07.22.22 | 7 min read

What is Flash? Ensuring top-quality plastic molding

If you played with little green toy soldiers as a kid, you’ve probably come across your fair share of soldiers that weren’t fit for battle—seemingly in every pack there would be one or two duds that couldn’t stand up on their own because their plastic bases weren’t completely flat—thanks to excess plastic stuck on the bottom from production. In the world of play, there aren’t any words for these leftover globs of plastic, it was just a fact of life. But in manufacturing, we call it flash.

While toy soldiers are a thing of the past, you may still notice those thin ridges of excess plastic across many products you use today. In fact, flash continues to be the most common defect in plastic injection molding.

What is Flash in Plastic Molding

Flash is a deformity that occurs naturally with the use of molds and is an inherent part of the plastic molding process. Flash is created when excess material is squeezed outside the bounds of the mold cavity. To better understand the various ways this can happen, let’s first dig into the anatomy of a mold.

Traditional molds are split into two halves that are tightly clamped together during the plastic injection molding process. To fill the mold evenly, molten plastic is pushed through a nozzle into an external opening of the mold (the sprue), through built-in channels (called runners) and fed into the gates of each individual cavity. Molds also contain vents that help maintain a certain pressure within the mold and allow for plastic molten to flow throughout. At the end of the molding process, built-in ejector pins release the object from the mold.

Insert molding and overmolding processes work similarly, except they also involve a metal component that the plastic is being molded onto, such as a seat belt buckle.

Each moving part of the mold helps to keep the plastic molten contained within the bounds of the mold cavity until it has solidified. When tooling gets damaged or process parameters are out of balance (such as temperature of the molten plastic, injection speed or shot size), plastic can begin to backflow through gates and runners or even blow the mold open.

Why Flash is Bad

The sometimes sharp, uneven ridges caused by flash can significantly affect the appearance and the function of a product. For example, flash on baby bottles can be an irritant against infants’ skin and it can also compromise the function of the bottle itself; other products like Lego wouldn’t fit together properly with flash present; and the poultry industry also heavily relies on zero-flash manufacturing. When it comes to plastic chain pieces in chicken feeders, for example, flash would cause too much variance from piece to piece and disrupt the metered distribution of chicken feed.

These are just a few of many products in the world that have zero tolerance for flash due to the consequences it has for the product and its end users. However, other products like little toy soldiers can have slightly looser tolerances because it won’t impact the end user nearly as much.

Avoiding Flash at All Costs: Prevention and Removal

Generally, there are two approaches that can be taken for controlling flash: prevention and removal. The preferred approach is to fine-tune the plastic molding parameters and prevent flash altogether. This saves time in the supply chain and can sometimes reduce long-term manufacturing costs.

Flash can also be removed after plastic injection molding in secondary processes; however, this is known to be labor and cost intensive if steps haven’t been taken during the plastic molding process to minimize flash.

Quality Control in Plastic Molding

Flash is challenging to control because there’s no single root cause, and therefore no universal solution. A combination of process safeguards and parameter adjustments are needed to avoid it completely and meet tight tolerances. Here’s a look at some of the biggest contributors to flash in plastic injection molding.

Tool Wear and Mold Contamination

Tooling naturally wears and becomes dirty over time. Worn sprue bushings and surface wear, along with plastic residue and dust accumulation that causes mismatched parting lines, are all common issues that occur over time. Left unchecked, these factors can compromise the seal that’s achieved when the two mold halves are clamped together. That’s why regular tool inspection and parting line maintenance is so important.

Nozzle Parameters: Temperature and Shot Size

During the injection phase of plastic molding, the nozzle not only delivers the shot of molten plastic into the mold, but it also controls the temperature. Higher temperatures yield lower viscosity. If the viscosity gets too low, the plastic flows more quickly and can inflate the pressure within the cavity. When that internal pressure overcomes the external clamping pressure, the mold blows open and flash occurs.

Shot size is also a factor. The volume of plastic molten being injected needs to match the mold’s capacity. Shots that are too big or too small affect pressure within the cavity during injection and result in parts that are not fully formed or that have significant flash. Both outcomes could ultimately render a part unusable.

Internal PSI

Injection and packing pressure play a huge role in how a mold is filled and the quality of the final product. When set too high at either stage of the plastic molding process, the internal pressure could potentially overcome the external clamping pressure and cause the mold to blow open. On the other hand, pressure that is set too low could cause internal backflow of molten plastic. Both scenarios involve plastic escaping the mold cavity to create flash.

Design for Manufacturability

Another big contributor to flash in plastic molding is when the design isn’t optimized for the manufacturing process or the specific machines a supplier uses. Designing for manufacturability involves working with the supplier team and engineers to run injection simulations to make improvements to the part prior to manufacturing. Finding the best position for the parting line is especially important during this process to ensure the cavities are balanced and to minimize flash extension and thickness. This also gives engineers an opportunity to determine the level of flash that can occur on a product before it impedes on the performance or appearance of a product. These simulations allow engineers to communicate to the supplier what level of flash they will accept from the start, which ultimately saves time and money in manufacturing.

While DFM may involve higher cost upfront, it’s a vital step for improving plastic molding quality and reducing time and money spent on troubleshooting in the manufacturing stage.

100% Flash-Free Molding

Some companies go beyond DFM and optimize process parameters by investing in 100% flash-free tooling and processes. The first manufacturer to produce anti-colic baby bottles, for example, needed a way to produce high-quality baby bottles that helped reduce colic in infants caused by the ingestion of air. A key part of this design was that no amount of flash was acceptable. Plastic injection blow molding (a specialized version of traditional injection molding) provided a way for 100% flash-free molding to be achieved.

Manufacturers who have extremely tight tolerances often need specialty processes and experienced plastic molding partners that can identify potential problems and fine-tune parameters for repeatable, reliable results.

If you’re looking for a trusted supplier to partner with on your next project, start a conversation with one of our experts today and learn more about our production testing and quality control processes for plastic injection molding and insert molding.

Article | Plastics

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