Injection molding is a widely used plastic processing method. However, flash frequently occurs during the injection molding process, affecting not only the product's appearance and dimensional accuracy but also increasing post-processing costs. In-depth investigation of the causes of flash and finding effective solutions are crucial for improving the quality and production efficiency of injection molded products.
I. Causes of Flash in Injection Molded Products
(I) Injection Molding Process Parameters
**Excessive Injection Pressure:** Excessive injection pressure causes the molten plastic to fill the mold cavity with excessive impact force. When the cavity pressure exceeds the clamping force of the mold parting surface, the melt overflows from the parting surface and other gaps, forming flash.
**Excessive Injection Speed:** Rapid injection speed causes the melt to fill the cavity in a short time, without sufficient time for even distribution. This easily creates high-pressure zones in weak areas of the mold, leading to flash formation.
**Excessive Holding Pressure or Holding Time:** Excessive holding pressure and holding time cause too much molten plastic to be squeezed into the cavity, increasing the likelihood of flash formation.
Excessive Barrel Temperature: Excessive temperature reduces the viscosity of the molten plastic, increases its fluidity, and makes it prone to overflowing at mold gaps.
(II) Mold Design and Manufacturing Aspects
Uneven Mold Parting Surface: If the parting surface is not machined precisely enough during mold manufacturing, resulting in unevenness, it will prevent the mold from closing tightly, causing flash.
Excessive Mold Clearance: Excessive clearance between the slider and groove, and between the ejector pin and ejector hole, provides an overflow channel for the molten plastic, causing flash.
Insufficient Mold Strength: During injection molding, the mold is subjected to significant pressure. If the mold structure design is unreasonable or the material selection is inappropriate, resulting in insufficient mold strength, the mold will deform under high pressure, producing flash.
(III) Raw Material Aspects
Differences in Plastic Particle Flowability: Different batches or brands of plastic particles may have different flowability. If the flowability is too high, flash is more likely to occur under the same injection molding conditions.
Impurities in Plastics: Impurities mixed into plastics during storage or transportation may alter their properties, causing abnormal local flowability and resulting in flash.
(IV) Equipment Aspects
Insufficient Clamping Force of Injection Molding Machine: The clamping force of the injection molding machine cannot meet the mold expansion force requirements during injection, causing the mold parting surface to open and producing flash.
Insufficient Platen Parallelism: Poor platen parallelism in the injection molding machine will cause uneven force distribution on the mold during mold closing, resulting in excessive gaps in some areas and thus producing flash.
II. Solutions to Flash Problems in Injection Molded Products
(I) Optimizing Injection Molding Process Parameters
Adjusting Injection Pressure: By gradually reducing the injection pressure and observing the product molding process, the optimal injection pressure can be found that ensures the product fills the cavity without producing flash. Multi-stage injection can generally be used, using a lower pressure in the initial filling stage, and then appropriately increasing the pressure for holding after the cavity is basically filled.
Controlling Injection Speed: Adjust the injection speed reasonably according to the shape, size, and mold structure of the product. For products with complex shapes, a slow-fast-slow injection speed curve can be used to avoid turbulence and high-pressure zones in the melt within the mold cavity.
Optimize holding pressure parameters: Determine appropriate holding pressure and holding time through experimentation to ensure dimensional accuracy and surface quality of the product, while avoiding flash. The holding pressure should be gradually reduced, and the holding time should not be too long.
Adjust barrel temperature: Set the barrel temperature reasonably according to the characteristics of the plastic. While ensuring sufficient plasticization, appropriately lower the temperature to increase the melt viscosity and reduce flash generation.
(II) Improve mold design and manufacturing
Improve the machining accuracy of the mold parting surface: During mold manufacturing, use high-precision machining equipment and processes to ensure the flatness and smoothness of the parting surface. For molds with flash problems, the parting surface can be ground and polished to ensure a tight fit.
Control mold clearance: For parts such as the slider and groove, and ejector pins and ejector pin holes, strictly control the clearance dimensions and adopt reasonable fit tolerances. If the gap is too large, it can be repaired by methods such as inserting bushings or electroplating.
Enhancing mold strength: During the mold design stage, strength calculations and analyses should be performed to optimize the mold structure and select suitable mold materials. For large molds or molds subjected to high pressure, reinforcing ribs or other structures can be added to improve the mold's resistance to deformation.
(III) Standardizing Raw Material Management
Selecting suitable plastic granules: Based on product requirements and injection molding process conditions, select plastic granules with appropriate flowability. When changing plastic suppliers or batches, conduct thorough trial molding and performance testing to ensure the stability of raw materials.
Strengthening raw material inspection: Strictly inspect purchased plastic granules to check for impurities. During production, regularly sample and test raw materials to ensure that their quality meets requirements.
(IV) Maintenance and Inspection of Injection Molding Equipment
Checking the clamping force of the injection molding machine: Regularly test and adjust the clamping force of the injection molding machine to ensure that the clamping force meets the mold requirements. Changes in clamping force can be monitored in real time by installing pressure sensors or other equipment.
Correcting Template Parallelism: Use professional measuring tools to regularly check the parallelism of the injection molding machine templates and correct any problems promptly. Simultaneously, pay attention to the daily maintenance and upkeep of the injection molding machine to ensure its normal operation.
III. Case Analysis
(I) Case 1: Flash Problem in Injection Molding of an Electronic Product Shell
Problem Description: During the production of an electronic product shell, a large amount of flash was found at the parting surface around the product, severely affecting the product's appearance and assembly performance.
Cause Analysis: Inspection of the injection molding process parameters revealed that the injection pressure was too high and the holding time was too long. Additionally, slight wear was present on the mold parting surface, leading to increased clearance.
Solution: Reduce the injection pressure and holding time, and grind and repair the mold parting surface. After adjustments, the flash problem was effectively resolved, and the product quality met requirements.
(II) Case 2: Flash Problem in Injection Molding of an Automotive Part
Problem Description: Flash appeared at the ejector pin area of the produced automotive parts, resulting in a high product scrap rate.
Root Cause Analysis: Inspection revealed that the gap between the ejector pin and the ejector pin hole was too large, causing molten plastic to overflow from the gap. Simultaneously, the parallelism of the injection molding machine's template was off, resulting in uneven stress on the mold.
Solutions: The ejector pin and ejector pin hole were re-machined to reduce the gap. The injection molding machine's template was aligned to ensure even stress distribution during mold closing. After these improvements, the flash problem was resolved, and production efficiency and product quality were improved.
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