Excess flash

Excess flash—Indicates the compound flow was excessive during the mold closing process (shelf loaded compression molding) [3]. 

Figure 6 Percent excess flash by elastomer type for 11-s mold closure.

Accuracy and reproducibility are of vital importance to industry. Quite apart from the effect of these factors on the final product, several plant items are frequently links in a continuous chain of production processes. A sizing error in one machine, for example, could overstress and damage the succeeding machinery. Similarly, an error in a press may increase stress on the tool and could necessitate an additional operation to remove excessive flash . Wear in a material preparation unit could allow oversize material to be passed to a molding machine, creating an overload situation with consequent damage. 

Excess flash Excessive stock loading, improper mould aligning, low pressure and improper mould design. Loading blanks of correct weight, increase moulding pressure and replace loose dowel pins in the mould. 

Gating and venting must be done with an eye to the effects of reinforcement orientation and bubble track formation downstream of flow obstructions. The size and number of vents required depends upon the volume of the part and the rate of fill used in addition to the shape of the part itself. Preferred vent geometry is a groove. 003-.005 inches deep and 3/8 inches wide. This does not permit excessive flash and polymerizes closed rather quickly. 

The mold quality as well closing speed and force strongly affect product quality. For example, increasing the mold closing speed reduces the gravitational sag effects, thus increases the wall thickness. To avoid excessive flash, mold closing pressure P = 7-14 MPa is normally used. In addition, mold surface quality will dictate that of the hnished part. 

Surface indentation arises from excessive flash being extruded through the clearance space between the shoulder and clamping ring. 

Unbalanced runners Use a short shot and weigh the amount in each cavity Examine the runners for excessive flash... 

Parison size is very important—too small a parison can result in a thick pinch-off, which can cause rocker bottoms and warpage, and too high blow ratios to make the product without excessive thinning or blowouts. Too large a parison may result in a thicker wall than wanted at the ends of the pinch-off, excessive flashing, trimming problems and excessive cooling requirements. The correct parison lies somewhere between these two scenarios. 

Fiber separation Excessive flash Dieseling Porosity... 

Mounting one mold on the arm is fairly straightforward. Multiple molds require careful design for mounting on a single spider. Improper mounting can result in an abnormally high rate of wear and tear on the mold and excessive flash formation on the part. 

Fig. 10. Close-up of a 26 x 26 in area moulding tool making twelve Lucas Headlamp Surrounds per cycle. The press operator is seen blowing excess flash from the tool with an air gun before loading with powder. (Courtesy of Daniels Hamilton Ltd.)...

Problem—Blistering, Flat Spots, and Excessive Flash IN Compression, Transfer, or Injection Molding... 

Hot, rich glycol at reduced pressure generates flash gas that can be used as a fuel gas to the regenerator reboiler or as a stripping gas or for both. Excess flash gas is normally vented to the flare system. The flash separator pressure is maintained through a back-pressure control valve on the gas outlet line from the separator. The liquid flow is maintained through a level-control valve. 

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