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How to solve the problem of flash in injection molding?

2025/10/08 By le zhan

What is flash during injection molding

Mid-sized injection molding plants lose tens, if not hundreds, of thousands of dollars annually due to flash—the thin, excess plastic that seeps through mold crevices. This translates into not only scrapped parts but also rework time to trim parts, premature mold wear, and rejected orders from quality-conscious customers. As a professional injection molding machine manufacturer, I hate to see my clients’ factories turn profits into losses due to flash, which often stems from overlooked mold misalignment and excessive injection molding machine pressure. Therefore, it’s crucial to identify, resolve, and prevent flash during injection molding by addressing mold alignment and machine pressure.

What is flash during injection molding?

Flash during injection molding refers to thin, gooey, or lumpy plastic that seeps into tiny gaps between mold halves, around ejector pins, or along parting lines during the injection molding process. Flash is not only unsightly but also creates a host of other problems. First, it results in scrap. If flash cannot be trimmed, you may lose 1–3% of parts per injection, and trimming increases labor costs. Secondly, mold damage can occur: over time, excess plastic can wear away mold edges or block vents, leading to expensive rework or replacement. Quality defects can also occur, with parts with flash likely failing dimensional inspection or assembly, leading to customer returns. While improper resin selection or mold design can cause flash issues, we diagnose 70% of flash problems as resulting from improper mold alignment or excessive pressure in the injection molding machine.

flash during injection molding

Mold Alignment Issues in Injection Molding

The first step in resolving flash issues is to determine whether mold misalignment is the cause. The following methods can diagnose this. First, perform a visual inspection. With the mold closed (but not pressurized), check for uneven gaps along the parting line. A properly aligned mold should have a consistent, fine gap. Next, test with a feeler gauge. Insert a thin feeler gauge at 4-6 points along the parting line (top, bottom, left, right, and diagonal).

If the feeler gauge slides in easily at one point but not at others, the mold is misaligned. Next, test run 10-20 parts using a clear, low-viscosity resin. If flash occurs only on one side of the part or at a specific portion of the parting line, misalignment is likely present. Also, ensure that you evenly bolt the mold to the machine’s movable and fixed platens before the actual injection molding process begins. Loose bolts or uneven torque can cause mold misalignment.

Mold Alignment Issues in Injection Molding

Fixing Mold Alignment Issues and Eliminating Injection Molding Flash

Once you confirm mold alignment, you can correct it using the following methods. First, adjust the tie rods. Most injection molding machines use four tie rods to maintain parallel platens. If the gauge indicates the mold is off-center, loosen the locknuts on the tie rods and rotate the adjustment screws to move the movable platen left or right or up and down. Once aligned, tighten the locknuts and retest with a feeler gauge.

Also, replace worn mold bushings/guides. The mold uses bushings and guides to maintain alignment during the closing process. If these parts are worn, replace them with OEM parts provided by the mold supplier. Wear of bushings is also a major cause of gradual misalignment. If adjusting the tie rods and replacing the bushings doesn’t work (for example, if the machine’s platen has warped), consult a technician from your injection molding machine manufacturer.

Excessive injection molding machine pressure during injection molding

If mold alignment checks are correct, another possible cause is excessive pressure. Injection molding machines use two main pressures to cause flash: clamping pressure (which holds the mold closed) and injection pressure (which pushes the resin into the mold). Diagnosing excessive pressure can be done in the following ways.

You can check the clamping pressure: Most machines display the clamping pressure on the HMI (human-machine interface). If the clamping pressure exceeds the maximum recommended by the injection molding machine manufacturer for your mold size, there’s a problem. If flash appears on all parts (not just one side) and the mold is aligned correctly, excessive injection pressure or speed may cause the resin to enter the tiny mold gap faster than the clamping force can maintain the mold closed. Look for “spraying” near the gate, a classic sign of excessive injection speed/pressure.

Excessive injection molding machine pressure during injection molding

Alternatively, you can conduct a part consistency test by reducing the clamping pressure by 5% and the injection pressure by 5%, producing 20 parts. If flash decreases or disappears, it indicates a pressure issue. If parts are short, the pressure is too low, and you need to find the optimal balance between full fill and no flash.

Adjusting Pressure to Prevent Injection Flash

If you discover that the injection molding machine’s pressure is too high, you can adjust two settings: the clamping pressure adjustment. First, reduce the clamping pressure in 5% increments (for example, from 100 tons to 95 tons), molding 10 parts at a time. Stop when the flash disappears, but the part is still filled. For most molds, the optimal clamping pressure should be 10-15% higher than the “minimum pressure to prevent flash.” Next, adjust the injection pressure and speed. If the flash is uniform, first reduce the injection pressure. If the flash is close to the gate, reduce the injection speed; a faster speed will force resin into the gap before the mold seals. You can also monitor the injection molding machine’s “pressure-time” graph; a smooth curve indicates stable pressure.

Fixing flash leads to high-quality products

By focusing on mold alignment and injection molding machine pressure, you can address approximately 70% of flash causes with tools and steps that can be implemented today. Use feeler gauges to diagnose alignment, adjust tie rods or replace bushings for repairs, and leverage HMI data to diagnose pressure, making minor adjustments and verifying them with a trial run. As a professional injection molding machine manufacturer, I help customers improve production efficiency, produce high-quality parts, reduce flash-related waste by 60% to 90%, and ultimately increase profits.

 

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