Screw Bosses Design Guide For Custom Plastic Parts
In today’s world, injection molded parts are more commonplace than ever. They have applications in most industries including automotive, electronics, kids toys, health care, and more.
Plastic injection molding is a popular method of fabricating plastic parts. At its simplest, it involves injecting molten plastic into a mold cavity where it then cools and solidifies acquiring the shape of the mold.
Sounds easy, right? Not quite.
When designing plastic components, you need to adhere to specific design guidelines. Failure to observe these guiding principles results in cosmetic flaws, structural failure, and overall ineffective parts.
In this blog, I share all you need to know about designing screw bosses for custom plastic parts. Learn the technology’s basic principles and actionable design tips that will save you time and cut costs. I’ll also cover the purpose and applications of screw bosses.
What are screw bosses in injection molding?
Before we get into the design principles of screw bosses, we need to define this injection molded part. So, what exactly are screw bosses?
Screw bosses are cylindrical protrusions with holes at the center. They’re a common design element that is typically positioned on a part’s wall. Screw bosses serve various functions including aligning parts, facilitating fastening, and strengthening parts.
Now that we’re more familiar with the basics of this feature, I’ll take you through the design guidelines of plastic bosses.
Screw Boss of Injection Molding Design Guide
Designing bosses is a fairly straightforward process. That being said, you’ll need to follow a set of specific design guidelines to ensure optimal performance and reduce the risk of cosmetic flaws.
When designing screw bosses, we need to consider factors such as wall thickness, boss height, draft angles, minimum radius, and more. In this section, I’ll guide you through the basic design guidelines to help you incorporate bosses into your part design successfully.
Minimum Radius At Base Of Boss
It’s advisable to have a slight fillet at the base of a boss to minimize stress. The junction between the base of the boss and the nominal wall is usually under a lot of stress. And just by having radii at the base, you can significantly decrease the stress concentration. However, if the radius goes beyond a maximum value, it creates overly thick sections.
In addition to reducing stress concentration, a radius at the base of the boss will improve strength and create enough draft for easy withdrawal from the mold.
The radius at the base of a boss should range between a quarter to half of the nominal wall thickness.
Spacing Between Bosses
If a part requires multiple bosses, you’ll need to consider the spacing between the boss features. This is because bosses that are too close to each other create thin regions that are challenging to cool down. Further, close proximity of boss features affects the efficiency and overall quality of the product.
An overly thin mold wall will also create a cooling challenge. Further, this type of component is hard to manufacture and does not last long due to differential cooling.
I’d recommend ensuring that the spacing between bosses is greater than or equal to 2 times the nominal wall thickness.
Radius At Base of Hole In Boss
A radius on the core pin is necessary to prevent sharp corners. This design feature serves two main purposes – it aids in the molding process while also decreasing the stress concentration. The value of the radius at the base of a hole in the boss should be between 0.25 and 0.5 times the nominal wall thickness.
Minimum Draft for Boss Outer Diameter
A draft angle refers to the degree of taper of the side wall of the boss from the vertical axis. Applying the right draft (or taper) to the outer diameter of a boss allows for easy removal of the part from a mold.
Without any taper or in case of inadequate draft, parts are often scratched during ejection and the mold is subjected to unnecessary wear and tear. This affects the surface finish of the part and the tool life of the mold.
To avoid these issues, I’d suggest applying a draft angle of 0.5 degrees or more on the external walls of the boss.
Minimum Draft for Boss Inner Diameter
An appropriate taper on the inner diameter of a boss facilitates easy withdrawal from the mold. You should apply a minimum draft on the hole in the boss of 0.25 degrees or more to mitigate the risk of damaging the part during ejection from the mold.
Boss Height To Outer Diameter Ratio
If a boss is too tall, it creates a material mass and a thick section at the base. Further, a tall boss causes challenges with cooling the core pin. These cooling difficulties will impact the cored hole dimensionally and increase cycle times.
To avoid these issues, you should limit the boss height to less than 3 times the outer diameter.
Minimum Radius At Tip Of Boss
Bosses are often used as points for attachment and assembly. This means that bosses are usually subjected to loadings not experienced in other areas of a component. To minimize the resultant stress, you’ll need to equip the tip of the boss with a fillet of a specific minimum radius value.
Chamfer At Top Of Boss
A chamfer positioned at the top of a boss serves a range of purposes:
It facilitates the assembly of parts by providing a good lead-in for fasteners.
The chamfer plays an important role in softening sharp edges.
This design element also provides a distinct look and therefore contributes to the aesthetics of the part.
With such benefits, there is no questioning the significance of a chamfer on a boss.
