Molding defects & Solutions – UPMOLD

Plastic Injection molding TROUBLESHOOTING

Upmold — Tue, 25 Sep 2018 13:33:02 +0000

The post Plastic Injection molding TROUBLESHOOTING appeared first on UPMOLD.

Injection moulding problems and solutions

Upmold — Fri, 18 Aug 2017 09:49:09 +0000

Plastic injection moulding problems and solutions chart

Plastic molding defects definition, reasons and solutions.

Molding Defects	Alternative Name	Descriptions	Causes
Blister	Blistering	Raised or layered zone on surface of the Plastic part	Tool or material is too hot, often caused by a lack of cooling around the tool or a faulty heater
Burn marks	Air Burn/Gas Burn	Black or brown burnt areas on the plastic part located at furthest points from gate	Tool lacks venting, injection speed is too high
Color streaks (US)		Localized change of color	Plastic material and colorant isn’t mixing properly, or the material has run out and it’s starting to come through as natural only
Delamination		Thin mica like layers formed in part wall	Contamination of the material e.g. PP mixed with ABS, very dangerous if the part is being used for a safety critical application as the material has very little strength when delaminated as the materials cannot bond
Flash	Burrs	Excess material in thin layer exceeding normal part geometry	Tool damage, too much injection speed/material injected, clamping force too low. Can also be caused by dirt and contaminants around tooling surfaces.
Embedded contaminates	Embedded particulates	Foreign particle (burnt material or other) embedded in the part	Particles on the tool surface, contaminated material or foreign debris in the barrel, or too much shear heat burning the material prior to injection
Flow marks	Flow lines	Directionally “off tone” wavy lines or patterns	Injection speeds too slow (the plastic has cooled down too much during injection, injection speeds must be set as fast as you can get away with at all times)
Jetting		Deformed part by turbulent flow of material	Poor tool design, gate position or runner. Injection speed set too high.
Polymer degradation		polymer breakdown from oxidation, etc.	Excess water in the granules, excessive temperatures in barrel
Sink marks		Localized depression (In thicker zones)	Holding time/pressure too low, cooling time too short, with sprueless hot runners this can also be caused by the gate temperature being set too high
Short shot	Non-Fill/Short Mold	Partial part	Lack of material, injection speed or pressure too low
Splay marks	Splash Mark/Silver Streaks	Circular pattern around gate caused by hot gas	Moisture in the material, usually when resins are dried improperly
Stringiness	Stringing	String like remain from previous shot transfer in new shot	Nozzle temperature too high. Gate hasn’t frozen off
Voids		Empty space within part (Air pocket)	Lack of holding pressure (holding pressure is used to pack out the part during the holding time). Also mold may be out of registration (when the two halves don’t center properly and part walls are not the same thickness).
Weld line	Knit Line/Meld Line	Discolored line where two flow fronts meet	Mold/material temperatures set too low (the material is cold when they meet, so they don’t bond)
Warping	Twisting Part	Distorted part	Cooling is too short, material is too hot, lack of cooling around the tool, incorrect water temperatures (the parts bow inwards towards the hot side of the tool)

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Injection Molding Defects And Solutions

Upmold — Fri, 18 Aug 2017 08:42:43 +0000

Plastic Injection Molding Defects And Solutions

molding flash

Molding Flash of Injection Molding defect

Solving molding flash problems
Definition
molding flash occurs when a thin layer of material is forced out of the mold cavity at the parting line or ejector pins location. This excess material remains attached to the molded article, and normally has to be manually removed.

Causes
Worn or poorly fitting cavity/mold plates
Including, mold plate deformations and obstructions (grease, dirt, debris)http://upmold.com/wp-admin/post-new.php?post_type=page#

Insufficient clamp force
The machine clamp force must be greater than the pressure in the cavity (that is, clamp opening force), to sufficiently hold the mold plates shut.

Overpacking
Overpacked sections cause increased localized pressure.

Non-optimal molding conditions
Including, material viscosity, injection rate, and runner system. For example, high melt temperature, which makes a less viscous melt.

Improper venting
An improperly designed venting system, a very poor venting system, or a venting system that is too deep.

Remedies
Ensure correctly fitting mold plates
Set up the mold to seal properly. Clean the machine from any obstructions. Add pillar support or thicken the mold plates if there is any deformation of the mold plate during the molding process.

