No abstract theories or formulas. Each defect is addressed following logic of "Symptom → Quick Assessment → Troubleshooting Steps → Solution". You can save this on your phone and refer to it directly when you encounter a problem.

Symptom: Partially incomplete filling of product, with irregular gaps at the end of cavity or in thin-walled areas.
Quick Assessment: First, check if venting channels are blocked by oil, then check if gate is too small. These two points account for more than 60% of short shot causes.
Troubleshooting Steps:
Check Mold Venting: Check for oil or carbon deposits on parting surface, insert gaps, and venting channels. Wipe parting surface with a clean cotton cloth to check for black sludge; Check Mold Temperature: A low mold temperature will cause melt to solidify before reaching the end. Touch far end of mold; if it feels noticeably cool, prioritize increasing mold temperature; Check gate size: Is gate too small or blocked? Remove hot runner or check cold runner for foreign objects in gate; Check material quantity: Confirm that injection quantity is sufficient and that screw residual weight is not too low (residual weight <5mm may cause metering instability); Check back pressure: Insufficient back pressure will result in insufficient melt density and insufficient actual injected material.
Solutions: Increase mold temperature (+10℃ each time, observe effect); Reduce injection speed (too fast will cause spraying, and material front will cool too quickly); Increase back pressure (usually 0.5-2MPa, depending on material); Enlarge gate size or add an auxiliary gate; Clean venting channels
Mnemonic: Increase mold temperature first, check venting; a blocked gate is the most troublesome problem. Sufficient back pressure and material quantity will solve material shortage problem.

 

Symptom: Thin flakes of excess material are present at parting line, ejector pin holes, and insert gaps.
Quick Diagnosis: First, distinguish between "insufficient clamping force" and "excessive cavity pressure." Touch flash—if flash is thin and spread across parting line, it's mostly due to insufficient clamping force; if flash is concentrated near gate and thicker, it indicates excessive injection pressure.
Troubleshooting Steps:
Check Clamping Force: Calculate product's projected area × cavity pressure (generally 30-50 MPa) and compare it to machine's clamping force. If it's close to or exceeds rated value, flash is inevitable; Check Mold Fit: Check for foreign objects (material debris, oil, rust) at parting line. Use carbon paper to check fit of parting line; Check Injection Pressure and Speed: Excessive first-stage injection pressure or speed leads to excessively high peak cavity pressure; Check holding pressure parameters: Excessive holding pressure or time can force melt into mold even after cooling and sealing; Check mold deformation: Are support pillars properly tightened? Mold "opening" under high pressure is a hidden cause of flash.
Solutions: Reduce injection pressure and speed; Shorten holding time and reduce holding pressure; Check and add support pillars (support heads); Clean foreign objects from parting surface; Replace with a larger tonnage machine if necessary.
Misconception Correction: Many operators increase clamping force as soon as they see flash, even locking machine to over 95% of its rated value. This is a Pyrrhic victory—mold guide pillars will deform, and machine elbows will wear down. Correct approach is to reduce injection pressure, not to indiscriminately increase clamping force.

Symptom: Surface depressions appear in thick-walled areas of product, commonly found on the back of reinforcing ribs, at the base of screw posts, at the junction of thick and thin sections.
Quick Diagnosis: First, check if shrinkage is located in a thick-walled area. If so, core cause is either "insufficient pressure holding for shrinkage compensation" or "uneven cooling."
Troubleshooting Steps:
Check wall thickness differences: Does product exhibit significant abrupt changes in wall thickness (e.g., from 3mm to 8mm)? This is a design issue; manufacturing process can only mitigate it, not cure it. Check pressure holding parameters: Is pressure holding sufficient? Does pressure holding time cover gate solidification time? Check gate size: A gate that is too small or too thin will solidify and seal prematurely, leading to pressure holding failure. Check material temperature and mold temperature: Excessively high material or mold temperature increases material shrinkage, exacerbating shrinkage. Check cooling water channels: Does mold area corresponding to thick-walled region have independent cooling water channels? Is temperature difference between inlet and outlet water greater than 5℃?
Solutions: Increase holding pressure (gradually increase by 5-10 Bar each time); Extend holding time (based on gate solidification time, which can be determined by weighing); Lower mold temperature (especially local mold temperature in shrinkage areas); Enlarge gate size and delay sealing time; Optimize product wall thickness design (wall thickness difference controlled within 1.5 times)
Key takeaway: For shrinkage, first check thick-walled areas, holding time and pressure. A small gate seals too early; enlarging gate is key.

