During the vertical injection molding process, flash (also commonly referred to as burrs, overflows, or spew) is a frequent defect that causes major headaches for many processing plants. It not only severely impacts the product’s appearance and dimensional accuracy but also leads to a significant waste of raw materials, and can even damage high-precision molds.
So, what exactly causes the molten plastic to “leak” out of the mold cavity? As a vertical injection molding machine manufacturer with years of deep industry experience, MINHUI brings you this comprehensive troubleshooting and optimization guide, covering everything from core root causes to practical solutions.
What Exactly Causes Flashing in Vertical Injection Molding Machines?
In essence, flashing occurs when the molten plastic, under injection pressure, breaks through the clamping resistance of the mold parting line (PL line) or other mating gaps. This is typically caused by a combination of factors across the following four dimensions:
1. Clamping Force Cannot Overcome Injection Tension
If the clamping force of the injection molding machine itself is too low, the immense longitudinal projected tension generated as the molten plastic is injected at high speed and pressure will directly push the mold open, causing mold expansion and flashing.
2. Physical Gaps in Mold Alignment
The mold dictates the final shape of the product. If the mold structure lacks rigidity and undergoes physical deformation, if the parting line itself is not tightly sealed, if the machining accuracy fails to match the machine specifications, or if foreign matter (such as residual plastic debris) is trapped on the parting line, it will result in incomplete contact on one side or a localized area of the mold, thereby exposing a gap.
3. Improper Molding Process Conditions
The “fluidity” of molten plastic is a double-edged sword. If the technicians set the melt temperature too high, or if the injection pressure and injection speed are set too high, the viscosity of the plastic melt drops sharply, causing its fluidity to skyrocket. Highly fluid plastic behaves like water, easily penetrating into the tiniest mold gaps that would otherwise be sealed.
4. Excessive Venting System Design
To avoid product burning or air trapping, molds are designed with venting slots. However, if the venting slots are designed too deep (exceeding the flash limitation value of that specific plastic), or if the venting locations are inappropriate, the venting slots can inadvertently turn into overflow channels.
Vertical Injection Molding Machine Manufacturer Guide: How to Thoroughly Resolve This via Equipment and Molds
Once the root cause is identified, we must apply the right remedy. Drawing from MINHUI‘s years of experience in machine debugging and customer service, we recommend prioritizing targeted inspections from the following key equipment and physical structure levels:
🔧 1. Strictly Match and Calibrate Mechanical Clamping Force
- Scientific Selection: When choosing a vertical injection molding machine, you must ensure that the rated clamping force of the machine is higher than the tension formed during injection by the longitudinal projected area of the molded product. If the product’s projected area is large while the machine’s clamping force is insufficient, flashing will be inevitable.
- Inspect the Toggle Mechanism: If the clamping components are poorly adjusted or the toggle mechanism does not fully straighten during clamping, it will cause uneven distribution of the clamping force. This triggers a phenomenon where one side of the mold is tightly clamped while the other side is loose, inevitably producing flash on the loose side during injection.
📐 2. Conduct a Deep Check on Mold and Platen Parallelism
- Poor parallelism of the mold itself, improper or unparallel installation on the vertical slide table or rotary table, or parallelism deviations in the machine’s upper and lower platens will break the balance of force.
- Furthermore, if the tie bars (tie columns) of the injection molding machine are unevenly stressed or undergo uneven deformation over long-term use, it will cause a slight tilt when the mold closes, leading to incomplete mold sealing.
⚙️ 3. Promptly Maintain and Replace Worn Machine Parts
Flash that appears repeatedly and irregularly is often a signal of worn mechanical components. Please focus your inspection on the following points:
- Check Ring (Non-Return Valve) & Screw Assembly: Severe wear on the check ring or an excessive gap between the barrel and the screw will cause backflow of the melt during injection, making pressure control unstable.
- Nozzle & Feeding Zone: Failure of the spring nozzle’s spring will lead to drooling and material leakage. Additionally, if the cooling system at the feed throat fails, it will cause a “bridging” phenomenon, resulting in unstable material feeding, insufficient cushion, and a chain reaction of flashing.
Conclusion
Resolving flash and overflow issues on vertical injection molding machines requires both precise equipment hardware support and patient fine-tuning of process parameters. By executing a step-by-step troubleshooting process—from machine clamping force and mold parallelism to component wear—you can eliminate the vast majority of flash-related troubles.
If you encounter other stubborn molding process challenges during the operation of your vertical injection molding machines, or if you are looking for high-rigidity, high-precision vertical injection molding equipment, please feel free to contact the MINHUI team. Our technical experts are ready to provide you with tailored technical support and molding solutions.
