Simulation of the Gas-Assisted Injection Molding Process

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Journal of Materials Processing Technology 178 (2006) 350357

Simulation of the gas-assisted injection molding process using amid-plane model of a contained-channel part

A. Marcilla , A. Odjo-Omoniyi, R. Ruiz-Femenia, J.C. Garca-Quesada

Chemical Engineering Department, University of Alicante, San Vicente del Raspeig, 03690 Alicante, Spain

Received 5 April 2005; received in revised form 5 April 2005; accepted 4 April 2006

Abstract

Computer-aided engineering (CAE) simulation and experimental studies have been carried out on the cavity filling and gas packing steps in

the gas-assisted injection molding of a contained-channel part. A mid-plane model of the three-dimensional geometry of the mold cavity has

been proposed to be analyzed by the finite element method. The shot size, the distribution of the gas bubble and the residual wall thickness were

calculated using a commercial simulation software (Moldflow Plastics Insight Version 4.1). The outcomes predicted by simulation were compared

with the experimental results indicating the good predictive capability of the proposed model.

2006 Elsevier B.V. All rights reserved.

Keywords: Gas-assisted injection molding; Computer simulation; CAE analysis; Model; Mold filling

1. Introduction

Nowadays gas-assisted injection molding (GAIM) is not aninnovative technology for manufacturing hollowed plastic parts

with thefirst patentdating from 1978 [1]. However,the establish-

ment as a common molding process has not yet been achieved

in the polymer industry. This is a result of some pitfalls in the

practical application of the GAIM process due to the inherent

gas instability that implies a complex relationship between the

parameters which control the gas flow and the quality of the

molded parts. Its higher limitation is the understanding of the

characteristics of the process, specially with respect to the typi-

cal flow phenomenon.

There are four different processes to produce GAIM parts:

short-shot process, full-shot process, back-to-screw process and

mold with retractable cores [2]. In this work, we have studied the

first process which is also called the standard GAIM. The short-

2. After a short delay period, compressed nitrogen gas cores

out the molten polymer. The penetrating gas leaves behind

a polymer layer at the mold walls, attaining a molded partwith a polymer skin and an inner gas channel ( Fig. 1b). Gas

penetration which takes place at this step is denominated

primary gas penetration. This step finishes when the mold

cavity has been entirely filled (Fig. 1c).

3. Gas is continuously injected to transmit the packing pres-

sure to the polymer. At this stage the polymer shrinkage is

counteracted by a growth of the gas core. The gas packing

pressure remains until all polymer material has solidified.

Gas penetration during post-filling step is termed secondary

gas penetration (Fig. 1d).

In full-shot GAIM the first step of the short-shot process is

extended until the shot weight of molten polymer fills entirely

the mold cavity. The effects of the processing conditions on
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A. Marcilla et al. / Journal of Materials Processing Technology 178 (2006) 350357 351

Fig. 1. Description of the steps of the short-shot process (standard GAIM): (a)

polymer short-shot; (b) start of primary gas penetration; (c) end of primary gaspenetration; (d) secondary gas penetration.

GAIM offers a considerable number of advantages over con-

ventional injection molding. These advantages mainly originate

from the near constant value of the gas pressure throughout the

gas core as a result of the pressure drop in the gas core is insignif-

icant compared with the pressure drop in an equivalent molten

polymer[5].The advantages over conventional injection mold-ing are reduced part weight, injection pressure, clamp force,

shrinkage, warpage and residual stress, apart from better sur-

face finish[6].Moreover GAIM enhanced design possibilities

and thick-sectioned components are possible without excessive

warpage, shrinkage and cooling time[7].

All these advantages are exploited only if the design and

processing parameters are perfectly understood. GAIM appli-

cation is more critical than conventional injection molding

due to the additional parameters which are introduced, suchas the numbers and location of gas injection points, the

amount of molten polymer injected (shot size), the gas delay

time, the injected gas pressure and the holding time for gas

injection.