 

Symptom: A visible linear mark forms where two or more melt streams merge, resulting in reduced strength and poor appearance.
Quick assessment: First, check if weld line is located at melt merge point. If so, core problem is "insufficient temperature at merge point" or "insufficient merging pressure."
Troubleshooting Steps:
Check Melt Temperature: Is melt temperature too low? Higher temperatures result in stronger molecular diffusion during melt fusion, making weld lines less noticeable. Check Mold Temperature: A low mold temperature causes rapid cooling of surface layer at fusion point, preventing molecular chains from fusing. Check Injection Speed: If speed is too slow, melt will cool excessively before fusion. Check Venting: Are there venting channels at fusion point? Trapped air will hinder fusion of two melt streams. Check Gate Location: Is gate location appropriate? Can weld line be moved to a non-visible surface or a low-stress area by adjusting gate?
Solutions: Increase melt temperature and mold temperature (most direct and effective); Increase injection speed (allowing melt to converge at a higher temperature); Add venting channels or overflow wells at convergence point; Adjust gate position, moving weld line to a non-critical area; For exterior parts, local heating can be applied to weld line (e.g., hot runner needle valve timing control)
Key takeaway: Weld line temperature is low, increasing temperature and speed is paramount. Poor venting leads to overflow, gate position must be reasonable.

Phenomenon: Fine silver-white streaks appear on product surface, resembling earthworms crawling.
Quick identification: There are three types of silver streaks—moisture silver streaks, gas silver streaks, and decomposed silver streaks. First, determine type:
Moisture-induced silver streaks: Evenly distributed from start to finish, as fine as hair. Gas-induced silver streaks: Concentrated near the gate or at the end, radiating outwards. Decomposition-induced silver streaks: Accompanied by yellowing, black spots, and a burnt smell.
Troubleshooting steps:
Check raw material drying: Was it dried at specified temperature and time? Was hopper kept warm? Did exposure time exceed 30 minutes? Check back pressure and screw speed: Excessive back pressure or excessive screw speed can cause shear heat, leading to material decomposition. Check barrel temperature: Does it exceed material's tolerance limit? Especially nozzle and front section temperatures. Check venting: Poor mold venting allows compressed gas to mix into melt surface. Check recycled material ratio: Is there too much recycled material or is it mixed with impurities?
Solutions: Re-dry raw material (the only solution for moisture-induced silver streaks); Reduce back pressure and screw speed (decompose silver streaks); Reduce barrel temperature (decompose silver streaks); Clean venting channels (for gas-induced silver streaks); Reduce proportion of recycled material to ensure raw material purity
Key distinction: Treatment methods for moisture-induced silver streaks and decomposed silver streaks are completely opposite—the former requires increasing drying temperature and time, while the latter requires reducing barrel temperature. Doing it wrong way will only make things worse.

 

Symptoms: Black or brownish-black burn marks appear at the end of product or in areas where air is trapped, often accompanied by a burnt smell.
Quick diagnosis: First, check if burnt location is at the farthest end of mold or in parting line gap. If so, 90% of time it's due to poor venting causing compressed and ignited gas.
Troubleshooting steps:
Check venting channels: Are there venting channels on parting line, insert gaps, and ejector pin holes? Is injection depth appropriate (generally 0.02-0.04mm)? Is it clogged with oil or carbon deposits? Check injection speed: If speed is too fast, melt pushes air to the end, which cannot escape in time and is compressed and ignited. Check barrel and screw: Are there any dead zones where material accumulates? Is check ring worn? Old material will carbonize if it stays at high temperatures for a long time. Check hot runner: Is nozzle temperature too high? Is needle valve not closing tightly, causing drool carbonization?
Solutions: Clean and deepen venting channels (most direct and effective); Reduce injection speed, especially end speed; Clean barrel and screw regularly (recommended every 3-6 months); Check hot runner temperature control system and calibrate thermocouples; Shorten melt residence time to avoid prolonged heating after shutdown
Mnemonic: If burning occurs, check venting channels first; slow down speed to let air escape. Clean carbon buildup on barrel regularly, and calibrate hot runner temperature.

