This presentation helps plastic injection moulding machine operation moulders to resolve burn mark problems

1. Prepared By: K Karthigeyan
ritikaadhanuish@gmail.com

2. Burning Mark
 Burn marks is an oxidized, discolorations spot on moulding, usually rust colored or black colored,
that appear on the surface of the injection molded Parts
Categories of Burn Mark
Air Burn
Material
Degradation
Burn
Material
Contamination
Burn
FR Burn
Material Shear
Burn

3. Air Burn:
 A patch or streak of brown or black material on the component caused by air or gases, its normally occurred at part end
filling area and/or opposite of the gate point.

4. Air Burn Remedies
 To accomplish this air burn mark, vents are machined into the parting line, end filling area of
component to evacuate air.
 Venting is also easily achieved around ejector pins and core slides by the sufficient clearance
 The dimensions of air vents are typically 0.013-0.025 mm in depth and 1.6-12.7 mm in
width.
 Note: It is very important to the consistent production of high quality products.
Possible Root Cause:
Normally it’s Caused by trapped gasses in a poor or non vented area of the mould, the gasses may gets ignite by pressure &
temperature.
Air Burn:

5. Material Degradation Burn
 The material becomes oxidized & charred inside the barrel by over heat & this kind of burn can be noticed
on purging material & feed system too.

6. Possible Root Causes
1. Pitting mark on screw & or barrel metal
2. Screw tip assembly worn out,
3. Nozzle & sprue orifice mismatch in hot sprue mould
4. Abnormalities on barrel assembly
5. Higher Heater wattage
6. Barrel temperature too high
7. Low Screw stroke utilization
8. Higher material residence time.
9. Processing with lower specific injection pressure machine
10. Material slippage inside the barrel
Material Degradation Burn

7.  Standard Moulding process - 20 to 70%
 Fast cycle Moulding process - 15 to 30%
 Precision parts moulding - 30 to 60%
Residence time (𝑠) =
2 ∗ 𝐶𝑦𝑙𝑖𝑛𝑑𝑒𝑟 𝐻𝑒𝑎𝑑 𝑉𝑜𝑙𝑢𝑚𝑒 𝑐𝑚3
∗ 𝑀𝑒𝑙𝑡 𝐷𝑒𝑛𝑠𝑖𝑡𝑦(
𝑔
𝑐𝑚3)
𝑆ℎ𝑜𝑡 𝑊𝑒𝑖𝑔ℎ𝑡 (𝑔)
∗ 𝐶𝑦𝑐𝑙𝑒 𝑇𝑖𝑚𝑒 (𝑠)
 Barrel residence time is the time taken by the plastics material dwell inside the
barrel, from the entry of plastics granules from the hopper to the exit through
the nozzle tip. It will be expressed in Sec or Min.
Screw stroke utilization
Residence time
Recommended Residence time:
 Commodity Plastic like PP, LDPE,HDPE,PS : Max. 11 Min
 Engineering Plastic like PC,PA,POM,ABS & etc., : Max. 6 Min
Material Degradation Burn

8. Material Contamination Burn
 The contaminated material get burns faster inside the barrel caused burn mark on the component, its appear on the
component at different places like black or brown wave lines & art shapes
Possible Root Causes
1. Dust content on virgin or RG material
2. Foreign particles on Virgin or RG material
3. Mixing of high percentage of RG material
4. Processing with contaminated raw materials

9. 1. The Virgin Raw Material should be free from dust & foreign particles
2. Regrind must be clean, low dust content, not thermally degraded & free from foreign particles
Suggestions
Material Contamination Burn

10. Material Shear Burn
 The internally generated heat when a polymer melt flows through restricted passages in the mould
 The shear burn appears on the components like a brown or black colored line shapes.
Possible Root Cause:
1. Higher injection Speed
2. Lower Melt Temperature
3. Lower Sprue or Nozzle orifice diameter
4. Thin wall parts
5. Air vent not sufficient
6. Longer flow path component

11. Possible Root Cause:
1. Lower Screw stroke utilization & Longer Residence time
2. Use of Worn out Screw, Barrel & screw tip assembly
3. Higher barrel temperature
4. Higher compression ratio screw
5. Higher screw rotation speed (Dosing – RPM) & Higher Injection Speed
6. Use of RG with dust, thermally degraded & higher percentage
7. Difference on set & actual barrel temperature
8. Suspect on Virgin Raw material.
FR - Burn
 The flame retardant grade raw materials are thermally degraded inside the barrel
 We can judge the place & quantity of thermally degraded material inside the barrel from defective burned component
because this kind of burns normally occurs same place & size on moulding products & it can be noticed on purging
material too.