Avoid overpacking

Select machine with higher clamp force

Vent appropriately
Use the material supplier recommended venting size.

Optimize processing conditions
Reduce pressures and shot size to the minimum required.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding air trap

Molding Air Trap of Injection Molding Defects

Solving molding air trap problems
Definition
Air traps occur when converging flow fronts surround and trap a bubble of air. The trapped air can cause incomplete filling and packing, and will often cause a surface blemish in the final part. Air trapped in pockets may compress, heat up and cause burn marks.
Causes
Racetrack effect
Hesitation
Unbalanced flow paths
Flow paths do not need the racetrack effect or hesitation to have unbalanced flow. In a part with uniform thickness, the physical length of flow paths may vary, and again air traps may occur.
Inadequate venting
Lack of vents or undersized vents in these last-to-fill areas are a common cause of air traps.

Remedies
Balance flow paths
Avoid hesitation and racetrack effects
Balance runners
Changing the runner system can alter the filling pattern in such a way that the last-to-fill areas are located at the proper venting locations.
Vent appropriately
If air traps do exist, they should be positioned in regions that can be easily vented or ejection and/or vent pins added so that air can be removed.
Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding warpage

Molding Warpage of Injection Molding Defects

Solving molding warpage problems

Definition
Warpage occurs when there are variations of internal stresses in the material caused by a variation in shrinkage. Warped parts may not be functional or visually acceptable.

Causes
Non-uniform cooling
Temperature differences from one side of the mold to the other can lead to layers freezing and shrinking at different times and generating internal stresses.
Inconsistent shrinkage
Resulting from: a) Material variations such as property variations, varying moisture content, inconsistent melt and pigmentation; b) Process conditions variations such as inconsistent packing and varying mold and melt temperatures; c) Machine variations such as a damaged check ring and unstable controller.
(Animations not available More…)

Remedies
Minimize differential shrinkage
Minimize orientation effects
Position gates for uni-directional flow, and modify part thickness.
Change part geometry
Add features such as stiffening ribs to the design. Alter part design to avoid thick sections and reduce the thickness of any features that intersect with the main surface.
Use thinner wall sections with ribs
Thicken only those wall sections that require extra material for structural stability and that cannot be strengthened using another method.
Change material
Semi-crystalline have naturally higher shrinkage and hence are more prone to warpage.
Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding burn

Molding Burn of Injection Moulding Defects

Solving molding burn problems

Definition
Burn marks are small, dark or black spots on the part surface. This phenomena is also often referred to as dark streaks or specks.

Causes
Adiabatically heated trapped air (see Air Traps)
Air trapped in pockets may compress, heat up and cause burn marks.
(Animations not available More…)
Material degradation
This can be caused by excessive injection speed, residence time or melt temperature. Improper screw or runner system design may also lead to material degradation.

Remedies
Eliminate air traps
To prevent burn marks, you should move air traps to places which can be vented, or where ejector pins can be added.
Optimize the runner system design
Restrictive sprue, runner, gate, or even part design could cause excessive shear heating that aggravates an already overheated material, causing material degradation.
Modify screw design
Contact material/machine suppliers to get the right screw design information to avoid improper melt mix or overheating that leads to material degradation.
Select machine with smaller shot size
Optimize melt temperature
Reduce temperature to avoid material degradation from overheating, or increase it to limit residual stress.
Optimize back pressure, screw rotation speed, or injection speed
Balance shear heat against residual stress.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding hesitation

Molding hesitation of Injection molded defects

Solving molding hesitation problems
Definition
Hesitation is when flow slows down or stops along a particular flow path.

Hesitation
If plastic filling a cavity has the option of filling either a thin section or a thick section, the plastic will tend to fill the thick section first as this route offers less resistance to flow. This can result in plastic in the thin section stopping or slowing significantly. Once the plastic starts to slow down, it will cool more rapidly, so the viscosity will increase. This higher viscosity will inhibit flow further causing even faster cooling and so the problem is self propagating.
Hesitation can occur in ribs and in thin section of parts that have significant changes in wall thickness.
In the image above, the rib circled in red, offers a higher resistance to flow because it is much thinner than the rest of the part.
Look at these animations to see how hesitation in a part can be avoided by using a different injection location. When there is no alternative route available, the flow in the rib will be continuous and not hesitate as seen in the right-hand example.

