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Standards
for Flash |
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- Introduction
- It is the purpose of this section to list and discuss many of the factors that have an
effect on the amount of flash, to describe the basic methods by which flash can be
removed, and furnish the means by which the designer can designate on the product drawing
the flash location and flash variation permissible.
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- Definition
- (A) Flash
- Flash is excess rubber on a molded product. It results from cavity overflow and is
common to most molding operations. Flash has two dimensions - Extension and Thickness.
- (B) Flash Extension
- Flash extension is the film of rubber projecting from the part along the parting line of
the mold.
- (C) Flash Thickness
- Flash thickness is measured perpendicular to the mold parting line. Variations in flash
thickness are normally included in closure tolerances.
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- General Information
- A method for designation permissible flash extension and thickness on a molded product
will result in better understanding between rubber manufacturer and consumer and benefit
both. This method must permit the designation of a surface where no parting line is
permissible. It must also designate areas where a parting line is permissible and define
the amount of flash extension tolerable in such areas. The designer, without specific
rubber processing knowledge, should be able to specify flash extension limits in any given
area on his drawing. Use of RMA Drawing Designation provided in this section will provide
this capability, however, the designer should not specify the amount of flash extension
which can be tolerated without impairing product function or appearance. A method
designating areas permitting flash and describing flash extension tolerance will result in
the following benefits:
- (A) Avoid errors in mold design concerning parting line location.
- (B) Uniformity in appearance and function of molded products supplied by more that one
source.
- (C) Simplification of inspection procedures.
- (D) Reduce over-finishing or under-finishing products.
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- Molding techniques have been developed to produce "flashless" products. The
mold parting line, depending on location on the product, is barely discernible with no
measurable thickness or extension. Initial cost and maintenance of this tooling and
equipment is high and very close manufacturing control is required.
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- In instances where flash extension is not a problem or where it is otherwise
advantageous, parts are shipped as molded with no flash removal necessary.
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- Methods for removing flash from products with metal or other inserts are approximately
the same as the methods for non-inserted rubber products. Rubber flash adhering tightly to
inserts is generally acceptable. If it must be removed, it is done by mechanical means
such as wire brushing, abrasive belts or spot facing. If adhered rubber flash is not
permissible, is should be so specified on the drawing.
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- Flash removal is an important cost factor in producing finished molded rubber products.
Cost conscious designers will permit the widest possible latitude in flash thickness,
flash extension, and in location of flash consistent with adequate function and appearance
of the product.
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- FACTORS TO BE CONSIDERED ON SETTING STANDARDS ON FLASH
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- Flash Location
- Parting lines (flash lines) must be located to facilitate part removal from the mold
cavity after curing.
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- Flash Thickness
- Flash thickness is determined in the molding operation and may vary with mold design,
closing pressure, with weight and shape of preform, and type of compound used - and many
lesser factors. Normal variations in flash thickness have been taken into account in the
tables set up for closer tolerance, and will receive no further consideration.
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- Flash Extension
- There are many methods by which flash extension on rubber products can be removed. The
particular method selected will be determined by the degree of flash extension permitted
as well as by the flash location, flash thickness, and other factors. Some of the more
common methods of flash removal are as follows:
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- Buffing
A moving abrasive surface material is applied to the rubber part to remove excess
rubber by abrasive action.
- Die Trim
A cutting tool, shaped to the contour of the molded product at the parting line, is
applied with a force perpendicular to the flash extension and against either a flat plate
or a fitted shape. This creates a shearing or pinching action removing the excess flash.
Die trim can be done with a hand or machine mounted die. Machine mounted dies are often
used for multiple trimming of small uniformly shaped products from multi-cavity molds.
- Machine Trim
Flash is removed by passing the rubber part through machine mounted rotating or
reciprocating cutting tools. These devices are customarily adapted to a particular product
and may shear, saw, or skive the flash away.
- Tumble Trim
There are two basic types of tumble trimming. Both utilize a rotating barrel or drum
in which the heavier rubber sections strike the thinner and more fragile flash breaking it
free. Dry tumbling at room temperature is most effective with the higher durometer
"hard" compounds. The other type of tumbling utilizes carbon dioxide or other
refrigerant to freeze the molded parts, thus making the compound more brittle so the flash
will break more readily. Any tumbling operation will have an effect on surface finish.
- Mechanical Deflashing
Modern deflashing machines utilize an abrasive medium, tumbling, and a refrigerant for
quick freezing. The time and temperature is closely controlled while the parts are
agitated in an enclosed barrel. Refrigerant (usually carbon dioxide or nitrogen) is
metered into the deflashing chamber while the parts are being impinged with a mechanically
agitated abrasive medium. The flash, being thin, freezes first and is broken away by the
abrasive medium and the tumbling action before the heavier rubber part itself has lost its
resiliency. Some loss of surface finish may be expected and some of the abrasive medium
may adhere to the molded parts.
- Pull Trim or Tear Trim
A very thin flash extension is molded immediately adjacent to the part and a thicker
flash is molded adjacent to the thin flash but farther from the part. When the flash is
pulled from the molded part, it separates at its thinnest point adjacent to the molded
part. This method
- Method of Designation of Flash
- Extension
- The symbol "T" with a notation in thousandths of an inch for the maximum
extension shall be used. Example: T .032 (.032 in. maximum extension permitted.) IF METRIC
DIMENSION THE DRAWING DESIGNATION WILL BE FOLLOWED BY mm INDICATING mm OF FLASH PERMITTED.
Example T .80mm (.80mm maximum extension permitted.)
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- Thickness
- The flash thickness may be specified following the extension limit if it is critical to
the function of the part. Closure tolerances will apply as in tables 1, 2, 3, and 4.
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- Location
- An arc enclosing the actual area included by this designation and a leader to the trim
symbol, designates the maximum allowable flash extension and thickness thus enclosed. If
no flash can be tolerated in a given area, the symbol "T" .000 is used.
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- Standards
- The designer may indicate on his drawing any amount of maximum flash extension
permissible. However, as a matter of simplicity, a progression of flash extension Drawing
Designations is suggested in Table 6. Only those areas requiring such a designation should
be specified. The use of a standard note can frequently be used with no further notation.
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