April 27, 2019

Plastic Boss Design on Molded Parts

 

 
 
 
 
 
 
 
Princeton’s WordNet defines “emboss” as “raise in relief,” and that’s exactly what a boss is: a feature raised above a surface. In plastic parts, bosses are typically used to assist in assembly, as a receptacle for a screw or threaded insert or as the locator for a mating pin on another part.
Because of its function, a boss must have sufficient strength to do its job. This dictates a minimum size for the feature. At the same time, because a boss rises from a surface, it thickens the surface at that point raising the risk of sink or development of voids as the part cools. The challenge: bosses should be big enough to do their job but not big enough to cause avoidable sink in the surface from where they rise.
Design Cube features several plastic boss designs

The Design Cube illustrates various boss features.
Boss Configurations Vary on Plastic Parts

A typical boss is an open-topped cylinder, essentially a round rib. Standard guidelines suggest that its wall thickness be between 40 and 60 percent of the thickness of the wall from which the boss rises. If your design requires more strength than this guideline would provide, you should consider ways to strengthen the boss without thickening its walls. The most common of these is to surround the boss with gussets to support and strengthen its walls.
If a boss is part of a vertical wall, it should not create a thick area in the wall. Similarly, if a boss is located close to a vertical wall, it may be tempting to tie the boss to the wall by filling the space between the boss and the wall, resulting in a thick area. A better way to tie the boss to the wall is with one or more ribs.
In some cases, Protolabs’ milling process may require that the walls of the boss be thicker than the standard 40 to 60 percent mentioned above. Unlike steel tools, which can use steel core pins to form the inside diameter of a boss, Protolabs forms both the outside diameter and the inside diameter of a boss with features of the aluminum mold. When cutting a mold for a relatively tall boss, we may not be able to use a sufficiently small-diameter cutter to create walls that thin. You can avoid the problem by reducing the height of the boss, allowing the use of smaller-diameter cutter. Or you may find that a small amount of sink in the wall surface opposite the boss is acceptable.

Other Considerations for Plastic Boss Design

There are two additional points to consider in designing bosses. As mentioned earlier, a boss is a circular rib, and like any rib, its walls—both inside and out—must be drafted to facilitate ejection. Depending on the height of the boss, this draft can be anywhere from a half degree to 3 degrees. If draft of the inside diameter of the boss is not acceptable, the boss can be molded as a solid and the inside diameter drilled, by you, as a secondary operation. Also, because a boss is formed by a blind hole in the mold body, we may have to add vent pins to the rim of the boss to allow the escape of trapped gas during mold filling. Otherwise shorts or burns may form on the rim.

roduct designers build strength into their injection-molded parts in different ways and for different reasons. Dissecting part application is a good first step in determining if additional strength should get designed into a part. How will customers be using your product and what environment will it live in? You may need parts that can withstand repetitive impact, resist wear, or bear heavy loads. It may be as simple as integrating ribs or gussets into your design, or a more complicated combination of design elements involving support features, material, wall thickness, and more. Finding the proper balance of design considerations will help address your part’s need for strength and stability.
Features that make stronger molded parts on Design Cube

Our design cube illustrates thick and thin ribs, proper use of gussets, well-designed bosses, and other considerations to be mindful of when building strength into parts.*
Ribs and Gussets

Ribs are thin, wall-like features typically designed into the geometry of a part to add internal support to walls or other features like bosses. In a similar fashion, gussets are support features that reinforce areas such as walls or bosses to the floor. Just as bridge beams and columns are supported at their vertex with gussets to add critical strength to the structure, the same concept applies to plastic injection molding.
Both ribs and gussets provide stability in parts without having to increase wall thickness, and are particularly beneficial for parts with already thin walls that could potentially be compromised by end user wear and tear. It’s important to note that ribs and gussets should be no more than 60 percent of nominal wall thickness. These features are kept thinner than the primary walls in an effort to avoid overly thick sections where ribs and gussets intersect with the wall. When you have a surplus of material meeting at internal rib-to-wall intersections, sink marks can appear on the visible side of the part.
Product designers can play with different ribbed formations to create square, rectangle, diamond, triangle, or honeycomb patterns that stiffen the part. A pattern of ribs is equivalent to coring out unneeded material, leaving only the rib support system—it also reduces the weight and cost of the part. But, remember to not remove surfaces and features that interface with the other parts in the product assembly.

Fillets and Radii

Because sharp corners weaken parts, fillets—curved faces where ribs meet walls—can also be designed into part geometry to eliminate additional mechanical stress concentrations on a finished part. Much like gussets, a fillet that is too small will fail to accomplish its task of stress reduction, but a fillet that is too large can once again create sink. Identifying the proper size and locations of fillets (and ribs and gussets) are important. When adding a fillet to the inside of a corner, also add a radius to the outside of the corner, if possible. If the risk of sink is too high in some sections, then other strength-building methods should be considered.

