Typical defects and countermeasures against defects are shown below. Contact us if you cannot solve problems for undescribed causes, in case of equipment problems, or if you do not know what measures to take for unknown causes.
| Symptoms |
Causes |
Countermeasures |
| 1. Streaky patterns (flow marks) appear along the flow direction of resin. |
1. Inappropriate molding conditions (sprue marks) |
1. Set metering conditions appropriately.
a. Raise screw rotation rate to recommended value.
b. Lower back pressure to recommended value.
2. Set injection conditions appropriately.
a. Raise temperature of rear of cylinder.
b. Raise mold temperature to recommended value.
c. Raise injection speed. |
| 2. Jetting patterns produced when resin passes through gate remain on surface. |
Primary measures: Prevent jetting.
1. Slow down early-stage gate passing speed.
2. Scale up gate. |
Secondary measures: Make patterns difficult to remain even if jetting occurs.
1. Change grade for better flowability.
2. Raise resin temperature.
3. Raise mold temperature. Raise dwelling force.
4. Change gate position (Shorten straight-ahead distance after passing through gate.)
a. Change gate to position where flow butts
core, etc., in direction that butts side.
b. Use a tab gate. |
| 3. Flow patterns produced by change in flow speed when resin passes through corners and uneven-thickness parts (thin-walled parts ⇔ thick-walled parts) remain on surface. |
1. Round corners.
2. Gradually slope or round parts where wall thickness is changed. |
| 4. Mixing of different resins |
1. Investigate location of mixing and take measures. |
| 2. Flow marks |
1. Resin is in poor contact with cavity because pressure in cavity is insufficient. |
1. Raise dwelling force. Extend dwelling time.
2. Scale up runner and gate.
3. Raise mold temperature. Raise resin temperature.
4. Raise injection speed. |
| 2. Gas venting is inadequate. |
1. Enhance gas venting.
2. Do not raise resin temperature excessively.
3. Dry material sufficiently. |
| 3. Discoloration |
1. Oxidation degradation of resin |
1. Set drying conditions as recommended.
2. Set cylinder temperature in recommended range.
3. Nitrogen-purge or evacuate the hopper.
4. Change design of places of oxidation degradation of resin (hot runners, etc.) appropriately. |
| 4. Black spots |
1. Oxidation degradation of resin (burn) |
1. Set drying conditions as recommended.
2. Set cylinder temperature in recommended range.
3. Nitrogen-purge or evacuate hopper.
4. Clean places of oxidation degradation of resin (screw, hot runner, etc.).
5.Change the design of the places of oxidation degradation of resin (screws, hot runners, etc.) appropriately.. |
| 2. Mixing of foreign matter (dust) |
1. Investigate location of mixing and take measures. |
| 5. Surface delamination |
1. Mixing of different materials, such as PE and PP |
1. Investigate location of mixing and take measures. |
| 2. Adhesion to mold |
1. Lower mold temperature.
2. Lower injection speed.
3. Lower resin temperature.
4. Lower peak pressure for filling.
5. Scale up gate. |
| 6. Stringiness, drooling |
1. The viscosity of molten resin at nozzle tip is too low. |
1. Take a suck-back.
2. Set metering delay or suck-back delay so that sucking back is performed immediately before mold opens.
3. Lower nozzle tip temperature.
4. Replace nozzle with one having a smaller nozzle tip diameter. |
| 7. Voids |
1. Vacuum void : If surface solidification is fast on thick-walled products, central resin is drawn into surface in association with cooling and shrinkage. As a result, central part is insufficiently filled. |
1. Position gate at thickest portion of molded products.
2. Scale up gate, runner, sprue, and nozzle adequately to molded product thickness. Set gate thickness 50 to 60% or more of molded product thickness.
3. Raise dwelling force. Extend dwelling time. Leave a cushion until gate sealing.
4. Secure functioning of backflow prevention valve to prevent back-flow during dwelling.
5. Lower injection speed.
6. Change to higher-viscosity grade. |
| 2. Mixing of bubbles |
1. Reduce sucking back.
2. Raise back pressure. |
| 8. Sink marks |
1.Surface is drawn in and dented by internal shrinking because thick-walled parts and ribs are insufficiently cooled or pressure in cavity is insufficient. |
1. Lower mold temperature.
2. Scale up sprue, runner, and gate.
3. Raise dwelling force. Extend dwelling time.
4. Leave a cushion until gate sealing.
5. Make rib thickness approximately 1/3 of base thickness.
6. Thin walls. |
| 9. Cracks |
1. Stress during mold release (occurring immediately after molding) |
1. Improve parts having mold release problems (undercuts and parts on which a stress is exerted during mold release).
2. Review ejector system. |
| 2. Adherence of oil, etc. (occurring immediately after molding or in several days) |
1. Clean mold (rust inhibitor, slide grease, etc.)
2. Use clean gloves (against adherence of sebum). |
| 3. Internal distortion: stress crack (occurring several days after molding) |
1. Change shape so as not to leave internal distortion.
2. Perform annealing. |
| 4. Stress distortion (occurring after assembly or during use) |
1. Reduce stress exerted on molded products.
2. Anneal in assembled condition. |
| 5. Chemical stress: chemical crack (occurring immediately after contact with substances or in several days) |
1. Stop use of substances and chemicals that cause cracking.
2. Coat molded products.
3. Reduce internal and external distortion. |
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