Using Core-outs to Reduce the Weight of Plastic Parts

What’s a Core-out?

Core-outs are sections of a plastic part where a thick section of solid material has been replaced by square, rectangle, or hexagonal sections. This effectively replaces that solid material with empty voids surrounded by walls and ribs of uniform thickness. The core-outs themselves are usually placed on non-cosmetic surfaces, with the open areas facing down (like the underside of a handle) so that you don’t inadvertently create cubbies for dirt, dust, fluids or other unwanted material.

Carefully removing sections of a solid object is not an idea specific to injection molding of plastic parts. For example, many 3D printers (and their associated modeling software) have options for “sparse fill”: using a honeycomb or other structure with plenty of hollow space in places that would normally have solid material. In fused deposition modeling (FDM) 3D printers—where objects are built up layer by layer through extruding molten plastic through a small nozzle—this drastically shortens the amount of time need to print an object and reduces the amount of filament consumed during the print.

Sparse fill in 3D printing, however is different from core-outs in injection molded parts in one critical aspect: volumes which are sparse filled are fully enclosed and thus those internal voids are not visible from the outside nor subject to having outside material enter them. Core-outs always have one open side, a necessity due to the fact that the part needs to be releasable from the mold. As we discuss later on, this feature of core-outs has practical implications.

This same idea is behind the steel I-beam’s “I” shape, where the chief advantage is much lower weight compared to a solid beam of steel, while preserving almost all the beam’s strength. Like sparse fill in 3D printing, the I-beam’s shape allows a fixed amount of steel to make more I-beams compared to a solid steal beam, due to the subtracted volume.

Why Put in Core-Outs?

The main advantage of putting core-outs in injection molding parts is lower part weight, just like in those I-beams. Secondary advantages include reduced cycle time and reduced part price. The price reduction on each part means that in high-volume production runs, the extra time (and thus cost) needed to design in core-outs into a part is more than returned, especially if that work is already mostly done for you by your contract manufacturer as part of a DFM analysis.

Core-outs can also be used to avoid sink marks (also called shrink marks) which can arise in areas of a part which are very thick. Although usually a minor cosmetic issue if they occur on a customer-visible surface, sink marks can be a functional problem if the surface they’re on needs to be flat (e.g. some liquid might pool in the depression instead of running off).

There are other ways to address these above issues without core-outs, such as splitting the part into two or more separate pieces, switching to a different material which is more forgiving with thick walls, using gas-assistance molding, or even just adjusting the injection molding condition. The problem with almost all of these is that they’re more costly and time-consuming than hollowing out a few areas using core-outs.

When to put in Core-outs?

Core-outs are added to a part during the design phase, usually with input from the injection molding contract manufacturer, as we mentioned earlier. Although your contract manufacturer may offer good suggestions for the location and dimensions of possible core-outs, it’s ultimately up to you to make sure your part still has the necessary strength and rigidity to do its job. Also, remember that your injection molding contract manufacturer is basing their recommendations on the only context that really matters to them: the individual part. The location and dimensions of possible core-outs must not interfere with the form, fit or function of that part when it’s assembled into the final product and used by the customer.  In medical and food preparation settings, objects with exposed cavities where food or other contaminants can gather can become a health and safety risk. In those special cases, one of the above alternatives to core-outs (such as switching the material to Nylon which can tolerate thicker walls without sink marks) might be your best bet.

Coring-out sections of your injection molded part can reduce it’s weight, cycle time, and cost without degrading its form, fit or function. To realize all these benefits, however, you must keep the full picture in mind as you work closely with your contract manufacturer on core-outs and any other design changes intended to make your part more manufacturable.

Providence Enterprise, with it’s large in-house engineering staff, is able to assist you with core-outs or any other manufacturing hurdles. Learn more at


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