Hesitation can reduce part quality due to variation in surface appearance, poor packing, high stresses and non-uniform orientation of the plastic molecules. Alternatively, if the hesitation allows the flow front to freeze completely, part of the cavity may remain unfilled (short shot).
Viewing the fill time and temperature results may help explain why the hesitation occurred.

The fill time plot will show hesitation by a very narrow spacing of fill time contours. The temperature plot will show hesitation by a low temperature and a large temperature gradient.
Note: Hesitation can lead to asymmetrical and unpredictable flow patterns.

What to do
Move the polymer injection location away from the area of hesitation so that the bulk of the cavity fills before the melt reaches the thin area. The absence of alternative flow paths will give less time for the polymer to hesitate.
Move the polymer injection location to a place that will cause greater pressure to be applied where the hesitation occurred. It is useful to have thin ribs/bosses as the last point to fill, so all the injection pressure is applied at this point.
Increase the wall thickness where the hesitation occurred, to reduce the resistance to flow.
Use a less viscous material (that is, a material with a higher melt flow index).
Inject more quickly to reduce the potential of hesitation time.
Increase the melt temp so that it flows into the thin area more readily.

molding weld and meld line

Molding weld and meld line of Injection moulded defects

Solving molding weld and meld line problems
Definition
A weld or meld line on plastic parts can cause structural problems and/or be visibly unacceptable.
Difference between weld and meld lines

The difference between a weld and meld line is simply determined by the angle at which the converging flow fronts meet.

In the above diagram, the converging flow fronts (indicated by red arrows) meet. If the angle, , is greater than 135?a meld line will form. If is less than 135?a weld line will form.

Weld Lines
When a weld line forms, the thin frozen layers at the front of each flow path meet, melt, and then freeze again with the rest of the plastic. The orientation of the plastic at the weld is therefore perpendicular to the flow path. The following animation shows plastic filling a cavity. The weld line occurs where two flow fronts meet, and the polymer molecules are misaligned.
It is the sharp difference in molecular orientation at the weld which causes the significant decrease in strength at this point.

Meld Lines
A meld line occurs when two flow fronts blend together at an oblique angle. The orientation of the plastic molecules is therefore more uniform than the orientation after a weld line has formed. The following diagram shows the length of a part where a meld line forms.
The red arrows show the direction of plastic flow. The white lines represent the orientation of the polymer molecules after the meld line has formed.
Meld lines are normally stronger than weld lines and are often much less visible.

Note: The term weld line is often used to mean both weld and meld lines.

Weld and meld lines on a plastic part can cause structural problems and be visually unacceptable. (A line, notch and/or color change can appear.) Therefore weld and meld lines should be avoided if possible (when the cavity has unbalanced flow paths unnecessary weld and meld lines can occur).
If it is not possible to remove a weld/meld line, it should be positioned in the least sensitive area possible. Avoid weld lines in areas which need strength, or which need to appear smooth. This can be done by changing the polymer injection location or altering wall thicknesses to set up a different fill time. In a different fill time, flow fronts may meet at a different location and therefore the weld/meld line will move.
Moving:
Alter gate positions
Change part thickness
Improving the quality:
Increase melt & mold temperature
This will allow the flow fronts to weld to each other better.
Increase ram speed
Optimize runner system design
Reduce runner dimensions while maintaining the same flow rate to make use of frictional heating.
Note: The processing conditions help to determine the quality of the weld or meld line that has formed. A good weld occurs when the melt temperature is no lower than 20°C below the injection temperature.
Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding overpacking

Molding overpacking of injection plastic molded defects

Solving molding overpacking problems

Definition
Overpacking is when extra material is compressed in one flow path while other flow paths are still filling.

Overpacking
Overpacking occurs when the easiest (shortest/thickest) flow paths fill first. Once this flow path has filled, it will still be under pressure as extra plastic is injected into the cavity to fill the remaining flow paths. This pressure will push more material into the already full flow path, causing it to have a higher density and lower shrinkage than other regions. The overpacked fill path will have frozen under pressure, so stresses will be frozen in.
Note: The key result used to identify overpacking is the fill time result. Display the fill time at 100% fill and look for any flow paths that do not finish at the same time as the first pat
In the above diagram, the white lines represent the polymer molecules. Note that the flow paths are not balanced and overpacking will occur in the left of the model.
Overpacking generally occurs in sections with the shortest fill time. It can cause a range of problems including warpage due to non-uniform shrinkage, increased part weight due to wasted material and non-uniform density distribution throughout the part.

