Insufficient filling, also known as short shot, is one of the most common and troublesome quality defects in injection molding production. Its typical manifestation is that molten plastic cools and solidifies before completely filling mold cavity, resulting in incomplete ends, uneven edges, localized depressions, or holes in product. This problem is particularly prominent in thin-walled parts—when wall thickness drops below 0.5mm, flow resistance increases exponentially, and insufficient filling rate can soar from 2% to 20% for regular parts.
Insufficient filling not only affects appearance quality of product but also directly damages its structural integrity and mechanical properties. Case studies show that a home appliance parts factory experienced a scrap rate as high as 35% due to insufficient filling when producing washing machine control panels. Causes of short-filling molding are multifaceted, involving mold design, injection molding machine, plastic materials, and machine setup processes. This article will systematically analyze causes of short-filling molding from these four dimensions and propose targeted solutions.

 

Molding Before delving into causes from each dimension, it is necessary to understand physical mechanism of short-filling molding. During injection molding, molten plastic is propelled by screw through nozzle, runner, and gate into mold cavity. As melt flows within cavity, it continuously contacts cooler mold wall, resulting in continuous heat dissipation. When temperature of melt front drops below solidification point, flow stops. Essence of short-filling molding is that melt flow front loses its fluidity before cavity is completely filled.
Root causes of this result can be summarized into three categories: insufficient pressure to propel melt forward, melt temperature dropping below solidification point prematurely, and air or other gases within cavity blocking flow channels. These three categories of causes correspond to issues related to machine capability, process parameters, and mold design, respectively. This forms logical basis for analysis presented in this paper, which unfolds from four dimensions.

Mold is channel for melt flow and space for molding. Rationality of mold design directly determines whether melt can successfully fill cavity. Mold-related short-filling problems are often the most stubborn—because once mold is manufactured, modifications are costly and difficult.
(I) Cause Analysis
1. Inappropriate Gating System Design: Gating system includes main runner, branch runners, and gate. If runners are too small, too thin, or too long, pressure loss along flow path of melt will be too great, obstructing flow and easily leading to poor filling. Especially, a gate size that is too small will significantly increase flow resistance. Case studies show that when diameter of submersible gate's entry point is too small, injection pressure is lost excessively when passing through gate, easily resulting in short-filling at tail of product. For multi-cavity molds, differences in runner length or gate size between cavities can lead to uneven filling—cavities with shorter runners fill first, while cavities with longer runners may experience short fills.
2. Poor Mold Venting: If air cannot be expelled from mold cavities in time, it will form airlocks at cavity ends, hindering continued flow of melt. Short fills caused by poor venting are often accompanied by burning. Depth of venting channels should generally be controlled between 0.02-0.05 mm. Deep cavities and complex structures are particularly prone to air trapping; without venting channels, short fill problems will be very pronounced.
3. Insufficient or Uneven Mold Temperature: If mold temperature is too low, melt cools rapidly upon entering cavity, and flow front solidifies before cavity is fully filled. Uneven temperature across different areas of mold can also cause premature cooling and solidification in some areas. Studies indicate that approximately 40% of short fill cases can ultimately be traced back to improper mold temperature settings. Once melt comes into contact with cold mold wall, a "frozen layer" forms on the surface within 0.1 seconds. This solidified skin continues to grow inward, compressing effective runner cross-section.
4. Product Structure Design Issues: Excessively thin product walls or abrupt changes in wall thickness will cause excessive flow resistance. For thin-walled parts, flow resistance increases geometrically.
5. Improper Cold Slug Cavity Design: If cold slug cavity is too small or improperly located, it cannot effectively capture cold slug generated during injection molding. Once inside cavity, cold slug will block flow channels.
(II) Solutions
1. Optimize Gating System: Enlarge gate size and runner cross-sectional area; For excessively long runners, runner can be thickened or shortened appropriately; For multi-cavity molds, runner balance should be checked, and gate size should be adjusted as needed to ensure even filling; Auxiliary runners can be added where filling is insufficient to increase flow channels for plastic melt; Multi-point feeding or changing the gate position can be used if necessary.
2. Improve Venting System: Add venting channels in areas with insufficient filler material and areas prone to air trapping; Clean clogged venting channels; Venting channel depth is generally controlled between 0.02-0.05mm; Create venting channels on parting surface, or add venting inserts in areas prone to air trapping; Reducing clamping force appropriately can improve venting performance.
3. Set Mold Temperature Appropriately: Set an appropriate mold temperature based on material characteristics. For example, for high-viscosity materials such as PC or PPSU, mold temperature should be at least 120℃; for PA66 with 30% glass fiber, a mold temperature of 80-100℃ is recommended; for ABS, 50-80℃ is recommended. Ensure mold temperature controller is working properly, and control temperature difference on mold surface within ±5℃. Insulate mold plate with a heat insulation plate to reduce heat loss.
4. Improve Product Structure Design: Increase wall thickness appropriately, especially in thin-walled areas prone to insufficient filler material. To improve flow, reinforcing ribs should be added to areas where flow is difficult, without affecting functionality. Avoid abrupt changes in wall thickness.
5. Optimize cold slug well design: Increase size of cold slug well. Ensure cold slug well is positioned appropriately to effectively capture cold material.

