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International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 4, July–Aug 2016, pp.249–255, Article ID: IJMET_07_04_027

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ISSN Print: 0976-6340 and ISSN Online: 0976-6359

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MODELING THE FILLING PHASE OF INJECTION

PROCESS OF TECHNICAL PARTS WITH

THERMOPLASTICS COMPOSITES BASED ON HEMP

FIBERS

Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari

Laboratory of Industrial Management and Energy and Technology of Plastics and Composites–ENSEM–

Hassan II University – Casablanca Morocco

ABSTRACT

The injected thermoplastics composites grow considerably in different industrial fields and

provide increasing performance allowing manufacturers to consider innovative and competitive

technical solutions. The extent of their physical and mechanical and aesthetic performance is

constantly developed. However, the level of control of production technologies must be sufficiently

high to better meet the specifications in terms of quality and performance and productivity, but also

costs.

In this paper, we present a numerical simulation of thermoplastic composites injection molding,

in order to better predict the behavior of these materials during processing and propose operating

conditions to give the best results and optimize the production. We studied the evolution of certain

defects depending on implementation parameters: Temperature and pressure and speed of

injection. The tested material is polypropylene reinforced at different rates (10 to 30%) of hemp

fibers.

Key words: Thermoplastic Composites, Injection Molding, Experimental, Short Hemp Fibers,

Parts Quality, Processing Parameters.

Cite this Article: Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari,

Modeling The Filling Phase of Injection Process of Technical Parts with Thermoplastics

Composites Based on Hemp Fibers. International Journal of Mechanical Engineering and

Technology, 7(4), 2016, pp. 249–255.

http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=7&IType=4

1. INTRODUCTION

Modeling of the filling the mold cavity during an injection cycle is difficult. On one hand, the geometry of

the molded parts is often complex: multi-cavity mold, curved surfaces, three-dimensional parts. On the

other hand, the filling thermorheology depends also on the polymer characteristics. It exists in the literature

many studies on filling molds with simple geometry: rectangular cavity, cylinder, disc injected through the

center Pearson, Kamal and Koenig [1-2-13] modeled the flow in disk geometry with pseudoplastic and

thermo-dependent behavior law without considering the thickness solidified and neglecting the unsteady

Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari

http://www.iaeme.com/IJMET/index.asp 250 editor@iaeme.com

terms. Wu and al [4] have used the same geometry with consideration of the solidified thickness and

thermal-mechanical coupling.

Modeling flow in flat plate geometry was the subject of many studies. White then Titomanlio [5-6]

proposes one-dimensional models. Van Wijngaarden and al [7] studied a two-dimensional model: The inlet

flow rate is constant until a limit value of the pressure at the start of the plate. In addition, the calculation is

carried out at constant pressure (and rate flow decreases with time). The retained behavior law is a thermo-

dependent power law.

This work consists in modeling the behavior of a thermoplastic composite based on short hemp fiber

during the filling phase of the injection molding process. Rheological tests on the online rheometer at

different temperatures allowed us to determine a thermo-dependent behavior law for each composite

formulation in industrial conditions. This law will be used thereafter as input data for more accurate

modeling of the filling phase.

2. EXPERIMENTAL PROCEDURE

2.1. Materials

For industrial applications, the polypropylene homopolymer is most used. It offers the best mechanical and

thermal characteristics. The characteristics of the polypropylene used are: density = 900kg/m3, melt index I

= 25g /10min, melting temperature T = 168°C, thermal conductivity at 25°C is λ = 0.3W/mK.

In order to improve their properties to use them as reinforcements for composite materials, Hemp fibers

have been treated with alkaline solution. The used fibers have a diameter of 250 μm and a length ranging

from 2 to 2.5 mm. For better fiber matrix interfacial adhesion, we have added to the mixture 5% of

polypropylene grafted maleic anhydride. The granules of polypropylene reinforced at various rates of

hemp fibers were produced by the extrusion process.

2.2. Geometry of the mold cavity

The mold cavity allows molding complex part for technical use in the automotive field. The finite element

mesh components are triangles, which helped to limit the calculation time.

The actual injection conditions (time and pressure) are considered in the simulation.

Figure 1 The studied geometry

2.3. Simulation approach

The originality of our simulation work is the introduction of a thermo-dependent rheological behavior law

determined experimentally using rheological measurements on an online rheometer developed in our

laboratory [8-9].

Modeling The Filling Phase of Injection Process of Technical Parts with Thermoplastics Composites Based on

Hemp Fibers

http://www.iaeme.com/IJMET/index.asp 251 editor@iaeme.com

The determination of the thermo-dependent power law is made from measurements at three

temperatures. Through the curves equations determined above, we deduce the coefficients of power law:

)exp( CTAB

wrγη &= (1)

The plot log η= f(logγ& c) determines by a linear regression the viscosity behavior law according to γ& c

at each temperature.

The representative power law of the rheological behavior of polypropylene reinforced with 25% of

weight short hemp fibers in the studied temperature range can be written:

)013,0exp(81,2063 62.0Twr

−= γη & (2)

This relationship will be used to model the composite flow in a mold cavity.

Following the same approach, it was determined behavior laws for pure PP and PP with different rates

of fiber.