What to do
To solve problems caused by overpacking, balance the flow paths.
Thicken or thin parts of the model to act as flow leaders or deflectors.
Move the injection location to a position that will define similar length flow paths.
Divide the cavity into imaginary sections, and use one injection location for each section.
Remove unnecessary gates.

molding flow line

Molding flow line of injection moulding defects

Solving molding flow line problems
Definition
A flow mark or halo, is a surface defect in which circular ripples or wavelets appear near the gate. Ripples, a similar defect, appear as small fingerprint-like waves near the edge or at the end of the flow.

Causes
Material freezing near the gate
Low melt and mold temperature and low ram speed can result in cold material entering the cavity. The partly solidified material takes on the form of the flow pattern.

where:
a = normal fountain flow with no ripples
b = flow causing ripples(R)
Insufficient material compensation
Early gate freeze-off or low packing pressure may not pack the cavity properly. The material near the gate then freezes while maintaining the form of the flow pattern.

Remedies
Optimize the cold well
Design the cold well in the runner system to trap the cold material during the filling phase. The proper length of the cold well is usually equal to that of the runner diameter.
Optimize the runner system design
Restrictive runner system design can result in premature gate freeze-off. It can however, increase shear heating for better melt flow.
Increase mold & melt temperature
Optimize packing pressure

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding sink mark and void

Molding sink mark and void of plastic molded defects

Solving molding sink mark and void problems
Definition
Sink marks and voids both result from localized shrinkage of the material at thick sections without sufficient compensation.
Sink Marks
Sink marks appear as depressions on the surface of a molded part. These depressions are typically very small; however they are often quite visible, because they reflect light in different directions to the part. The visibility of sink marks is a function of the color of the part as well as its surface texture so depth is only one criterion. Although sink marks do not affect part strength or function, they are perceived to be severe quality defects.
Voids
Voids are holes enclosed inside a part. These can be a single hole or a group of smaller holes. Voids are caused when the outer skin of the part is stiff enough to resist the shrinkage forces thus preventing a surface depression. Instead, the material core will shrink, creating voids inside the part. Voids may have severe impact on the structural performance of the part.

Causes
Sink marks are caused mainly by thermal contraction (shrinkage) during cooling. After the material on the outside has cooled and solidified, the core material starts to cool. Its shrinkage pulls the surface of the main wall inward, causing a sink mark. If the skin is rigid enough, deformation of the skin may be replaced by formation of a void in the core.
Localized geometric features
sink marks typically occur in moldings with thicker sections, or at locations opposite from ribs, bosses or internal fillets.
High volumetric shrinkage
Insufficient material compensation
Early gate freeze-off or low packing pressure may not pack the cavity properly.
Short packing or cooling time
High melt and/or mold temperatures

Remedies
Optimize packing profile
As sink marks occur during packing, the most effective way to reduce or eliminate them is to control the packing pressure correctly. To determine the effects of packing on sink marks, use a simulation package such as Moldflow Plastics Insight.
Change part geometry
Alter part design to avoid thick sections and reduce the thickness of any features that intersect with the main surface.
Reduce volumetric shrinkage
Relocate gates to problem areas
This allows these sections to be packed before the thinner sections between the gate and the problem areas freeze.
Optimize the runner system design
Restrictive runner system design can result in premature gate freeze-off.
Change material

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding high volumetric shrinkage

Molding high volumetric shrinkage of molded plastic defects

Solving molding high volumetric shrinkage problems
Volumetric shrinkage is the contraction of polymer due to the change in temperature from melt temperature to ambient temperature.
High volumetric shrinkage can cause part warpage, sink marks, critical dimensions that are too small, and internal voids.
Excessive wall thickness and inadequate packing can both contribute to high volumetric shrinkage in a part.
The key result to use to identify high volumetric shrinkage are the shrinkage results.
To reduce volumetric shrinkage, you can alter:
Part design (wall thickness)
Mold design (gate positions)
Processing conditions (increase packing pressure)

molding racetrack effect

Molding racetrack effect of plastic molding defects

Solving molding racetrack effect problems

Definition
The racetrack effect occurs when flow races through thick sections of the cavity before the thin sections have filled.
Note: Thick sections offer less resistance to flow than thin sections.

