Mould And Tool Maker

Sunday, November 2, 2008
Correcting Molding Defects caused by improper Mold Design
In the following number of molding defects mostly caused by faulty mold design are listed, along with steps to remedy them. It is assumed that the material has been properly melted in the plasticating unit of injection molding machine and processing parameters such as injection, holding, and boost pressure, injection rate, clamping pressure etc., are optimally set.

# Molding with visible kit lines
It caused by poor position of gating, poor mold venting and non-uniform mold cooling.
It can be solved by :
- Objectionable type of gate
- Detrimental gate cross section
- Enlarge gate
- Add air venting
- Raise mold temperature by use e.g., mold temperature control (MTC)

# Mold with Srink Marks
It caused by runner and gate too small
Try these action to solve :
- System solidifies before holding pressure has became efective
- Check mold temperature
- Considerable differences in wall thickness

# Burned spots at molding
- Material is overheated in narrow gates
- Mold is duficiently vented
- Adverse flow of material

# Distorsion of Mold
- Poor location of gating
- Poor type of gate
- Non-uniform mold temperature
- Wrong Mold temperature
- Considerable difference in wall thickness more than one circuits.

# Sprue can not be demolded, break off
- Sprue has an undercut because :
1. Radii of nozzle and sprue bushing do not match
2. Nozzle and sprue bushing are misaligned
3. Nozzle orifice is larger than orifice of sprue bushing
4. Sprue bushing is insufficiently polished, grooves in circumferential direction

- Sprue is not yet Solidified because :
1. Sprue bushing is too large and sprue too thick
2. Area of sprue bushing is adequately cooled, check mold temperature

# Mold Flashes
- Poor fit at parting line
- Matching surface damage by e.g., remnants of material
- Rework surface
- Clamping force is insufficient, protected area of part too large. Core shifting has caused a considerable differential in wall thickness.
- Mold temperature too high

# Melts leaks between nozzle and sprue bushing
- Orifices of nozzle and spue bushing are misaligned (Do : Check locating ring and alignment)
- Contact pressure between nozzle and sprue bushing insufficient(Do : increase pressure)
- Check uniform pressure with thin paper
- Proper contact result in uniform indentation
- Radii of nozzle and sprue bushing do not match
- Nozzle orifice is larger tnah orifice of sprue bushing

# Part can not be molded
a) Molding sticks in cavity
- Sprue or runner system have undercut
- Check radii of nozzle and sprue bushing
- Check surface of cavity for imperfections ( Do :Repolish it and round sharp corner)
- Cavity temperature is too low
- Formation of vaccum
- Check taper

b) Molding sis multilated:
- Undercut too large
- Ejector are placed in location with detrimental to proper transmision of ejector force resulting in unaccetable pressure peaks.
- Check cavity surface for imperfection
- Formation of vaccum
- Check taper

# Incomplete mold filling
- Runners too long or too small or both
- Flow in cavity is resticted by lenght or wall thickness
- Open gate
- Core shifting
- Insufficient mold venting
- Mold temperature too low

# Molding has mat or steaky surface
- Mold surface in adequately polished
- Poor location of gating
- Poor type of gate
- Increase size of runners and gates and polish
- Cold slug well needed
- Mold too cold (condensed humidity)
- Overheating of material from insufficient venting or poor flow path

# Flaking (scaling) of surface
- Excessive temperature differential between melt and mold
- Raise mold temperature
- Enlarge mold temperature

# Molding with poor surface glass
- Check mold temperature
- Enlarge runners and gate
- Round sharp corner
- Repolish mold

# Discoloration near gate
- Mold temperature too high
- Enlarge runners and gate
- Cold slug well needed

# Brittle molding
- Check mold temperature
- Enlarge runners and gate
- Cold slug well needed

# Rippled molding surface
- Reduce mold temperature


manges/mohren; how to make injection molds
Hansen Publisher, Munich Vienna New York
posted by Rudy @ 3:07 PM  
1 Comments:
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