Using molded effectively to make the right design and processing decisions early

by Andreas Wonisch

With molded we built an easy-to-use web app which can help you to design for manufacturing (DFM) of your plastic parts made by injection molding. While the injection molding process itself is quite complex the usage of molded is easy and intuitive. Even if you’re a CAD designer, plastics engineer or project manager without prior simulation knowledge you can use it without difficulty and gain many insights. Thanks to our intuitive user interface it is possible to understand how the cavity is being filled by the plastic melt and how it will affect the manufacturing and possible issues like weld lines, prolonged cycle times or voids. Because of the super-fast results which are available within seconds to minutes all of this can be done quite early in the product development cycle so you can make informed decisions early and without relying on expensive CAE tools.

In our first blog post we will give you some tips when and how to use molded more effectively. We will also give you an example how you can use the results to find design issues and improve the processing of your part.

The plastic product development process

The process to make a new plastic part by injection molding is quite complex and can take a long time as many different steps and back-and-forth between these steps are necessary. Any issues related to the part design, the chosen injection location or selected material can be quite costly to correct, especially if they are only discovered late in the process. In the worst-case scenario the mold has already been manufactured when problems are being discovered, requiring a very expensive re-design of the mold. Therefore CAE (computer aided engineering) tools have been used in injection molding for more than 40 years now. Even the simple tools from decades ago already provided some benefits which reduced risks and could potentially save a lot of costs and time. These days simulation is common practice when making new plastic parts.

However, even though the used expert CAE tools are extremely capable these days, they still mainly target the engineering phase. Also, they normally require expert users to operate, are expensive to license and takes several hours and in some cases even several days to finish. With molded it is possible to use simulation even earlier in the product development process, starting already with the part design phase. This way, part designers, product engineers and even product managers can check early designs for manufacturability and identify and correct issues which would normally only be discovered in the engineering phase. This accelerates the development process and leads to less back-and-forth later in the process. The expert CAE tools are still useful of course, and we actually recommend performing them before going to the prototype or mold design phase.

Case study: ECU housing

To show you how you can use molded during the part design process we designed a demonstrator part. In this case study you will learn how to use molded to come up with a better plastic part design, an optimized injection location and a suitable BASF material for our demonstrator part. The ECU housing (electronic control housing, see image above) is a typical plastic housing which could hold circuits and other electronic elements. In this case it resembles a housing often use for automotive applications and it contains four holes for screws to fix it to another component.

The part has been quickly designed by our CAD engineer and then exported as a STEP file. He only exported the plastic component itself and not any of the overmolded circuits as molded needs a single plastic cavity to perform the analysis. After creating a new project the CAD file can be directly uploaded. molded will then perform the analysis by automatically selecting an injection location. Note that you might already select a BASF material during the initial project creation, but it is not required. Here the results are available within approximately 30 s for the initial analysis.

Check most important metrics first

After the initial analysis is finished you will immediately notice some metrics on the right-hand side of the screen. They are designed in such a way to give you a very quick initial indication about your part, its design features and some processing information. Note that this information is only approximative and can be used as a guide when exploring different versions.

For processing the most important metrics are the cycle time and the required pressure demand. The cycle time result gives you an indication if the part can be produced cost effectively. For our demonstrator part the cycle time shows an unusual broad range of 25 – 52 s. This normally means that there are some issues in the part design which prolong the cycle time (see next chapter). The estimated pressure supply of ~19 MPa is modest but could potentially increase when using a material with poorer flow characteristics (i.e. higher viscosity). This is something to watch for when comparing different materials. Additional insights can be gained from the flow length which is 91 mm with a base wall thickness of 2 mm for this part. Note if the ratio between flow lengths and base wall thickness becomes larger than ~100 this can pose problems (depending on the material class). You don’t need to calculate this ratio yourself though as it would be also visible in a significantly higher pressure demand.

