For previous reading, please refer to Daily Share: A Comprehensive Analysis and Precise Solutions for Injection Molding Shrinkage and Sink.
In injection molding production, gas trapping and burning is one of the most challenging defects during mold trial phase–it can appear suddenly, leading to batch rejection, or it can remain latent for a long time, worsening with process fluctuations. Its essence is that during melt filling, gas in mold cavity (air, material decomposition gases, water vapor) cannot be discharged in time, is violently compressed and heated (reaching material decomposition temperature), ultimately igniting material or causing carbonization defects. This article will, through four major modules–nature of problem, troubleshooting logic, practical solutions, long-term prevention–and combined with real-world cases, deconstruct a comprehensive strategy for addressing this problem.

 

Gas trapping is not an isolated phenomenon, but rather result of an imbalance in three stages of gas generation, discharge, and compression. A precise understanding of underlying logic is necessary to accurately pinpoint root cause of problem.

There are three core causes of gas generation:
Material moisture absorption/decomposition: When hygroscopic materials (such as PA, PC, PET) are not sufficiently dried, moisture vaporizes at high temperatures in barrel (e.g., 280-300℃ for PA), forming a large amount of water vapor. One case involved a factory testing a PA66 gear mold; product end showed burning, and testing revealed a material moisture content of 0.2% (standard ≤0.1%). After drying, problem disappeared–water vaporization was main cause. If material temperature is too high (e.g., PVC above 190℃, POM above 240℃) or residence time is too long, thermal degradation of material will produce low-molecular-weight gases (e.g., PVC decomposes into HCl, POM decomposes into formaldehyde), leading to burning accompanied by a pungent odor.
Residual air in mold cavity: In deep cavities, thin-walled or complex molds (such as automotive instrument panel frames), air in mold cavity before it is completely filled with melt cannot escape, especially when gate is far from the end. Air is "pushed" by melt into blind areas, creating localized high pressure.

Ineffective gas venting often stems from mold design or fit issues:
Vent groove defects: Incorrect location (not placed in "dead zone" where melt fills last, such as product end or bottom of ribs), insufficient depth (ABS requires 0.02-0.03mm, PC requires 0.01-0.02mm, soft PP can be as shallow as 0.01mm), or insufficient width (slow gas flow rate, low venting efficiency). During a trial run of a mobile phone case, scorching was observed fixed to edge of product. Upon disassembly, it was found that original vent groove depth was only 0.01mm (ABS standard 0.02mm). Problem was solved after deepening groove.
Overly tight mold fit: Excessive grinding of parting surface (contact rate > 95%), or clearance between sliders/ejector pins and guide rails < 0.01mm (design is usually 0.01-0.02mm), preventing gas from escaping through gaps. In one case, scorching was observed at corresponding position of slider; measurement showed a clearance of only 0.003mm. After grinding and restoring clearance to 0.015mm, scorching disappeared.
Unreasonable gate/runner design: Gate is too small (e.g., point gate diameter < 0.8mm) or located too far away, causing melt to jet at high speed ("snake flow"), leading to severe compression of gas at the front. In a trial run of a home appliance casing, gate diameter was 0.6mm. After adjusting it to 0.8mm and moving it forward, filling was smoother, and scorching was reduced.

Process parameter settings can directly amplify gas problems:
Excessive injection speed: Front of melt acts like a "piston," compressing gas, which doesn't have time to escape before being compressed and heated. For example, scorching occurred at the end of an ABS casing. After reducing injection speed of last 30% from 90% to 50%, gas had time to escape through vent grooves, and scorching was reduced.
Excessive holding pressure/time: During holding pressure stage, melt continues to fill tiny gaps, further compressing residual gas. Short shot experiments showed that after reducing holding pressure from 80 bar to 60 bar, carbonization no longer appeared at the end of product. Excessive barrel temperature: This exacerbates material decomposition and generates additional gas. In a PET preform mold trial, reducing barrel temperature from 290℃ to 275℃ reduced decomposition gas and alleviated scorching.