 

Injection molding machines provide injection power and plasticizing capacity. Improper machine selection, component wear, or malfunction can all lead to insufficient plasticizing.
(I) Cause Analysis
1. Improper Injection Molding Machine Selection, Insufficient Plasticizing Capacity: This is the most common cause related to machine. When product quality exceeds actual maximum injection capacity of injection molding machine, machine cannot provide sufficient melt volume. Even if product quality is close to actual injection capacity of injection molding machine, insufficient plasticizing may still occur—material is not heated enough in barrel, failing to provide adequate melt to mold in time. Producing large-sized products using a "small horse pulling a large cart" approach almost inevitably results in insufficient plasticizing.
2. Screw or Barrel Wear: Wear of screw, barrel, or injection nozzle can cause material backflow, resulting in insufficient actual mold filling. Wear also leads to decreased plasticizing efficiency. Check ring failure is also a common problem—a damaged check ring causes molten plastic to flow back, resulting in unstable injection volume.
3. Nozzle Problems: A nozzle with an inner orifice that is too small increases flow resistance, causing injection pressure loss. Poor fit between nozzle and mold's main runner inlet can lead to material leakage or cold material blockage. Nozzle blockage also directly affects injection.
4. Heating System Malfunction: A malfunctioning heating system, such as a thermocouple malfunction, can cause inaccurate temperature readings—a reading that is actually high but actual material temperature is too low.
5. Injection Cycle Too Short: When injection cycle is too short, material temperature cannot keep up, also causing insufficient material.
(II) Solutions
1. Appropriate Machine Selection and Replacement: Ensure that plasticizing capacity and injection volume of injection molding machine meet product requirements. It is generally recommended to have a certain margin of error in injection capacity. If machine capacity is insufficient, a larger tonnage injection molding machine should be used. Case studies show that replacing a 120-ton machine with a 160-ton machine significantly reduced short-fill defect rate from 35%.
2. Inspect and Replace Worn Parts: Regularly inspect screw and barrel for wear and replace them as necessary. Check check ring for failure. If molten material backflow or unstable injection volume is observed, replace check ring promptly.
3. Clean and Adjust Nozzle: Disassemble nozzle for cleaning. Check if nozzle matches mold inlet. Appropriately increase nozzle tip diameter. Adjust nozzle center position to ensure good alignment with main runner inlet.
4. Overhaul Heating System: Check if thermocouples are working properly. Confirm that temperature display is accurate, avoiding situations where displayed temperature is high but actual temperature is low.
5. Adjust Injection Cycle: Appropriately extend injection cycle to ensure material temperature keeps up. Ensure sufficient cooling water flows through hopper throat to maintain correct temperature.

