What is a Burr?A burr refers to the excess material that protrudes around the perimeter or edges of a manufactured product. The term originates from the English word "burr," which means an overhang or protrusion.
It is a term widely used in industries such as manufacturing and construction.
In injection molding, burrs typically occur where the molten resin fills the mold along the mating surfaces of the cavity core (parting line/PL surface), forming at the edges of parts or boundaries of mold components.
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When molten resin is filled beyond necessity inside the mold, flash occurs at the mating surfaces of the cavity core (PL surface).
Injection molding allows for mass production of identical products using a mold. Over tens of thousands to millions of cycles, the mold ages and wears out. High filling pressures and sliding parts wear out, resulting in flash.
During startup or when issues arise in injection molding, the product may not eject from the mold properly. Inexperienced handling or careless actions can damage the mold. Dents in the mold can cause convex-shaped flash on the product.
While the specific locations susceptible to flash can vary with mold design, there are common areas where flash typically occurs:
The mold is clamped under high pressure according to its size. Due to the high pressures involved in filling,
Complex molded products require molds with complex shapes. Typically, molds have a nested structure, where multiple parts are fitted into the main body of the mold (mother mold). This nested structure allows for minimal manual correction of partial mold wear or issues. However, the joints between the nested parts can become flash points.
In the mold structure, parts that interfere with mold opening or product ejection actions are made using slide nests. Slide nests can directly become flash points if the alignment precision of the mold is low. Since slide nests slide, they are provided with clearances. Wear of these clearances from cumulative shots can lead to gaps where flash occurs.
The gate is the entrance through which molten resin is filled into the product part of the mold. It experiences high pressure during the filling and holding stages. Gates are severed in conjunction with the mold opening actions. If the gate is cut on the runner side, gate flash occurs. Changing the holding pressure in stages or altering the mold opening speed can improve gate separation. Since the gate is integral to the flow, the mold shape cannot be easily structurally modified. If gate flash cannot be improved by condition adjustments, additional nipper cutting is used as a solution.
When excessive filling pressure and holding pressure are applied to the runner or sprue pin, rather than the product part, flash occurs. If flash at the runner mating surface becomes large, it can lead to ejection failures due to runner release issues. Excessive holding pressure and time are unnecessary.
For each of the typical causes of flash mentioned above, the following countermeasures are proposed:
In this case, adjusting the molding conditions is the main countermeasure. To avoid overfilling, using the short shot method is effective. During startup or during interruptions like chocolate stops, create intentional short shots instead of full fills from the first shot, and gradually increase the fill amount while monitoring the condition of the molded product. This is effective in addressing flash. Additionally, attention is needed when switching material lots, even if the material is the same. When switching to a better-flowing material lot, the flow behavior changes even at the same filling speed. Especially under molding conditions involving multi-speed filling in complex molds, a narrow range of acceptable conditions can exacerbate flash. Check the product shape at the material lot switch point.
Mold wear is unavoidable. Understand the performance of the molded product and the quality required by customers, and strive to produce optimal molded products. The clamping force should be sufficient at the minimum value necessary for the parting line to clamp, based on the lowest detectable pressure. Proceed cautiously with setting and adjusting conditions, especially fill pressure, fill speed, holding pressure, holding time, and barrel temperature. While faster mold opening and ejection actions can enhance the cycle, not damaging the mold is paramount. Overly aggressive conditions can cause flash and accelerate mold wear. Mold under appropriate conditions.
In injection molding, the quality of the mold significantly affects the quality of the molded product—it is often said that "the mold is 90%." Handle the mold with utmost care. Prohibit inexperienced personnel from working on mold-related troubles, and have experienced personnel or those knowledgeable about molds create procedures to prevent damage.
Up to this point, we have discussed the main causes of flash and countermeasures. However, completely eliminating flash is challenging. It is important to focus on the following aspects for accurate detection of flash:
Initial inspection during startup is crucial. Typically, product variability occurs during startup because the mold temperature has not stabilized. Begin collecting good parts only after the startup variability has stabilized and each shot is confirmed to be of good quality. Even if the startup is conducted under the same molding conditions as previous production, it does not guarantee good parts. First, compare with a prepared standard sample and ensure there is no flash.
Detecting flash due to mold wear in advance is difficult. As a countermeasure, take time samples periodically during production and monitor their progression. If the size or thickness of the flash gradually increases, consult with the customer to understand the limits. Most molded products are assembled into a final product. Ensure that flash does not prevent assembly or cause secondary damage by interfering with movement.
Perform regular maintenance on the mold every tens of thousands of shots. Daily maintenance accessible on the molding machine is insufficient. Over time, soot accumulates, and grease wears off in molds. Disassemble the mold for thorough cleaning and greasing (known as a mold overhaul). Separate the cavity cores, and clean and grease the nests, slide nests, ejector pins, stripper plates, guide pins, and cooling tubes, and replace O-rings and gaskets. Regular maintenance helps avoid downtime.
We have summarized the points to consider when addressing defects and improving productivity in injection molding factories, especially regarding flash. The main causes of flash are broadly threefold: overfilling, mold wear, and mold handling. Manage these factors and aim to establish higher molding processing techniques.
