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Finite element simulation of a PET bottle blow molding, top load and internal pressure tests

Area of expertise:
Industry:
Keywords PET, Polyethylene terephthalate, bottle, plastic, injection stretch blow molding, preform, internal pressure test, top load test, buckling
Programs in use ANSYS, LS-DYNA, Blow molding simulation software

In 2008 FE multiple modeling and analysis of a PET bottle was performed in CompMechLab.

PET bottle was first manufactured in 1977. In order to reduce 2l glass bottles turnout companies Pepsi and Du Pont developed a new concept of plastic bottles made of Polyethylene terephthalate (PET) for the Pepsi Cola beverage. Coca-Cola company started using PET bottles for their soft drinks just two years later. By 1990th PET bottles became very popular and in many cases almost substituted 0.5l and 0.33l glass bottles.

Strength, leakprofness, light weight made PET bottle so popular among consumers; low production cost and manufacturing simplicity - among producers. Another important issue was PET bottle full recyclability.

PET bottles are manufactured by blow molding with use of special machines. At the initial stage the preforms are made of dry and pre-heated polymer - the PET resin is injected into a tube-shaped mould to make structurally amorphous preforms. Preforms may be of different shape, size, color, weight and neck design.

 

 ПЭТ преформы  ПЭТ бутылки

PET preforms

PET bottles

  Image - courtesy of http://www.alphaplast.ru/

By means of conveyor preformes are put into the infrared oven and heated above the glass transition temperature. After heating they go to the blow molding part of the machine.

 

 Выдувная машина  Выдувная машина

Blow molding machine SIAPI EASP20.200/2

Image - courtesy of www.tma1992.ru

Injection stretch–blow moulding (ISBM) is one of the main blow moulding processes used to make thin-walled PET bottles. During the ISBM the preform is simultaneously stretched with a stretch rod and blown up with high-pressure air inside, to create bottles with desired shapes. Due to this technology almost uniform material distribution is achieved.

blow molding

Blow molding process
Image courtesy of Ottmar Brandau, OB Plastics Consulting

Numerical simulation of the blow molding as well as of subsequent mechanical bottle testing is a very topical issue nowadays. Application of CAE technologies at the early stages of new bottle design development and preform manufacturing can help to optimize bottle weight, increase mechanical characteristics, predict bottle damage. It also becomes possible to substitute costy experiments on physical prototypes (internal pressure test, top load test, drop-test, conveyor test) with numerical simulation.

In the current article application of CAE technologies to PET bottle simulation is presented with use of specific example. At the first stage of analysis the CAD-model of the bottle was created based on real prototype. Also the preform CAD model was built based on a drawing.

 

          pepsi light PET bottle           Bottle CAD model      Чертеж ПЭТ преформы   

Physical prototype

CAD model (CompMechLab)

Preform drawing

Injection stretch blow molding simulation was carried out then. Parameters of the process - pressure and stretch rod displacement vs time are shown on a plot below.

Параметры процессы выдувания

Finite element model contains 49 033 nodes and 97 900 elements. Preform is simulated as a viscoelastic material with temperature-dependent properties. It is assumed that the preform is uniformly heated at the initial moment. Transient non-linear problem of thermo-viscoelasticity with contact interaction is solved then. Simulation results are presented at the pictures below and in the AVI-Gallery.

 

                                   

Blow molding stages (CompMechLab)

During the blow molding process due to thermomechanical processes (that are depending on the PET properties and environment) temperature distribution is changing.  Also residual stresses are appearing. The Von Mises  stress is an equivalent stress that should be compared with yield strength in order to estimate whether material may break. The residual stress distribution should be taken into consideration in the subsequent structural analysis of the bottle (top load, internal pressure, drop-test). Bottle final shape and wall thickness are main results of blow molding simulation along with residual stress distribution.

 

     

Bottle thickness distribution

Residual temperature

Residual von Mises stress

During the internal pressure test the bottle is loaded by uniform internal pressure. The initial stress distribution originating after the blow-molding is taken into consideration. The von Mises stress distribution allows to understand which zones are more loaded (red) and what is the maximum equivalent stress value.

Non-linear buckling analysis of the empty PET bottle under top load was also caried out. Main results of this analysis are the post-buckling bottle shape and value of critical top load.

 

PET bottle radial disp           PET bottle stress         PET bottle buckling

Internal pressure test

Top load test
Radial displacement

Equivalent stress 

Total displacement

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