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Troubleshooting-Injection Molding


Flow marks, troubleshooting
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-li

ke waves near the edge or at the end of the flow.

Causes
? Material freezing near the gate. Low melt or mold temperature, and low ram speed can result in cold material entering the cavity. This can cause the partly solidified material to take on the form of the flow pattern.

Ripples caused by low temperature . 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. A restrictive runner system design can result in premature gate freeze-off. It can however, increase shear heating for better melt flow. Increase the mold and 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.

Average velocity result
The Average velocity result shows the average magnitude of velocity of the polymer inside the mold cavity over time.

The magnitude of the flow velocity is a straight average through the thickness (but only the melt is considered, not the frozen layer).
Note: The Average velocity result is an intermediate result, meaning its animation by default is through time and the scale by default is the minimum to maximum of the entire range of the result.

Using this result
The Average velocity result can be used to determine areas with a high flow rate. High velocity values for a particular model section could indicate a high flow rate, meaning there could be filling problems such as overpacking or flash. This could also mean that polymer flow is unbalanced, where polymer flows fast through one section and flows slowly through another section of the part.

Note: Right-clicking the result in the Study Tasks pane and selecting Properties allows you to plot velocity in either the x, y, or z directions, and to specify the velocity dart length. When the Average velocity result is created as an XY plot, the direction in which velocity is displayed is indicated by the legend values in the top right. For example, E3_X means x component of element 3. Combined with fill time information, velocity result plots can help you to determine gate locations, runner sizes, and part thickness to achieve a balanced mold and runner design.

Things to look for
? ? ? ? Overpacking. Hesitation. Racetrack Effect. Unbalanced Flow.

Jetting, troubleshooting
Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runners, or gates; or 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

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.

Overlapping gate to minimise jetting . 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.

Short shots, troubleshooting
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 the flow paths have completely 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 also, ""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 areas that can be easily vented or ejection pins added so that air can be removed. Increase mold and melt temperature. This will decrease the viscosity of the melt, making it easier for the plastic to flow through the part. Increase ram speed. This can cause greater shear heating, which decreases the viscosity of the melt, making it easier for the plastic to flow through the part. Change the 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. Use a different 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 specification.

Sink marks and voids, troubleshooting
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 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.

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.

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 Autodesk Moldflow Insight. Change the part geometry. Change the part design to minimize 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. Use a different 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.

Weld lines and meld lines, troubleshooting
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. Weld lines can be caused by material flowing around holes or inserts in the part, multiple injection gates or variable wall thickness where hesitation or "race tracking" can occur. If the different flow fronts have cooled before meeting, they don't interfuse well and can cause a weakness in the molded part. A line, notch and/or color change can appear.
Note: The Weld lines result in the Study Tasks pane may not show all weld lines if the model mesh is too coarse.

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

In the above diagram, the converging flow fronts (indicated by black 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.

Remedies
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. With a different fill time, flow fronts may meet at a different location and therefore the weld/meld line will move. ? Moving: Change the gate positions. Change the part thickness. ? Improving the quality: o Increase melt and mold temperature. This will allow the flow fronts to interfuse more. o Increase ram speed. o Optimize runner system design. Reduce runner dimensions and maintain the same flow rate to use shear heating to increase the melt temperature at the flow front. o o 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 requires consideration of all relevant aspects of the mold design specification.


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