Injection Moulding Troubleshooting

Black spots and brown streaks appear as dark spots or streaks in the moulded part and are usually caused by thermal damage to the melt.

Possible Solutions

• Check the material for contamination.
• Decrease the melt temperature.
• Decrease the overall cycle time.
• Purge and/or clean the screw and barrel.
• Decrease the screw speed. High screw speeds may cause the material to degrade.
• Material may have too much regrind content.
• Material may be over dried. Decrease drying time/temperature. Refer to drying instructions provided by the material supplier.
• Material may be prone to thermal degradation. It may be necessary to use a more thermally stable material.
• Dead spots may be occurring, ensure that the alignment between the machine nozzle and mould sprue is correct.
• Residence time may be too long, or the shot size may be too small for the machine. It may be necessary to move the mould to a machine with less injection capacity.

Blisters are hollows created on or in the moulded 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 mould temperature.
• Ensure regrind is not too coarse.
• Provide additional mould vents.
• Relocate gate.

A brittle moulded part has a tendency to break or crack. Brittleness results from shorter molecular chain length (thus lower molecular weight). As a result, the toughness of the part is substantially less than the specification.

Possible Solutions

• Set proper drying conditions before moulding.
• Reduce regrind polymer. Polymer suppliers can provide optimum drying conditions and regrind percentage for the specific polymer.
• Change to a tougher grade or different polymer.
• Optimize the runner system design. Restrictive sprue, runner, gate, or even part design could cause excessive shear heating that aggravates an already overheated polymer, causing polymer degradation.
• Modify screw design, to avoid improper melt mix or overheating that leads to polymer degradation.
• Select machine with smaller shot size, minimizing residence time reduces polymer degradation.
• Reduce residual stress and strengthen weld lines by increasing melt temperature, within limits.
• Do not overheat the polymer.

Bubbles are similar to blisters in that there is air entrapped in the moulded 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.

Burn Marks or Dieseling show up on the finish moulded 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 moulding 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 mould temperature.
• Improve mould 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 mould 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.

Cracking or Crazing is caused by high internal moulded 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 moulded.

Possible Solutions

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

Delamination occurs when single surface layers start flaking off the moulded 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 mould temperature.
• Insufficient Blending. Check melt homogeneity and plasticizing performance.

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 mould to smaller shot-size press.
• Provide additional vents in mould.
• Purge heating cylinder.
• Shorten overall cycle.

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 mould temperature.
• Increase clamp pressure.
• Check mould 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.

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

Possible Solutions

• Decrease injection speed.
• Increase cold slug area in size or number.
• Increase injection pressure.
• Increase melt temperature.
• Increase mould temperature.
• Increase nozzle temperature.
• Increase size of sprue/runner/gate.

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.

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 plasticising capacity of machine or use machine with large plasticising capacity.

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.

Material Leakage is usually caused by material forces overcoming the structural strength of the mould. 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 mould. 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 mould 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.

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 mould temperature.

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

Possible Solutions

• Check mould for undercuts and/or insufficient draft.
• Decrease booster time.
• Decrease cylinder and nozzle temperature.
• Decrease injection pressure.
• Decrease injection-hold.
• Decrease mould cavity temperature.
• Increase clamp pressure.
• Increase mould-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 mould.
• If possible add undercuts to the core to allow the part to pull out of the cavity.

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 mould temperature.
• Increase nozzle temperature. Ensure that the manifold and nozzles have reached the set temperature.
• Increase shot size and confirm cushion.
• Make sure mould 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 mould 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.

Sink Marks occur during the cooling process if certain areas of the part are not cooled sufficiently causing them to contract.

Possible Solutions

• Decrease amount of regrind use.
• Decrease back pressure.
• Confirm that the non-return valve being used is not leaking excessively.
• Decrease melt temperature. Do this if the sink marks are near the gate or thick walled areas.
• Decrease mould temperature. Do this if the sink marks are near the gate or thick walled areas.
• Decrease injection rate. Do this if the sink marks are near the gate or thick walled areas.
• Dry material.
• Increase injection pressure. Do this if the sink marks are away from the gate or in thin walled areas.
• Increase injection speed. Do this if the sink marks are away from the gate or in thin walled areas.
• Increase mould temperature. Do this if the sink marks are away from the gate or in thin walled areas.
• Increase injection-hold.
• Increase shot size and confirm that the a cushion is being maintained.
• Increase size of sprue and/or runners and/or gates.
• Relocate gates on or as near as possible to thick sections.
• Increase cooling time.
• If possible change the mould design to maintain an even wall thickness throughout the part.

