2/11/2021
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`By Design: Part design 106 - Corner radiuses
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`By Design: Part design 106 - Corner radiuses
`
`In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special perspective on
`issues important to design engineers and the molding industry.
`
`A properly designed injection molded part is characterized by having uniform wall thickness, draft angles, and rounded
`corners. These rounded corners have two primary functions. They improve the flow of the melt around a corner,
`allowing the cavity to be filled at lower injection pressures and resulting in less molded-in stress throughout the part.
`Corner radiuses also make molded parts stronger by distributing any stress on the corner over a broader area on the
`part. In other words, there is more plastic material to absorb the load.
`
`The injection molding process is unforgiving of sharp inside corners on 3-D parts. Figure 1 shows a cross-section of a
`portion of a mold for a 3-D box-shaped part. The left side of the part has sharp corners. On the right side, the corners
`have been radiused.
`
`During the cooling portion of the molding cycle, the top of the part shrinks and attempts to become smaller. The steel
`core inside of the part does not shrink. This condition creates a high level of molded-in stress as the material is pulled
`tightly against the sharp corner on the left side of the core pin. There is the same amount of stress on the right side of
`the part, but that stress is spread over a broader area by the radiuses in that location.
`
`If the right angle part with sharp corners shown in Figure 2 were loaded by pushing the vertical wall to the left, that load
`would be concentrated at the sharp corner where the vertical and horizontal walls meet. This is the same type of sharp
`corner that would be highly stressed during the molding process for the reasons just described. These two factors
`combine to produce an unnecessarily weak part.
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`Radiuses Increase Strength
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`For example, a .125-inch-thick polycarbonate part of the same shape, with a .010-inch inside corner radius, would, on
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`https://www.plasticstoday.com/print/19338
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`1/3
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`Exhibit 2007
`IPR2020-00878
`Page 1 of 3
`
`In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd. (Libertyville, IL) shares his special
`perspective on issues important to design engineers and the molding industry.
`Glenn Beall | Nov 28, 1999
`

`

`By Design: Part design 106 - Corner radiuses
`2/11/2021
`the average, withstand an impact load of only 2.5 ft-lb. Increasing the radius to .020 inch would increase this part’s
`impact strength to 20.2 ft-lb. In other words, doubling the size of the radius results in an eightfold increase in impact
`strength. This is the fundamental reason why design engineers are so diligent in radiusing the corners on molded
`parts.
`
`The injection molding process produces parts with high levels of molded-in stress on sharp inside corners. Plastic
`materials such as nylon and polycarbonate are notch sensitive, and they become more highly stressed during molding
`than some of the less sensitive materials, such as ABS and polyethylene. All plastic materials respond differently to
`sharp inside corners, but it is generally agreed that the graph in Figure 3 is a good average representation of this
`situation.
`
`With an inside radius of less than 25 percent of the nominal wall thickness, there is a rapid buildup in stress on inside
`corners during the cooling part of the molding cycle. This stress will be progressively reduced by increasing the size of
`the radius. A radius larger than 75 percent of the nominal wall thickness provides a negligible reduction in stress.
`
`This graph can also be interpreted to indicate that, as the level of molded-in stress increases, there will be a reduction
`in the part’s ability to resist a rapidly applied load.
`
`Radius Rules
`
`A good rule to remember is that whenever the inside radius on a 90° corner is less than 25 percent of the nominal wall,
`there will be a high concentration of stress at the corner.
`
`The junction between two walls can be progressively strengthened by increasing the inside radius to 75 percent of the
`nominal wall. Larger radiuses can be specified; however, they will not significantly strengthen the junction between the
`two walls. An acceptable average radius for an inside corner would be 50 percent of the nominal wall.
`
`The part shown in Figure 4 has a 50 percent inside radius and a sharp outside corner. The inside radius will reduce the
`molded-in stress and make the part stronger by distributing any load on the part over a broader area.
`
`This combination of sharp and radiused corners produces a 30 percent increase in wall thickness at the corner. This
`thicker wall will take longer to cool than the rest of the part. And this part will have a higher mold shrinkage factor on
`the outside than on the inside. In some cases there will be sink marks due to the increase in shrinkage in the thick
`section on both sides of the corner, approximately two-thirds of a wall thickness away from the corner, as depicted in
`Figure 5. The fact that there is more linear shrinkage on the outside surface of the part than on the inside surface will
`encourage the corner to warp to an angle of greater than 90°.
`
`The recommended proportions for injection molded corner radiuses are shown in Figure 6. The inside radius is half the
`nominal wall. The outside radius is one and one-half times the nominal wall. These proportions produce a uniform wall
`thickness around the corner. Shrinkage will be the same on both sides of the corner and there will be no sink marks.
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`https://www.plasticstoday.com/print/19338
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`Exhibit 2007
`IPR2020-00878
`Page 2 of 3
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`By Design: Part design 106 - Corner radiuses
`2/11/2021
`As the melt flows around a properly proportioned corner such as this, there will be no increase in area and no abrupt
`change in direction. The melt is gently directed around the corner and there is a minimum loss in cavity packing
`pressure. These proportions will result in a good, strong corner with a reduced tendency towards postmold warpage. A
`corner of this type will be dimensionally stable.
`
`Specifying Corner Radiuses
`
`Experienced designers are aware of the fact that all corners should be radiused. This has resulted in the common
`practice of specifying corner radiuses as a special drawing note, such as “radius all corners .030 inch” or “break all
`sharp edges.” This is an unsatisfactory approach to specifying corner radiuses. Rarely is one radius size suitable for all
`of the corners on a molded part. Inside and outside corner radiuses should not be the same size. Each corner should
`be given careful consideration and radiused accordingly.
`
`The note to “radius all corners or break all sharp edges” is not practical in the injection molding process. Some corners,
`such as those at parting lines, shutoffs, or other mold fitments cannot be radiused without creating costly or fragile
`elements in the mold.
`
`If the part is being designed as a detailed engineering drawing on the board or in 2-D CAD, each corner radius should
`be drawn and specified in the traditional manner. If the part is drawn as a 3-D solid model database, the part must be
`drawn with properly proportioned corner radiuses. If the radiuses are not in the database, they will not be in the cutter
`path program or the cavity. Corner radiuses are too important to the success of an injection molded part to be relegated
`to a drawing or database note.
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`Cost vs. Benefit
`
`Everything that has ever been written about designing injection molded parts always contains the statements “maintain
`a uniform wall thickness” and “radius all of the corners.” These are good design guidelines to follow. However, there
`are instances when the location of a radius can significantly increase mold cost without providing any benefit to the
`performance of the molded part.
`
`The often repeated rule to radius all of the corners on injection molded parts should be changed to “radius all corners
`where practical.” In this case, “practical” means the radius provides some functional or molding benefit without
`adversely affecting the cost of the mold.
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`Source URL: https://www.plasticstoday.com/design-part-design-106-corner-radiuses
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`https://www.plasticstoday.com/print/19338
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`Exhibit 2007
`IPR2020-00878
`Page 3 of 3
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