Introduction
Injection molding is the main manufacturing process for connector insulating bases (plastic housings). By injecting molten engineering plastic into the mold cavity and cooling to solidify, plastic parts of the required shape are obtained. Connector plastic housings usually have characteristics such as small size, high precision, thin walls, and complex structures, making injection molding relatively difficult and prone to various appearance and dimensional defects. These defects not only affect product appearance but may also lead to poor assembly, reduced performance, or even functional failure. This article will analyze in detail the causes of the three most common defects in connector injection molding: sink marks, flash, and bubbles, and discuss corresponding solutions.
1. Sink Marks: The Most Common and Challenging Appearance Defect
Sink marks, also known as sink marks or dents, refer to local depressions appearing on the surface of injection molded parts, usually occurring at positions with thicker wall thickness, such as the root of ribs, the back of cylindrical bosses, and intersections of wall thickness. Sink marks are one of the most common defects in connector injection molding and are also a major factor affecting product appearance quality.
Root Causes of Sink Mark Formation
The essence of sink marks is that plastic undergoes volume shrinkage during cooling and solidification, while the already solidified outer surface cannot follow the internal melt to shrink synchronously, causing the surface layer to be pulled inward by internal tension. Specifically, there are the following main causes:
1. Uneven Wall Thickness: This is the most fundamental cause of sink marks. In areas with thicker wall thickness, plastic cools more slowly and has greater solidification shrinkage; while areas with thinner wall thickness cool faster and solidify first. When the interior of the thick-walled area continues to shrink, the already solidified surface is pulled inward, forming a depression. Structures such as ribs, BOSS columns, and latches on connector plastic housings often produce sink marks on the outer surface at corresponding positions.
2. Insufficient Holding Pressure: The holding pressure stage is the key stage of supplementing melt into the cavity to compensate for shrinkage. If the holding pressure is insufficient, the holding time is too short, or the gate solidifies too early, it cannot effectively compensate for the volume gap caused by plastic cooling shrinkage, leading to sink marks on the surface.
3. Excessive Melt Temperature: Excessive barrel temperature increases the volume expansion of the plastic melt, and the shrinkage after cooling also increases accordingly, making sink marks more likely to occur. At the same time, excessively high melt temperature also extends cooling time and reduces production efficiency.
4. Uneven Mold Temperature: Uneven mold temperature distribution causes different plastic cooling rates in various areas. Areas with high temperature cool slowly and have large shrinkage, prone to sink marks.
5. Improper Injection Speed: Too slow injection speed causes the melt temperature to drop more during filling, increasing viscosity, which may make it difficult for holding pressure to effectively transfer to the distal end of the cavity, and distal positions are prone to sink marks.
Methods to Solve Sink Marks
1. Product Design Optimization
- Minimize wall thickness as much as possible and maintain uniform wall thickness. The typical wall thickness of connector plastic housings is generally between 0.4mm and 1.0mm, and the wall thickness difference should not exceed 30%.
- The thickness of ribs and BOSS columns should not exceed 60%-70% of the main wall thickness, and rounded transitions should be made at the root to reduce local wall thickness.
- For taller BOSS columns, consider making them hollow or with a hollowed bottom structure to reduce the problem of excessive wall thickness.
- Design sink mark pockets on the back of the sink mark position, using design to transfer the sink mark position and improve appearance.
2. Mold Design Optimization
- Optimize gate position and size, placing the gate at the position with the largest wall thickness to ensure holding pressure can be effectively transmitted.
- Appropriately increase gate cross-section and runner size, delaying gate solidification time to ensure sufficient holding and feeding.
- Optimize cooling water channel design to make mold temperature distribution uniform, reducing uneven shrinkage caused by temperature differences.
3. Process Parameter Adjustment
- Increase holding pressure and extend holding time, so that more melt is supplemented into the cavity during the holding stage to compensate for shrinkage.
- Appropriately reduce melt temperature to reduce the overall shrinkage of the melt. But also note that the melt temperature should not be too low, otherwise it may lead to insufficient filling.
- Optimize injection speed, use segmented injection control, appropriately decelerate at the end of filling, reduce shear heat and internal stress, and ensure effective holding pressure.
- Appropriately increase mold temperature, although it may extend cooling time, it can improve melt flowability and surface quality, and for some materials helps reduce sink marks.
- Extend cooling time to ensure the product is fully cooled and solidified before mold opening and ejection, avoiding deformation caused by ejection.
