Faults Cause and Remedies

Faults Cause and Remedies:

Short Short: A short shot occurs when the flow of molten material doesn't completely fill the cavities in a mold. The result is that the molded component is incomplete after cooling

Possible Reason:

1. Molten Plastic is not reaching the mould cavity section.

Possible Solution:

1. Increase melt temperature

2. Increase mould temperature

3. Increase injection speed

4. Increase injection pressure if process is pressure limited

5. Check if the mould is vented in the area of short shots

6. Increase gate and runner sizes

Flash:

Flash is a molding defect that occurs when some molten plastic escapes from the mold cavity. Typical routes for escape are through the parting line or ejector pin locations. Additionally, excessive injection pressure may force the plastic out through the route of least resistance.

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Flashing of a part can occur for several reasons from variations in the process or material to tooling trouble . Flash appears on the part ' sedge along the parting line of the mold or anyplace where the mold has metal meeting metal to form a boundary of the part .

Possible Reason:

1. Molten Plastic is flowing into unwanted sections of the mould cavity.

Possible Solution:

1. Check for Mould shut-off and mould damage

2. Decrease melt temperature

3. Decrease mould temperature

4. Decrease injection speed

Sink:

Sink marks in injection molded plastic parts develop when material in the region of thick features such as ribs or bosses shrinks more than the material in the adjacent wall. ... Differential rates of cooling result in a depression on the adjacent surface that is known as a sink mark.

Possible Reason:

Plastic is shrinking as it cools but additional plastic cannot be for further compensation of the shrinkage.

Possible Solution:

1. Increase pack and hold pressures

2. Increase pack and hold times

3. Decrease mould temperatures

4. Decrease melt temperatures

Splay:

Splay is a cosmetic defect that occurs in plastic injection molding, when silver or white streaks appear on the surface of the part. It can be difficult to determine what is truly splay, as its appearance is very similar to other defects.

Possible Reason:

A layer/ streak of a unwanted gaseous byproduct from the melt or moisture in the material comes in between the melt flow and the cavity walls preventing the texture from being picked up and in addition eventually leaving a residue

Possible Solution:

1. Dry plastic to suggested moisture levels

2. Decrease injection speeds

3. Decrease melt temperature

4. Decrease screw rotation speeds

5. Decrease back pressure

6. Increase mould temperatures

7. Increase venting

8. Increase gate sizes

Warpage:

Warping (or warpage) is the deformation that occurs when there is uneven shrinkage in the different parts of the molded component. The result is a twisted, uneven, or bent shape where one was not intended. Causes: Warping is usually caused by non-uniform cooling of the mold material.

Possible Reason:

A differential cooling rate of the melt in two sections of the moulded product.

Possible Solution:

1. Decrease melt temperature

2. Experiment with differential mould temperatures on the fixed and movable sides

3. Increase pack and hold pressures

4. Increase pack and hold times

5. Increase cooling time

Burn Marks:

Burn marks typically appear as black or rust-colored discoloration on an edge or surface of a molded plastic part. Burn marks generally don't affect part integrity, unless the plastic is burned to the extent of degradation.

Possible Reason:

When air and gasses get trapped inside the mould cavity during plastic injection, the high pressure results in the dieseling of the plastic resulting in the burning of the plastic.

Possible Solution:

1. Increase venting in the mould

2. Decrease injection speed

3. Decrease melt temperature

4. Decrease Screw Rotation Speeds

Contamination/Black Specs:

Brown specks or streaks refer to the same type of defect, except the burning or discoloration is not as severe. Black specks and black streaks are caused by overheated (degraded, burned) material or by contamination of the resin. Overheated materials can degrade and lead to black streaks.

Possible Reason:

This can be degraded plastic and/or foreign material that can get mixed with the plastic that needs to be moulded.

Possible Solution:

1. Reduce melt temps

2. Reduce injection speeds

3. Reduce screw speeds

4. Find source of foreign material

Voids:

Vacuum voids are often caused by uneven solidification between the surface and the inner sections of the prototype. This can be aggravated when the holding pressure is insufficient to condense the molten plastic in the mold (and thereby force out air that would otherwise get trapped).

Voids tend to appear on thicker part surfaces and can occur when the outside layer of the part cools off and solidifies faster than the internal which can in turn cause a void.

Possible Reason:

Usually occur with the parts are thick. The walls solidify and the plastic melt shrinks towards the wall and therefore sucks a vacuum void on the inside of the part.

Possible Solution:

1. Reduce melt temperature

2. Reduce mould temperature

3. Reduce injection speed

4. Increase pack and hold pressures

5. Increase pack and hold times

Bubbles:

Plastic Molding Defects. Time is money and one small issue that can be costly in injection molding is bubbling. Bubbles have two primary causes: gas pockets or vacuum voids.

By using a hot air gun or small lighter, warm the area of the part that has the bubble until it begins to soften up.

Possible Reason:

When moisture and/or a gaseous byproduct gets mixed with the melt and is injected in the mould cavity this moisture or gas if embedded inside the melt can show up as bubbles.