Phenomenon: After demolding, product's shape deviates from design dimensions; flat surfaces become bent, right angles become skewed, and the overall shape twists.
Quick Judgment: First, observe direction of deformation. If it's "bending towards gate," it's due to internal stress caused by excessive holding pressure; if it's "uneven warping," it's due to uneven cooling or shrinkage differences caused by molecular orientation.
Troubleshooting Steps:
Check cooling uniformity: Is temperature difference between mold's inlet and outlet water > 5℃? Does cooling water channel cover all areas? Especially, are there independent water channels in thick-walled areas? Check holding pressure parameters: Excessive holding pressure or time leads to excessive internal stress. Check molecular orientation: Does gate location cause unidirectional melt flow? Unidirectional flow produces strong molecular orientation, resulting in greater shrinkage along orientation direction after cooling. Check ejection balance: Are ejector pins evenly distributed? Is there excessive ejector pin force causing localized deformation? Check mold temperature distribution: Is temperature consistent across all areas of mold? Scan cavity surface with an infrared thermometer.
Solutions: Optimize cooling water circuit to ensure inlet and outlet water temperature difference is <3℃; Reduce holding pressure and time; Increase number of gates to reduce unidirectional flow; Adjust ejector pin layout to ensure balanced ejection; Annealing treatment if necessary (especially for stress-prone materials such as PC and PA).
Design Warning: If product wall thickness difference exceeds 2 times (e.g., 3mm and 7mm coexist), process methods can only alleviate warping, not cure it. Wall thickness uniformity must be optimized from product design stage.

Phenomenon: Product cracks during use or assembly, or breaks directly upon demolding.
Quick Assessment: First determine whether it is "brittle fracture" or "stress cracking". Brittle fracture surfaces are smooth and without deformation; stress-induced fracture surfaces are rough and show signs of streaking.
Troubleshooting Steps:
Check internal stress: Use glacial acetic acid immersion method or polarized light method to test residual stress. Check demolding system: Are there enough ejector pins? Is ejection balanced? Is draft angle sufficient (generally >1°)? Check holding pressure parameters: Is holding pressure too high? Over-holding pressure is number one process-related cause of stress cracking. Check material compatibility: Has material come into contact with chemicals (solvents, cleaning agents, greases)? Some materials (such as PC) are extremely sensitive to stress cracking. Check proportion of recycled materials: Excessive recycled materials will reduce material's toughness.
Solutions: Reduce holding pressure and time; Increase number and area of ejector pins to ensure balanced ejection; Increase draft angle and polish cavity; Annealing (essential for transparent materials such as PC and PMMA); Control proportion of recycled material (≤30%); Avoid product contact with organic solvents.
Mnemonic: Check internal stress first when cracking occurs; reducing holding pressure is the first step. Add more ejector pins if needed; annealing is a good supplement.

 

Phenomenon: Localized haziness and inconsistent gloss on product surface, resembling a layer of water mist.
Quick Assessment: First, check if hazy area is in a thick-walled section far from gate or near weld lines. If so, it's usually due to uneven cooling or an excessively low melt flow front temperature.
Troubleshooting Steps:
Check mold temperature: Is mold temperature too low? Especially localized mold temperature in hazy area. Check mold surface: Are there scratches, corrosion, or insufficient polishing in cavity? Transparent materials such as PMMA and PC are extremely sensitive to mold surface. Check injection speed: If speed is too slow, surface of melt cools too quickly during filling, making it impossible to replicate mold's mirror finish. Check melt temperature: Is melt temperature too low? High melt viscosity results in large surface shear during flow, reducing gloss. Check venting: Trapped air can cause localized dullness.
Solutions: Increase mold temperature (especially critical for transparent materials); Increase injection speed (allowing melt to contact mold wall at a higher temperature); Increase melt temperature (reduce viscosity); Repolish mold cavity; Clean venting channels.

