3D Printing VS. Injection Molding: Technologies for Success. Part 2

Enclosure design

The first part dealt with the problem statement, specifically the functionality of the enclosure, the cost per batch, and a comparison of 3D printing and plastic injection molding based on the functionality and production cost criteria. This time, we'll elaborate on how to turn goals and problems into custom enclosure design and go through the benefits that each technology offers at this stage. However, everything is in order.

Elaborating the product for the production technology

Products for 3D printing and injection molding are designed differently. After working out the issues we covered in the previous part, it is recommended to give yourself a firm response as to what technology you will employ at the start of the design process. Every method has a unique set of “techniques”. Everything appears stunning as long as it is on your CAD system’s screen. Yet, the moment you start to ignore one or more process features, you begin to produce defective items. Both 3D printing and injection molding fall under this. While this is not as important in 3D printing, it can be challenging and, in many cases, impossible to modify a mold once it has been made.

Every technology has its own drawbacks which cast a shadow over seemingly flawless design sketches. 

To the basic design features they generally include:

1. Process Slopes. They are essential for molding, particularly plastic injection molding, to ensure that parts may be removed from the mold without being hindered and to make it easier for molten material to flow. Technological slope of the inner surfaces and holes of the parts shall be greater than the slope of the outer surfaces. The value of the slope has a significant impact on the ultimate precision of the part elements. When selecting the optimal slope angle, the kind of the part surface, the degree of mechanical strength of the elements or part as a whole, the mechanical strength of the polymer employed, the manner of part ejection, and the level of cleanliness of the molding surfaces should all be taken into consideration. When utilizing 3D printing, there is no need to resort to slopes; on the contrary, slopes might reduce the quality of prints. If the slope is not for design purposes, it is preferable to leave walls straight when constructing an enclosure for 3D printing.

2. Walls. Plastic injection molding wall thickness influences the incidence of internal tensions that result in bloat, cracking, and warping during component soaking in the mold. Although the ratio of wall thickness to part length must have a definite value, it is ultimately decided after taking into account other factors. Polycarbonate, polysulfone, and stiff polyvinyl chloride are examples of low-fluidity plastics that can be used to create parts with low, thick walls, whereas polyamide, polyethylene, and polypropylene are instances of high-fluidity plastics that can be used to create high structures with thin walls. It is not advisable to give solid section thicknesses greater than 10 to 12 mm, except for extraordinary cases. Wall thickness can be reduced using stiffening ribs. It is important to maintain the same wall thickness throughout the part. Various cooling rates for components with varying thickness result in warping of glaws brought on by the production of gas bubbles or surface densities. The design guidelines are less rigid when creating an enclosure for 3D printing:

First, you must follow the rule that the wall thickness must be a multiple of the nozzle diameter. The wall thickness can be 2 mm, 2.4 mm, etc. when employing a nozzle with a diameter of 0.2 mm, for instance. When employing a nozzle with a thickness of 0.3 diameter, the wall thickness may be 2.1 mm, 2.4 mm, etc.

Second, it’s crucial to take the product's height into account when determining the wall thickness. For instance, if the product has a wall of 2 mm and a height of 200 mm, the print head will eventually start to wobble the product, ruining the print and forcing a reprint.‍

3. Rounding Radii. Roundings are included on the outside and inside of products when they are being designed for plastic injection molding. These roundings help to:

1. Boost the part’s overall or individual mechanical strength.
2. Eliminate or reduce internal stresses that cause warping and other types of deviations from proper geometry.
3. Reduce the value and shrinkage fluctuations in the part, which enhances accuracy.

Additionally, roundings facilitate the flow of mass in the mold during molding, make it easier to remove parts from molds and enhance the aesthetic of the finished product, simplify the mold production and reduce the wear and tear on the mold. For thermoset parts, the minimum radius should be 0.8 mm, and for thermoplastic parts– 1-1.5 mm. Not all of these approaches are necessary for 3D printed objects, and in some circumstances they may even be harmful. Figure 1 illustrates situations when the design for 3D printing and injection molding differs. On the left, you can observe how the item should be designed for injection molding, while the right column contains the instances of the right design for injection molding. 3D printing does not always necessitate the same wall thickness throughout the part, thus stress is not concentrated in these areas. Sharp corners should be avoided when 3D printing as well. The component may easily deform if this happens.