Figure 2 Simulation approach [10]

Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari

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3. RESULTS AND DISCUSSION

3.1. Evolution of the pressure and temperature during the filling of the mold cavity

We are interested in analyzing the evolution of the pressure during filling especially at the injection

location. This pressure determines the complete filling of the mold cavity and the fiber orientation which

entails the performance and appearance of the final part.

Figure 3 Pressure evolution at the injection location during the filling phase

During filling, the volume of the mold cavity is filled at a relatively low pressure. The pressure curve

mainly depends on the material resistance to flow into the mold and in particular: the reinforcement

content - the section of the channels and cavity - the distance traversed by the material in the mold- the

viscosity of the molten material - the injection speed – the material temperature - the mold temperature.

Figure 4 shows a prediction of the temperature evolution in the mold cavity.

Figure 4 Evolution of temperature in the mold cavity

Modeling The Filling Phase of Injection Process of Technical Parts with Thermoplastics Composites Based on

Hemp Fibers

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The evolution of the temperature in the mold governs the cooling time. This also depends on the part

thickness and the material temperature and his thermal diffusivity.

The cooling time must be determined for a temperature in the core of the part, giving no deformation at

mold release and for a residual pressure value in the part causing no depression when the part is no longer

in contact with mold walls. This phase is crucial to the quality of the manufactured part and productivity.

3.2. Effect of the injection location on the filling time

We are interested to estimating the filling time of the mold cavity depending on the position of the

injection location.

Figure 5 Prediction of the part filling time at two different positions of the injection location.

We note a difference of 0,18s, allowing a significant gain on the cycle time, especially if we think on

an industrial scale on high rates. Furthermore, it remains to analyze the presence of defects on both

configurations in particular weld lines.

3.3. Prediction of defects in the part

Several types of defects can occur in the molded parts, in this section we will focus on the influence of the

position of the injection location on the appearance of certain defects such as air traps and weld lines.

Figure 6 Prediction of air traps occurrence in the part at different positions of the injection location

Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Hamid Bakhtari

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After observation of Figure 6, it is observed the presence of many air traps in the second design, but

generally this problem is encountered in the processing of composites with vegetable fibers. The fibers

water content is one of the limiting parameters for hemp fiber, which has a direct influence on the molding:

the viscosity of the material and the mechanical properties and surface appearance of molded parts.

The presence of humidity in the material can cause poor adhesion fiber / matrix and contribute to the

formation of internal bubbles in molded parts [11, 12]. Specific treatment for hemp fiber is required,

(steaming the fibers before mixing).

The appearance of a weld line is a major problem both aesthetically and mechanically in the design of

injected part with thermoplastic composites.

Figure 7 Prediction of weld lines appearance in the part at different positions of the injection location

Figure 7 shows that the injection location has an influence on the appearance of weld lines in the room,

in particular in areas with irregular geometry. The first design is still the best.

Weld lines are formed when two melt fronts come into contact at low speed. Especially in areas of

geometric variations: Change in thickness – holes - the presence of insert [13-14]. These weld lines cause

in addition to the drop in the mechanical properties of the parts, optical imperfections in particular when

using high shine material [15].

5. CONCLUSION

We conducted a numerical simulation of filling of an industrial part for technical use in the automotive

industry. Our simulations were based on a behavior law of the material experimentally determined. [16]

We have shown the influence of the injection location on the part quality and on the presence of certain

defects such as air traps and weld lines and finally on the filling time. Our results are in good agreement

with experimental results. [17]

Moreover, this work has allowed us, at first, to validate the part design, then, analyze the impact of the

production cycle parameters on both the quality of the produced parts and productivity.

REFERENCES

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[2] M.R. Kamal, S. Kenig. Polym.eng.sci, Chap.13, pp.102. 1973

[3] M.R. Kamal, S. Kenig. polym. eng. sci 12, 4. 1982

[4] Wu C.F Huang, C.G Gogos; polym Eng sci 14, 223. 1974

[5] J.L. White, H.B Dee. Poly Eng.Sci 3, 14. 1974

Modeling The Filling Phase of Injection Process of Technical Parts with Thermoplastics Composites Based on

Hemp Fibers

http://www.iaeme.com/IJMET/index.asp 255 editor@iaeme.com

[6] G. Titomanlio. Polym. Eng. Sci 22, 589. 1982

[7] Wijngaarden, J.F, Dijkman, P.Wesseling 11.175. 1982

[8] A. Haddout, G. Villoutreix ; Brevet N/REF 233 489D.14028DL. (1992).

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[13] Robert A. Mallo. Plastic Part Design for Injection Molding. Hanser Publishers. 1994

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[16] Basma Benhadou, Abdellah Haddout, Mariam Benhadou, Houssam Ourchid, Study of thermorheological

behavior of polypropylene composites reinforced with short hemp fibers during industrial injection

molding process. International Journal of Mechanical Engineering and Technology, 7(4), 2016, pp. 29–

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[17] B. Benhadou, A. Haddout, M. Benhadou, H. Bakhtari, Study and optimization of the injection molding

of composites based on short hemp fibers. International Journal of Mechanical Engineering and

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