Splay Marks, Silver Streaks are usually caused by water vapour blisters at the flow front burst and freeze on the wall of the moulding surface.

Possible Solutions

• Check for contamination.
• Decrease melt temperature.
• Decrease nozzle temperature.
• Dry resin pellets before use. As per the manufacturers recommendations.
• Incorrect storage of pellets. Moisture on the pellets could be transferred into the melt, especially if the resin is not normally pre-dried.
• Raise mould temperature. This will prevent condensation on the mould walls from being carried into the melt.
• Ensure the mould is not leaking water onto the cores or cavities. Again this will prevent condensation on the mould walls from being carried into the melt.
• Relocate gates on or as near as possible to thick sections.
• Shorten overall cycle.

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

Possible Solutions

• Check mould for undercuts and/or insufficient draft.
• Decrease booster time.
• Decrease injection pressure.
• Decrease injection speed.
• Decrease injection-hold.
• Decrease mould 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.

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

Possible Solutions

• Clean mould surface.
• If the part design allows increase the polish of the moulding surface.
• Increase cylinder temperature. This applies to moulds that have a polished surface.
• Increase injection pressure. This applies to moulds that have a polished surface.
• Increase injection speed. This applies to moulds that have a polished surface.
• Increase mould temperature. This applies to moulds that have a polished surface.
• Decrease cylinder temperature. This applies to moulds that have a textured surface.
• Decrease injection pressure. This applies to moulds that have a textured surface.
• Decrease injection speed. This applies to moulds that have a textured surface.
• Decrease mould temperature. This applies to moulds that have a textured surface.
• Increase melt temperature.
• Make sure venting is adequate.

Surface Finish (Scars, Wrinkles). Is the appearance of the ripples or wrinkles on the surface of the moulded 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 mould for surface defects.

Part is too small when compared to the drawing specifications.

Possible Solutions

• Decrease mould temperature.
• Increase booster time.
• Increase cylinder temperature.
• Increase hold-time.
• Increase holding pressure.
• Increase injection pressure.
• Increase injection speed.
• Inspect mould for surface defects.

Valve pin does not close properly. This will leave the gate protruding from the part. This may also occur if the valve pin is too hot, the material may stick to the valve pin.

Possible Solutions

• Valve pin is too short. Check and replace if necessary.
• alve pin fit. Ensure that the valve pin is lapped to the gate steel when appropriate.
• Damaged gate. Check if valve pin is too long, rework if necessary. Also check to ensure that the valve pin is concentric with the gate, if not replace it.
• Hydraulic / Pneumatic seals may be worn. Replace as necessary.
• Insufficient pin/land area in the gate area of the mould. Increase the gate area cooling, or increase the valve pin land contact.
• Insufficient hydraulic or air pressure. Increase the pressure up to but not beyond the maximum rating of the unit being used.
• Excessive hold time. Decrease the hold time.

Voids are hollows created in the part. They are normally found in thick sectioned parts caused by material being pulled away from the hot centre section towards cold mould walls leaving a void in the centre.

Possible Solutions

• Clean vents.
• Decrease injection speed.
• Decrease melt temperature.
• Dry material.
• Increase injection pressure.
• Increase injection-hold.
• Increase mould temperature.
• Increase shot length.
• Increase size of gate.
• Increase size of sprue and/or runners and/or gates.

Warping, Part Distortion is shows up as parts being bowed, warped, bent or twisted beyond the normal specification outlined on the drawing.

Possible Solutions

• Adjust melt Temperature (increase to relieve moulded-in stress, decrease to avoid overpacking). stress, decrease to avoid over packing). stress, decrease to avoid over packing).
• Check gates for proper location and adequate size.
• Check mould knockout mechanism for proper design and operation.
• Equalize/balance mould temperature of both halves.
• Increase injection-hold.
• Increase mould cooling time.
• Relocate gates on or as near as possible to thick sections.
• Try increasing or decreasing injection pressure.

Weld Lines are created when two or more melt flow fronts meet possibly causing a cosmetically visible line. It can also create a weakened area in the finished moulded part especially with filled resins.

Possible Solutions

• Increase injection pressure.
• Increase injection speed.
• Increase injection hold.
• Increase melt temperature.
• Increase mould temperature.
• Make sure part contains no sharp variation in cross-sections.
• Vent cavity in the weld area.