Flash Repair Techniques

When it comes to eliminating flash from the periphery of a molded part, there are at least three different approaches that can be taken. The choice is based largely on amount and severity of the flash.


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“There’s more than one way to skin a cat” applies to more than just the exfoliation of felines; it’s also applicable to mold repairs. Although the goal is always to make an effective repair, often there is more than one way to accomplish it. As in anything that has to be managed, the best choice is the one that balances effectiveness and efficiency.

When it comes to eliminating flash from the periphery of a molded part, there are at least three different approaches that can be taken. The choice of which method to employ is based largely on the amount and severity of the flash.

There are four basic repair methods used for eliminating P/L flash: (1) peening, (2) spot welding, (3) spotting and (4) tig welding. The trick is to determine which method may be best for the application.

The first step is to assess how bad the situation really is. To do that, ask yourself the following questions.

  • How much of the periphery of the part is affected?
  • Is it a spot here and there or large sections of the part?
  • What is the cause of the flash? Don’t forget at this point to consider whether the flash is due to some cause other than cavity edge deterioration.
  • Is it possible that there is an alignment issue?
  • Could it be a build up of debris on the PL or could the flash be the result of process parameters out of spec, material too hot, clamp tonnage too low, injection pressure too high?


1. Peening

Peening is the displacement of material by means of compression. Smack a steel plate with the ball of a ballpein hammer and you’ll create an indentation, but not only will you make a dent in the steel, you’ll also raise a ring of steel around the indentation. Run a bench stone over the spot and you’ll see the circle shine up first. This is raised material being stoned off. This characteristic of steel comes in handy for adjusting the fit of a core pin or shrinking the diameter of an ejector pin hole to eliminate fingernail flash or pin flash. It also can be used very judiciously for minor PL flash. The trick is to know how and where to strike the steel. The danger lies in the fact that you’re swinging a hammer within a whisper of a delicate cavity edge and the potential damage could be worse than the original problem. Peening is also effective at closing small vertical gaps between pockets and inserted cores or lifters.


2. Spot Welding

Spot welding is a valuable tool, sometimes. The applicability of this method depends somewhat on the appearance requirements of the part. If the flash is a fit and function issue and eliminating the flash is the only consideration, it may prove to be a great tool. Conversely, if appearance of the finished part is critical, the surface finish left by this method may not be acceptable. The repaired area will always be somewhat porous. The technology is, in essence, spot welding. You may create hundreds or even thousands of tightly grouped spot welds, but it will never be the same as melting the base metal into a pool and adding filler material to make a homogenous repair. The added material whether it is shim, wire or powder will always have the potential to flake off of the base metal. On the upside, this method is well suited for non-show areas on appearance critical parts and re-establishing worn edges.


3. Spotting

In cases where the flash covers a large percentage of the periphery of the part, spotting may be the most effective measure. It is a method whereby the cavity half PL is coated with bluing and the halves are pressed together to transfer the bluing on the high spots. These areas are handworked to remove minute amounts of material and the process is repeated. With each subsequent round, more of the surface area comes in contact until bluing is transferred from one half to the other around the entire perimeter of the part. Shops that perform this service are equipped with spotting presses to facilitate the opening, tilting (for accessibility) and clamping of mold halves together. Bench spotting can be accomplished in a limited fashion with a hoist, some large clamps and a heavy piece of brass—not a preferred practice and not nearly as effective as work done in a spotting press.


4. Welding

Welding may be the only option in cases where the damage is severe enough that none of the previous methods will suffice. Welding carries with it its own problems: sink around the affected area that may leave an unacceptable appearance issue. Also, tig welding can alter the chemistry of the steel to the point that the base metal and the welded area may not take a polish the same way. This too can, in some cases, be seen on the finished molded part. Micro welding offers a significant advantage over conventional tig welding in that the weld is much smaller (hence micro) and precisely located; therefore, minimizing the amount of handwork required to restore the repaired surfaces. Laser welding adds to microwelding the advantage of reducing the heat affected zone, which in turn, minimizes the probability of weld sink.



Each of these methods has their place and advantages. Having options is always preferable to being limited by only one strategy. Some repairs may even require the use of several options. The important thing is either to develop the skills in each technique or to develop working relationships with shops that offer the service so you have access to it when needed.