Trouble Shooting

Trouble Shooting Topics

• Gate Stringing, Drooling
• Gels
• Jetting
• Material Leakage
• Oversized Part
• Part Sticking
• Short Shot (incomplete Filled Parts)
• Sprue Sticking
• Surface Finish (Low Gloss)
• Surface Finish (Scars, Wrinkles)

Topics

2. Blisters (Air Entrapment)

Description
Blisters are hollows created on or in the molded part. In contrast to a void (vacuum) this entrapped gas can also appear near the walls.

Possible Solutions
-Decrease melt temperature.
-Decrease screw speed.
-Dry material.
-Increase back pressure.
-Increase mold temperature.
-Ensure regrind is not too coarse.
-Provide additional mold vents.
-Relocate gate.

3. Brittleness

Description
Brittleness is a condition where the part cracks or breaks at a much lower stress level than would normally be expected based on the virgin material properties.

Possible Solutions
-Check for material contamination.
-Decrease amount of regrind use.
-Decrease back pressure.
-Decrease injection pressure.
-Decrease screw speed.
-Increase melt temperature.
-Dry material. Refer to the drying instructions provided by the material supplier.

4. Bubbles

Description
Bubbles are similar to blisters in that there is air entrapped in the molded part.

Possible Solutions
-Decrease injection speed.
-Decrease injection temperature.
-Dry material further.
-Increase injection pressure.
-Increase number and/or size of vents.
-Increase shot size.

5. Burn Marks, Dieseling

Description
Burn Marks or Dieseling show up on the finish molded parts as charred or dark plastic caused by trapped gas and is usually accompanied by a distinctive burnt smell.
Note: If this problem is allowed to continue without fixing the root cause it will very quickly cause damage to the molding surface.

Possible Solutions
-Alter gate position and/or increase gate size.
-Check for heater malfunction.
-Decrease booster time.
-Decrease injection pressure.
-Decrease injection speed.
-Decrease melt and/or mold temperature.
-Improve mold cavity venting. Vents may become smaller over time due to wear and they will need to be brought back to their original depth.
-Reduce clamp force to improve venting. Vents may become smaller because they are being crushed by the clamping force. If it is possible to reduce the clamping force without causing flash then this should be done. Note: This is always good practice to minimize wear on the mold and machine.
-Improve venting at the burn location. Burn marks often occur on deep ribs that have no venting. If possible it may be helpful to put an ejector pin or sleeve at the burnt area to allow the trapped gas to escape to atmosphere.

6. Cracking, Crazing

Description
Cracking or Crazing is caused by high internal molded in stress or by an external force imposed upon the part. They can also be caused by an incompatible external chemical being applied to the finished parts The cracks often don’t appear until days or weeks after the parts have been molded.

Possible Solutions
-Decrease injection pressure.
-Dry material.
-Increase cylinder temperature.
-Increase mold temperature.
-Increase nozzle temperature.
-Modify injection speed.
-If the material is partially crystalline then it may help to reduce the mold and/or melt temperature.
-If the material is amorphous then it may help to increase the mold and/or melt temperature.

7. Delamination

Description
Delamination occurs when single surface layers start flaking off the molded part.

Possible Solutions
-Adjust injection speed.
-Check for material contamination. Incompatible resins or colorants may have been accidently mixed causing this condition to be seen.
-Dry material.
-Increase melt temperature.
-Increase mold temperature.
-Insufficient Blending. Check melt homogeneity and plasticizing performance.

8. Discoloration

Description
Discoloration is similar to burn marks or brown streaks but generally not as dark or severe. It may cause the part to be a darker shade than the virgin pellets and is often found nearest the gate area, however it can also appear as dark streaks throughout the part.

Possible Solutions
-Check hopper and feed zone for contamination.
-Decrease back pressure.
-Decrease melt temperature.
-Decrease nozzle temperature.
-Move mold to smaller shot-size press.
-Provide additional vents in mold.
-Purge heating cylinder.
-Shorten overall cycle.

9. Excessive Flash

Description
Excessive Flash is often seen near sealing faces, out of vent grooves, or down ejector pins. It appears as thin or sometimes thick sections of plastic where it would not be on a normal part.
Note: Flash can very quickly (within a few cycles) damage the parting line surfaces.

Possible Solutions
-Decrease back pressure.
-Decrease cylinder temperature.
-Decrease injection hold time.
-Decrease injection pressure.
-Decrease injection speed.
-Decrease mold temperature.
-Increase clamp pressure.
-Check mold venting. Vents may have been ground too deep for the material being used.
-Check sealing surfaces to ensure that they seal off properly by “blueing” them in under clamp tonnage.
-Check ejector pin bore diameter to pin diameter tolerances. The tolerances may be too large allowing plastic to flash down the opening. The tolerances may be too large for the material being used and can occur due to wear over time.

10. Flow, Halo, Blush Marks

Description
Flow, Halo, Blush Marks are marks seen on the part due to flow of the molten plastic across the molding surface.

Possible Solutions
-Decrease injection speed.
-Increase cold slug area in size or number.
-Increase injection pressure.
-Increase melt temperature.
-Increase mold temperature.
-Increase nozzle temperature.
-Increase size of sprue/runner/gate.

11. Gate Stringing, Drooling

Description
The part does not break cleanly from the gate area.

