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.

Draft angles are necessary design considerations in structural foam molding and since lower pressures are applied in this technique, the draft angles can be smaller. You should however; note that higher cavity pressures make it difficult to release the part from the mold which is why the draft angle should be larger if the wall thickness is thin.

The process of structural foam molding primarily involves the addition of a blowing agent and the modification of process parameters to create a sandwich construction. This particular construction is solid on the outer layers and has a foam core center.

If varying wall thicknesses are needed, then transitions from thick to thin should be tapered with radii and fillets. It is recommended to include a gate in the thinner section and let the material flow into the thicker sections.

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.

The honeycomb structure that is created during the structural foam molding process enables a very high strength-to-weight ratio as compared to the parts manufactured via traditional injection molding techniques.

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.

A structural foam is a type of composite material that is created when a polymer combines an inert physical gas, such as nitrogen, or a chemical blowing agent during the molding process. It has a low-density, cellular network at its core and is combined with a high-density solid skin. The cellular core decreases the overall weight while the solid surface enables the part to stay strong and impact resistant.

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.

Structural foam molding also allows greater flexibility in terms of resin selection since even commodity resins can be used in this technique, thus enabling manufacturers to lean towards cost-effective resins if possible.

Injection molding is carried out in two stages. The first stage is an injection stage, where the molten resin is injected into a mold, and the second stage is the packing stage, where pressure is built and the molten resin takes the mold shape to form the plastic part. In structural foam molding, the injection stage is similar to the traditional injection molding technique, however in the latter stage, the resin is mixed with a blowing agent that is activated by the molten resin’s heat. At this stage, the blowing agent expands the resin and creates a cellular foamed structure which takes the mold shape and forms the part.

You should note that there is a higher resistance in wall sections that are less than 0.250” / 6.35 mm however; this can be compensated with increased injection pressure.

Today, structural foam molding is a highly evolved technique that can create lighter, stronger, and more resilient plastic parts than traditional injection molded parts. In this article, we discuss the structural foam molding technique in-depth, its design considerations, and key differences with traditional injection molding techniques.

Just like any manufacturing technique, engineers must consider the design criteria early on in the process to manufacture parts efficiently via structural foam molding. Let’s understand the various design considerations for structural foam molding:

Structural foam molding is also a competitive technique in terms of cycle times when compared to injection molding and other techniques thus, making this technique a cost-effective as well as time-saving process.

In the structural foam molding process, two components are stored in separate containers in liquid form which are then mixed to form a resin. This resin mix is injected into a pre-prepared mold which is then cured via chemical reaction. In this process, lesser quantities of the resin mix are used so that the resin itself does not fill the mold.

Only thermoplastics can be used to manufacture plastic parts via the structural foam molding technique and thus, this technique is extremely limited to the type of materials that can be used for manufacturing.

Traditional injection molding techniques are extremely expansive and inclusive to various project requirements however; structural foam molding techniques pose certain advantages w.r.t design and operational requirements. Let’s understand when should you choose structural foam molding over traditional injection molding techniques:

In structural foam molding, bosses are frequently used to insert and attach fasteners to assemble multiple structural foams, and ribs are commonly used to increase part rigidity and load-bearing capability. Ribs are also incorporated instead of increasing wall thickness.

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.

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.

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.

Structural foam molding is a low-pressure injection molding process which is why aluminum molds are suitable and can be used for this technique. In addition, structural foam injection molding can incorporate other types of materials such as metal and wood.

The blowing agent in this process is activated by the reaction between the two components and it further expands to fill the surplus space of the mold with foam which creates the honeycomb texture in the center. The surface cells collapse as they come in contact with the mold wall thereby creating a solid or rigid surface.

This is a low-pressure injection molding process and the intended aim is to reduce the density and thus, the weight of the finished part.

The parts that are manufactured via structural foam molding have improved electrical insulation characteristics and better heat insulation as the cellular core of structural foam parts demonstrates excellent insulation properties, thus, making this technique apt for parts that require thermal or acoustic insulation.

At VEM-Tooling, we have an experienced team of design experts and engineers who can help you choose the right solution for your project!

The parts that are manufactured via the structural foam molding technique are much more impact-resistant and can be applied to manufacture large parts due to lower pressure and in metal-to-plastic replacement.

While we cannot do foam molding, you can still reach out to us to have a look at your part. In many assemblies, there are parts that can be manufactured by injection molding, which is our core competency for decades.

Christian is the Executive Director of Operations at StudioRed, a longstanding design studio with over 40 years of experience, known for its dedication to innovation, as evidenced by a diverse portfolio of 4000+ completed projects, 200+ awards, and the successful launch of 450+ products to market. With a background in Economics and Technology Management, Christian helps bring a fresh perspective to the engineering and design team throughout the development process, all while keeping things organized.

Structural foam molding is a type of manufacturing process in which, the packing stage involves mixing the molten resin with a blowing agent to create thermoplastic foam. The chemical blowing agent is subjected to heat and it works by creating a microcellular structure to expand the material and form the part against the mold.

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.

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.

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.

The wall thickness in structural foam molding can be as low as 0.180” / 4.5 mm, and up to 0.500” / 12.7 mm or thicker. The optimal thickness of wall sections should be 0.250” / 6.35 mm.

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.

Structural foam molding techniques can manufacture thick-walled parts and intricately detailed parts without causing any depressions or sink marks and it is thus more efficient than traditional injection molding techniques. In addition, the low-pressure injection process of structural molding reduces stress and plastic warpage issues.

Structural foam molding is more economical than injection molding and other techniques. Since this technique can incorporate Aluminium molds instead of Steel molds, it gives the technique an edge in being cost-effective. The low pressure and clamping forces in this technique minimize the wear and tear.  In addition, if Aluminum molds are maintained well, then their lifespan can be lengthened.

This technique can be used to manufacture parts that have a certain range of wall thicknesses, and thus, is again limited to only certain types of parts.

Parts that are manufactured via the structural foam molding technique are lightweight, strong, and durable. Structural foam molding may be the right design choice if you’re seeking to manufacture a lower-weight high-strength part.

As the cooling occurs, a honeycomb structure develops to form the interiors whereas a solid skin is formed against the mold. The honeycomb structure of the interiors has various advantages such as preventing shrinkage and reducing product weight without compromising structural integrity.

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.

Since the base is a cellular network and not a solid structure, the base material that is typically used to create a structural foam is a thermoplastic polymer rather than a thermoset. Some of the most commonly used thermoplastics in structural foam molding are polyurethane, polycarbonate, polyphenylene oxide, and acrylonitrile butadiene styrene.

You should note that although Aluminum molds are cost-effective, however; they aren’t long-lasting, especially when subjected to the high pressures of traditional injection molding techniques.

The parts manufactured via structural foam techniques aren’t as dense as similar parts that are manufactured using other processes and are thus lighter. The finished part manufactured via structural foam molding is typically 10 – 40% lighter than an equivalent solid part.

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.

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.

You should note that in structural foam molding, the part walls must be thicker as thin walls can deter the required reaction to create the cellular structure.

Since the structural foam molding process uses a blowing agent, it restricts the manufacturing of various part types and thus, has limited design flexibility.

Structural foam molding is a type of manufacturing technique that has a unique approach and distinct advantages. It is a popular technique to create stronger, more rigid plastic parts and the means to reduce potential part weight for larger parts.

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.

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