Possible Solution:

1. Dry material to suggested moisture levels

2. Increase back pressure

3. Reduce melt temperature

Gate Blush: 

Gate is too small or the wrong style for the material. The gate restricts flow as the part fills. But, if it is too restrictive for the required flow, the material will react and blush will appear. The fix: review the gate size against the nominal wall thickness and material.

Injection fill speed is too high. The speed at which resin enters the mold has a big impact on finished parts. If the mold is filled too quickly, the molten plastic will react to the flow restriction at the gate. The fix: adjust the fill speed until the optimal rate is achieved to prevent blushing.  Careful—slowing the fill speed down too much may introduce other problems.

Injection pressure is too low. In order for a part to be formed properly, the resin must enter the mold with enough pressure that a consistent speed is achieved. If the pressure limit is set too low, then the flow may hesitate and accelerate. The fix: increase the fill pressure limit.

Melt temperature is too high or too low. Success in plastic injection molding relies, in large part, on achieving the right flow. Plastic that is either too hot or too cold will not flow properly, which means it won’t fill the mold properly. The fix: find the ideal barrel heat for the material and the mold you are using.

Nozzle diameter is too small. When the nozzle diameter is too small, it compounds the flow restriction in downstream runners and gates. The fix: enlarge the nozzle diameter. Ideally, the nozzle tip should be the same size as, or slightly smaller than, the sprue bushing opening.

Gates are not in the right locations. We’ve blogged about gate location in the past. If gates are positioned such that thin areas fill first, the plastic there will begin to harden before thicker areas receive material. The fix: ensure that gates are positioned so that thicker-walled areas are filled first.

Possible Reason:

Shows up at the gate when the material is sheared differently compared to the rest of the part.

Possible Solution:

1. Slow down the injection speeds in the gate area

2. Profile the injection speeds if necessary

3. Experiment with increasing and decreasing melt temperatures or hot tip temperatures in case of hot runner moulds

Jetting:

Jetting refers to a situation where molten plastic fails to stick to the mold surface due to the speed of injection.

Causes: Jetting occurs mostly when the melt temperature is too low and the viscosity of the molten plastic becomes too high, thereby increasing the resistance of its flow through the mold.

Possible Reason:

Usually seen with the part is thick causing the injected plastic to ‘snake fall’ on to the cold mould surface and start freezing immediately. When the fresh plastic come in it does not blend well with the first material causing the jetting marks.

Possible Solution:

1. Reduce injection speed

2. Increase melt temperature

3. Change gate location

Weld Lines:

A weld or meld line on plastic parts can cause structural problems and/or be visibly unacceptable. A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process.

Possible Reason:

The melt flow front is usually cold due to the exposure to the cold cavity. When two flow fronts meet as in a flow around a mould pin they do not fuse uniformly causing a weld line. Sometimes air can also get trapped in to form the defect.

Possible Solution:

1. Increase melt temperature

2. Increase mould temperatures

3. Increase injection speeds

4. Increase venting

Flow line:

Flow lines are streaks, patterns, or lines - commonly off-toned in color - that show up on the prototype part as a consequence of the physical path and cooling profile of the molten plastic as it flows into the injection mold tooling cavity. Injection molded plastic begins its journey through the part tooling via an entry section called a “gate.” It then flows through the tool cavity and cools (eventually hardening into a solid).

Causes: Flow line defects are caused by the varying speed at which the molten plastic flows as it changes direction through the contours and bends inside the mold tool. They also occur when the plastic flows through sections with varying wall thickness, or when the injection speed is too low causing the plastic to solidify at different speeds.

Remedies:

1. Increase injection speeds and pressure to the optimal level, which will ensure the cavities are filled properly (while not allowing the molten plastic time to start cooling in the wrong spot). The temperature of the molten plastic or the mold itself can also be elevated to ensure the plastic does not cool down sufficiently to cause the defect.

2. Round corners and locations where the wall thickness changes to avoid sudden changes in direction and flow rate.

3. Locate the gate at a spot in the tool cavity with thin walls.

Surface Delamination:

Surface delamination is a condition where thin surface layers appear on the part due to a contaminant material. These layers appear like coatings and can usually be peeled off (i.e. “delaminate”). Delamination, which may also be called lamination or layering, is a defect where the surface layers of a molded part can be peeled off. Delamination often occurs in the gate area, but can occur in any area of the molded part and can be a small or large defect.

Causes: Foreign materials that find their way into the molten plastic separate from the finished product because the contaminant and the plastic cannot bond. The fact that they cannot bond not only has an affect on the appearance of the prototype, but also on its strength. The contaminant acts as a localized fault trapped within the plastic. An over-dependence on mold release agents can also cause delamination.

Remedies:

1. Pre-dry the plastic properly before molding.

2. Increase the mold temperature.

3. Smooth out the corners and sharp turns in the mold design to avoid sudden changes in melt flow.

4. Focus more on the ejection mechanism in the mold design to reduce or eliminate the dependence on mold release agents.