How to detect and correct design issues

After checking the metrics it is a good idea to go through the different DMF and simulation results which can be assessed by the tabs at the top of the screen. The “Thickness” results will show you how the thickness varies throughout the part. We already noticed that the demonstrator part has an unusually high range of possible cycle times which often indicates a wide range of wall thickness. Indeed, as you can see from the thickness results even though the base wall thickness of the part is only 2 mm there are some regions which go up to 8 mm. They can be found around the screw fixtures. Another way to point out these mass accumulations is to use the plastic design guide at the right-hand side of the screen, go to thickness there as well and click on “show spots”. It will point out the areas which we identified from the thickness result. The fixtures have not been designed well and should be re-designed to avoid excessive plastic accumulation there as it will increase the danger of internal voids and can lead to high amount of shrinkage and warpage in these regions.

By going to the “Undercuts” result we can also check if we have any undercuts in our model which increase mold complexity. In our case there is an undercut in the small side connector region which would need slides for demolding. There are also small undercuts at the snap fits and at the BASF logo. As they are small, they can be neglected during the current design iterations. Note that like the wall thickness you can also check for undercuts by using the “show spots” function in the guide.

Find the optimum injection location

Clicking on the filling result you will be able to see how the polymer melt fills up the cavity. Without having set an injection location molded will automatically determine an injection location it deems suitable. However, this might not be the best one depending on your requirements and the accessibility inside the mold. In the demonstrator part an injection location at the base a little bit off-center, closer to the side wall with the BASF logo has been chosen. However, for housings it often makes sense to choose an injection location in the center of the part to fill up the side walls at the same time. This will usually lead to less amount of warpage and – if it’s a fiber-filled material – to a more homogenous orientation. By clicking on the “Changes” button we can select a new injection location with the right mouse button at the center of the base. After starting the new analysis, a new version will be created which will allow us to compare both injection locations scenarios.

Comparing the two injection locations we can observe that with the new version (left hand-side) the pressure level increases slightly, and the filling pattern is more homogenous with the side walls of the main housing element being filled up roughly at the same time. However, the smaller side connector has a more diagonal filling pattern now. For the larger connector it is more evenly in comparison. As the filling pattern can affect both fiber orientation and warpage it is important to determine which of those two connectors is more important regarding dimensional tolerances and then select the more appropriate injection location. Other things to look out for are weld lines and pressure demand.

Find the best BASF material

Finally, molded also helps you to find a suitable BASF material for best processing performance. In this case we initially chose a 30% glass fiber filled PBT material (Ultradur B 4300 G6). If for your application you need a material with better flowability you can check one of the “High Speed” grades and compare the pressure supply. If we switch to Ultradur B4300 G6 High Speed the pressure will be reduced by more than 50%. This would help to be able to use a smaller injection molding machine. Another option is to check if a material with some enhances properties like e.g. the Ultradur B 4330 G6 HR which is Hydrolysis resistant. When choosing that material pressure will increase slightly to ~21 MPa. You should note that all of these pressure values offer only rough guidance and are not meant to be correct in absolute terms. In any case, they also do not consider the gating system which would lead to higher pressure requirements. However, these values give you an indication what to expect and allows a comparison between different materials as shown here.

Conclusions

As you can see after uploading the CAD data of your plastic design it is possible to quickly check the most important metrics like cycle time or pressure demand, identify and then fix any issues with your design like undercuts or mass accumulations and choose a optimal injection location based on your requirements. And of course, you can also use molded to try out different BASF materials and see what effects they have on the injection molding process. Since the results are usually available within less than a minute it makes sense to explore different options and compare them. While normally every simulation would be quite time-consuming and expensive, molded allows you to quickly compare different options – even those that you normally wouldn’t simulate.

For our demonstrator part we now gained several new findings: We can go back to our CAD designer and improve the geometry of the fixture holes. We can already propose an optimized injection location for subsequent more detailed simulation or the tooling. And we know how different Ultradur materials will perform and that we are generally fine with any of those we compared. Of course, it’s also possible to dig even deeper into the results and there are still more things to learn from molded. We will keep you updated in subsequent blog entries.

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