Flash – What Is It and What Is Acceptable?

Flash is plastic that flows into areas where mold components meet, which could be on parting lines, around inserts or ejector pins or the shutoffs of core features. Source (all images): DJC Plastic Consulting

Flash has been an issue since the first metalsmith poured molten copper between the two halves of his first clay mold. It may be one of the oldest and most persistent challenges in molding, but understanding what causes it, and when it’s truly unacceptable, remains essential to maintaining quality and efficiency in modern manufacturing. Let’s review this topic by answering some common questions.

What is flash? Flash is any plastic that has flowed and solidified in areas where mold metal comes together, that is attached to the plastic part, but is not a design feature of that part.

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Is there such a thing as acceptable flash and what makes it unacceptable? I have been accused by my family of walking through stores, randomly picking up plastic commodity items, commenting on all the flash and asking how they can sell something with all that flash? If the item sells just as well with flash, doesn’t impede the manufacturing process or affect the end consumer, then all that flash didn’t really matter, except to plastic nerds like me!

Greater flash may be allowed in non-critical areas than in certain critical areas. In this case, the slot is more critical than other areas.

Flash is acceptable if it meets the customer's specifications. However, are the specifications written to produce usable, functioning parts at the lowest cost with minimum rejects?

Flash will forever be a part of injection molding. Understanding and agreeing on what is acceptable is crucial for a successful molding program.

How should the flash specifications be documented? A general note on a part drawing, such as “No flash allowed,” may be a designer’s lazy way of dealing with flash, but it is often meaningless because it can be interpreted in too many ways. I have literally had a customer say the “No flash allowed” note on their drawing meant that if they can’t see it at arm’s length, it is acceptable. Of course, their receiving inspection person might interpret it quite differently (or have a different length arm!), causing rejected shipments and unnecessary mold repairs.

Example of a boss formed on the side of a part if flash is allowed on the exterior surface of the boss, formed by stepping parting line. (top) When no flash is allowed on a boss on the side of the part it may need to be formed with solid steel using a side action. (bottom)

Other interpretations I have seen of “no flash” are, “So it isn’t too obvious,” or “If I can’t feel it, it’s acceptable” or often they mean truly, “No flash at all is allowed!” Some parts have no flash specification on the part drawing, but may have it covered in a separate document. This document may or may not be referenced on the drawing. Make sure to get all the specifications and review them, not only for flash, but also for any other requirements that could affect the part.

How much magnification is required to determine if there is any flash? 5X, 10X, 100X? A concrete specification is better than a vague one. What is deemed acceptable to designers and project managers during development may not be seen that way as it is handed off to the production group. I would rather have a specification that says, “0.001 inch (0.025mm) maximum flash,” rather than “No flash allowed.” It may be difficult or impossible to achieve, but it is a concrete starting point for negotiations.

A note for flash allowance inside a small orifice may state something like, “0.003 Inch Maximum Flash.” This part would be rejected.

Are the flash specifications too tight? Can the flash allowance be loosened? Often, it is necessary to push back on unreasonable specifications. One engineer for a medical part truly didn’t want any flash on their part, being afraid flash would cut the glove of a surgeon — on a part made of a soft TPE! It took discussions by some members of their own team to explain that there was no danger of the rubbery part cutting a glove.

What about varying flash tolerances? Allowing greater maximum flash in

non-critical areas will give the molder a greater chance of success initially and as the mold wears. It also allows them to focus on areas where excessive flash would affect function or otherwise be detrimental to the user. As I mentioned earlier, sometimes it is good to push back on specifications. If the customer has what appears to be an unreasonable specification, ask whether it is required for an acceptable part. If they state that it is, ask if it would be possible to increase the allowance in certain, non-critical areas, explaining how it would benefit them.

Very often, I have found customers amenable to revising flash specifications in non-critical areas. Knowing critical flash areas can even affect how the mold is built: forming features in solid steel rather than with a witness or parting line or inserting areas to keep parting lines sharp.

What if flash specifications are nonexistent? Occasionally, I have seen

A note for flash inside a small orifice may state, “006 Inch Maximum Flash Height. Open Area Interior To Flash Must Be 0.0010 Inch Squared or Greater.” This part would be accepted with no detrimental effect to its function.

drawings that have no requirements for flash. These drawings are usually produced by companies that typically work with machined rather than molded parts, or by designers unfamiliar with molded plastics. It is best to get quality requirements solidified before production starts.

If there are no specifications, it is best to ask the customer to add a maximum flash allowance to the drawing. My recommendation is to request an allowance as generous as possible without affecting the function. For example, if the vent land is 0.040 inch (1 mm), then ask for a flash allowance of 0.031 inch (0.8mm).

This may be excessive, and the customer may object to your recommendation, but at the early stages of a project, it is a negotiation. The goal is not to create a specification that will cause you or the customer pain in the future, but to produce something concrete that both parties can objectively measure and agree on.

What if the customer won’t add a flash allowance to their part drawing? I have occasionally seen this happen, typically when building a new mold to replace an older mold. These molds are usually ordered by a buyer who couldn’t get anyone in development to revise the legacy drawing. If they insist on leaving off a flash specification or keeping the “No flash allowed” note on the drawing, carefully document what you agreed upon with the customer.

In these cases, the safest approach is to include that documentation for flash and cosmetics in the acknowledgement of the purchase order. Always keep a copy with your quality documentation as well. That way, if the customer ever updates their drawing or wants to hold you to a tighter specification than originally agreed upon, you have documentation proving the original agreement.

Flash will forever be a part of injection molding. Understanding and agreeing on what is acceptable is crucial for a successful molding program.