main differences

are in a used material and product building

technologies. Between this technologies belong, for

example; stereolithography (SL), laminated object

manufacturing (LOM), fused deposition modeling

(FDM), selective laser sintering (SLS) and direct

metal laser sintering (DMLS) [4-15]. All these

technologies are based on creation of real product

directly from 3D CAD data in a few hours; this

results in speeding up process planning and tooling

design, because of possibility to provide a real

product at an earlier design stage [1, 4, 5, 21].

Laser sintering is one of the leading commercial

processes for rapid fabrication of functional

prototypes and tools. The process creates solid

three-dimensional objects by bonding powdered

materials using laser energy [6, 23, 24].

Direct metal laser sintering (DMLS) allows

creating fully functional metal part without using

any tools and without any shape restrictions. The

parts produced by this technology have mechanical

properties fully comparable with cast or machined

parts. Benefits of this technology increases with

shape complexity; that means the more complex

part, the more economical the process becomes [9-

11, 17-23].

Principle of this technology is based on melting

very thin layers of metal powder. The process

begins by applying first layer onto a steel platform

and melting required contour. Then is another layer

applied and process continuous until the whole part

is made. Minimal thickness of each layer is 20 µm.

It is also necessary to use supporting structure,

which is applied simultaneously with base material.

The supports are necessary because the powder

itself is not sufficient enough to hold in place the

liquid phase created by melting required contour

[11, 16-22].

Once the part is created then it goes through

some finishing operations including support

removal, shot peening and polishing. Between

advantages belong besides shape complexity and

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cost and time savings also possibility of recyclation

of about 98% of not used powder. It also has some

disadvantages such as limited working space, high

acquisition costs and high porosity [11-12, 25].

2 Design Child seat was chosen as a sample of the injected

part. This product was chosen because it is

complicated enough to use Direct Metal Laser

Sintering technology. Its basic dimensions are

359*399*374 mm (height*width*depth).

Fig.2 Injected part

2.1 Material Polypropylene was chosen as a product material due

to its advantageous combination of processing

properties, high stiffness, low price and availability.

For analysis purposes was an actual polypropylene

chosen, its trademark is Daplen BH 345 MO and the

supplier is Borealis company. Its basic properties

can be seen in table 1.

Table 1 Material properties

MATERIAL PROPERTY VALUE

Elastic modulus E [MPa] 1340

Shear modulus G [MPa] 481.3

Melt flow index MFI [g/10min] 45

Shrinkage [%] 1.34

Hardness [HRC – R-scale] 89

2.2 Injection Mold Design of injection mold was made in consideration

of interchangeability, this means that can be, for

example; unconventionally manufactured cavity

used together with conventionally manufactured

core without any change in injection mold, however;

comparison was carried out only with standard

organization – conventional versus unconventional

cavities organization.

The injection mold itself was specifically designed

due to the shape of injected product, where could

not be neither ejector nor gate traces visible on the

exposed side. Therefore injection and ejection

systems are on the same side – in this case on the

right. Hot injection system with three hot nozzles

was chosen to carry out an injection of melted

polymer. Ejection was carried out by eight standard

ejection pins and control of this system was

implemented by four external hydraulic pullers.

Partially original parts and partially HASCO

standards were used for designing whole mold.

Fig.3 Injection mold – left side

Fig.4 Injection mold – right side

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2.3 Cavities Cavities were divided into several smaller parts for

easier manufacturing and possible repairs. These

parts were manufactured by using only conventional

methods of machining in case of conventionally

manufactured cavities and in case of

unconventionally manufactured cavities were their

parts manufactured partially by conventional

methods and partially by using Direct Metal Laser

Sintering technology (marked parts), due to its very

expensive manufacturing process. This can be seen

in figures 5 and 6. All parts were assembled with

screws and centered with pins.

Fig.5 Core

Fig.6 Cavity

2.4 Cooling Systems Cores and cavities are the same on the first sight; the

difference is in the cooling channels inside them.

