Causes of Plastic Scorching in Injection Molding and How to Prevent It
Plastic scorching is a common injection molding defect that can affect product appearance, material performance, and production stability. It usually occurs when resin is exposed to excessive heat, long residence time, contamination, or poor process control inside the barrel, screw, or nozzle area.
1. Common Signs of Plastic Scorching
Scorching defects in injection molding usually show the following symptoms:
- Abnormally high processing temperature or temperature control failure
- Smoke, irritating odor, or unusual fumes from the nozzle or barrel area
- Black specks or burnt particles appearing on the plastic surface
- Continuous air marks or burn-related voids near weld lines or gate areas
These signs usually indicate that part of the plastic material has overheated, degraded, or burned inside the machine before injection.
2. Main Causes of Plastic Scorching
Plastic scorching can be caused by several processing and maintenance issues during injection molding production.
Temperature Control Problems
If the barrel, nozzle, or heating system temperature is set too high, or if the temperature controller fails, the resin may overheat and burn. Excessive melt temperature is one of the most direct causes of scorching defects.
Burnt Material Accumulation in the Screw and Barrel
If the screw and barrel are not cleaned regularly, degraded plastic may remain inside the machine. Over time, this burnt residue can mix with fresh material and enter the molded part, causing black spots, discoloration, and scorch marks.
Excessive Heating Time
If plastic stays in the heated barrel for too long, especially during interruptions or low-cycle production, the resin may start to degrade. Long residence time inside the barrel is a major cause of material burning and performance loss.
Long Machine Stoppage Without Purging
When the machine is stopped for a long time without cleaning the screw and nozzle, the plastic left inside may decompose and carbonize. Once production restarts, the degraded material can be injected into the product and cause scorching defects.
Frequent Material or Color Changes
Repeated mold changes or color changes without proper cleaning can leave mixed material or old resin inside the barrel. This increases the chance of contamination, color streaks, and thermal degradation.
Poor Nozzle or Head Sealing
If the machine head, nozzle, or related sealing parts are not properly tightened, plastic may leak into small gaps and remain there under heat. Over time, this trapped material can degrade and burn, then contaminate the melt flow.
3. How to Eliminate Plastic Scorching
To prevent and eliminate plastic scorching in injection molding, the following actions are recommended:
- Check whether the heating system and temperature control units are working normally
- Clean the screw, barrel, and nozzle regularly and thoroughly
- Control heating time properly and avoid excessive residence time of resin in the barrel
- Purge the machine promptly during shutdowns, mold changes, and material changes
- Clean the barrel carefully after color changes to avoid contamination and resin degradation
- Ensure the nozzle and machine head are properly tightened to prevent resin leakage into gaps
- Immediately clean the screw and nozzle area once scorching is detected
4. Importance of Stable Process Control
Preventing scorching defects requires stable processing temperature, correct machine maintenance, proper purging procedures, and careful control of material residence time. By improving machine cleaning routines and maintaining the heating system in good condition, injection molding manufacturers can reduce burnt material defects and improve part quality.