Blow molding is a manufacturing process that is used to create hollow plastic parts by inflating a heated plastic tube until it fills a mold and forms the desired shape. This process is highly economical to manufacture high volume, one-piece hollow objects. In general, there are three main types of blow molding: extrusion blow molding, injection blow molding, and injection stretch blow molding.
The blow molding process begins with melting down the plastic and forming it into a parison or, in the case of injection and injection stretch blow molding (ISB), a preform. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can pass. The parison is then clamped right into a mold and air is blown into it. The air pressure then pushes the plastic out to match the mold. Once the plastic has cooled and hardened the mold opens up and the part is ejected. Water channels are curved inside the mold to assist in cooling.
Why Virtual Simulation?
Manufacturer often have to design and produce package for a new product within a limited design-to-delivery time and cost. Designers often create innovative concepts, but retreat to conventional shapes due to limited time and resources for physical prototyping and testing.
Simulation enables designers to
- Virtually test new innovative concepts,
- Reduces design time and expensive cost of physical testing,
- Lowers material cost and improves sustainability by light-weighting,
- Reduces damage cost during production and transport by eliminating bad designs quickly,
- Improves confidence in quality by predicting package usage performance.
Output of Interest
The main objective from a blow molding simulation is to predict the accurate thickness distribution throughout the final product.
This helps designer to understand:
- which regions required excessive or less amount of material
- if the mold cavity pressure during manufacturing process is enough to capture the desired final shape accurately
- mass flow rate inside the cavity
- induced residual stresses after the cooling
- and lot many other custom parameters of interest.
Once the thickness distribution is captured accurately, many other questions related to the performance of the final shaped part can be answered through different class of simulation as shown in the below flowchart.
Blow molding simulation involves all the three classes of nonlinearity: material, geometric (high deformation of initial shape) and boundary (complex contact and distributed pressure loading). Out of these, the most critical one is the material modeling of the heated preform material. The material property is dependent on temperature, biaxial stretching, strain rate, etc. A quick approximation of the material characterization can be made by using hyper-elastic or elastic plastic material. But it may not capture the final thickness distribution very accurately. Initially, Isight data matching component will help to approximate the material model quickly and efficiently with the help of some testing data.
Generally, the preform has varying thickness throughout its length. So, the mid-surface extraction is little tricky but can be extracted easily using Abaqus sketch module (constructing spline) by taking the reference of the cut geometry of the solid preform as shown in the image. The contact nonlinearity can be tackled efficiently by using the general contact algorithm.
Simulation Analysis Approach:
Abaqus/Explicit solver is the best in class solver to simulate such kind of complex nonlinear problem. In the explicit step, first the stretching rod is pushed through the heated preform and simultaneously mass flow rate is introduced in the fluid cavity. The blow molding process is fast enough to assume it to be an adiabatic process. Thus, in the course of blow molding simulation no effect of temperature need to be captured. Once the stretching rod reaches the end of the mold cavity it is held at the same position by using an amplitude function used to define the boundary condition. During the course of the analysis the cavity pressure is monitored. Once the desired cavity pressure is reached, the thickness distribution and the other parameters of interest can be post-processed or further an Abaqus/standard analysis can be performed to calculate the residual stresses. Below video shows the video of blow molding simulation and the changing thickness distribution throughout the simulation.[KGVID]https://www.viascorp.com/wp-content/uploads/2020/10/Thickness_Change_Animation_1-Converted.mov[/KGVID]
To meet the dynamic, competitive landscape of the consumer packaged goods (CPG) industry to produce a wider variety of top-quality, innovative containers in ever shorter time periods and at lower unit prices SIMULIA Abaqus Unified FEA is one of the best in class numerical package. Abqus explicit solver, automated meshing, complex material characterization library, robust general contact algorithm helps the engineer to confidently design and deliver the new innovative products. The above simulation took few hours to model and simulate the whole blow molding process. The model is just a representative demo of similar kind of simulation. However, this kind of simulations requires lot many parameters to be calibrated with the experimental data for first time to setup the whole process.
If you want to have a better understanding of your product during/after the blow molding process under different loading scenario, please do not hesitate to contact us.
Subhadip Maiti, M.Tech., is a Senior Simulation Consultant at VIAS – He is a Mechanical Engineer with more than eight years of professional experience of strong Industrial and consulting experience in linear and non-linear FEA, structural design & optimization, fatigue and fracture mechanics, noise & vibration analysis, solid mechanics and design etc . He also has a strong background in experimental testing and simulation correlation. Mr. Maiti has worked with multiple consulting/MNC companies in developing simulation workflows, process automation, new product design, SOP for experimental testing etc. He holds Master’s degree in Mechanical Systems Design from Indian Institute of Technology, Kharagpur. He can be reached at [email protected].
1] J. Zimmer, M. Stommel J. Denria, “Method for the evaluation of stretch blow molding simulations with free blow trials”, IOP Conference Series: Materials Science and Engineering.
2] Simulia ebook “Create package designs faster, better and cheaper with realistic simulation”.