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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. The melted plastic takes the desired shape after cooling.
Today, it's almost impossible to do anything without using injection molded parts. 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. Wall thickness, Draft, and Textures, Ribs & Bosses, Holes/Depressions, etc. are some of the important elements to be considered in the design of plastic parts. 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.
10 Boss Design Guidelines one should follow in Plastic Parts Design
1. Minimum Radius at Base of Boss
A fillet of a certain minimum radius should be provided at the base of a boss to reduce stress. 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 0.25 to 0.5 times the nominal wall thickness.
2. Spacing between Bosses
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.
3. Radius at Base of Hole in Boss
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 0.25 to 0.5 times the nominal wall thickness.
4. Minimum Draft for Boss OD
An appropriate draft on the outer 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 0.5 degrees.
5. Minimum Draft for Boss ID
Designs may require a minimum taper on the ID of a boss for proper engagement with a fastener. The draft is required on the walls of a boss to permit easy withdrawal from the mold. It is recommended that minimum draft on the hole in boss should be greater than or equal to 0.25 degree.
6. Boss Height to OD Ratio
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.
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7. Minimum Radius at Tip of Boss
Under service conditions, bosses are often subjected to loadings not encountered in other sections of a component. A fillet of certain a minimum radius value should be provided at the tip of a boss to reduce stress.
8. Chamfer at top of Boss
Boss should have chamfer on top. A chamfer at top of boss is good lead in for the fasteners.
9. Wall Thickness of Boss
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.
10. Standalone Boss
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. For better rigidity and material flow, a boss should be connected to the nearest side wall.
How do you design your injection molded parts? Let us know by entering in comments section below.
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Designing for plastic injection molding is like raising teenagers'some parts (and some teens) are more challenging than others. But by following a few simple rules, even the most difficult of these challenges can be overcome (parenting aside).
For injection molding, there are some common troublemakers during part design:
Look familiar? This list just happens to correspond with some of the more engaging features you can incorporate into your part designs, which is one reason why it's so important to master them. Another is that, depending on your application, these features often provide more functional and aesthetic parts while reducing production costs. The following tips and techniques will help you do just that.
There are plenty of examples where molded parts need a clip or snap fit. The cover for an electronics device. A snap-on lid for a tool set. Both rely on a flexible, hook-like piece of material that extends into and catches on a female pocket or slot in the mating part. In either case, probably the best way to create these features is with a sliding shutoff, which as the name implies slides into the mold and prevents material from flowing into the desired area. This design also requires a hole in the part beneath the snap, the portion of the mold that passes through the part shutting off with mating features, which creates the snap fits.
However, this approach requires that the part feature is aligned with the direction of mold opening and that a relief hole at the base of the clip is allowable. Additional draft may also be required. If none of these are feasible, a side-action might be possible, but this must be aligned with the mold's parting line and oriented perpendicular to the pull direction. Additionally, the side action must be accessible and remain in constant contact with the part.
Finally, hand-loaded inserts can be used. These are small chunks of machined metal that must be manually inserted and removed from the mold at each cycle. And consider the material'a flexible polymer-like ABS makes a better clip than does acrylic or PEEK, although some clever design tweaks such as adjusting the clip length or its geometry can get around many material constraints. Check with Protolabs' applications engineers for other design tweak ideas (see customer service number and address below).
Note: This whole discussion on clips applies equally well to any undercut or recessed part feature. This includes O-rings, side holes and pockets, windows, and so on. Based on the orientation of the part feature relative to the mold pull and parting line, various advanced molding techniques such as side-actions, telescoping shutoffs, inserts, and pins might be needed. All are perfectly acceptable and well-understood solutions, although, due to the added complexity, some of these resolutions may have an effect on molding and part cost.
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