Input and output channels are same for both variants

because of keeping interchangeability. Diameter of

cooling channels varies from 6 to 12 mm. Water

was chosen as a coolant liquid. Its temperature was

30 °C and its pressure was 3.5 bar. In both cases

was effort to design as good cooling system as

possible to determine whether investment in to the

direct metal laser sintering technology will even pay

off at this product.

Fig.7 Core cooling channels

Fig.8 Cavity cooling channels

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As can be seen from figures 7 and 8 most of the

channels are completely the same for both variants

and only at the most problematic areas was the

direct metal laser sintering technology used for

creation more complex cooling channels. This

should also result in significant cost savings.

3 Analysis Flow and mechanical analysis was done to compare

both variants.

3.1 Flow Analysis This analysis was done in Autodesk Moldflow

Insight 2011 under same process parameters, which

can be seen in table 2.

Table 2 Process parameters

PROCESS PARAMETERS VALUE

Mold surface temperature [°C] 35

Melt temperature [°C] 200

Mold-open time [s] 5

Coolant temperature [°C] 30

Coolant pressure [bar] 3.5

Ejection temperature [°C] 115

There were only few different results among many

obtained. The most significant of them was time to

reach ejection temperature, where the difference

between conventional and unconventional cooling

system was 16 seconds. Ejection temperature was

set to 115 °C according to the material list; this

temperature was achieved after 65.74 s in case of

conventional cooling and after 49.97 s in case of

unconventional one (figures 9 and 10). However this

time was necessary only in places which does not

restrict ejection of the part, therefore was ejection

time set to 55 s in conventional cooling system and

to 40 s in unconventional cooling system.

Fig.9 Time to reach ejection temperature – conv.

Fig.10 Time to reach ejection temperature – unconv.

Filling time was another of the examined results

and it was found out that both cavities are filled

almost in the same time. Conventionally

manufactured cavity was filled in 4.582 s and

unconventionally manufactured cavity was filled in

4.557 s. The difference in both variants is minimal

because there is very little influence of cooling

system on this parameter.

Fig.11 Filling time – conv.

Fig.9 Filling time – unconv.

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3.2 Mechanical analysis Mechanical analysis was done in Catia V5R18 only

at one cavity which was chosen core. Investigated

injection pressure was approximately 47 MPa in

both cases, but loading pressure was increased by

safety coefficient to 50 MPa. For analysis purposes

were both cavities clamped exactly the same like in

the injection mold frame.

Calculated von Misses stress was 502 MPa in

case of conventionally manufactured core and 817

MPa in case of unconventionally manufactured core

(figures 13 and 14). Despite higher stress in the

second case the safe stress (1267 MPa) was not

exceeded, therefore both variants comply.

Fig.13 Von misses stress – conv.

Fig.14 Von misses stress – unconv.

4 Economical evaluation

Economical evaluation was done according to the

pricing of the manufacturing process of both

variants, furthermore according to fixed costs and

cycle length (table 3).

Table 3 Costs

Figure 15 shows that despite higher investment

costs to the direct metal laser sintering technology;

it will be paid off after 73512 cycles due to a shorter

injection molding cycle. This means that this variant

will be better choice if the planned production is

higher than 73512 cycles.

Fig.15 Cost comparison

5 Conclusion In this research paper were designed injection mold

and two types of cavities for one injected product –

child seat. Both cavities were compared from flow

and mechanical points of view and economically

evaluated at the end.

Investigation of flow analysis results showed that

both variants are equally suitable for chosen injected

product, because the only significant difference was

shorter injection molding cycle at unconventionally

manufactured cavities. Mechanical analysis showed

that both variants comply because neither of

variants exceeded allowed von misses stress.

Economical evaluation showed that using

unconventional variant is eligible only for higher

productions.

Acknowledgement:

This paper is supported by the internal grant of TBU

in Zlin No. IGA/FT/2012/041 funded from the

resources of specific university research and by the

European Regional Development Fund under the

project CEBIA-Tech No. CZ.1.05/2.1.00/03.0089.

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