Durable Thermoplastics

Material selection also plays a role in the stiffness, durability, toughness and other characteristics of parts; balancing the relationship between those material properties and part functionality is key. For example, product designers can choose a thermoplastic resin that will make a stiff, rigid part, but if its application demands a high degree of impact resistance, the brittleness of an inflexible part may cause it to break. Material properties differ from resin to resin—here’s a glance at some of our more frequently used resins:

  • ABS is a good, stable consumer-grade resin that is tough and impact-resistant in daily use environments. It’s commonly used in housings for remote controls, battery-powered tools, and body panels for monitors, printers, and copiers. There may be chemical resistance concerns with ABS.
  • Polycarbonate is more impact-resistant than ABS, and good for lenses and parts that require more shine. It is susceptible to stress cracking, and risks crazing/hazing due to chemical compatibility concerns.
  • Unfilled nylon is pliable and impact-resistant with good lubricity for wear. Glass-fiber filler increases nylon’s stiffness and compressive strength, but the material becomes more brittle on impact. Glass-fiber filler helps increase heat deflection.
  • Acetal is an excellent self-lubricating bearing material with great wear properties and good stiffness. It’s not good for cosmetic parts or parts that require pad printing, paint or decals.
  • TPEs are great for dust seals and padding corners for impact resistance, and used in overmolding applications for grip features. They’re not always good in dynamic applications; static applications are best. There may be chemical resistance concerns with TPE.

Reinforcing materials with an additive can also build strength into parts. Long and short glass fibers make resins stronger and stiffer (but more brittle), and carbon fibers can make resins even stiffer yet. Minerals such as talc and clay are often used as fillers to increase the hardness of finished parts; and glass beads and mica flakes are used to stiffen a part and reduce warping and shrink.

Wall Thickness

Sometimes, if you want to add strength to a part, you just have to increase overall wall thickness. Protolabs provides a list of recommended wall thicknesses based on resin type to help design parts that are neither too thin nor too thick. The larger that parts get, the more attention needs to be paid to ribs, gussets, materials, and other factors that improve strength. As usual, our applications engineers are available to discuss part geometry and design, and look for the interactive moldability analysis that comes with every quoted part at Protolabs.

Press fits for injection-molded parts can be challenging. A well-designed injection-molded part will usually have draft, but the same draft that helps eject a part from the mold may also keep a press-fit part from staying firmly engaged. Consider a gear pressed onto a shaft (see Figure 1). In this case, the shaft has a “D”-shaped profile that acts as a drive flat and an orientation feature for assembly.
The drive flat keeps the gear from spinning freely on the shaft under load. Your molder will likely ask for draft on the D-hole to help release the feature from the mold. Draft is needed since as the resin cools, it shrinks onto the core that creates the hole. The request for draft is reasonable, but what if your design intent does not allow for draft? Here are a couple of options.
1. Leave the hole as-is and require the molder to support zero draft. This is a risky request. If the hole is shallow enough you might not have any problems, but as the hole gets deeper, more stress is applied to the core of the mold during cooling and ejection. Increased force required for ejection could result in “pin punch” on the part, and increased ejection forces could result in breaking the core or ejector pins. The molder may have to tweak process parameters to prevent damage to the mold. This can increase the likelihood of imperfections like sink, porosity, and weak knit lines. Considering all the possible consequences, it makes sense to consider other options instead.
Illustration of drive shaft part

Figure 1: D-shaped through-hole in center of gear part accepts the drive shaft.
Press fits for injection-molded parts can be challenging. A well-designed injection-molded part will usually have draft, but the
Draft and assembly function

Figure 2: The cross section shows a portion of the assembly cut in half. Notice the draft on the purple plastic through-hole toward the bottom of the image. This could allow wobble in the finished assembly.
2. Add draft to the hole. Draft allows the part to release from the mold because the shrinking part does not have to be forced along the same diameter shaft for the depth of the hole. If the hole is drafted, once the ejector system gives a slight bump to the part, the part releases from the mold because the draft falls away from the part wall. This reduces stress on both the part and the mold. Having draft allows the molder the flexibility to use process tweaks to effectively address other geometric and cosmetic concerns. Draft solves the production problems, but your responsibility as the designer is to ensure that the draft doesn’t adversely affect the function of your assembly. For example, you would not want to have slop in the fit that could cause gear wobble (see Figure 2).
3. A good compromise option is to add crush ribs. Crush ribs can give you the best of both approaches: draft for the molder and the same alignment you’d get with a straight-sided hole. The main hole is drafted for ease of ejection and protection of the mold, and three or more undrafted ribs along the length of the hole create a tight fit and good alignment with the shaft (see Figure 3).
Injection molding crush ribs illustration

