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Countermeasures for Molding Defects

Contents Introduction Safety Molding Machines Molding Conditions Recycling
Molding Characteristics Product Design Mold Design Countermeasures for Defects

 

List of DURACON® POM grades This page's PDF(0.97MB)

9. Countermeasures for Molding Defects

 

Table 9-1 Causes and measures against molding defects

Defect phenomenon Causes Measures
1. Flow marks (1) When jetting occurs, the string of jetting is pushed on the surface of molding and leave the pattern.

<Primary measure>

Prevent jetting generation.

a. Decrease initial gate-passing speed.

b. Enlarge gate size.

c. Change material to a higher flow grade.

d. Elevate material temperature.

<Secondary measure>

Prevent showing up to the surface if jetting occurs.

e. Elevate mold temperature and increase holding pressure.

f. Alter gate position. (Shorten flow-length from gate to mold-wall that resin first contact). e.g.,:
•Section where resin bump into core quickly.
•Thin wall section.

g. Use a tub gate.

(2) Flow rate change generate flow pattern. The rate change occurs at corner sections or non-uniform thickness section. (thin wall ↔ thick wall).

a. Make comers rounded.

b. Make thickness change sections gently inclined and rounded.

(3) Insufficient degassing Enlarge gas vent.
2. Pit marks, wrinkles (1) Insufficient adhesion of resin to cavity due to lack of inner pressure in cavity.

a. Increase the holding pressure and time.

b. Enlarge runners and gates.

c. Raise mold temperature and material temperature.

d. Increase injection speed.

(2) Insufficient degassing

a. Enlarge gas vent.

b. Do not elevate material temperature too high.

c. Dry the material sufficiently.

3. Weld marks Insufficient melting adhesion of flow front

a. Raise mold temperature. (buried cartridge heaters in weld sections).

b. Increase injection speed.

c. Change material to a higher flow grade.

d. Enlarge gas vent at weld area.

e. Provide weld escape.

f. Change flow pattern to welds by adjustment of thickness.

4. Silver marks (1) Volatile such as moisture and decomposed gas from material. Entrapped air during plasticization.

a. Dry the material sufficiently (100°C or higher).

b. Do not raise material temperature too high.

c. Increase screw back pressure.

d. Enlarge gas vent.

e. Degas from runners.

(2) Air pocket generation by unbalanced flow in cavity.
(3) Mixing with foreign matter such as PE, PP, etc. Purge the matter sufficiently from the cylinder.
5. Sink marks Surface sink with shrinkage of inner part due to insufficient cooling of thick wall and rib section and insufficient inner pressure in the cavity.

a. Lower mold temperature.

b. Enlarge sprue, runners and gates.

c. Raise holding pressure and prolong holding time.

d. Keep cushion amount until gate sealing.

e. Minimize the rib thickness to approximately one-third of the base thickness.

f. Decrease the thickness of thick wall sections.

6. Surface delamination (1) Mixing with foreign matter such as PE, PP, etc. Purge the matter sufficiently from the cylinder.
(2) Shear delamination

a. Raise mold temperature.

b. Lower injection speed.

c. Enlarge gates.

(3) Oil separation of oil filled grades

a. Decrease initial gate passing speed.

b. Prevent air trapping by poor feeding during plasticization. (Control cylinder temperature).

7. Surface roughness (1) Mold deposit

<Measures against mold deposit>

a. Dry the material sufficiently (100°C or higher).

b. Do not raise material temperature too high.

c. Enlarge gas vent.

d. Raise mold temperature.

<Cleaning cavities>

e. Ultrasonic cleaning of core in a solvent.

(2) Insufficient adhesion to cavity

a. Raise mold temperature and injection speed.

b. Increase holding pressure and holding time.

c. Enlarge sprue, runners and gates.

d. Enlarge gas vent grooves.

8. Vacuum voids In case of thick moldings and the surface solidifies rapidly, inner molten resin is pulled up to the surface with cooling and shrinking. Accordingly, vacuum voids is generated inside of molding.

a. Locate gate at the thickest section of molding.

b. Enlarge sprue, runners, gates and nozzle to balance with molding thickness. Gate thickness should be more than 50%-60% of molding thickness.

c. Increase holding pressure and holding time. Keep cushion amount until gate sealing.

d. Ensure that non-return valve work properly to prevent back-flow during holding time.

e. Decrease injection speed.

f. Change the material to high viscosity grade.

 


Contents Introduction Safety Molding Machines Molding Conditions Recycling
Molding Characteristics Product Design Mold Design Countermeasures for Defects