Racetrack Effect
The racetrack effect indicates unbalanced flow paths and can often cause unnecessary weld lines and air traps. The following diagram shows a part with a thick rim.
The flow of plastic (red arrows) races around the rim trapping a pocket of air (blue circle).
What to do
A large difference in wall thickness throughout a part can cause problems, but is sometimes necessary from a design point of view. However, in the previous example, the racetrack effect through the thick regions is not actually the problem. The problem is unbalanced flow that allows the racetrack effect to occur. If the plastic reached all parts of the thick rim at the same time, the racetrack effect would not occur.
Flow path 1 is shorter than flow path 2. However, by slightly thickening flow path 2 or thinning flow path 1 (see flow leaders and deflectors), the plastic could be forced to reach all parts of the thick rim at the same time. This would result in balanced flows.
The above example of racetrack in a symmetrical part with a thick rim is often easy to solve. In more complicated parts, thick walls may need some thinning, the polymer injection location may need to be altered, or multiple polymer injection locations may need to be used.

molding Unbalanced flow

Molding Unbalanced flow of plastic injection defects

Solving molding Unbalanced flow problems

Definition
Unbalanced flow is plastic completely filling some flow paths in the mold before other flow paths have filled.

Unbalanced flow
Unbalanced flow can be the cause of many molding problems such as flashing, short shots, high cycle time, density differences throughout the part, warpage, air traps and extra weld lines.

Flow is balanced when all the extremities of the mold fill at the same time.

To recognize unbalanced flow, you need to recognize the different flow paths in the mold. These are the different routes that the plastic takes throughout the cavity.
The following part contains three fundamental flow paths (shown as red arrows).

Each flow path is of a different length. Therefore, if the part has uniform thickness, flow path 1 will fill first, followed by flow path 2, followed by flow path 3.
The key result used to identify unbalanced flow is the fill time result.
What to do
By altering the thickness of regions within the part, flow can be hastened or delayed in certain directions to help balance flows. In the above diagram, varying the part thickness and creating flow leaders and deflectors, thinning flow path 1 and thickening flow path 3, is the answer. These variations in thickness are known as Flow Leaders or Flow Deflectors.

In other examples it is often necessary to consider the position of the polymer injection location, or the number of polymer injection locations. For example, if you choose a single injection location that defines some flow paths to be three or four times the length of others, then it is almost impossible to balance flows. Try moving the polymer injection location to a position that will define similar length flow paths. Alternatively, visualize the cavity in smaller, more manageable sections. Then use multiple injection locations, one per sub-section.
For a multi-cavity part, balance flows in each cavity first, then proceed to alter runner dimensions to ensure that:
All cavities fill at approximately the same time, with the same pressure
The temperature at the end of fill shows uniform distribution in each cavity, predicting uniform shrinkage, and acceptable weld line quality.
The shear stress in each cavity (ignore runners) is less than the recommended limit for the material chosen.

molding Underflow

Molding Underflow of molding plastic defects

Solving molding Underflow problems

Definition
Underflow is when a flow front reverses direction.

Underflow
Underflow occurs when flow fronts from two directions meet, pause momentarily, then one of the flows reverses direction and flows back between the outer frozen layers. When the flow reverses direction the frozen layer partly re-melts due to frictional heating.
In the animation below, the gate in the center has a much smaller volume to fill than the other two gates. When the center gate’s volume is filled, the other volumes are still filling, so the flow front from the left gate has a lower pressure than the center gate. When the two flow fronts meet, the left flow front reverses direction.
The arrow shows the direction of the underflow.
This flow reversal gives poor part quality, both from surface appearance and structural viewpoints.

What to do
Inspect the filling pattern to assess if underflow is likely to occur. Displaying the fill time result at 100% will not indicate the presence of underflow. Play the fill time animation from beginning to end and watch for any flow fronts meeting, then consider the geometry surrounding this point.

molding brittleness

Molding brittleness of processing injection defects

Solving molding brittleness problems

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

Causes
Material degradation
This can be caused by excessive injection speed, residence time or melt temperature. Improper screw or runner system design may also lead to material degradation.

Weld line weaknesses
Non-optimal crystallinity
High residual stress
Incompatible materials blended together
Too much regrind
Improper drying conditions
Excessive drying either drives off volatiles in the plastic, making it more sensitive to processing, or degrades the material by reducing the molecular weight.