Symptoms: Granular bumps or blemishes appear on product surface, with a different color than base material, resembling an embedded "cold bean".
Quick Assessment: Observe location of cold slug marks—if near gate, it indicates nozzle drooling or insufficient cold slug well; if randomly distributed on product surface, it indicates excessively low barrel tip temperature or unstable screw metering.
Troubleshooting Steps:
Check Nozzle Temperature: Is nozzle temperature too low? Is there drooling? Check Cold Slug Well: Is there a sufficiently large cold slug well at the end of main runner? Diameter of cold slug well should be 1.5-2 times diameter of main runner. Check Barrel Tip Temperature: Is tip temperature too low? Melt cools while staying at the tip of barrel. Check Back Pressure: Is back pressure too low? Insufficient melt density makes it easy for cold slug to mix into tip. Check Injection Retraction (Squeezing): Is injection retraction amount sufficient? Prevent residual material from dripping from nozzle.
Solutions: Increase nozzle and barrel tip temperature; Increase cold slug well size; Increase back pressure (increase melt density); Adjust injection depth (prevent drooling); Regularly clean nozzles and runners.

Symptom: Products produced from same mold exhibit inconsistent dimensions, exceeding tolerance limits.
Quick Diagnosis: First, observe pattern of fluctuation. If anomaly occurs every few mold cycles, it may be due to a worn check ring or screw. If fluctuation is random, it's likely due to unstable process parameters or batch variations in raw materials.
Troubleshooting Steps:
Check check ring: Is there melt backflow during injection? Check if check ring is worn or stuck. This is the most common cause of dimensional fluctuations. Check screw wear: Is metering section worn? Wear will lead to inaccurate metering, resulting in different injection quantities each time. Check material temperature stability: Is barrel temperature control accurate? Are thermocouples aged? Measure nozzle temperature with a temperature gun. Check mold temperature stability: Is mold temperature controller accurate? Is inlet and outlet water temperature difference stable? Check raw material batches: Are melt flow index and moisture content consistent across different batches of raw material? Check holding pressure parameters: Are holding pressure and time stable? Voltage fluctuations can cause unstable hydraulic system pressure.
Solutions: Replace check ring and worn screw; Calibrate temperature control systems of barrel and mold temperature controller; Standardize raw material batches and ensure proper incoming material inspection; Check stability of hydraulic system pressure and flow; Add monitoring and alarms for material storage stroke.
Hidden Killer: Worn check rings are "number one killer" of dimensional instability, but are most easily overlooked. Diagnosis Method: After injection, observe whether screw continues to advance (if so, it indicates a poor check ring seal).

Symptoms: Product surface at ejector pin location is white, raised, or sunken.
Quick Diagnosis: White marks indicate "excessive ejection force or insufficient ejector pin area"; sunken marks indicate "ejector pin not retracting properly or ejector pin design being too short"; sunken marks indicate "ejector pin being too long or product sticking to mold".
Troubleshooting Steps:
Check number and distribution of ejector pins: Are there enough ejector pins? Are they concentrated in areas of high stress on product? Check draft angle: Is draft angle sufficient? Generally, >1° for outer surfaces and >1.5° for inner surfaces. Check mold polishing: Is cavity insufficiently polished? Especially in deep rib areas and inside screw pillars. Check holding pressure parameters: Excessive holding pressure leads to excessive product clamping force, making ejection difficult. Check ejection speed and pressure: Is ejection too fast or too forceful?
Solutions: Increase number and area of ejector pins (especially on the back of rib areas); Increase draft angle; Polish cavity and rib areas; Reduce holding pressure and time; Adjust ejection speed and pressure (slow at first, then faster); Add mold release agent (use with caution, may affect subsequent painting or bonding)