Inherent properties of plastic materials directly affect melt flow behavior. Inappropriate material selection or inadequate pretreatment can both contribute to insufficient melt flow.
(I) Cause Analysis
1. Poor Material Flowability: Different plastics have significantly different flowability. If flowability of selected material cannot meet filling requirements of product—especially for thin-walled, long-flow complex products—short melt flow is likely to occur. Poor material flowability means that, under same temperature and pressure, melt filling length is shorter.
2. Excessive Moisture Content: Many plastics (such as ABS, PC, PA, etc.) are hygroscopic. If material is not sufficiently dried, moisture will vaporize at high temperatures, producing bubbles and gases that hinder melt flow. A case study shows that a medical device factory producing transparent PC shells experienced a 28% defect rate of silver streaks and insufficient melt flow due to a raw material moisture content of 0.2% (PC requires ≤0.02%).
3. Improper Lubricant Addition: Too much or too little lubricant will affect filling. Too much lubricant may cause melt slippage and unstable flow; too little lubricant will increase frictional resistance. For cases requiring flow aids, addition rate is generally 0.05-0.1% (by weight).
4. Foreign Matter in Material: Foreign matter mixed in material can clog runner or gate, causing localized short-filling.
5. Excessive Use of Sprue Residue: Excessive use of sprue residue and recycled material will lead to decreased material performance and poor flowability.
(II) Solutions
1. Appropriate Material Selection: For thin-walled, long-flow products, prioritize materials with good flowability. Under guidance of material supplier, select a grade with an appropriate melt index. If necessary, add an appropriate amount of flow aids (such as lubricants and dispersants) to reduce melt viscosity.
2. Strict Drying Treatment: Set drying temperature and time according to material characteristics. For example, hygroscopic materials such as ABS and PC need to be dried at 80-120℃ for 2-4 hours. PC materials require a moisture content ≤0.02%, and in actual production, it should be controlled below 0.015%. Use a drying hopper or dehumidifying dryer to ensure material remains dry during injection molding.
3. Control Lubricant Usage: Strictly follow formula when adding lubricant, avoiding excessive or insufficient amounts. Ensure lubricant is evenly dispersed in material.
4. Ensure Material Purity: Strengthen raw material storage management to prevent foreign matter contamination. Check material for foreign matter before use.
5. Control Sprue Ratio: Increase proportion of virgin material. Reduce number of times sprue material is recycled, generally not exceeding three times.

 

Machine adjustment is "bridge" connecting mold, machine, and material. Even if all three are functioning correctly, improper process parameter settings can still lead to insufficient molding. In practice, most melt shortage problems are initially resolved through process adjustments.
(I) Cause Analysis
1. Insufficient Injection Volume: Improperly set feed rate or insufficient screw metering stroke result in insufficient molten melt reserve at the barrel tip. This is the most easily overlooked yet also the easiest to resolve cause.
2. Injection Pressure Too Low: Injection pressure is direct driving force propelling melt forward. When pressure is too low, melt is unable to overcome flow resistance to reach cavity end. For thin-walled parts, injection speed is often more critical than injection pressure—if it's too slow, even if final pressure is high enough, melt front will have already solidified en route.
3. Injection Speed Too Slow: Injection speed determines speed at which melt advances within cavity. When speed is too slow, melt front cools and solidifies before reaching cavity end. For thin-walled parts (wall thickness 0.5-0.8mm), injection speed should be set at 75%-90% of actual achievable maximum injection speed.
4. Low Melt Temperature (Material Temperature): When material temperature is too low, melt viscosity is high and flowability is poor. Low temperature at the front of barrel causes molten material entering cavity to reach a point where it is difficult to flow earlier due to cooling effect of mold. After a period of production, molten plastic continuously carries away heat, causing melt temperature to drop and viscosity to increase, leading to insufficient filling.
5. Low Mold Temperature: A low mold temperature causes melt to cool and solidify prematurely. Mold temperature and filling length are approximately directly proportional.
6. Insufficient Holding Pressure and Time: Purpose of holding pressure stage is to compensate for shrinkage in cavity before gate solidifies. Insufficient holding pressure or too short a holding time will result in incomplete filling at the end of cavity.
7. Improper Back Pressure Setting: Excessive back pressure will over-shear melt, causing uneven material temperature and premature cooling. Insufficient back pressure may lead to uneven plasticization and inconsistent melt density. For thin-walled parts, a back pressure of 0.5-1.0 MPa is generally sufficient.
8. Improper V/P Switching Point Location: Switching speed/pressure switching point (V/P point) too early will cause filling stage to transition to holding pressure before completion, resulting in insufficient filler.
9. Temperature Fluctuations: Excessive temperature fluctuations during injection molding can lead to unstable conditions, such as intermittent insufficient filler and flash. Melt temperature fluctuations should be controlled within ±3℃ (±1.5℃ for precision injection molding).
(II) Solutions
1. Adjust Injection Volume: Increase injection volume and adjust screw metering stroke; Ensure sufficient melt reserve at the front of barrel; Increase buffer volume.
2. Optimize Injection Pressure and Speed: Appropriately increase injection pressure; Adjust in stages to avoid excessive pressure leading to flash; Increase injection speed, especially for thin-walled parts. When adjusting, select one parameter first, observe effect, and then decide on next step; Use multi-stage injection: such as high-speed filling of most of cavity followed by low-speed injection to prevent flash.
3. Increase melt temperature: Appropriately increase barrel temperature, but within material's tolerance range to avoid degradation; Increase nozzle temperature; Gradually increase barrel temperature after a period of production to compensate for heat loss; Using a hot runner system can reduce melt residence time by 30-50%.
4. Increase mold temperature: Use a mold temperature controller to raise mold temperature to material's recommended process range; For high-viscosity materials such as PC, mold temperature should be at least 120℃.
5. Optimize holding pressure parameters: Appropriately increase holding pressure; Extend holding time to ensure that shrinkage compensation is completed before gate solidifies; For thin-walled parts, it is recommended to set V/P switching point at 98% of fill volume.
6. Set back pressure appropriately: Back pressure is generally set at 0.5-1.0 MPa; Avoid excessive back pressure leading to excessive shearing. Increasing back pressure appropriately can increase shear heat, which is beneficial for plasticization.
7. Controlling Temperature Stability: Monitor screw torque to control melt temperature fluctuations within ±2℃; Use a mold temperature controller to ensure mold temperature stability.
8. Adjusting Injection Cycle: Appropriately extend injection time. Ensure a stable molding cycle and avoid temperature changes caused by cycle fluctuations.