4. Material Selection
- Choose plastic materials with lower shrinkage rates, or add appropriate proportions of glass fiber and mineral fillers to reduce shrinkage.
- Pay attention to material drying, as excessively high moisture content may exacerbate uneven shrinkage and other defects.
2. Flash: A Common Problem Affecting Assembly and Dimensional Accuracy
Flash, also known as burrs or overflow, refers to excess thin edges formed at product edges when plastic melt overflows from mold parting surfaces or ejection mechanism gaps during the injection process. Connector plastic housings have small sizes and high fitting accuracy, and even slight flash can cause assembly difficulties or functional failure.
Causes of Flash Generation
The root cause of flash generation is the mold parting surface not fitting closely, causing melt to overflow from the gap under injection pressure. Specific causes can be divided into the following categories:
1. Mold Issues
- Insufficient parting surface machining precision and flatness, with gaps present.
- Mold guide mechanism worn or fit clearance too large, causing mold misalignment.
- Excessive fit clearance of moving parts such as inserts, ejector pins, and sliders.
- Foreign objects or residual plastic on the mold parting surface, causing incomplete mold closure.
- Mold vent slots cut too deep or too wide, exceeding the flash value of the material.
- After long-term use, the mold parting surface wears, with collapse or deformation.
2. Equipment Issues
- Insufficient injection molding machine clamping force, causing the mold to be pushed open under injection and holding pressure, creating gaps at the parting surface.
- Poor parallelism of injection molding machine platens, causing uneven mold stress and local gaps.
- Inaccurate barrel or nozzle temperature control, leading to excessively high melt temperature.
3. Process Parameter Issues
- Excessive injection pressure or holding pressure, exceeding the clamping force capacity, causing the mold to expand.
- Excessive injection speed, causing excessive impact pressure after the melt fills the cavity instantaneously.
- Excessive melt temperature, reduced plastic melt viscosity, too good flowability, making it easier to overflow from tiny gaps.
- Excessive shot size, with excess melt being squeezed into mold gaps under high pressure.
4. Material Issues
- Too good material flowability (such as materials with excessively high melt flow index), prone to flash generation.
- Excessive lubricant addition, reducing melt viscosity and increasing the risk of flash.
Methods to Solve Flash
1. Mold Repair and Optimization
- Inspect and repair the parting surface, improve the fit of the parting surface through lapping, polishing, and other methods.
- Inspect and repair the guide mechanism to ensure good fit of guide pillars and guide bushes with accurate positioning.
- Check the fit clearance of moving parts such as inserts and ejector pins, and timely replace or repair those out of tolerance.
- Adjust vent slot depth to ensure smooth venting without producing flash. Different materials have different flash values, and low-viscosity materials such as LCP have smaller flash values, so vent slots should be shallower.
- Perform regular mold maintenance, timely clean parting surface residue, and check for wear conditions.
2. Equipment Adjustment and Inspection
- Calculate the required clamping force to ensure sufficient injection molding machine clamping force. Clamping force is generally calculated based on product projected area multiplied by cavity pressure. Connector products usually take 300-500 kg/cm-squared of cavity pressure.
- Check the parallelism of the injection molding machine platens, and adjust if there is deviation.
- Regularly calibrate barrel temperature to ensure accurate temperature control.
3. Process Parameter Adjustment
- Reduce injection pressure and holding pressure, using the lowest possible pressure while ensuring product filling and dimensions.
- Optimize injection speed, use multi-stage injection, decelerate when filling is nearly complete, reduce cavity pressure impact.
- Appropriately reduce melt temperature, increase melt viscosity, and reduce flash tendency. But pay attention to balancing filling quality.
- Adjust shot size to avoid excessive feeding.
- Appropriately increase mold temperature, although it sounds contradictory, in some cases increasing mold temperature can reduce injection pressure requirements, which actually helps reduce flash.
4. Post-Processing
- For slight flash, it can be removed through post-processing methods such as tumbling, sandblasting, and manual trimming. But this increases costs and is not a fundamental solution. Priority should be given to solving from the process and mold.
- For flash problems in mass production, consider adding a de-flashing process, such as cryogenic deflashing.
3. Bubbles and Voids: Internal Defects Affecting Performance
Bubbles refer to voids or cavities inside the injection molded part. Bubbles can be divided into two types: one is gas bubbles formed by trapped air, moisture, or volatile matter; the other is vacuum voids formed by uneven shrinkage during the cooling process. Bubbles not only affect appearance but may also reduce mechanical strength and even lead to product functional failure.
