RADII AND CHAMFERS

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GUIDELINES:

Avoid a sharp three-sided corner by using a radius or chamfer. The radius at the bottom of the draw is most critical. The deeper the part the larger the radius or chamfer required.

OVERVIEW:

The key to good part design in thermoforming is understanding the need for a proper size radius or chamfer. These features are typically needed to allow for part strength, retention of material thickness, and/or esthetics.

SPECIFICS:

One of the most difficult features in thermoforming is the three-sided sharp corner in a female mold. This feature accentuates the draw ratio because it forces the material to over the three walls as it is pushed into the corner. The material appears to stretch and thin out at a geometric rate, usually causing the material to either thin to an unacceptable ending gauge or actually tear and create a hole in the part. A quick check to see if this condition is occurring is to hold a part up to a light source and inspect the corners to see if the gauge is so thin that light can be transmitted through the part. Many times the tool is constructed ignoring this problem, causing either a heavier starting gauge to be used or an adhesive filler applied to the inside of the part backing up the corner and adding strength. These "cures" will add significant cost to the part and should be avoided if at all possible.

A common design technique is to use radii and/or a chamfers on the part, preventing the material from having to continue deeper into the corner, thus arresting the thinning hat would normally occur. The other advantage of radii and chamfers is that they distribute stress over a larger area than a sharp 90 degree corner. A chamfer does not distribute the stress as well as a radius, but it gives the designer the option of sharp corners at the transition points of the chamfer. Where a three-sided corner does occur, one large radius with a chamfer or smaller radius on the other edges is often sufficient to solve the thinning and strength problems that occur.

As the draw ratio gets larger the radii will almost always have to be increased. Use this chart as a very rough rule of thumb to help determine the approximate radius you may need:

Sometimes it may be necessary to prototype a particular corner or feature of the part prior to the start of mold construction. This is usually a quick method of answering any questions regarding material thickness questions.