Flash in Injection Molding: Causes, Effects, and Solutions

Table of Contents

Flash in injection molding
Flash in injection molding

I. Introduction

One of the nightmares of any manufacturer is to produce a part that is sub-par or has any defects. Consequently, doing everything to avoid this situation is a major concern. In the ever-changing world of plastic injection molding, understanding the concept of flash in injection molding is comparable to mastering a tricky balance between precision and perfection.

As one of the common defects that injection molding manufacturers sometimes face in production, flash often results from the excess that finds its way out of the molds through the mold edges. However, there are ways and solutions for this subtle yet impactful excesses and one of the aims of this article is to show you.

We’ll provide a comprehensive overview that encompasses its definition down to its occurrence and subsequent consequences. So, if you’re ready, let’s get started.

II. What Is Flash in Injection Molding?

To begin with, flash in injection molding is never a palatable thing both for the product integrity and quality as well as the reputation of the injection molding manufacturer.

In injection molding, the process of melting the plastic resin to form a fluid that can flow into the farthest corners of the molds helps in forming the product. However, it also has the potential to flow out of the molds through the edges of the molds.

The thing is, molds come in halves that are clamped together to form the whole mold and an ineffective clamping often leaves a loophole for the molten plastic to escape. This intricate detail is where quality inspection takes center stage to ensure that these two halves are firmly clamped to avoid the leakage of molten plastic.

Additionally, molten plastic often undergoes extreme pressure when passed into the mold, and mold designers often consider this pressure during the design stage to specify the required clamping force.

Interestingly, the mold complexities also contribute to the occurrence of flash considering that complex mold geometries have areas that are hard to reach as molten plastic flows through them. The consequence is some blockages along the plastic flow path.

This obstruction constitutes a pressure build-up at this point resulting in a leak of the plastic material eventually becoming a defect known as flash. The effects of this defect is far reaching impacting both the aesthetics and the product’s functionality.

Flash in injection molding Injection molded parts 1
Flash in injection molding Injection molded parts 1

III. The Injection Molding Process

A quick look at the injection molding process can give a better understanding of how defects like flash come about and can also give manufacturers insight into how to avoid their occurrence.

The whole process falls simply under four major activities including mold clamping, plastic injection, cooling phase, and the product ejection. However, the parameters that ensure the quality of these process, including melting temperature, injection pressure, and flow speed are very critical and needs close monitoring.

It all begins with the selection of a suitable plastic material with a known melting temperature as well as the behavior of this material under temperature and pressure. This plastic material is melted in the injection machine chambers and is ready for the injection phase. However, the mold must have been adequately prepared. This preparation can include high-quality CNC machining to manufacture the mold precisely and clamping the two halves appropriately to receive the molten plastic successfully.

Think of this process as a hook on a cliff helping a mountain climber reach the top where one hook cannot be used except the previous ones are properly executed. Every stage in the injection molding process is a cumulative contribution to the success of the production.

The consequences of any missteps along the way come with dire consequences for the product’s quality and aesthetics. Hence, the reason why any error that can lead to a flash in injection molding should be avoided at all costs.

IV. Causes of Flash in Injection Molding

In the high-quality mold manufacturing industry, understanding the causes of flash helps injection molding engineers quickly find lasting solutions to it. Let’s examine these causes that lead to flash in injection molding.

First and the most common cause of flash is a misalignment of the two halves of the mold. Once a shift occurs while placing the mold together, the end result is almost certain to come down with a flash.

Another common cause of flash injection molding is applying inadequate clamping force to the mold during its preparation for plastic injection. With this inadequate clamping force, what you’ll get is a leakage of the molten plastic through this weak point leading to a flash in the final product.

Excessive injection pressure is yet another reason for the occurrence of flash considering that the injection molding process needs a significantly high injection pressure to ensure the plastic reaches every corner of the mold. The result is often a leakage of the plastic through the gap created by this pressure and eventually flash appears.

When the venting system provided for an injection molding process is inadequate, what you get is a buildup of pressure often caused by the trapped air further resulting in seepage of molten plastic. This pressure differentials from the trapped air reduces the efficiency of the clamped mold leading to flash in the final product.

Having a thorough knowledge of these causes helps plastic engineers and injection molding professionals to easily find the solution to flash occurrence during injection molding.

V. Effects of Flash on Injection Molding

When looking at a defect such as flash in injection molding it is in order to examine its effects as both qualitative and quantitative. Qualitative affects the way it is perceived by end users while quantitative defines the costs from the prototypes using  3D printing technology and other expenditures in the production

One qualitative effect is aesthetics. Flash is often the cause of unpleasant visual effects on the plastic part produced. The resulting flappy addition to the initial design of the plastic part poses a blemish and makes it less desired by end users. This also reduces the quality as it is perceived as below par relative to other parts of the same design but without such defects as flash.

While some end users might overlook the aesthetic effects of Flash dismissing it as mere cosmetic, most will still find it problematic considering that Flash can also have functional effects on the end product. Many see flash as an unwelcome appendage that reduces the effectiveness of the parts for its designed functionality. Hence, the functional effect of flash often renders it unfit.

Then comes the quantities effects of flash and this can include time, material, and cost losses. All these are huge, especially with the cash expended and the downtime that impacts the amount of revenue lost.