Phenomenon: Vacuum bubbles are visible inside thick-walled product after cutting. Bubbles may not be visible on the surface, but can be detected by X-ray or sectioning.
Quick identification: First, observe location of bubbles. If they are in the center of a thick-walled structure, it's a shrinkage bubble (insufficient holding pressure); if they are near the surface, it's a trapped bubble (poor venting or moisture).
Troubleshooting steps:
Differentiate types: Shrinkage bubbles are spherical or elliptical with smooth inner walls; trapped bubbles are irregularly shaped with rough inner walls. Check holding pressure parameters: Are holding pressure and time sufficient? Was gate sealed during holding pressure stage? Check wall thickness design: Are there any abrupt changes in wall thickness? Thick-walled areas cool slowly, resulting in greater internal shrinkage. Check venting: Trapped bubbles require checking if mold venting is sufficient. Check raw material drying: Moisture bubbles require raw material to be re-dried.
Solutions: Increase holding pressure and time (to reduce shrinkage bubbles); Extend cooling time (to allow thick walls to solidify fully); Increase gate size (to delay sealing time); Increase venting (to trap air bubbles); Re-dry raw material (to remove moisture bubbles)

 

Phenomenon: Melt flows from gate into mold cavity, meandering like a snake, leaving a snake-skin or earthworm-like pattern on the surface.
Quick Assessment: Check for curved flow marks near gate. Jetting is caused by melt passing through a small gate at high speed and directly entering mold cavity; it is common in side gates or point gates.
Troubleshooting Steps:
Check gate size: Is gate too small? Is the thickness too thin? Check injection speed: Is injection speed too fast when passing through gate? Check gate location: Is gate directly facing an empty area of mold cavity? If facing an open area, melt will spray directly without resistance. Check the mold structure: Can spraying be avoided by changing gate type (e.g., fan gate, ear gate)?
Solutions: Reduce injection speed at gate (most critical); Increase gate size; Use a fan gate or ear gate; Move gate to a non-visible surface; Install a baffle structure opposite gate, allowing melt to impact before spreading.

Symptom: Product color deviates from standard sample, or there are color bands or spots on the surface.
Quick Judgment: First, check type of color difference. If the overall color is darker or lighter, it's a problem with color masterbatch ratio or temperature; if it's color bands or spots, it's due to uneven mixing or degradation.
Troubleshooting Steps:
Check color masterbatch ratio: Is amount of color masterbatch added accurate? Is it mixed evenly? Check barrel temperature: Is temperature too high, causing degradation of color powder/masterbatch? Darker masterbatches are especially sensitive to temperature. Check screw type: Is a screw suitable for masterbatch dispersion being used? Is mixing section sufficient? Check recycled material: Is recycled material ratio consistent? Different batches of recycled material may have color differences. Check barrel cleanliness: Was machine thoroughly cleaned before changing colors? Residual old material can cause color streaks.
Solutions: Calibrate masterbatch addition ratio; Lower barrel temperature (especially important for darker materials); Extend mixing time or increase back pressure; Standardize recycled material ratio; Use a dedicated cleaning agent to thoroughly clean machine when changing colors
Appendix: Defect Troubleshooting Decision Tree (Golden Three-Step Method)
When encountering a problem, don't panic. Troubleshoot in this order:
Step 1: Observe phenomenon and determine direction
Is it an appearance problem or a size problem? Is it a localized problem or a systemic problem? Is it an occasional or persistent issue?
Step Two: Check Three Key Factors
Temperature: Material temperature, mold temperature, ambient temperature. Pressure: Injection pressure, holding pressure, back pressure. Speed: Injection speed, screw speed, ejection speed
Step Three: Check Mold and Raw Material
Mold: Venting, gate, cooling, ejection. Raw Material: Drying, batch, recycled material ratio
In conclusion:
This manual covers 15 of the most common defects in injection molding. Every solution comes from hands-on experience, not just textbook platitudes. We recommend you save it; next time you encounter a problem, refer to it before adjusting the machine.
Remember: Machine adjustment is not trial and error, but a logical process of troubleshooting.