Plastic screw boss design guide

With so many ways to formulate plastic to get just the right combination of color, texture and durability, it's easy to forget how the parts will be assembled.

However, if the parts will be assembled with screws, overlooking such parameters as thread style, driver speed and boss design could spell disaster on the assembly line. Souza, fastening consultant with Quality Screw and Nut Co. Leominster, MA. Many types of mechanical fasteners can be used to assemble plastic parts, including rivets, spring clips, and machine screws with mating nuts or inserts.

But the most common mechanical fasteners for plastics are thread-forming and thread-cutting screws. Thread-forming screws create threads by using the screw as a forming tool to push the walls of the plastic hole into the shape of a thread, says Bruno Marbacher, engineering manager at Bossard USA Portsmouth, NH.

The threads at the tip of the screw are smaller in diameter, so the pressure on the wall of the boss is applied gradually. Thread-forming screws can be safely used with plastics that have a flexural modulus ofpsi or less. With a large enough hole, these screws can also be used in plastics with a flexural modulus as high aspsi.

Thread-cutting screws are required for harder, less ductile plastics. Thread-cutting screws remove material as they are installed, reducing stress on the boss. However, these screws cannot be installed and removed repeatably. A thread-forming screw will crack the material. One of the biggest differences is in the flank angle. Standard screws have a flank angle of 60 degrees.

In contrast, screws for plastics have a flank angle of 30 to 48 degrees. The narrower flank angle exerts less outward force on the boss. This reduces the likelihood that the boss will crack and allows engineers to make the boss as much as 30 percent thinner.

The narrow flank angle also means the threads are longer, so more plastic is captured between each thread.

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The shear load can be distributed over a larger amount of plastic. And, the lower profile helps reduce installation torque. Another difference is thread spacing, says Souza. Whereas a No.Such features are designed into the latest release by model train specialist Bachrus Inc. The model train controller product has some nice architecture features built into the electronic housing to help the form fit with the function — the outside that is seen in the creative exploration of shape and style.

As the goal here was to wow the end user and add value to the product line, but with that said the inside was meticulously engineered to give that outer appearance a good first impression.

Because to get those right means that there is added strength without added wall thickness — as surface blemishes due to sink marks in the plastic show up more with an increase in plastic. So there are some minor tweaks that can make a design even better if you find the right source. However, I thought I would compile a little bit of information on the topic here in a handy, easy-to-access blog report.

The preferred thermoforming machine screw, which is engineered for plastic, is a thread forming screw like the EJOT PT cross recessed raised cheese head style screw — sourced in many places such as the Fabory Catalog. The key advantages of this thread forming fastener are many:. Finally its recessed thread root design gives optimal material flow into the cavity recess between the threads on the screw as it cuts into the boss.

plastic screw boss design guide

Companies like Acument, of Sterling Heights, MI, give some general guidelines online to create the qualifiable boss design you will need see the chart below or look at: www. Acument recommends to keep-in-mind that specific applications need to allow for various molding conditions, tool design, weld lines, and proximity to any injector gates, etc.

With all that said though, the laboratory testing and in the field applications have produced great results for product designers to rely on, instead of the educated guess approach or the simple tube with draft. Also a designer needs to notice the different recommendations for the inner diameter i.

Design Guide

As each plastic has its own molecular composition, some polymers are prone to cracking under expansion stress, while others are more capable of resisting those pressures when the screw is driven into place. Knowing your product material early on in the design process helps you create the optimal boss dimensions. The surface finish on the final part might be high-gloss and capable to reflect subtle changes in the surface. So go ahead and get the right boss design built into your parts and you will be on your way to making great designs.

Contact David Duncan d. This is the second contribution of his monthly blog for Design Product News. More of his blogs are available at www. For his previous DPN blog, click here.Most high-volume plastic parts for the consumer market will ultimately be manufactured with a plastic injection molding process due to its unmatched economy for high volume. The 1-dimensional manufacturing physics of this process often dictate that a structure be built in multiple parts that are snapped or screwed together.

Therefore, many printed parts during the design cycle will be fabricated according to plastic injection molding design rules and will often require metal fasteners to assemble. The technologies high fidelity, surface quality, and isotropic material properties make it a great tool for all aspects of the design process including visual mockups, engineering prototypes, functional parts, jigs and fixtures, and manufacturing tools, etc To achieve accurate dimensions without geometric defects, design rules for plastic injection molding require parts with relatively thin and constant wall thickness.