The environmental impact of flash in injection molding is also quantifiable and has dire repercussions considering the impact of the sheer quantities of wastage introduced into the environment.

Flash in injection molding parting line
Flash in injection molding parting line

VI. Strategies for Flash Prevention

Fortunately, flash is a preventable defect and seasoned engineers and manufacturers often overcome this challenge. Now how do they prevent the occurrence of this unwanted flaw? The answer lies in following rather basic but critical prevention strategies highlighted below.

It all starts with a meticulous and detailed approach to high-quality mold manufacturing especially at the mold design stage. The design should encompass the proper alignment of the mold and ensure that every detail is met with precision. This guarantees the seamless coupling and fitting of each half of the mold during clamping and mold injection.

Secondly, the clamping of the mold needs adequate force to ensure that it stays in play before, during, and after the injection of the molten plastic. The clamping force should grip the molds firmly to avoid molten plastic leakage during the injection process.

Again, adequate monitoring of critical injection molding parameters especially the injection pressure to ensure that it doesn’t exceed the specified design pressure. Also, the venting system in place should be adequate to allow free flow of air out of the molds preventing the undesirable pressure differentials that often lead to flash.

The place of operators is also vital to the prevention of flash as experienced injection molding professionals usually have the expertise to prevent flash. They know the signs and symptoms that point to flash during the injection molding process and can quickly arrest the situation before it goes haywire.

VII. Detecting and Removing Flash

While you may have done your best to prevent flash injection molding, sometimes it still finds a way to creep into the process and you might be left holding the short end of the stick. However, there is still hope and these step skills can help you both detect and remove flash to save the quality of your product.

First is the number one quality assessment tool for every quality examiner – visual inspection. Detecting flash will require that you visually inspect every aspect of the product for the clandestine presence of flash. You’ll scrutinize each part using perceptible checks, looking for any anomalies that might betray the concealed flash.

If by chance you find any presence of flash on the parts, you will then need to move on to the removal stage. Just like a surgeon removing unwanted growths in humans, trimming the flash from the parts will require precision. Once the flash is removed the molded parts are then ready to take center stage.

In all of this, it is essential that maintaining the integrity of the molded part should be at the forefront of the flash removal process. Any action that might jeopardize this integrity should be aborted to avoid a waste of more resources and time. Hence, the knowledge and experience are required to guarantee that the final product is flawless without any trace of flash.

VIII. Quality Control and Inspection

While detecting flaws such as flash in any product or part is the purview of the quality control and inspection department, their job description entails much more. It is responsible for ensuring the quality of the product according to design often starting long before there is any mold on the ground.

Before and during the design the quality control management system is often set up to create a set of criteria that determines if the quality objectives of the product are met. These criteria are the yardstick by which quality inspection of the final product is done. The aim is to ensure that the injection molding process has not left behind any imperfections that could compromise the integrity of the final product.

One of the tools engaged in the quality inspection process for flash is the visual inspection as highlighted previously in this article. However, other sophisticated tools including coordinate measuring machines (CMM) and optical tools help quality inspectors to conduct a thorough examination. They use these devices not just to identify the visible traces of flash but to delve deeper into the very soul of the molded part

Additionally, industry-recognized quality specifications including ISO 9001 standards and certifications provide a high level of quality assurance for both the end users and the manufacturers.

In the end, quality control and inspection ensure that the injection molding process attains the desired precision while striking the right balance required.

Flash in injection molding trimming 1
Flash in injection molding trimming 1

IX. Case Studies and Real-World Examples

As stated initially in this article, flash is a common flaw many injection molding manufacturers encounter and try to prevent at all costs in every production.

One real-world example is in the automotive plastic injection molding by TDL. The challenge was to take every measure to ensure the precision of the designed part without the occurrence of flash on any part. TDL did just that.

Started with creating a comprehensive checklist to inspect and ensure that the final product met design specifications right from the design stage. Then, a veteran plastic operator was in charge of the injection molding process making sure that the mold was adequately prepared and aligned before the injection began.

Additionally, they monitored the injection pressure to ascertain that it didn’t exceed the design pressure. And lastly, the vent system was adequate to allow the free flow of air that could have been trapped in the mold which would have resulted in a pressure build-up and the occurrence of flash.

The understanding that flash can and will compromise the functionality and aesthetics of the molded part helped TDL put these measures in place and helped them achieve a flawless, flash-free part.

X. Conclusion

In a nutshell, flash in injection molding often has grave consequences for the quality of the molded part especially for its visual appeal and its usage. It costs the manufacturer so much in cost, time, and reputation, and the end user in ineffective functionality.

However, following the preventive steps outlined in this article such as meticulous design, adequate clamping force, effective vents, and controlled pressure can save you from a lot of headaches.

Likewise, employing visual inspection and other sophisticated quality inspection tools like CMM and optical analytical tools can help with quality control including detecting flash.

Ultimately, an injection molding manufacturer’s reputation is tied to the integrity of their product, and taking every step to avoid defects like flash will go a long way to save your reputation in the industry. So, what’s your next move?


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

Contact Our Experts

Send us a Email, we will feedback to you ASAP!