When air entrapment and scorching occur during mold trials, follow logic of "outside to inside, process first then mold, simple first then complex" to avoid wasting time on blind mold modifications.
Step 1: Verify Material and Drying (Can be completed within 5 minutes)
Check state of pellets: Check if pellets are sticking together or discolored (due to moisture or decomposition), and smell for any rancid odor (indicating material decomposition).
Measure moisture content: Use a rapid moisture analyzer (e.g., PA requires <0.1%, ABS <0.05%). If it exceeds limit, extend drying time (PA should be dried at 120℃ for more than 4 hours), or install a heated drying device in hopper (to prevent secondary moisture absorption).
Temporary verification: Use dried material from same batch for mold trials. If scorching disappears, problem is related to material.
Step 2: Adjust Process Parameters (10-15 minutes of trial and error)
Prioritize reducing speed: Reduce injection speed (especially in last 30% stage) to allow time for gas to escape. For example, in a PC lampshade scorching problem, reducing injection speed from 100% in three stages to 80%-70%-50% shifted scorching from end to near gate (indicating that gas at the end was not expelled at original speed).
Lowering temperature as an aid: Lower barrel/nozzle temperature (e.g., from 300℃ to 280℃ for PC) to reduce decomposition gas. Note: When lowering temperature, observe whether flow is smooth to avoid underfilling.
Reduce holding pressure: Gradually reduce holding pressure (e.g., from 80bar→60bar→40bar) and observe effect of holding pressure on scorching. If scorching is reduced after lowering holding pressure, it indicates that gas was excessively compressed during packing stage.
Step 3: Check Mold Venting System (30 minutes - 1 hour)
Locate scorching position: Fixed position scorching → poor mold venting; random position → material/process problem.
Cleaning vent grooves: Use a brass brush/compressed air to remove mold release agent and plastic residue from grooves (especially important for hot runner molds).
Measuring parameters: Use a depth gauge to measure depth of vent grooves (matching material flowability), and use a feeler gauge to measure gap between slider/parting surface (ensure ≥0.01mm).
Temporary remedy: If vent grooves are too shallow, they can be lightly sanded with fine sandpaper to enlarge them (pay attention to uniformity), or breathable tape can be applied (for short-term emergency use).
Step 4: Verifying mold structure (1-2 hours)
Blue dye test of parting surface: Apply blue dye to fixed mold, and observe contact marks on moving mold after closing mold. If there is local non-contact (<90%), it indicates that parting surface is too tight, and gas cannot escape.
Manual testing of slider: Push slider and feel fit clearance (normally 0.01-0.02mm). If it is stuck or clearance is too small (<0.005mm), it needs to be repaired.
Short shot experiment: Inject 50%-80% of melt and observe whether end of flow coincides with burnt area. If they coincide, it means that this area is a "dead zone," and vent grooves or porous steel need to be added.

 

Based on troubleshooting results, address issues specifically:
If main cause is material/process:
Adjust drying parameters (e.g., extend drying time for PA), reduce injection speed/melt temperature, and reduce holding pressure. For example, a PP part was scorched due to a barrel temperature of 240℃ (PP recommended 220℃). After lowering temperature, decomposition gas decreased, and scorching disappeared.
If main cause is mold venting:
Add vent grooves: Process 0.02-0.03mm deep vent grooves at burnt location (such as end of product), with a width of 5-10mm (depending on product size).
Replace with porous steel: Embed PM-35 porous steel in deep cavity area (permeability > 300 permeability units), utilizing porous structure of metal for venting. In a trial molding of an automotive trim panel, scorching occurred at the bottom of deep cavity. After embedding porous steel, gas was discharged through gaps between steel particles, and scorching problem was solved.
Adjusting slider gap: Grind slider or guide rail to ensure a clearance of 0.01-0.02mm, allowing for gas escape.

Ultimate solution to gas entrapment and scorching requires establishing error prevention mechanisms before, during, and after mold trials:
1. Before mold trial: Prediction and design optimization
DFM analysis: During product design phase, identify areas prone to gas entrapment, such as deep cavities, thin walls, and ribs, and mark them on drawings as "requiring venting" (e.g., reserving a 0.5mm venting groove space at the end).
Mold design review: Confirm venting plan with mold manufacturer (venting groove location, depth, width, and whether porous steel is used) to avoid "design defects" being carried into mold trial.
2. During mold trial: Data recording and rapid verification
Record process window: Record in detail degree of gas entrapment under different injection speeds, temperatures, holding pressures, and create a "parameter-defect" comparison table. Optimal parameters can then be directly used in subsequent production.
Short shot + venting verification: During mold trials, first perform a short shot (50%-80%) and mark flow end; then mold a full product and observe whether scorching coincides with short shot end to quickly locate venting needs.
3. After mold trial: Experience accumulation and mold improvement
Problem review: If multiple process adjustments are ineffective, mold needs to be modified (e.g., adding venting grooves, increasing slider gap), and improvement plan should be recorded in mold file.
Standardized training: Incorporate gas entrapment troubleshooting process (material → process → mold) into mold trial operator training manual to avoid repeating mistakes.

 

Essence of gas entrapment and scorching is that gas discharge is obstructed. Troubleshooting requires systematic verification from three aspects: material drying, process parameters, and mold venting. Prioritize adjusting process (low cost, quick results), then perform targeted mold modifications (to solve fundamental problem). The key to long-term error prevention is "design prediction + mold trial verification + experience accumulation," reducing gas generation and compression from source, ultimately achieving "first-time mold trial success and stable production without scorching."
Core principles: If scorching occurs, first check if material is dry; try reducing speed and temperature to adjust process; ensure vent grooves are deep and correctly positioned; avoid overly tight mold tolerances; anticipate potential problems during design and keep records to prevent recurring errors.