How to Get Your Injection Molding Simulation Right

It was the month of April in 2012 when we have released the first SOLIDWORKS Plastics product to our great community of passionate designers. The plastics injection process is a complex mix of many parameters such as Material, Temperature, Injection Location, Injection Time, Pressure, Packing Time and many more.

Everyone involved in either plastic part design, mold design or mold manufacturing does confirm that they expect and plan to have several mold rework cycles to get their plastic parts done right for their quality, form and function requirements.

It got around slowly that individuals and companies using injection molding simulation software can minimize mold rework, by identifying potential issues early in the plastic part design and/or mold design cycle to eliminate them before they become really expensive.

In this article I want to share with you some experience I have gathered by helping dozens of resellers and end customers predicting their injection molding process with SOLIDWORKS Plastics. Anyways, this article is not a replacement to a full product training on SOLIDWORKS Plastics. If you are interested in learning more make sure to book one of the training classes with your authorized Reseller.

Basically, there are three main pitfalls I have come across frequently, that might cover 95% of all recommendations I have shared with users:

1. Make sure you have 5 elements through your wall thickness

The model is discretized by finite volumes as known from computational fluid dynamics solutions like SOLIDWORKS Flow Simulation. The most accuracy you will get if you can mesh the model the way that you have at least 5 elements through your wall thickness everywhere in your model. This recommendation is valid for any solid mesh type, but here is where the powerful meshing option in SOLIDWORKS Plastics come into play with its Hybrid Meshing technology. This technology will efficiently mesh your model with prism elements on the skin and will fill up the core of your geometry with tetrahedral elements. However, you should double check using the “Solid Mesh View” or carefully slide through your mesh during the mesh generation and modify the “Min. Layer Thickness” value if necessary.

injection molding

Achieving this will help to accurately represent the fountain flow inside the cavity. The plastic cools first at the mold wall and the frozen layer grows from the outside inwards leaving the last molten area at the center of the part. Having not enough elements, the flow resistance is too low, leading to a wrong meld front calculation and too low pressure predictions. Also other important results will be wrong like temperature as shown below. All the results are used for the (Packing) next stage and therefore having an impact on the accuracy. Also for the last step, the warping calculation, those errors will be carried over as an input to the warping calculation and will lead to unrealistic warping results.

2-temp-dis

2. Always perform a Cooling Analysis before predicting part warpage

Ultimately, we want to have a warpage prediction as accurate as possible. There are two reasons why a part is deformed, one is the “In-mold Residual Stress” that is the portion of stress that develops during filling and packing, until just before the part is ejected. The other one is the “Quenching Thermal Stress” that is associated with the thermal contraction of the part as it is cooled down to room temperature.

When not performing a Cooling Analysis before you go and calculate Filling and Packing Stage, your assumption is that you have a perfect uniform mold wall temperature across all walls – which is the ideal cooling situation that happens rarely. It is more likely that you have a non-uniform temperature at the mold wall that lead to a unbalanced cooling. Such unbalanced cooling will result in an asymmetric tension-compression pattern across the part, causing a bending moment that will cause part warpage.

For a better illustration, I have created a simple flat plate part (200mm x 40mm x 2mm) that undergoes warpage calculation, first with a uniform mold wall temperature and second and third with a temperature difference at 10° and 20° Celsius.

3-warp-z

3. Your results can only be as accurate as your input parameters

SOLIDWORKS Plastics is pretty smart and gives you a good estimation on process parameters like Filling Time, Packing Time and Melt Temperature by taking into account your geometry and type of material you have selected. What does this mean to you? If you have the same Parameters used for your real Plastics Injection Process, your results will correlate pretty good to your physical injection. Or looking at this from the other side: If you have a mold that produces parts that does not fulfill your quality standards and you want to see if a Simulation can give you some answers why this happens, you have to know all the parameters used in your real life injection molding process to transfer them to your study setup.

Again, the more you go towards an accurate warping calculation the more sensitive is your calculation to realistic parameter input. The closer you will stay to a simple fill calculation only for predicting melt front behavior, weld lines, air traps, the less you have to worry about your inputs as the default parameters are very good to have a realistic filling prediction.

Originally posted in the SOLIDWORKS Tech Blog.

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