Figure 3: Traditional crush ribs. The sharp points of the plastic are undrafted ribs, while the main internal diameter of the hole has draft.
Because the ribs present only a small surface area to the mold, their lack of draft creates little resistance to ejection and less risk of damage to the mold. The narrow points where the ribs meet the shaft (or other mating part) allow the ribs to deform during press-fit to ensure a tight fit without creating a lot of stress on the part. Unfortunately, the sharp “V” features cannot be directly milled into the mold and thus require electrical discharge machining (EDM) or other extra processing during mold-making.
All is not lost, however. Protolabs suggests an alternative rib shape that can be directly machined into the mold, minimizing cost. In place of the sharp point of the traditional “V”-shaped rib, consider a rounded rib (see Figure 4). A rounded rib can be directly created by milling, eliminating the need for EDM electrodes. It is faster and less expensive than a “V”-shaped alternative, and still provides the narrow, “crushable” point of contact with the mating part.
Crush rib design example

Figure 4: Suggested Crush Rib Design. The contact points of this design are formed by the radius of the end mill, allowing the inside diameter to be drafted while the ribs remain undrafted.
The same form can be used for ribs designed to create “standoff” where parts mate, for example where an air gap is needed. While Protolabs offers EDM in some applications, we believe the full-radius rib created by an end mill is just as effective and reduces cost by minimizing mold complexity and manufacturing time. This reduction in time allows you to receive parts faster, test sooner, and bring your ideas to market before your competition.

 

Everyone wants to save money on manufactured parts. It sounds simple, but one of the easiest ways to reduce price-per-piece cost in injection molding is by increasing part quantity. That is because the initial upfront cost to design and machine the mold amortizes over more parts. At Protolabs, for example, up to 25,000 parts or more can be molded from the same tool.
However, maybe your molding project calls for only a handful of parts. No worries. Protolabs builds cost-effective molds for production quantities as low as 25 pieces, often within a few days of ordering.

Undercut illustration

Undercuts, such as those shown, can complicate and, in some cases, prevent part ejection, so eliminate them if possible.
In addition to per-unit costs, consider the material. Many plastics overlap in strength and functionality, but some are inherently easier to mold, driving down part costs. You can experiment with different materials in the interactive quote you receive when you upload your design to Protolabs.
Here are 11 injection molding design considerations to stretch your manufacturing dollar:
1. Eliminate undercuts
2. Get rid of unnecessary features
3. Use a core cavity approach
4. Reduce cosmetic finishes and appearances
5. Design self-mating parts
6. Modify and reuse molds
7. Pay attention to DFM analysis
8. Use a multi-cavity or family mold
9. Choose on-demand production option
10. Consider part size
11. Use overmolding

Undercuts

Undercut features complicate and, in some cases, prevent part ejection. Get rid of them if you can, but maybe that’s not possible, if, for example, you need a side action, sliding shutoff or pick out. One alternative may be using sliding shutoffs and pass-through cores, or by changing the parting line and draft angles to provide an easier mold build. These reduce tooling costs as you avoid additional pieces to the mold that add to manufacturing costs. In addition to the rise in manufacturing costs of using hand-loaded inserts, this also may have an impact on your piece part price because of longer cycle times and manual mold operation.

core cavity illustration

Using a core cavity, as shown, can be a cost-effective method of molding tall walls and ribbed surfaces.
Unnecessary Features

Textured surfaces, molded part numbers, and company logos look cool, but be prepared to pay a bit extra for these and other non-mission critical features. That said, permanent part numbers are a requirement for many aerospace and military applications. Use a mill-friendly font such as Century Gothic Bold, Arial, or Verdana (san-serif fonts), keep it above 20 pt., and don’t go much deeper than 0.010 to 0.015 inch. Also, be prepared to increase draft if part ejection is a concern.

Core Cavity

If you need an electronics housing or similar box-shaped part, you can either sink the wall cavities deep into the mold base, requiring long thin tools to machine ribs into the mold, or machine the aluminum material down around the core and mold the part around it. The latter approach is known as a core cavity, and is a far more cost-effective method of molding tall walls and ribbed surfaces. Better yet, this makes it easier to provide smooth surface finishes, adequate venting, improved ejection, and can eliminate the need for super-steep draft angles.