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4. Stiffening ribs. It is advised to include stiffening ribs into the design of the product in order to increase stiffness and strength, to reinforce particularly stressed areas or protruding parts. By adding stiffening ribs, it is possible to minimize the cross sections of the part's individual components, to lessen internal stresses at the junctions of different cross-sectional walls, and to avoid warping or even cracking. The many stiffening ribs include:

• reinforcing - they increase the part’s strength;
• distributing - they take on loads and distribute them over a greater surface area;
• delivering - they deliver the same wall thickness throughout the part.

Stiffening ribs are frequently unnecessary when creating a product for 3D printing. The reason for this is that 3D printing technique develops an item in stages. Therefore, stiffening ribs only minimally or do not at all absorb loads.

5. Holes. Hole configurations are due to their different purposes (technological, to lighten the product and make wall thickness the same throughout the part, for fastening, etc.) The placement of the holes on the surface of the parts, as well as their types and configurations, determine accuracy, shrinkage, and the amount of stress in the part’s material. In injection molding, there should be at least the diameter of the hole between any holes that are next to one another or between a hole and the product’s edge. If the hole is placed close to the edge, the latter must have the same shape as the nearby hole. The minimal hole size for 3D printing depends on the materials and parameters. The 3D-printed hole must have a minimum diameter of 2 mm. If obtaining an exact-sized hole is required, it is advised to print a hole with a smaller diameter and then, during post-processing, drill the holes to the required size. This is because the hole could get smaller if the product shrinks after printing. 

Topology optimization

As we have previously deduced, the injection molding process, particularly custom plastic molding, imposes numerous restrictions on the part's design, while additive manufacturing helps produce complexly configured ones. Topological optimization can be used to optimize an injection molded part by lowering the structure's total weight without sacrificing strength. Topological optimization is a computer-aided design technique that allows for the creation of a product's ideal shape under specific operating circumstances. The technique enables the creation of new, more effective engineering structure topologies with provided target functions and applied constraints.

Software for topological optimization doesn't create an object model from scratch. The part's geometric model is loaded into it, and then we indicate the locations that shouldn't be modified, which are typically the fasteners. The application has the ability to modify the shape of the remaining areas of the part. The loads that the product would experience during use are then applied, and the computer will start to optimize the geometry of the provided model based on this data. It happens thanks to FEA (Finite element analysis). The item is split into areas. The application then calculates the load value in each area, taking into account the nearby areas. Upon calculation, the application disposes of the areas that do not take on loads, giving us the revised geometry. By using this technology, the finished product's weight can be decreased without sacrificing functionality. Once optimized, injection molding is frequently all but impossible due to the product's shape. While additive manufacturing can be done without any issues.

In closing,

In the early stages of part development, EnCata works with the Customer to consider different manufacturing techniques. This aids in adapting the product design to the specific process during development. Given the high cost of injection molding tooling, it is also necessary to factor in the cost of the preproduction process. The part's shape plays a crucial role. For example, we would employ 3D printing or, failing that, plastic or metal machining to assemble a phone prototype in one or two copies. Although milling is labor- and resource-intensive, the prototype closely resembles the finished item, which will be produced by injection molding in a mass production setting. For small batches, EnCata most certainly won’t provide injection molding because it’s not the least expensive option and not everyone can utilize it. However, there are some exceptions. As an illustration, we developed the Smart Cork project (Fig.3).

The body of the electronic module for this gadget was created using 3D printing, while the remaining components were designed using injection molding. Food grade materials should be used for the product to interact with beverages, which 3D printing cannot ensure. In other words, employing 3D printing to create all the components would have prevented the device from being sold and the project from being commercialized. Because of this, we opted for extra expenses when we made the first 200 prototypes. After giving the product to real users for testing, it was used in actual business operations. Then we got orders for more units, so we made back our investment. In this project, it was reasonable to employ injection molding at the prototype stage.