Article by Protomold
When you start looking into injection molding, one of the first things you notice is that everyone talks about the “A-side” and the “B-side.” These terms refer to one side or the other of an injection mold. When a mold is manufactured to make your part, some of your part’s outer surface is created by the A-side and some by the B-side. A-side and B-side are such fundamental concepts for injection molding that nobody bothers to explain what these two terms really mean. There is a complex web of factors and consequences around which side of your part is assigned to the A-side, and which to the B-side. Understanding the implications can help you design better parts and to avoid unpleasant surprises down the road. Most of the constraints and attributes around “sidedness” in injection molding track back two basic factors. First is physics: plastic shrinks as it cools. Second is the conventional design of injection molding machines. Almost all injection molding presses are built so they inject molten plastic into one side of a mold (conventionally called the A-side), and have the part ejection system in the other side of the mold (conventionally called the B-side). In Figure 1, the injection unit of the press is on the left side, and the clamping unit (which incorporates the ejection system) is on the right side.
One of the main drivers for choosing which side of your part is the A-side and which is the B-side is ejection. It seems plausible that when the mold opens after forming your part, it will just tumble out to make room for the next cycle. Such is not the case, however. Since the plastic shrinks as it cools, it shrinks around any convex parts of the mold, and it holds on tight. And, except for a few rare geometries, your part will hang on to both sides of the mold. Injection molding presses are designed with this in mind, though, and the press can use quite a bit of force to pull the mold open. Your part will generally stay in the half of the mold that has the most convex surface area. If that side is the B-side, no problem, the ejector system will push the part out of the B-side to get ready for the next cycle. If that side is the A-side, then everything stops until the molding process technician can figure out how to pry the part out of the A-side without damaging the mold.
Part surfaces can be extremely convoluted, with ribs, bosses, cores, through-holes and other features adding to each side’s tendency to grip the mold. In some cases, identifying the A- and B-sides requires both software tools and a fair amount of experience and intuition on the part of Protomold design staff. Even so, sometimes minor changes to the mold design or manufacturing process are required to ensure the part will stick to the B-side.
Whether you design your part to have a particular A/B orientation or simply approve the orientation chosen by Protomold, the orientation of your part to the mold halves will make a difference in cosmetics.
Depending on gate type, the A-side of a part may show vestiges of the gate, particularly if a hot tip gate is used. This is significant because the A-side is often the cosmetic side of a part, e.g., the outside of a case or shell. These vestiges can be anticipated and camouflaged or covered, for example, with decals.
The B-side will typically show ejector marks. These are usually less critical, as the B-side is often the hidden, non-cosmetic side of a part. There are exceptions—a concave plastic tray designed to be set into a surface, for example—in which case, the marks can be anticipated and treated in the same way as gate vestiges.
There is sometimes latitude in part orientation, and simple changes to your part design will often allow different orientations. As always, questions about side assignment or any other aspect of part design can be answered by our Customer Service Engineers at 877.479.3680.
To learn more about specific mold and part features referred to in ProtoQuote interactive quotes, and the operation of the mold itself, sign up to receive a no-cost Protomold Demo Mold sample.
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