Injection Molding Flash: Top 9 Causes and How to Solve Them

Injection Molding Flash- Top 9 Causes and How to Solve Them

Injection molding is among the most commonly used processes in the manufacturing industry, used to produce numerous types of plastic parts at various production scales.

However, as with almost every manufacturing process, injection molding has its own set of undesirable defects. One of these is known as injection molding flash.

Flashing is caused by the accumulation of excess plastic on the surface of the part. Its removal through manual or automatic means adds to the cost of the process and can damage your mold.

Therefore, it is you may be interested in understanding what flash is, how it is caused, and what steps you can take to prevent it from affecting your production.

What is Injection Molding Flash?

An injection molding cycle involves injecting molten plastic into a mold cavity and allowing it to cool down before opening the mold and taking out the molded part.

Sometimes, a small portion of the plastic flows out of the mold during injection. As a result, it solidifies outside the cavity and forms an unwanted attachment to the part, which is called flashing.

Plastic mold flash is usually observed at the parting line of the mold, which is where the two halves of the mold meet. However, other locations, such as the ejector pins, can also experience flash due to the same reasons.

Industry experts view flashing as a common injection molding defect and, hence, are always looking for ways to reduce or eliminate it. However, doing so comes at an increased cost of production, as special molds that decrease the probability of flashing are expensive to make.

As a result, eliminating flashing requires a trade-off between the extra costs and the quality and aesthetic appearance of the product.

The Main Causes of Injection Molding Flash

As injection molding is a multi-stage process, you can pinpoint several steps in which flash molding defects can occur. Therefore, it is important to understand the various ways this phenomenon occurs before you plan to reduce its occurrence.

The most commonly encountered causes of injection molding flash are outlined below:

Mold Flash During the Injection Phase

Mold Flash During the Injection Phase

In the injection phase, the material is inserted from the hopper into the mold using a reciprocating screw.

1. Mismatch of the Parting Line

Mismatch of the Parting Line

This is considered to be among the most common reasons for flash defects in injection molding. Mismatch occurs when dust, debris, or poor mold design prevents the complete sealing of the mold before the molten material is injected inside.

Consequently, the mold is slightly open, allowing the excess material to escape along the parting line, which separates the two halves of the mold. Upon solidification, this causes mold flash.

Older molds, especially, are more prone to this, as the wear experienced by them causes a mismatch of the parting line over time. Accordingly, molding experts suggest paying extra attention to the design features of your mold.

Even if you manage to keep it free of contaminants, you may experience flashing if the clamping pressure is incorrectly set or if you are producing complex parts.

Design features such as cavities lead to extra pressure on your mold, which causes material leakage, and eventually, flashing.

2. Improper Venting

As injection molding is a complex process, you have to ensure that temperature, pressure, and design features allow a smooth, defect-free production run.

Sometimes, the air is not able to exit the mold cavity during the injection phase. This leads to an increase in mold pressure due to the compressed air, forcing the material out of the mold.

Usually, vents are included to facilitate the escape of air and allow the plastic to fill the mold cavity. But the improper design of vents, for example, by making them too large or too small, prevents them from functioning effectively, leading to flashing.

Moreover, older or worn-out vents also hamper the air from escaping.

Mold Flash During Packing or Holding

The packing stage refers to the point where the pressure is adjusted and excess molten material is injected into the mold cavity to account for shrinkage, while the holding process involves maintaining the pressure until cooling.

3. Insufficient Clamping Pressure

Insufficient Clamping Pressure

Clamping pressure refers to the pressure required to hold the mold shut during injection. It needs to be set precisely to overcome the pressure present inside the mold.

If the clamping pressure of the molding machine is less than the pressure inside the mold, the two halves of the mold may separate. Without sufficient clamping pressure to counteract this, the material is allowed to escape and cause flashing.

4. Sprue Bushing Issue

Sprue bushings provide the opening needed to transfer the molten material from the barrel into the mold cavity. Over numerous production cycles, they become less able to withstand the injection pressure, leading to a loss of functionality.

This lack of pressure-handling ability leads to defects such as flashing.

Mold Flash During the Process Due to Flow Characteristics

Some characteristics of your injection molding process can also cause flashing.

5. Low Viscosity

Viscosity is a measure of a fluid’s ability to flow by overcoming the internal frictional force present in its adjacent layers. Factors that affect viscosity include temperature, pressure, speed of injection, and the inherent properties of the material.

Molten plastics with lower viscosities easily flow out of the mold, thereby causing injection molding flash.

6. Barrel and Nozzle Temperature

Barrel and Nozzle Temperature

As we mentioned before, temperature is one of the factors that affect material viscosity. Higher temperatures in the barrel and nozzle sections of the injection molding machine decrease the viscosity of the molten plastic, thus enabling it to flow out of the mold more easily.

