Methods to Avoid Injection Molding Flash

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Injection molding flash refers to excess plastic material overflowing from the mold cavity during the injection molding process and forming a thin, unnecessary covering along the mold seams. Flash can be caused by a variety of factors, such as excessive injection pressure, mold wear, or operating issues. Injection molding flash can cause cosmetic imperfections or functional issues in the final product.
In order to avoid injection molding flash problems, precise control of process parameters and regular mold maintenance are required to ensure that the produced plastic parts not only meet the design specifications, but also have a good appearance.

Injection Molding Flash

What Causes Plastic Injection Molding Flash?

In the field of plastic injection molding, the mold assembly comprises numerous metal components that interact both with each other and the flowing molten plastic. The mold itself is composed of two halves that converge along a designated parting line. The journey of the molten plastic resin material involves traversing through channels known as sprues and runners before being injected through specialized openings referred to as gates. Subsequent to the cooling process, ejector pins come into play, expelling the molded part(s), along with the runner, sprue, and gates, from the confines of the mold. At various junctures within this intricate mold structure, there exists the potential for a slender layer of plastic to escape the intended cavity, thereby giving rise to the phenomenon known as flash. Flash typically manifests along the parting line but can also occur at locations associated with ejector pins and elsewhere within the mold.

To address injection molding flash, Sungplastic uses Design for Manufacturability (DFM) principles to ensure potential issues are identified and resolved during the design phase before production begins.

Consequences of Flashing in Plastic Injection Molding

When flash occurs during the plastic injection molding process, it must be removed by the plastic injection molding manufacturer via deflashing. This process involves either skilled operators or robotic systems trimming away the unwanted material between molding cycles. However, the presence of injection molding flash can have multiple consequences that affect the manufacturing process and the final product.

    • Large Labor Force:
      The process of deflashing is labor-intensive, requiring skilled personnel or robotic equipment to perform precise trimming. This adds an extra step to the manufacturing process, potentially slowing down production.
    • Quality Compromises:
      Flashing can compromise the quality of the final product. It can result in irregular part dimensions, surface imperfections, and aesthetic flaws, which may not meet the desired standards.
    • Mold Damage:
      Repeated deflashing operations can put strain on the mold. Over time, this can lead to mold damage, necessitating costly repairs or replacements. Mold damage also contributes to longer downtime, affecting production efficiency.
    • Higher Costs:
      The need for deflashing, coupled with potential mold damage and compromised product quality, can lead to higher manufacturing costs. This includes labor expenses, maintenance costs, and potential material wastage.
    • Production Delays:
      Inefficient deflashing processes or mold issues caused by flashing can result in production delays, affecting order fulfillment and customer satisfaction.

Given these consequences, it becomes crucial for manufacturers to identify and address the root causes of injection molding flash. By doing so, they can reduce the need for deflashing, improve product quality, extend mold lifespan, and ultimately enhance cost-effectiveness and work efficiency in plastic injection molding operations.

How to Solve Injection Molding Flash

Effectively solving injection molding flash requires a systematic approach to identify and eliminate their root causes. By adopting these measures and maintaining a proactive approach to quality control and process optimization, the occurrence of injection molding flash can be significantly reduced or eliminated, ensuring higher product quality and production efficiency. Below are some key solutions for injection molding flash:

Parting Line Mismatch

Injection molding flash can result from a mismatch along the parting line within the mold. To prevent this issue:

    • Thoroughly clean the mold to remove any plastic residue, dirt, or contaminants, especially in areas like slides and leader pin bushings.
    • Ensure the proper fit of mold plates and mold seals.
    • Clear obstructions in the molding press.
    • Examine mold plates for deformation due to injection and packing pressures. Use pillar supports or adjust mold plate thickness accordingly.
    • Provide sufficient surface area on mold plates for even seal-off at the parting line, distributing clamping force uniformly.

Improper Venting

Inadequate venting within the mold can lead to flash. To address improper venting:

    • Check the mold vent size against the supplier’s recommendations for the plastic resin in use. Adjust vent sizes as needed.
    • Optimize vent depth to expel air as rapidly as viscosity allows. Tailor vent depth to the plastic material’s viscosity, employing deeper vents for stiffer materials and thinner vents for more fluid materials.