Possible Solutions
-Insufficient cooling time during the cycle.
-Excessive heat in the gate area. Check thermocouple in the nozzle or decrease the temperature of the hot runner manifold and nozzle
-Increase cooling at the gate area.Ensure that you have controllable turbulent flow in the gate area.

12. Gels

Description
Gels are bubbles, or blisters seen on or in the part due to poor melt quality.

Possible Solutions
-Change screw speed.
-Increase back pressure.
-Increase cylinder temperature.
-Increase overall cycle time.
-Increase plasticating capacity of machine or use machine with large plasticating capacity.

13. Jetting

Description
Jetting is caused by an undeveloped frontal flow of melt in the cavity. The uninterrupted plastic flows or “snakes” into the cavity and cools off enough so that it does not fuse homogeneously with the material that follows.

Possible Solutions
-Decrease injection speed.
-Change the melt temperature, up or down.
-Use higher compression screw.
-Increase the gate diameter.
-Move the gate so that when the plastic first enters the cavity it hits an obstruction such as a rib or wall.

14. Material Leakage

Description
Material Leakage is usually caused by material forces overcoming the structural strength of the mold.
NOTE: One sign that indicates that material has leaked is that the manifold reaches processing temperature very slowly.

Possible Solutions
-Manifold locator is oversize
-Processing temperature may be too low causing increased pressure in the manifold.
-Manifold locator may be hobbed into the mold. Decrease the force applied to the nozzle pad by the machine then repair the damaged area, then check and if necessary replace the locator.
-Insufficient number of mold assembly screws. Ensure that the quantity, type of screw, and the location of the screws correspond to the general assembly drawing.
-Nozzle may have overheated causing damage to the seal or gate. Check/replace the thermocouple in the nozzle, then check and if necessary repair the nozzle well area.
-Manifold may have overheated. Check and replace if necessary the following components; nozzle well area, thermocouple, valve disks, sprue disks, or pressure disks.

15. Oversized Part

Description
Part is too large when compared to the drawing specifications.

Possible Solutions
-Decrease booster time.
-Decrease cylinder temperature.
-Decrease holding pressure.
-Decrease injection pressure.
-Decrease injection speed.
-Decrease overall cycle time.
-Increase gate size and/or change gate location.
-Increase mold temperature.

16. Part Sticking

Description
Part is getting not pulling out of the cavity and in rarer circumstances cannot be ejected off the core.

Possible Solutions
-Check mold for undercuts and/or insufficient draft.
-Decrease booster time.
-Decrease cylinder and nozzle temperature.
-Decrease injection pressure.
-Decrease injection-hold.
-Decrease mold cavity temperature.
-Increase clamp pressure.
-Increase mold-close time.
-Texturing on part is too deep. The parts may stick in the cavity if a new texture or a retexturing has been performed on the cavity half of the mold.
-If possible add undercuts to the core to allow the part to pull out of the cavity.

 17. Short Shot (incomplete Filled Parts)

Description
Short Shots occur when the part does not completely fill.

Possible Solutions
-Increase back pressure.
-Increase injection pressure.
-Increase injection speed.
-Increase melt temperature.
-Increase mold temperature.
-Increase nozzle temperature. Ensure that the manifold and nozzles have reached the set temperature.
-Increase shot size and confirm cushion.
-Make sure mold is vented correctly and vents are clear.
-Confirm that the non-return valve used is not leaking excessively.
-Increase the switch over pressure, distance, or time (whichever method is being used) point from fill to hold so the fill stage is used longer.
-Change part design. Thin areas of the mold may not fill completely, especially if there is a thick to thin transition, or there is a long rib that cannot be vented very well. If the part design allows it, change in these areas can improve the situation.

18. Sprue Sticking

Description
Sprue Sticking generally occurs in a cold runner mold when the sprue is staying
in the mold.

Possible Solutions
-Check mold for undercuts and/or insufficient draft.
-Decrease booster time.
-Decrease injection pressure.
-Decrease injection speed.
-Decrease injection-hold.
-Decrease mold close time.
-Decrease nozzle temperature.
-Increase core temperature.
-Open the gates.
-Ensure that the correct design of nozzle tip for the material is being used.

19. Surface Finish (Low Gloss)

Description
Surface Finish (Low Gloss). Gloss is the appearance of the surface of the molded part when light is reflected off of it. Molds that are textured or resins that are filled have an inherently reduced level of gloss when compared to highly polished mold surfaces.

Possible Solutions
-Clean mold surface.
-If the part design allows increase the polish of the molding surface.
-Increase cylinder temperature. This applies to molds that have a polished surface.
-Increase injection pressure. This applies to molds that have a polished surface.
-Increase injection speed. This applies to molds that have a polished surface.
-Increase mold temperature. This applies to molds that have a polished surface.
-Decrease cylinder temperature. This applies to molds that have a textured surface.
-Decrease injection pressure. This applies to molds that have a textured surface.
-Decrease injection speed. This applies to molds that have a textured surface.
-Decrease mold temperature. This applies to molds that have a textured surface.
-Increase melt temperature.
-Make sure venting is adequate.

20. Surface Finish (Scars, Wrinkles)

Description
Surface Finish (Scars, Wrinkles). Is the appearance of the ripples or wrinkles
on the surface of the molded part.

Possible Solutions
-Decrease back pressure.
-Decrease nozzle temperature.
-Increase booster time.
-Increase the melt temperature.
-Increase injection pressure.
-Increase injection speed.
-Increase overall cycle time.
-Increase shot size.
-Inspect mold for surface defects.