Wall Thickness Of Boss
Sinking is a common challenge associated with injection molded parts. It occurs due to uneven cooling rates in thick regions of plastic parts, i.e., the material on the surface cools and solidifies quicker than material near the core.
Standard guidelines advise that you should keep the wall thickness of a boss at around 60 percent of the nominal wall. If boss walls are thicker than this value they’re prone to sink marks and voids.
If your design calls for more strength than this stipulated wall thickness can offer, you’ll need to consider alternative ways to strengthen the boss. A popular solution is incorporating gussets into the design.
However, if the boss is not in a visible area, you can opt for increased wall thickness capable of handling the increased stresses imposed by self-tapping screws. Sink marks are more acceptable in less visible areas.
Standalone Boss
Standalone bosses need to be cored and connected with the nearest side wall using ribs or gaskets. This practice creates better rigidity and material flow.
Counterbore Design
The hole in a boss is typically unthreaded. Therefore, threading a screw through the boss feature will displace some plastic material from the part. It’s best practice to leave extra space capable of accommodating the displaced material. This additional space is known as a counterbore.
Guidelines for Material and Thickness Selection
The injection molding process is compatible with a wide range of plastic materials. Different materials will have diverse properties including ductility, thermal expansion, and clamp retention. Such properties ultimately determine the efficiency of the plastic part.
When designing a plastic part, the ideal thickness of a section will depend on the type of material. For instance, a brittle material will require thicker bosses to withstand stress compared to a ductile material.
To achieve better part integrity and stability, it’s good practice to use the same material for the injection molded part and the screw boss.
This table provides an overview of different types of materials and their corresponding recommended thicknesses.
What Is the purpose of a screw boss?
Bosses are a common structure in injection molded parts and they serve a range of functions. In this section, I’ll cover some of the common purposes of a screw boss.
Prevents sink marks and warping
Sink marks are one of the most common defects during the manufacturing processes of plastic parts. While these small depressions do not affect the strength and function of the part, they interfere with its cosmetic finish. Warping is another common defect and it results in permanent bending.
A properly designed boss will reduce the risk of sinking and warping. By choosing thin rather than thick sections, the material cools evenly and you minimize sinking and warping.
Thread Integrity
Bosses are often used with threaded inserts, especially if the material of the boss doesn’t support threading. These components are inserted into the holes of bosses and they provide a sound and robust threading interface.
Sometimes, a boss design will include an undercut. This feature provides better thread engagement and minimizes the risk of loosening or falling out of the screws.
Strength and Durability
Bosses are often used with threaded inserts, especially if the material of the boss doesn’t support threading. These components are inserted into the holes of bosses and they provide a sound and robust threading interface.
Sometimes, a boss design will include an undercut. This feature provides better thread engagement and minimizes the risk of loosening or falling out of the screws.
Material Compatibility
As is common with other injection molded parts, bosses can be made using a wide range of materials with diverse physical properties. Apart from individual materials, it’s also possible to combine different materials and create blends (such as PC/ABS) that offer your preferred strength.
The extensive material compatibility of bosses means that they are useful for diverse applications across industries.
Convenient Assembly
A well-designed boss will not only aid in the assembly of parts but also make the whole process more efficient.
If a high-quality plastic part was to be disassembled for instance, it should be easy to put it back together. Bosses serve as a positioning aid as they align and show you how parts are meant to fit. With poor design, however, assembling becomes a more complex process.
Design elements such as a chamfer at the top of a boss also facilitate quicker assembly as they are a good lead in for fasteners. A rapid assembly process saves time and, therefore, cuts down costs.
Avoid Material Damage
Being attachment points, bosses typically carry significant loads. However, thanks to design elements such as incorporating radii, bosses allow for better stress distribution.
A key function of screw bosses is to strengthen vulnerable parts. This property mean that bosses play a crucial role in preventing material damage.
Prevent Loosening
Screw bosses are essential in securing parts together. They hold components in place during the assembly process and allow fasteners such as screws and threaded inserts to fix the parts in place. This ensures that components do not become loose or fall off.
Design Flexibility
Bosses come with a high degree of design flexibility. A boss can serve as a positioning aid, fixation point, or even a bearing surface where another part rotates about.
Further, bosses are made using a wide range of materials, and therefore, they’re available in different thicknesses. You can also add elements such as draft, radii, counterbore, and chamfers to achieve the your desired specifications.
Bosses allow for a variety of designs without compromising the look and functionality of parts.
Conclusion
Screw bosses are an important feature in injection molded parts. They serve as a positioning and fixation aid while also strengthening the part. Careful consideration of the basic design guidelines ensures that your bosses are durable, reliable, and free from any cosmetic flaws.
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