Remedies
Set proper drying conditions before molding
Material suppliers can provide optimum drying conditions for the specific materials.
Reduce regrind material
Contact material suppliers to get the recommended levels of regrind to use.
Change material
Optimize the runner system design
Restrictive sprue, runner, gate, or even part design could cause excessive shear heating that aggravates an already overheated material, causing material degradation.
Modify screw design
Contact material/machine suppliers to get the right screw design information to avoid improper melt mix or overheating that leads to material degradation.
Select machine with smaller shot size
Minimizing residence time reduces material degradation.
Reduce residual stress
Strengthen weld lines
Increase melt temperature within limits, not to overheat the material.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding crack

Molding crack of moulded plastic defects

Solving molding crack problems

Definition
Cracking can cause part failure, a short part life and be visually unacceptable.
Cracks may not be obvious until several days or weeks after production. Hence, it is better to recognize and remove the potential problem of cracking before production. Moldflow Plastics Insight provides detailed shear stress results.

Causes
High residual stresses
Cracks may occur in regions where internal shear stresses are frozen into the part.
Weld line weaknesses
Differential shrinkage
Differential orientation, packing and cooling cause differential shrinkage resulting in high internal stress levels being frozen in.

Remedies
Minimize residual stress
Program the ram speed or increase wall thickness to reduce flow induced stresses. Check for the recommended maximum shear stress value for the material (recorded in the Materials Database)
Minimize differential shrinkage
Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding delamination

Molding delamination of injection defects

Solving delamination problems

Definition
Delamination, sometimes referred to as lamination or layering, is a defect in which the surface of a molded part can be peeled off layer by layer.

Causes
High shear stress
Incompatible materials blended together
Excessive use of mold release agent
Excessive material moisture
Excessive moisture heats up and forms steam, which results in delamination on the surface.
Material degradation
This can be caused by excessive injection speed, residence time or melt temperature. Improper screw or runner system design may also lead to material degradation.

Remedies
Eliminate Degradation and Excessive Shear Stress
Reduce shear stress
Remove excessive moisture
Material suppliers can provide optimum drying conditions for the specific materials.
Reduce regrind material
Avoid excessive use of mold release agent
You should repair the ejection system or other problems to eliminate the difficulty of de-molding instead of over-using the mold release agent.
Avoid material contamination

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding dimensional variation

Molding dimensional variation of injection defects

Solving dimensional variation problems

Definition
Dimensional variation is characterized by the molded part dimension varying from batch to batch or from shot to shot while the machine settings remain the same.

Causes
Inconsistent shrinkage
Resulting from: a) Material variations such as property variations, varying moisture content, inconsistent melt and pigmentation; b) Process conditions variations such as inconsistent packing and varying mold and melt temperatures; c) Machine variations such as a damaged check ring and unstable controller.
Narrow molding window

Remedies
Remove excessive moisture
Material suppliers can provide optimum drying conditions for the specific materials.
Reduce regrind material
Contact material suppliers to get the recommended levels of regrind to use.
Optimize the runner system design
Poor design could cause material degradation through shear heating or inconsistent packing.
Replace the check ring if it is broken or worn out
Ensure uniform mold temperature
Make sure the mold temperature is uniform by checking the cooling system.
Set processing conditions within the molding window
Reduce differential shrinkage

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design

molding discoloration

Molding discoloration of plastic injection defects

Solving discoloration problems

Definition
Discoloration is a color defect characterized by a molded part’s color having changed from the original material color.

Causes
Material degradation
This can be caused by excessive injection speed, residence time or melt temperature. Improper screw or runner system design may also lead to material degradation.

Remedies
Optimize the runner system design
Restrictive sprue, runner, gate, or even part design could cause excessive shear heating that aggravates an already overheated material, causing material degradation.
Modify screw design
Contact material suppliers to get the right screw design information to avoid improper melt mix or overheating that leads to material degradation.
Select machine with smaller shot size
The typical shot size should be between 20 and 80 percent of machine injection capacity. For temperature-sensitive materials, the range should be narrowed down, depending on the material. Moldflow Plastics Insight (MPI) products can help you select the right size machine for a specific mold. This will help avoid material remaining in the heated barrel for prolonged periods of time.
Optimize melt temperature
Reduce temperature to avoid material degradation from overheating, or increase it to limit residual stress.
Optimize back pressure, screw rotation speed, or injection speed
Balance shear heat against residual stress.
Vent appropriately
Use the material supplier recommended venting size.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding excessive part weight

Molding excessive part weight of molded defects

Solving excessive part weight problems

Definition
In most cases, excessive part weight is an undesirable molding characteristic. It increases production cost due to the long cycle time required for cooling the excess material and the additional cost of the material.

