May 2011 Design Tip



Choosing Sides

When you start looking into injection moulding, one of the first things you notice is that everyone talks about the “Fixed half” or “A-side” and the “Moving half” or “B-side.” These terms refer to one side or the other of an injection mould. When a mould is manufactured to make your part, some of your part’s outer surface is created by the A-side (fixed half) and some by the B-side (moving half). A-side and B-side are such fundamental concepts for injection moulding 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 moulding track back two basic factors. First is physics: plastic shrinks as it cools. Second is the conventional design of injection moulding machines. Almost all injection moulding presses are built so they inject molten plastic into one side of a mould (conventionally called the A-side), and have the part ejection system in the other side of the mould (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.

Figure 1: Components of an injection moulding machine.

Figure 1: Components of an injection moulding machine.


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 mould 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 mould, and it holds on tight. And, except for a few rare geometries, your part will hang on to both sides of the mould. Injection moulding presses are designed with this in mind, though, and the press can use quite a bit of force to pull the mould open. Your part will generally stay in the half of the mould 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 moulding process technician can figure out how to pry the part out of the A-side without damaging the mould.


Part surfaces can be extremely convoluted, with ribs, bosses, cores, through-holes and other features adding to each side’s tendency to grip the mould. 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 mould 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 mould 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.


Your free ProtoQuote® interactive quote will provide a design analysis that will show how Protomold has decided to assign sides. In your quote, surfaces formed by the mould’s A-side are coloured green, those formed by the B-side are blue, and, if present, surfaces formed by side-actions will be shades of pink. You will be asked to sign off on these aspects of the design, along with the gate and ejector placement, as part of the order confirmation process.


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 +44 (0) 1952 683047.





To learn more about specific mould and part features referred to in ProtoQuote interactive quotes, and the operation of the mould itself, sign up to receive a no-cost Protomold Demo Mould sample.




(Figure 1 photo credit: Image created by Brendan Rockey, University of Alberta Industrial Design.)