Injection Molding Flash: Causes, Prevent and Fix Ways

Flash is one of the most prevalent injection molding flaws. It occurs when molten plastic escapes the mold cavity during the injection molding process and solidifies forming thin flaps on parts. A small extent of injection molding flash is to be expected – it happens every now and then. However excessive flash will affect the quality of injection molded parts and hinder proper seating and assembly. Therefore, it’s crucial to minimize or even eliminate flash as much as possible.

This article discusses the essentials of injection molding flash – its causes, preventative measures, and how to fix it. We’ve included helpful tips that will help you produce quality injection molded parts that are free from defects.

What Is Injection Molding Flash?

what is injection molding flash

Plastic injection molding flash refers to irregularly shaped protrusions that form on the surface of molded components. An injection mold will typically have a parting line. This allows a small amount of molten plastic to leak from the mold cavity. As it solidifies, the excess material remains attached to the molded part and is known as flash.

Injection molding flash often occurs along the mold’s parting line. However, it may also form on the mold’s edge or any other area where different geometries and surfaces of the mold meet.

What Causes Injection Molding Flash?

Now that we’re familiar with the basics of injection molding flash, let’s explore some of the main reasons you may experience this defect.

Parting line mismatches

Mold opening direction & Parting line

Injection molds have a parting line that separates the two mold halves. A parting line mismatch is one of the most common causes of mold flashing.

Misalignment of the parting line can be caused by design flaws or build-up of contaminants such as dust, dirt, and debris. A parting line mismatch means that the mold is not fully sealed. Therefore, molten material will inevitably seep out from the partially open mold and produce flash.

Older molds are more susceptible to parting line mismatch. With repeated use, tooling may become worn out resulting in leakages.

Improper venting

Injection molding quality is dependent on several process parameters including melt temperature, flow rate, nozzle and barrel temperatures, pressure, and lack of air pockets.

Venting is a crucial aspect of the injection molding process. It allows air to leave the mold cavity preventing air entrapment and formation of voids. Vents ensure that the molding process is effective and play a role in the appearance and integrity of the end product. Insufficient venting can not only cause flashing but also other injection molding defects such as incomplete filling and air pockets.

Low clamping pressure

Clamping pressure is the force needed to keep the mold closed during the injection molding process. This pressure must be high enough to withstand the internal pressure generated from injecting molten plastic into the mold.

If the injection molding machine has insufficient clamping force, the two mold halves may separate prematurely allowing plastic to escape and cause flash.

Low viscosity

Viscosity is a measure of a fluid’s thickness and its resistance to flowing. Molten plastics with low viscosities are more likely to seep out of the mold causing injection molding flash.

Different plastic materials will have varying viscosities when melted. For example, polyethylene (PE) and polypropylene (PP) have low melt viscosities while polyvinyl chloride (PVC)and Polymethyl Methacrylate (PMMA) have high viscosities. In general, thicker fluids are more viscous than their thinner counterparts. Other factors affecting plastic viscosity are temperature and pressure. High injection pressure encourages materials to flow more easily. This can facilitate injection mold flash if the material escapes the mold boundaries.

Uneven flow

Uneven flow of plastic can also cause injection molding flash. This occurs when some flow paths are shorter than others leading to unbalanced filling of the mold. In addition to flashing, uneven flow may also cause other injection molding defects such as air entrapment.

Overfilling the Mold

Injection molds are designed to hold a specific volume of plastic material depending on their size and shape. If the mold is overfilled, the clamping pressure will not be able to endure the injection pressure, and this results in leakages. As the excess plastic is forced to leave the mold cavity, your component is more susceptible to injection molding defects such as flash.

A machine operator needs to inject the right amount of plastic resin into the mold during each production cycle. An underfilled mold will cause unfinished parts due to material shrinkage upon cooling.

How To Prevent Flash In Injection Molding?

Mold flash affects both the aesthetics and functionality of a component. Fortunately, there are several solutions employed by engineers to prevent injection molding flash. They include;

Design for Manufacturability

Design Principles for Manufacturing and Assembly​

Design for manufacturability (DFM) is one of the most effective ways of preventing flash. It involves optimizing the product design to make it easier, cheaper, and faster to manufacture without sacrificing quality.

A DFM analysis can help you identify potential issues and optimize aspects such as wall thickness, parting line design, and mold gate design. A properly done DFM allows you to avoid flash during the fabrication process.

Slow Down the Injection Rate

The injection rate is the speed at which melted plastic enters the mold cavity. This crucial parameter influences other factors such as injection pressure, molding temperature, and viscosity. High injection speeds increase the temperature and injection pressure while lowering the material viscosity. This has the combined effect of increasing the chances of material escape and therefore flash formation.

When dealing with molding flash, industry experts often adjust the injection rate since this parameter has such a significant effect on flashing. That being said, a lower injection speed may translate to increased cycle times.

Specify a Flash-Free Mold

A flash-free mold is specially designed to eliminate flash in injection molding. It has proper alignment and very tight parting line tolerances, and this significantly minimizes the chances of flash.

Flash-free molds are an easy way of avoiding injection molding flash. However, they are way more expensive than standard molds.

Proper Mold Cleaning

As we’ve previously mentioned, contaminants such as dirt and debris on a mold can lead to a parting line mismatch. A simple way to minimize flash is to clean the mold meticulously paying special attention to the area behind slides and inside the leader pin bushings. A clean mold will seal better preventing leakage of molten plastic.

Proper Mold Maintenance and Process Improvements

Similar to other manufacturing tools, injection molds require regular maintenance. Using an injection mold that is in good condition helps prevent defects such as flashing.

Also, process variables such as clamp tonnage and clamp pressure need to be set accurately. If excessive force is applied, the mold may deform causing flashing.

How To Fix Injection Molding Flash?

Injection Molding Flash

If injection molding flash occurs, it’s still possible to remove the defect. Here are some of the most commonly used solutions.

Using Hot Air

Hot air is used in a wide range of industries for deflashing. This technique uses a focused, fast-moving stream of hot air to melt flash back onto the part. Hot air has proven very effective for thin and fine flash. However, applying this process on coarse flash would impact the appearance of the finished product. In such cases, it would be more effective to remove the flash mechanically and then smooth everything over using hot air.

Cryogenic Deflashing

Cryogenic deflashing is one of the most effective ways of getting rid of flash. It uses liquid nitrogen to cool the molded components. The molding flash will become brittle since it’s significantly thinner than the rest of the part.

The next step involves blasting the part with non-abrasive polycarbonate media which easily breaks off the brittle sections. Cryogenic deflashing is often preferred for its speed and excellent results. However, it can be more costly relative to other deflashing methods. Also, thin-walled parts require close attention as they can also be embrittled.

Manual Deflashing

This technique involves manually trimming the excess material along the parting line using cutting tools such as scissors, knives, and grinders. Manual deflashing is a highly versatile process that can be used for virtually all plastic materials. It does not apply undue stress on the material, and each component can be quickly inspected. On the downside, this process is slow compared to some of its alternatives.

Using Open Flame

At times, you can remove molding flash using an open flame. However, this method comes with some concerns. For starters, the flame may affect the final look of the part. Also, it can impede post-production treatments such as painting.

Choose Aria for Injection Molding Services

As a leading provider of injection molding services, Aria is committed to manufacturing functional, elegant, high-performance parts – on time and within budget. Our industry experts use advanced equipment and techniques to achieve precision in every aspect of the production process.

Thanks to our ISO-certified quality assurance processes, your project is guaranteed to meet your specific requirements. Upload your design today and get an instant quote and DFM feedback.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.