Clamping Pressure in the Packing/Holding Phase

Apart from its occurrence during the initial injection phase in plastic injection molding, flashing can also manifest during the subsequent packing or holding stages. During packing, adjustments are made to the pressure, and extra material is injected into the mold to account for material shrinkage and backflow. In the holding phase, the material is maintained under pressure equilibrium until gate freeze sets in, and the cooling process commences. In both these phases, if the clamping pressure is inadequate, it can result in the parting line of the mold being pushed apart, allowing molten plastic to escape and cause flash.

To address and potentially mitigate clamping pressure as a source of flashing:

    • Perform a First Stage Short Shot Test: If the product passes this test without any signs of flashing, it suggests that the issue arises after the injection phase, warranting consideration of clamping pressure as a potential cause.
    • Increase Clamp Tonnage: Ensure that the clamping pressure is robust enough to withstand the forces generated by the flowing plastic material within the mold. Properly match the mold size with the appropriate machine to guarantee the application of the necessary clamping pressure.

Challenges with Sprue Bushings

In injection molding, the hardened-steel sprue bushings within the mold play a pivotal role in facilitating the transfer of molten plastic into the mold cavity. However, issues related to sprue bushings can lead to the occurrence of flash, particularly when they lack adequate support to withstand the injection pressure or when the mold remains partially open.

To mitigate these concerns, consider the following measures:

    • Ensure Adequate Mold Support for Sprue Bushing: When the press nozzle seals onto the sprue bushing, it generates substantial forces ranging from 5 to 15 tons. If there is insufficient support within the mold structure for the sprue bushing, these pressures can transfer to the parting line, allowing plastic material to escape. To counter this, it is essential to incorporate the necessary support within the mold to bolster the sprue bushing.
    • Adjust the Length of the Sprue Bushing: If the surface of the sprue bushing creates a seal along the parting line, adjustments to the bushing’s length may be required. This ensures that nozzle pressure or thermal expansion does not inadvertently hold the mold partially open, permitting molten plastic to escape.

Addressing Process Variables Leading to Injection Molding Flash

Injection molding flash can result from various process variables. Below, we outline specific issues within process variables that can lead to flash and offer solutions to rectify them:

Low Material Viscosity:

Flash stemming from low material viscosity can often be resolved by addressing the following concerns:

    • Adjusting the melt temperature if it’s excessively high.
    • Minimizing excessive residence time to prevent material degradation.
    • Ensuring proper drying to eliminate moisture presence.
    • Reducing the amount of colorant containing lubricating agents.


Over-packing occurs when excess material is compressed in certain flow paths while others are still filling, causing localized pressure and potential flash. To address this issue, focus on optimizing molding conditions related to:

    • Material viscosity.
    • Injection rate.
    • Runner system design.

Injection and Packing Pressure:

Lowering pressure reduces the likelihood of flash. Excessive pressure can lead to mold opening issues. To mitigate this, reduce injection and packing pressure to reduce the required clamp force. Additionally, concentrate clamping force by reducing the contact area between mold halves.

Barrel and Nozzle Temperature:

Elevated barrel temperatures can increase material fluidity and susceptibility to flash. Higher temperatures may also enable material entry into small gaps. Reduce barrel and nozzle temperatures to align with recommended material temperatures, ensuring proper heating stages without flash occurrences.

Metering and Over-Fill:

When flash results from metering or over-filling:

    • Decrease the feed setting (stroke length).
    • Adjust cycle times during the cooling phase to prevent material overheating in the barrel.
    • Utilize materials with appropriate flow characteristics based on product requirements, referring to flow rate recommendations provided by resin manufacturers.

Reducing Injection Molding Flash with DFM

Manufacturers specializing in injection molding use design for manufacturability (DFM) as a proactive approach to preventing problems such as flash during the design phase. During this process, by carefully evaluating other potential problems such as injection molding flash, sink marks, weld lines, etc. of the project, timely design modifications, mold plan adjustments, etc. are made. Align design and manufacturing processes by adopting a DFM-driven approach, simplifying production and ensuring a high-quality end product.

Sungplastic is a long-established and experienced injection molding manufacturer. During the production process, our team of engineers solved numerous potential problems such as flash, warpage, etc. Our engineers simulate the injection molding process using specialized mold flow simulation software, creating 3D simulations of injection molding flow, heat flux and warpage, including complete molds with all details. We will work closely with you to solve flash and other related problems to ensure that the final product is both beautiful and functional, and fully meets your expectations for the product.

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