Sharp corners play a mandatory role in many instances. They look beautiful when light glints off the clean lines when the product revolves through space. Corners are distinct and precise. However, these sharp corners or edges can be terrible for plastic injection molding. They can create a lot of trouble during injection molding parts and every product designer must understand its importance. This is where plastic corner trim molding becomes vital.
Product designers are normally aware of all pitfalls linked with “going square” while manufacturing products without the appropriate amount of part accuracy, aesthetics suffer, strength, and corner rounding. Round and smooth corners are imperative, but some other factors also play their part even in the well-rounded part design.
Here are the most important factors that affect almost any part design and the product designer needs to be aware of them:
A few types of plastic are more sensitive to sharp-cornered components. Therefore, the designer needs to select the right material according to the application. Right material selection ensures accuracy appropriate functionality of parts.
You can strengthen adjacent walls to absorb strength linked with internal sharp edges. However, it can develop some other design issues in turn. So, it is another important factor that every product designer needs to know.
One particular part can simply be more moldable as compared to another part. To achieve appropriate function, fit, and form, it is important to ensure a perfect part design. Here, the designers use the plastic corner trim molding method to ensure a sound corner radius.
The injection molding process is still a difficult procedure, even after its extensive use in almost every industry. The thoughts that we have discussed above affect the corner radius amount needed in the design of any part. These factors also have a great impact on the effectiveness of the final product. By following these guidelines, you can achieve a great balance.
Assume that you need to design a case for carrying small components like servo motors for an electrical manufacturer using injection molding technology. The initial design will be a compartmentalized, shallow case. This initial design will look like a traditional tackle box that several compartments that keep contents for moving during transportation. Because of the heavyweight of the motors, you chose a glass-filled, rigid nylon body material. You will choose a transparent acrylic lid keeping in mind that you will be viewing the contents of the container without opening the cover.
Unluckily, the cover comes back unworkable. The bottom side is buckled, while the compartment walls are so warped that servo motors may not shift or fit in place. In turn, this will cause damage. The actual problem behind this is the sharp edges of the case that can be cut through the plastic corner trim molding technique. It is important to know that sharp corners or edges create pressure during the process of molding.
The radius is divided into two types; external and internal radius. The one that is available at the compartment’s bottom is a fillet radius. It is present at the point where the floor intersects walls. The designer should consider the radius equivalent to 0.5 times the thickness of the wall instead of leaving the edges sharp.
Similarly, you should round the top radiuses 3 times or 1.5 times to the closest wall. As a result, the molten plastic will flow easily. It will also eliminate any residual stress present which twists the workpiece of a thin wall. It also prevents cracking that may cause a premature failure of the product.
You may also notice some gaps alongside the compartments’ tops or the outside perimeter of the case. This gives a feeling that as if there was a lack of plastic availability for completing the part. Appropriate corner radius through plastic corner trim molding can be helpful in this situation. However, the raw material can act as a barrier in achieving this.
When you force the molten plastic through the injection mold intricacies, it doesn’t work like a non-crystalline solid. It contains a chain of molecules, creating links with its adjacent molecules. After creating links, it becomes difficult to cram into the tight edges or to move around the razor-sharp features, like wall intersections and posts. In turn, a situation ‘short shot’ occurs in which material finds it impossible to reach all crannies and nooks of a mold cavity. This is why we see such gaps in a final product.
Even if the material is unable to flow entirely, the residual strain establishes itself as a warp and then bends into the part. This is particularly factual for fiber-filled and glass-filled materials because of their greater molecular integrity and strength.
One solution to this thing is to change to an unfilled plastic or something like that. But, the part warp can be a concern if you do not ensure the appropriate mold design. Again, an appropriate radius is critical for avoiding such problems.
It is important to know that some machining limitations can affect these recommendations. This is because unless you use alternative manufacturing techniques, the internal edges in the mold cavity wouldn’t be smaller in comparison to the endmill radius that you use for machining them. This is a consideration that a Protolabs quote clearly illustrates.
To know these alternative manufacturing techniques, let’s assume a mold design of the insulator sleeve used in a medical device. The mold’s ‘male’ part is a popsicle-shaped aluminum insert with gentle tapers and curves.
The mold’s ‘female’ part is the piece to form the insulator sleeves outside. It is injection-molding-friendly, however a little challenging for producing through traditional techniques because of its thin cross-section and depth.
However, you cannot say that it is impractical but the production is going to cost you a little more. EDM (electrical-discharge-machining) utilizes high-energy flashes for eroding or burning away the metal. It needs tools, electrodes usually made with copper or graphite alloy machined in an inverse form.
Both the workpiece and electrode gradually disappear during the EDM process. This makes it mandatory to machine tools needed for completing most jobs.
Mold cavities like this are convenient to EDM. However, the flushing, orbiting, and arcing make it excessively slow for many molding scenarios. Due to this, many mold makers make bolt-on inserts for producing complex part geometries. In the example of a medical device, the taper parts and female section’s bullnose would probably mill on the CNC machine and fastened into the tool’s square pocket.
In the case of the electronics case, you would form the pockets through bolt-on aluminum chunks with thin channels. Both situations need you to apply proper radius throughout the diverse mold components.
Various parts profit from a reliable ribbing. Just take a flat, thin ABS plastic or polypropylene piece and twist it. It will twist pretty easily. Designers enhance plastic part’s strength by increasing wall thicknesses.
However, this may cause part sink, bubbles, and shrink, so you should avoid it. Introducing a honeycomb pattern is the right alternative. Or, you can also align short vertical ribs towards the bending forces of the part. This offers stiffness and improved structural strength to plastic parts.
In our example of the motor case, you can rib the product lid and base for making it sturdy yet lightweight. The contents of the product would also stay protected against any possible misfortune. Keep in mind that the rib corners should have an appropriate radius, both outside and inside.
If this all seems like trouble, you can get help from the CAD systems. You can also get professional advice from an expert about plastic corner trim molding for perfect product edges or corners.
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