Causes of Bubble Formation
1. Material Moisture and Volatiles: If the plastic is not sufficiently dried before processing, the moisture it contains will vaporize at high temperature, forming bubbles inside the product. Similarly, decomposition of additives in the material or residual monomers may also produce volatile gases.
2. Air Entrapment During Filling: During the injection filling process, if the melt advances too fast and the air in the cavity has no time to escape, it will be trapped, forming bubbles. This is especially common in products with complex shapes, thin walls, and large length-to-thickness ratios. Poor venting design is an important cause of air entrapment.
3. Vacuum Voids from Shrinkage: During the cooling and solidification process, if the surface has solidified but the interior is still in a molten state, continued shrinkage of the interior will form vacuum voids. This is essentially a type of internal sink mark, commonly found in thick-walled sections.
4. Degradation of the Material: Excessively high melt temperature or too long residence time in the barrel may cause thermal degradation of the material, producing gas and forming bubbles.
Methods to Solve Bubbles
1. Material Pre-Drying
- Fully dry the material before injection molding, strictly control moisture content within the range required by the material specification. Different materials have different drying requirements, such as nylon materials usually need drying at 80-120C for 4-6 hours, while PBT and LCP also need corresponding drying treatment.
- Use a dehumidifying dryer for engineering plastics with high drying requirements to ensure drying effect.
2. Optimize Mold Venting
- Reasonably design the gating system to control the filling flow direction so that air can be discharged from the parting surface or vent slots.
- Increase vent slots at positions prone to air entrapment to improve exhaust effect. The depth of vent slots should be controlled below the flash value of the material.
- For positions where it is difficult to set vent slots, consider using porous steel or adding vent pins.
3. Process Parameter Adjustment
- Reduce injection speed, especially the speed in the later stage of filling, to give air enough time to be discharged. Use multi-stage injection with low-speed filling, which helps reduce air entrapment.
- Appropriately increase mold temperature, improve melt flowability and surface quality, making it easier for gas to be discharged.
- Optimize holding parameters, appropriately increase holding pressure and extend holding time, which helps compact the melt and reduce vacuum voids.
- Appropriately reduce melt temperature to reduce material degradation and volatilization. Check whether the temperature settings of each section of the barrel are reasonable to avoid local overheating.
- Reduce the residence time of material in the barrel, avoiding long-term high-temperature residence leading to degradation.
4. Product and Mold Design Optimization
- Reduce wall thickness differences, avoid excessively thick sections, and reduce internal vacuum voids caused by uneven shrinkage.
- Optimize gate position and runner design to make melt filling stable and reduce turbulence and air entrapment.
- For positions prone to bubble generation, consider adding overflow wells or cold slug wells to guide the melt containing bubbles into the overflow well.
4. Brief Introduction to Other Common Injection Molding Defects
In addition to the three major defects mentioned above, connector injection molding often encounters the following problems:
Short Shot: The melt does not completely fill the cavity, resulting in an incomplete product. The main causes are insufficient injection pressure, too low melt temperature, too small runner gates, poor venting, etc.
Warpage: The product warps or twists after demolding. The causes are uneven internal stress, uneven shrinkage, uneven cooling, unbalanced ejection, etc. Deformation control of thin-walled precision connectors is particularly important.
Flow Marks: Wavy or streamline-like traces appear on the product surface. The causes are improper injection speed, too low mold temperature, uneven melt temperature, etc.
Ejection Marks/Ejection Damage: The product surface turns white or is pierced during ejection. The causes are excessive ejection force, insufficient draft angle, product sticking to the mold, insufficient cooling, etc.
Silver Streaks: Silver-gray stripes appear on the product surface, mainly caused by moisture and volatiles, belonging to a manifestation of bubble-related defects.
Conclusion
The occurrence of connector injection molding defects is often the result of multiple factors acting together, requiring systematic analysis and optimization from multiple dimensions such as product design, mold design, material selection, and process parameters. Sink marks originate from uneven volume shrinkage, flash originates from the imbalance between mold gap and melt pressure, and bubbles are related to moisture, venting, and shrinkage. Only by comprehensively considering all factors and systematically analyzing and optimizing can the injection molding quality of connectors be effectively improved.
RXSY attaches great importance to mold design and injection molding process control, with a professional engineering team and strict quality management system. The plastic parts of our connector products are carefully designed and verified to ensure excellent product quality and stability. For more information on connector products and manufacturing processes, please refer to the RXSY product series.