Optimization of molds and products by trial-and-error meth-

ods i e by modifying the mold and changing the process

cavity. This model should be optimized to apply on them the

numerical methods which are needed to carry out the sim-

ulation, i.e., finite difference method (FDM), finite element

method (FEM), boundary element method (BEM) and finitevolume method (FVM)[10].These numerical methods require

discretization of the physical domain (geometric model) into

finite sub-domains (elements) obtaining a mesh model of the

mold cavity. Through the discretization, the continuous govern-

ing equations are converted into a set of discretize algebraic

equations to be solved by the use of the computer. There are

three main different types of mesh models based on the type

of the element used to construct them: one-dimensional tubu-

lar elements (strip mesh), two-dimensional triangular elements

(mid-plane mesh) and tetrahedral elements (three-dimensional

mesh). According to the model used, different analysis can be

performed: 2, 2.5 and 3-dimensional simulations, respectively

[11,12]. Obviously, the higher the complexity of the element

used the higher the accuracy of the simulation, albeit computa-

tion time is considerably increased.

The major benefit of a two-dimensional analysis is speed

although their results are not equivalent to those produced withmore advanced analysis. In the case of GAIM simulation, strip

models lack the ability to predict an unsymmetrical polymer

wall distribution at any cross-section of the part.

Most of commercial injection molding simulation packages

are based on the so-called thin-film or HeleShaw approxima-

tion developed by Hieber and Shen[13],in which the fact that

most injection parts are thin-walled is used to decouple flow and

thermal effects in mid-plane and thickness directions; this has

led to what is known as the 2.5-dimensional approach[14].Asa mid-plane model is generated by a surface, which is a two-

dimensional geometry entity, to attain a representation of the

three-dimensional geometry of the part, a numerical attribute

called thickness is assigned to all the flat elements that compose

the mid-plane mesh. Many authors have investigated the relia-

bility of these commercial packages regarding to the simulation

of conventional injection molding [1520] and also concern-

ing GAIM for parts designed with gas channels [7,21,22].This

approach has become the state-of-the-art for the simulation ofinjection molding processes.

Some programs provide three-dimensional simulation of the

polymer GAIM process. However the analysis time is very

long,even usinghigh-speed parallel processingCPUs. The time-

consuming issue is still more important if we are dealing with

the mold design stage [23 24] Moreover nowadays these pro
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Table 1

Properties of polypropylene ISPLEN PB 130 G1M

Properties (unit) Method Value

Physical

Melt flow rate (230 C; 2.16 kg) (g/10 min) ISO 1133 1.3

Specific gravity (g/cm3) ISO 1183 0.905

Mechanical

Flexural modulus (MPa) ISO 178 1050

Izod notched impact strength (23 C) (kJ/m2) ISO 180 15

Izod unnotched impact strength (20 C) (kJ/m2) ISO 180 70

Thermal

Vicat softening temperature (C) ISO 306/A 140

Others

Shore hardness (Dscale) ISO 868 60

The aim of this work is to show the approximation achieved

using a mid-plane model (2.5-dimensional analysis) to simu-

late GAIM instead of using a complex three-dimensional model

which is high-computation time consuming and does not allow,

nowadays, the simulation of most of the analysis which can beexecuted with a mid-plane model of the part.

2. Experiments and simulation

2.1. Material specifications

The polymer used in this study was commercial polypropylene injection

molding grade ISPLEN PB 130 G1M supplied by Repsol-YPF.Table 1lists the

properties provided by the manufacturer.

2.2. Part geometry

According to the classification proposed by Avery[2]the geometry of our

mold cavity (Fig. 2)leads to a contained-channel part where the control of the

gas flow relies on the total part geometry. The part is circular in cross-section

with a diameter of 16 mm for the external cross-beams (labelled 1 and 4) and

Table 2

Constant process parameters used to produce the molded parts by GAIM

Parameter Value

Cylinder temperatures (C) 230, 225, 220, 215

Nozzle temperature (C) 235

Mold surface temperature (C) 40

Injection speed (cm3/s) 90

Gas delay time (s) 0

Gas pressure (bar) 150

Gas holding time (s) 18

Cooling time (s) 15

Inlet coolant temperature (C) 20

with a diameter of 14 mm for the two internal cross-beams (labelled 2 and 3).