From left to right the following features are shown: 1 Drilled and taped hole with hardware, 2 Press fit precision dowel pins. The MJP 3D printed diagnostics above and below show different sizes of screws that were driven into the simulated injection molded boss using an electrical drill. The boss wall thickness was 1 mm.

Standard machine screws from a hardware store can be used for injection molded bosses. However, special plastic thread forming or thread cutting screws have been developed and are commonly used for high volume injection molding applications.

Thread cutting screws have features that aid in cutting the threads into the plastic when inserted. They create less hoop stress but require less torque to become lose and less tensile force to be pulled out. Thread forming screws do not have a cutting tip and are designed to displace material in the plastic boss to create a mating thread. Thread forming screws can generates high levels of radial and hoop stress and may result in damage to a rapid prototype printed part.

The following 3D printed parts were done on a ProJet using M2R-CL and M2R-WT and show how sheet metal screws can be used with injection molded screw bosses with production-like wall thicknesses using holes that are standard for the given screw size. Sheetmetal or plastic screws can be used with both solid structures and injection molded screw bosses with production-like wall thicknesses using correct hole sizes for the given screw.

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Injection molded bosses must be strong enough to achieve the desired holding forces and yet not too thick so as to avoid part defects like surface sinks or voids in the plastic.

Correct hole size and wall thickness are both very important as there are failures on both ends of the sizing. This is true because threading into plastic with metal screws can result in a few different failures. Anytime the hole is oversize, the joint will fail due to a lack of thread engagement or the thread engagement will be stripped out completely as the screw is driven in.

When the hole is undersize, the smallest screw sizes might physically sheer off due to the torsion required for insertion or the screw driver engagement or thread engagement might be stripped out. The material type selected for these prototyping needs is important to consider along with the hole size and screw size choice.

The engineering material M2R-CL Armor was specially designed with both good stiffness and high elongation to work in injection molded boss applications with both thread cutting and thread forming screws.

The M2G-CL is likely the best material for complex engineering applications. Material selection is one of the most important factor for complex engineering applications.The illustrated moulded boss features a 1 Degree inclusive tapered hole. However, it is not a prerequisite. It is commonplace for these screws to be installed into drilled holes also.

To identify a suitable hole size for a selected screw diameter, choose a material from the boss geometry tables below closest to the application boss type and multiply the value in column A by the nominal external thread diameter. For example:- A 3. Where possible, the inclusion of a counter bore is recommended. A counter bore helps reduce the risk of damage to the top of the boss, reduce stresses in the boss, helps with screw alignment and acts as relief for any material extruded to the head of the screw, allowing flush finishes.

To calculate the counter bore diameter, add 0. The High Performance Plas-Tech 30 screw for plastic is a direct screw fastener for use in thermoplastics. It's design characteristics include a profiled thread root to assist plastic flow and stress reduction, a 30 Degree flank angle to reduce boss bursting forces and an 8 Degree pitch angle which combine to support low installation torques with high stripping torque values.

The Essentials of Better Plastic Part Design

Your browser doesn't support video.Bosses are used for the purpose of registration of mating parts or for attaching fasteners such as screws or accepting threaded inserts molded-in, press-fitted, ultrasonically or thermally inserted.

However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws. The boss can be strengthened by gussets at the base, and by attaching it to nearby walls with connecting ribs. High hoop stresses caused because of too much interference of the internal diameter with the insert or screw.

Knit lines -these are cold lines of flow meeting at the boss from opposite sides, causing weak bonds.

The thermoformed Plastic Boss: A How-to Guide

These can split easily when stress is applied. Knit lines should be relocated away from the boss, if possible. If not possible, then a supporting gusset should be added near the knit line. Bosses are used to facilitate the registration of mating parts, for attaching fasteners such as screws, or for accepting threaded inserts. Wall thicknesses for bosses should be less than 60 percent of the nominal wall to minimize sinking. The base radius should be a minimum of 0. Bosses can be strengthened by incorporating gussets at the base or by using connecting ribs attaching to nearby walls.

Links: Article content. Bosses Bosses are used for mounting purposes or to serve as reinforcement around holes. Good and poor design is shown in Figure 3. The same principles used in designing ribs pertain to designing bosses, that is, heavy sections should be avoided to prevent the formation of voids or sink marks and cycle time penalty.

Boss design recommendations are shown in Figure 3. You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Skip to content Home Boss design guidelines. Hoop stresses are imposed on the boss walls by press fitting or otherwise inserting inserts.