Cosmetics

Pretty parts are nice, but they often require bead blasting, EDM, or high mold polish to achieve an elevated level of cosmetic appearance. This drives up tooling costs. Anything greater than a PM-F0 (as machined) finish requires some level of hand work, up to an SPI-A2 mirror finish using Grade #2 diamond buff. Avoid fine finishes such as these unless they’re required for the job. One thing to consider regarding cosmetics is to let Protolabs know if you need the entire half of the mold polished or maybe just one small area. You could save costs by only polishing the area needed versus the entire side of a mold. When requesting a custom finish, just send Protolabs a color-coded image of the critical areas and desired finish level for each area.

Self-Mating Parts

Maybe you’re designing a snap-together case for some medical components, or two interlocking halves of a portable radio. Why build two mating parts when you can make one? Redesign the snaps so that the halves can be fit together from either direction, thus building a so-called “universal” part. Only one mold is needed, saving production expenses up front. And you can now mold twice as many of one part, instead of half the quantities of two.

Mold Mods

It is relatively easy to remove metal from an existing metal mold. Adding metal, on the other hand, can be difficult or, for all practical purposes, impossible with rapid injection molding. To look at this from the part perspective, you can add plastic, but you can’t take it away. Designing with this in mind is called “metal safe.”
Some injection-molded parts go through multiple iterations until a final, workable design emerges. Instead of purchasing a new mold for every revision, a little clever planning will allow the same mold to be used multiple times. Starting with the smallest, most basic part design, mold as many pieces as needed, then re-machine the mold to include additional part features, or a larger, taller version of the same part, and mold again. This is not an exact science, but given the right part, this re-use approach can save dollars on tooling development.

Moldability-DFM Analysis

Every quote for an injection-molded part at Protolabs is accompanied by a free design for manufacturability (DFM) analysis. This identifies potential problem areas, or opportunities for design improvement. Insufficient draft angles, “un-machinable” features, impossible geometries—these are just a few examples in which part designs can and should be improved before clicking the “accept” button. Be sure to review these suggestions thoroughly, and contact an applications engineer at Protolabs with any design-related questions.

Multi-Cavity and Family Molds

Maybe you are after a higher volume of parts? You can still achieve high volumes using aluminum tooling with two-, four-, or eight-cavity molds depending on size and part geometry that can reduce your piece part price, although this would impact your tooling costs.
Got a family of parts that all fit together? How about multiple molding projects at one time? There’s no reason to build a mold for each individual part, provided A) everything is made of the same plastic, B) each part is roughly the same size (e.g., have similar processing times), and C) can all be squeezed into the same cavity, while still allowing for proper mold functioning.
In addition, maybe you can join some of those parts with a living hinge? This method is a great way, for example, to mold two halves of a clamshell-style container. These parts would otherwise need a pin-type assembly to open and close. The only caveat here is that a flexible and tough material must be used, such as polypropylene (PP).

On-Demand Option

Still another way to reduce molding costs, depending on your part volumes, is to consider on-demand manufacturing. At Protolabs, two injection molding service options are available (see table below). One is best suited for those who need smaller part quantities, usually associated with prototyping. The other option, Protolabs calls it on-demand manufacturing, is a good fit for those who require slightly larger part quantities, typically up to 10,000-plus parts from aluminum molds. On-demand production can be a great option to manage demand volatility of your parts, reduce total cost of ownership, and leverage cost-efficient bridge tooling.

This is an example of one half of a self-mating part, which fits together in either direction with its other half, building a “universal” part.

 

This eight-cavity mold is used for a higher volume of parts (finished part is pictured above the mold illustration).

 

Here is an example of a family mold, used to produce the part pictured, which was a component of the SubQ It, a medical tool used to close surgical incisions with bio-absorbable fasteners.

 

PROTOTYPINGON-DEMAND MANUFACTURING
OBJECTIVEI need to validate my designI need the flexibility of on-demand production of end-use parts
BEST
WHEN
  • Lifetime volumes are likely below 2,000 parts
  • An affordable entry point for tooling is important
  • Part production is only needed for 1 year or less
  • Lifetime volumes exceed 2,000 parts
  • Lower part price is critical
  • Part production is needed for several years
MOLD
PRICE
LowerHigher
PART
PRICE
HigherLower

Not sure which service is right? Take a complete look at the service options or get quotes for both to compare.

Part Size

Always consider part extents. In molding-speak, that means how big is the part, and will it fit comfortably in the mold while allowing for sprues, runners, ejector pins, and all the other considerations needed to make a mold work. Protolabs’ maximum part size for injection molding is currently 18.9 in. (480mm) by 29.6 in. (751mm) with a maximum depth from the parting line of 4 in. (101mm) deep. However, larger parts like these, in turn, require a larger mold. This may have an impact on your mold and part costs.

Overmolding

Finally, consider an overmolding option. Depending on the part, it might cost you more upfront, but could potentially save you money later. For example, overmolding a gasket on a part upfront might be an added cost, but it can save costs later from having someone install a gasket manually.
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