Injection molded parts are a fantastic solution for medium to high volume products. Depending on the product, the target volume sales per year, cost of goods and the planned repair strategy, molded in plastic bosses offer a low cost way to align parts, and provide options for fasteners for mounting components and securing enclosures. However, proper consideration in the design and placement of the mounting bosses in a plastic part is required to avoid unnecessary cosmetic flaws and proper strength to avoid design failures.
There can be found a lot of injection mold part design suggestions for the creation of bosses, but all too often these guidelines are at odds with themselves. The recommendations can vary depending on whether they are made by a thread forming screw manufacturer, an injection molding contract manufacturer, the final assembly team, or even sometimes an industrial designer. The truth is that all of these inputs need to be considered, and the boss feature needs to perform and not cause cosmetic flaws that are visible on the part. But to achieve that a bit of a compromise needs to be met by the mechanical engineer designing the injection molded part. Of course all team members’ input needs to be considered, and in particular the injection molder needs to be worked with as they will refine the molding process and own the quality of the parts being produced.
So where does the problem begin and what are the problems? An oversized boss or improperly placed boss can cause sink marks on opposing walls of the injection molded part which may be key cosmetic surfaces. Sink in injection molded parts is where thick sections of plastic exist due to the intersections of internal features with an outer wall. If the thick section has too much more volume than the adjacent nominal wall thickness of the injection molded part, then the interior most plastic material will cool much more slowly and contract in the already cool and rigid plastic around it, creating dimples in plastic surfaces on the outside and stress in the part.
This can also cause warpage in an injection molded part, and, even worse, the boss features in the part may cause other mold flow problems affecting the injection molded part’s ability to fill out without other flaws.
On the other hand, an undersized boss may be a breeze to mold but might not retain the installed fastener causing parts and components to become loose and compromise the product, or worse the boss can bust out leading to part rejects in manufacturing and even full out product failure in the field.
To get it right it is kind of a balancing act to give the boss enough wall thickness so that as a fastener cuts or forms its way in to the pilot hole of the boss there is enough plastic material to handle those installation forces and enough to ensure that the threads of the fastener can rely on the boss for holding strength. Too little material and the plastic, if it does not break during installation, may cold flow and allow the fastener, and in turn the assembly, to loosen up, as the remaining material is not adequate to spread the stresses of the joint. These loads can vary depending on what is expected of the product, the pressures it sees, and even the range of temperatures it is used at.
To make the molding challenge even greater for the injection molded part, the boss will need to have draft to allow the part to be ejected from the steel mold tool, at least on its outer diameter walls. This means the cross section of the boss walls will get bigger at the base. It is usually good practice to have a slight fillet at this junction too. However, just like ribs, we talked about earlier, a boss can be thought of as a circular rib, and its wall section should be near 50 to 60 percent of the nominal wall it is bottoming out on, to prevent sink. It becomes difficult to maintain this constraint and have a thick enough wall section at the top of the boss such that the fastener engaging there does not just blow out the side of the boss’ wall, at least for anything other than very short bosses. Keep in mind very short plastic bosses do not allow for as much thread engagement by the fastener. For screws in plastic usually 2 to 2.5 times the nominal screw size is recommended for thread engagement. The number of threads engaged in a plastic bosses helps with strength more than what is conventional for a metal fastener in a steel nut.
To dial in the strongest possible boss, yet not have injection molding issues and cosmetic flaws like sink, a good design keeps the intent of the recommendations in mind but cheats a bit on both size and applies a couple of tricks.
By reducing the boss’ outside diameter from the overly thick walled recommendation of the screw manufacturer to a reasonably moldable thickness that still provides strength similar to the surrounding plastic walls, using minimal draft on smooth interior boss surfaces, and standing the boss away from outside walls allows for a good combination of strength and low risk of injection molding issues. Outside walls are kept to nearly a constant thickness and joining ribs are kept to 50 to 67 percent of nominal wall thickness. The pilot hole for the screw is sized near but not at the smallest diameter recommended, increased thread engagement depth ensures a strong joint in the plastic injection molded boss, spreading the loads for lower stress.
With the boss’ bore 30 percent deeper into the nominal base the boss sits on, and an outer surrounding trough at the base of the boss’ wall, the intersecting material sections is minimized which permits thicker boss walls than would be possible without these tricks.
Keep in mind that in the first injection parts produced out of the metal injection molding tool the bosses and specified fasteners can be further evaluated and confirmed for performance. There are many screw types available for applications in plastic with varied thread designs.
Additionally, a moderately designed screw boss in an injection molded part that has been designed with considerations for the assembly of the product and the part molding requirements will have a little leeway in either direction for further dial in. An approach where a bit more plastic is added to the injection molded part’s design to further thicken a boss will only require removing a bit more tool metal. This is called being metal or steel safe, and it is much easier and inexpensive to modify a metal injection mold tool by removing material than adding it back.
Changing boss bore size one way or the other may be fairly easy too as this may only require the replacement of a steel pin in the injection molding tool. And this can adjust fastener engagement, assembly torque values, and stresses in the plastic part. This usually is not an issue but is options if needed, say if the plastic alloy is varied during initial part shots.
Of course each individual design will have its own requirements and priorities. Some parts that are only for function and not seen on the outside of the product can accept molding flaws if they do not impart stresses that compromise overall strength. This is where the design team and the molder need to work together create the best parts possible.
Also, if the design of the injection molded part and its bosses have walked the line between the extremes, there is a lot that the molding partner can adjust via process, cycle time and cooling to reduce minor sinks so that cosmetic issues can be eliminated for the “as designed” injection molded part.
So work toward a balanced design, working with the hole development team to ensure the injection molded boss designs are easy enough to mold and strong enough to hold.