For those troubled by scorching during injection molding.

Technology & Services

For those troubled by burn marks during injection molding
~Supporting fundamental improvement of gas entrapment issues with mold maintenance services~
In injection molding, "burn marks" are a typical appearance defect where the product surface discolors to black or brown. They occur due to resin carbonization or gas burning, and can lead not only to a decrease in aesthetic quality but also to reduced strength and an increased defect rate.
They are particularly prone to occur in micro-molding of electronic component connectors and precision plastic parts, and in many cases, cannot be resolved by simply adjusting molding conditions. Such problems can sometimes be improved through mold maintenance, including the mold's venting structure and flow design.
■ What are Burn Marks?
Burn marks are defects that appear as black or brown scorch marks when air or resin decomposition gases trapped inside the mold are compressed and heated to high temperatures, causing the resin to carbonize locally.
They are particularly prone to occur in the following areas:
Last fill area of the resin
Tips of narrow flow paths
Around ribs where gas tends to accumulate
Deep parts of micro-pins and connector shapes
In such areas, if there is insufficient escape for air, instantaneous high temperatures occur, leading to burn marks.
■ Main Cause: Insufficient Escape for Air and Gas
Many burn defects occur due to a combination of the following factors:
Insufficient venting: Air and resin gas accumulate inside the cavity, leading to compression combustion.
Injection speed too fast: Rapid filling compresses air, causing a temperature rise.
Flow resistance due to fine features: Air is difficult to escape in fine pin shapes like connectors.
Dirt/gas accumulation inside the mold: Venting function deteriorates due to gas adhesion from long-term use.
Especially in precision part molding with small molding machines of 100t or less, even slight gas entrapment can lead to burn defects.
■ Main Solutions: Venting Improvement and Molding Condition Optimization
To fundamentally improve burn defects, measures are required from both the mold side and molding conditions.
Addition/reprocessing of vents: Add vent grooves to the cavity ends and gas accumulation points to improve gas discharge.
Optimization of injection speed: Suppress excessive high-speed filling to prevent compression heating.
Consideration of vacuum molding: Pre-evacuate air from the cavity to reduce the risk of burn marks.
Mold cleaning and maintenance: Remove gas deposits from vent grooves and cavity surfaces to restore exhaust function.
Such measures are not effective in isolation; optimization combining mold design and molding conditions is crucial.
■ Key Points for the Client to Understand
When requesting improvement for burn defects, sharing the following information will facilitate smoother cause analysis:
Location of burn marks (photos/drawings)
Type of resin used and its gas generation characteristics
Current injection speed and temperature conditions
Mold's vent structure and gate position
Product shape (presence/absence of ribs, fine features)
Especially for micro-parts and connector parts, venting design at the mold design stage is crucial, so improvements including design consulting are effective.
■ Features of SHINKO PRECISION VIETNAM's Mold Maintenance Service
SHINKO PRECISION VIETNAM comprehensively analyzes gas entrapment, flow resistance, and mold degradation that cause burn defects, and implements improvements including recurrence prevention.
Micro-machining technology specializing in precision molds of 100t or less
Maintenance proposals based on extending mold lifespan
Mold design consulting, including vent improvement and flow design optimization
Responsiveness through a collaborative network in Japan, China, Taiwan, and Vietnam
Production of high-precision mold parts and quick turnaround times
Burn marks don't improve, scorching occurs in micro-parts, there are limits to adjusting molding conditions alone...
Such problems can potentially be greatly improved through mold maintenance and design improvements.
If you are troubled by burn defects, please consult SHINKO PRECISION VIETNAM.
If you share the phenomenon and mold specifications, we will propose the optimal improvement plan.