 

In actual production, insufficient glue usage is often result of multiple factors working together. Therefore, establishing a systematic troubleshooting approach is crucial.
(I) Suggested Troubleshooting Sequence
Preliminary Inspection: Confirm whether material is sufficiently dry, whether injection volume setting is adequate, and whether mold venting is unobstructed. Process Parameter Adjustment: Follow "easy to difficult" approach—first increase injection volume, then gradually increase injection pressure/speed, then adjust material temperature and mold temperature. Change only one parameter at a time, observe effect, then proceed to next step. Mold Verification: Mark location of insufficient glue during trial molding and specifically check venting, gate, and cooling conditions at that location. Equipment Inspection: If insufficient glue usage occurs repeatedly, check whether mechanical components such as screw and check ring are worn.
(II) Comprehensive Case Study
A home appliance parts factory experienced a 35% defect rate due to insufficient glue usage when producing washing machine control panels. Systematic troubleshooting revealed:
Machinery: A 120-ton injection molding machine was used to produce 600g panels, resulting in insufficient injection capacity. Mold Issues: No venting grooves were designed in deep cavity of panel. Process Issues: Mold temperature was only 40℃, injection pressure was 80MPa.
Solutions Adopted:
Equipment: Replaced with a 160-ton high-capacity injection machine to ensure redundant injection capacity. Mold Issues: Added three 0.02mm×5mm venting grooves at corners of deep cavity of panel. Parameter Issues: Increased mold temperature from 40℃ to 60℃, increased injection pressure from 80MPa to 95MPa, and extended holding time by 2 seconds.
After above systematic improvements, defect rate of missing material decreased from 35% to an acceptable level.

Missing material in injection molded products is a systemic problem involving multiple coupled factors. From mold's gating system and venting design, to selection and maintenance of injection machine, from material flowability and drying treatment, to precise setting of process parameters—negligence in any link can become a cause of missing material.
Solving problem of missing material cannot be done piecemeal. Correct approach is to first understand physical mechanism of insufficient glue, then investigate each of four dimensions one by one to find root cause and develop a targeted solution. In practice, it is recommended to investigate in order of "process first, mold second, equipment third, and materials last"—because process adjustments are the lowest cost and fastest to show results, while mold modifications and machine replacements are more costly.
Injection molding is a highly practical engineering technology. Solving every insufficient glue problem is a comprehensive test of mold design, equipment maintenance, materials knowledge, and process control capabilities. Only by integrating knowledge from these four dimensions can accurate diagnosis and efficient resolution be achieved, keeping insufficient glue defect rate to a minimum.