7. High Pressure

Pressure is another one of the factors that affect viscosity. High pressures improve the flow properties of the material, facilitating its escape from the mold.

8. Overfilling the Mold

The volume of molten plastic that you need to insert inside the mold in any injection molding operation depends on the design of the mold. The machine operator has to ensure that the right amount of material enters the mold during a production cycle.

While insufficient filling of the mold can cause incomplete parts to be made due to shrinkage upon cooling, the feed settings must not also overfill the mold.

If this happens, the clamping pressure is not able to withstand the injection pressure, causing the material to leak. The excess material attaches to your final product, and mold flashing is observed.

9. Overpacking


If your mold design involves narrow pathways, the molten plastic can solidify in them, which creates a blockage to further flow.

A region of localized pressure develops there, affecting the molding process and causing a host of defects, including injection molding flash.

5 Effective Ways to Control Flash in Injection Molding

Now that we have understood the various reasons that cause injection molding flash, it is time to explore how such defects can be controlled or eliminated. Usually, preventing flash from occurring is better than removing it when it is present on your product.

Design for Manufacturability

Design for Manufacturability

Design for manufacturability (DFM) involves incorporating design elements that result in the easy production of high-quality parts at affordable rates. It focuses on optimizing the design without compromising on performance.

To minimize flash defects in injection molding, you can start by planning the mold design so that the parting line is located on the edge instead of on the surface.

This means that the parting line will not be visible on the edge, thus improving the appearance. The seam counteracts the parting line mismatch, thus preventing it from causing flashing.

Furthermore, design for manufacturability software also allows you to simulate the material flow in the mold so that you can foresee any other defects that are likely to occur.

You can then adjust parameters such as wall thickness and gate design to ensure a high-quality product.

Reduce the Injection Rate

Injection rate refers to the speed at which the molten plastic is fed into the mold cavity. High injection rates result in high pressures and temperatures inside the mold, causing a decrease in the material viscosity and, hence, flashing.

Thus, experts often look to decrease the injection rate so that these associated variables can be controlled and the material does not escape that easily.

However, doing so can come at the cost of an increase in the cycle time, which means that you may have to make a trade-off between the injection rate and the cycle time.

Specify a Flash-Free Mold

Using a flash-free mold is a quick way to eliminate flash. These molds have a higher injection molding tolerance than regular molds and are used where the mold’s functionality as well as aesthetics are important.

However, many facilities are unable to afford them due to their high cost, which affects their profits.

Proper Mold Cleaning

Proper Mold Cleaning

As we discussed before, the build-up of dust and debris on the mold can cause a parting line mismatch. Similar contaminants can also accumulate in the pathways and cause overpacking.

Therefore, you must ensure that the mold components are regularly and properly cleaned so that these issues are mitigated.

Proper Mold Maintenance and Process Improvements

You should also pay attention to the injection molding process parameters so that you obtain defect-free, high-quality results.

Parameters such as clamp tonnage and clamp pressure must be correctly set so that the right amount of force is exerted on the mold and it does not deform and result in flashing.

Kemal: Your Expert Partner for Your Injection Molding Needs

Kemal: Your Expert Partner for Your Injection Molding Needs

Sometimes, you may not find it easy to predict the results of your planned injection molding run. At other times, you may struggle with defects that affect your parts’ quality. Here is where an expert production partner can help you out.

Kemal takes pride in having successfully offered exceptional injection molding services to a wide-ranging clientele for 28 years. Our ISO-certified quality control processes help you quickly fulfill your global market demands at affordable rates.

Moreover, you get superior technical and manufacturing support from our dedicated team, which helps you streamline all aspects of your part at affordable rates.

Our online free quote service allows you to upload your design and send in your queries so that our team of industry professionals can review it and give DFM feedback.


Flashing takes place when excess molten material leaks out of the mold and accumulates on the surface of the part, affecting its functionality and appearance.

Causes of flash include mismatch of the parting line, improper venting features, low clamping pressure, issues with sprue bushing, low viscosity, high barrel and nozzle temperature, high pressure, overfilling of the mold, and overpacking.

The five common ways to prevent injection molding flash are incorporating design for manufacturability principles, reducing the injection rate, using a flash-free mold, ensuring regular mold cleaning and maintenance, and improving the process.

Flash is an injection molding defect that can be observed as excess plastic deposited on the finished product. It is caused by the escape of the mold material from the cavity.


As we have discussed above, injection molding flash can occur due to several reasons, and minimizing its occurrence requires keeping track of all stages and trade-offs from the design stage to the actual process specifications.

However, with an experienced manufacturer such as Kemal, you get the best service for your injection molding needs. Explore our website to learn all about our services, or contact us if you have any queries.

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