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Alternative: You are going to find slight difference here in the guidelines, however the principles are similar.Injection molding is one of the most widely used processes for fabrication of plastic parts.

The injection molding process requires the injection molding machine, raw plastic, and the mold. The raw plastic is melted in the injection molding machine and then poured into the mold where it is allowed to cool and solidify.

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The melted plastic takes the desired shape after cooling. They are used in medical equipment, automotive interior parts, electronic housings, housewares, compact discs, plastic toys,etc. Plastic parts design is a very complex process and it requires the involvement of many factors like functional requirements, process constraints, material selection and assembly or structural issues.

In this blog, we will look at some basic guidelines to be followed for designing bosses. Adhering to these basic design guidelines for injection molded parts can improve the moldability of the designs and the life of the mold, increase the operational life of the part and reduce manufacturing cost.

A boss feature finds use in many part designs as a point of attachment and assembly. The most common variety consists of cylindrical projections with holes designed to receive screws, threaded inserts, or other types of fastening hardware. Under service conditions, bosses are often subjected to loadings not encountered in other sections of a component. Provide a generous radius at the base of the boss for strength and ample draft for easy part removal from the mold.

A fillet of a certain minimum radius should be provided at the base of a boss to reduce stress.

plastic screw boss design guide

The intersection of the base of the boss with the nominal wall is typically stressed and stress concentration increases if no radii are provided. Also, the radius at the base of the boss should not exceed a maximum value to avoid thick sections. The radius at the base of boss provides strength and ample draft for easy removal from the mold. The recommended value for radius at the base of a boss is 0.

When bosses are placed very close to each other, it results in creating thin areas which are hard to cool and can affect the quality and productivity.

Also, if the mold wall is too thin, it is very difficult to manufacture and often results in a lower life for the mold, due to problems like hot blade creation and differential cooling. Hence, it is recommended that spacing between bosses should be at least 2 times the nominal wall thickness. Providing a radius on the core pin helps in avoiding a sharp corner. This not only helps to mold but also reduces stress concentration.

It is recommended that the radius at base of hole in boss should be 0. An appropriate draft on the oute r diameter of a boss helps easy ejection from the mold. The draft is required on the walls of a boss to permit easy withdrawal from the mold.

The recommended value for the minimum draft on the outer surface of the boss is greater than or equal to 0.

Designs may require a minimum taper on the ID of a boss for proper engagement with a fastener.

plastic screw boss design guide

It is recommended that minimum draft on the hole in boss should be greater than or equal to 0. A tall boss with the included draft will generate a material mass and thick section at the base.

In addition, the core pin will be difficult to cool, can extend the cycle time and affect the cored hole dimensionally. It is recommended that the height of boss should be less than 3 times of outer diameter.

A fillet of certain a minimum radius value should be provided at the tip of a boss to reduce stress.

Hi-Low Fastener Inch

Boss should have chamfer on top. Wall thicknesses for bosses should be around 60 percent of the nominal wall to minimize sinking. Wall thickness greater than this can create voids and sink marks and may also expand the cycle time. However, if the boss is not in a visible area, then the wall thickness can be increased to allow for increased stresses imposed by self-tapping screws. Bosses and other thick sections should be cored. It is good practice to attach the boss to the sidewall. In this case, the material flow is uniform and provides additional load distribution for the part.Fastening thermo plastic materials during construction and assembly as well as their load capacities prove to be challenging issues for industrial users on a regular basis.

This screw is the optimum way to fasten thermoplastics. It combines high-quality German engineering with a high savings potential for the user. For this reason, Screwerk has added this product exclusively to the Screwerk product-line as its standard screw with the name STP. With regard to its compatibility with other market standards the STP offers a problem-free substitution to various standards of similar use and applications.

The design of the boss geometry is of utmost importance for the function of the entire joint. The balance hole is especially important, as this prevents the boss from bursting. Through experimental trials it was proven that a value of 0.

The value can be increased up to 0. For optimal joints in thermoplastics, we recommend designing the bore geometry in relation with the screw diameters and materials. Please use the table below as a basis. Concept Engineers Buyer Retailer Platforms.

plastic screw boss design guide

Screwerk screws for plastic Screws for Plastic. Available dimensions top.

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Screwerk recommends the STP screw shown above for threaded connections in plastics. Many versions of the STP screw are continually kept in stock as part of our standard product program. Product information top. Construction Advice top. Advice about Boss Design The design of the boss geometry is of utmost importance for the function of the entire joint. Compatibility top.


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