Causes
Overpacking
Unnecessarily thick wall section

Remedies
Avoid overpacking
Use thinner wall sections with ribs
Thicken only those wall sections that require extra material for structural stability and that cannot be strengthened using another method.
Design a part to be made by gas injection molding
Note: When attempting to balance flows by altering the thickness along particular flow paths, try to use flow deflectors rather than flow leaders to keep part weight down.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding fish eye & air trap

Molding fish eye & air trap of injection molded defects

Solving fish eye problems

Definition
Fish eyes are a surface defect that results from unmelted material being pushed with the melt stream into the cavity and appearing on the surface of a molded part.

Causes
Low melt temperature
If the melt temperature is too low to melt the material completely, the unmelted pellets will merge with the melt stream, marring the surface of the part.
Too much regrind
The shape and size of regrind is irregular compared with original material, and can trap more air and cause the material to blend unevenly.
Incompatible materials blended together
Low screw rotation speed
If the screw rotation speed and the back pressure setting are set too low, there might not be enough frictional heating to melt the material completely in the barrel before the injection.

Remedies
Reduce regrind material
Contact material suppliers to get the recommended levels of regrind to use.
Optimize melt temperature
Modify screw design
Contact material suppliers to get the right screw design information to avoid improper melt mix or overheating that leads to material degradation.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding jetting

Molding jetting of molded plastic defects

Solving jetting problems

Definition
Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the mold wall. The buckled, snake-like jetting stream causes contact points to form between the folds of melt in the jet, creating small-scale “welds”.
Jetting leads to part weakness, surface blemishes, and a multiplicity of internal defects.

Causes
Excessive ram speed
Poor gate position
Lack of melt contact with the mold allows jetting to occur.
Inadequate hot runner system design

Remedies
Optimize gate design and position
Direct the melt against a metal surface by repositioning the gate or use an overlap or a submarine gate.
Use a tab or fan gate to slow down the melt with a gradually divergent flow area. This reduces the melt shear stress and shear rate.
where:
a = overlapping gate
Optimize the ram speed profile
Use an optimized ram-speed profile so that melt-front velocity is initially slow when the melt passes through the gate, then increases once a dispersed flow is achieved.

Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design specification.

molding short shot

Molding short shot of Injecton molding defects

Solving molding short shot problems

Definition
A short shot is the incomplete filling of a mold cavity which results in the production of an incomplete part. If a part short shots, the plastic does not fill the cavity. The flow freezes off before all of the flow paths have filled.
To ensure the finished part is of good quality, the part must also be adequately packed with plastic. Therefore the question to ask is not only, ‘Will the part fill?’, but, ‘Can a good quality part be made?’

Causes
Flow restrictions
Due to channels freezing or inadequate runner design.

Hesitation and long or complex flow paths

Inadequate venting

Back pressure due to unvented air traps can cause a short shot.
Low melt and/or mold temperatures
Insufficient material entering the cavity
An undersized machine, low shot volume, or inadequate ram speed.
Machine defects
Including an empty hopper, blocked feed throat, or a worn non-return (check) valve that causes loss of pressure or volume leakage.

Remedies
Before you try one of the methods listed below, check all of the other results, so that you know the exact cause of the short shot.
Avoid hesitation
Eliminate air traps
If air traps do exist, they should be positioned in regions that can be easily vented or ejection pins added so that air can be removed.
Increase mold & melt temperature
This will decrease the viscosity of the melt, making it easier to flow through the part.
Increase ram speed
This can cause greater shear heating, which can decrease the viscosity of the melt, making it easier to flow through the part.
Change part geometry
Balance flow paths so they fill in an equal time and an equal pressure. You may need to thicken thin sections, or reduce the complexity of a flow path.
Change material
Select a less viscous material (higher melt flow rate). By choosing a material with a higher melt flow rate, less injection pressure will be required to fill the part.
Increase the maximum injection pressure for this part
Solving one problem can often introduce other problems to the injection molding process. Each option hence requires consideration of all relevant aspects of the mold design

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