The total dimensions of the part are 200 mm 128 mm 60mm.

2.3. Injection molding machine and gas injection unit

The experiment was conducted using a 1300 kN Battenfeld TM1300/750

Unilog B4 injection molding machine equipped with a Airmould-Modular

systemwhichis based in a pressure-controlledprocessand comprisesfour mod-

ules: gas supply (by nitrogen bottles), compressor, gas pressure control and gas

injection. The injection molding machine has a screw diameter of 50 mm whichprovides a theoretical shot volume of 442 cm3 (polystyrene) with a maximum

specific pressure of 1714bar.

2.4. Process parameters

The fixed process parameters are shown inTable 2.They were chosen based

on material and equipment suppliers recommended processing ranges. The

quantity of molten polymer injected just before gas injection started was opti-

mized as explained below. Using the optimum conditions 50 parts were molded

in order to achieve the stabilization of the characteristics in the molded parts(weight was controlled). Finally, 20 parts more were produced and labelled to

be studied.

Thepolymeris injectedintothe mold cavity througha spruegateand a single

gas needle was placed in the runner system in a special antechamber behind the

gate and the cavity (see Fig. 3). If the gas needle is placed directly into the

cavity the polymer flow will be divided by the gas needle and weld lines will be

generated provoking a deficient propagation of the gas bubble[26].
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A. Marcilla et al. / Journal of Materials Processing Technology 178 (2006) 350357 353

Fig. 3. Feed system of the mold with locations for polymer and gas injection.

2.5. Characterization of the molded parts by GAIM

To determine the accuracy of the simulation outcomes the gas bubble pen-

etration in the experimental parts has been studied. Furthermore, it has been

measured the residual wall thickness, which is defined as the thickness of the

polymer layer that is left behind at the mold walls after the gas front has passed.

According to Poslinski et al.[27]the residual wall thickness in GAIM is deter-

mined by two phenomena: the penetration of a gas into a viscous liquid, and the

growth of the solid layer.

To evaluate the two characteristics above mentioned the molded parts were

sectionedby parallel planes to its symmetryplane at 20 mm intervals throughout

the complete length of the part (200 mm).

2.6. Computer simulation

Molded parts by GAIM were simulated using the commercial software,

Moldflow Plastics Insight (release 4.1). The first stage of any simulation by the

finite element method (FEM) is called pre-process. This is the most time-consuming step and consists of creating the geometric model of the mold cavity

to be analyzed. This can be performed by using either the simulation software

itself or one of the computer-aided design (CAD) computer programs. In this

work, an external CAD software, called GiD (developed by the International

Center for Numerical Methods in Engineering) has been used. It consists of

an interactive graphical user interface that provides pre-processing tools for the

numerical simulation of industrial and academic problems[28].Following, the

geometric model must be meshed using triangular elements. Afterwards, the

Fig. 5. Cross-section representation of the part using a mid-plane model con-

stituted by five sections (numbers into boxes are the thickness in millimeters of

each mid-plane section).

polymer is selected and the injection locations are set. To finish this first stage,it is required to introduce into the simulation software the process conditions.

In the second stage of a simulation process, fluid mechanical and heat transfer

calculations are conducted applying a finite element and finite difference analy-

sis. The GAIM simulation has been carried out using the MPI/gas module of the

simulation software. The last stage of a simulation is called post-process and

is where the experience of the analyst is required in order to extract the reliable

and mostimportant information from the multiplescolouredmap results offered

by the simulation.
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Simulation of the Gas-Assisted Injection Molding Process