SOLIDWORKS Plastics: Common Injection Molding Defects

SOLIDWORKS Plastics: Common Injection Molding Defects

Table of Contents


Injection molding is a complicated process and it can be difficult to predict
when defects will occur. Troubleshooting is often required to address issues
discovered after an initial trial run. Unfortunately, the mold has already
been manufactured by this point so adjustments are limited and costly.

It is much less problematic if issues can be sorted out during the design
stage. Fortunately, many defects can be avoided by following plastic part
design best practices and mold flow simulation software such as SOLIDWORKS
Plastics can provide additional insight. The table below includes a list of
common injection molding defects, how to detect them in SOLIDWORKS Plastics,
and suggested solutions.


Defect

Description

How to Detect in SOLIDWORKS Plastics

Suggested Solution
Burn Marks Burn marks appear when the air is trapped in the cavity and compressed
to combustion.
Select the Air Traps checkbox in the Flow Results. Burn marks might
occur at these locations if combustion occurs. A Venting Pressure plot
(SOLIDWORKS Plastics Professional) can provide a more detailed look at
the problem.
Add air vents to release the pressure. Decreasing the fill rate can give
the air more time to escape.
Flash Flash occurs when insufficient clamping force or mold imperfections
allow some molten plastic to escape the mold cavity.
The Flow Results Summary indicates the required clamping force to
successfully mold the part. If this value is greater than the clamping
force of your machine, flash can occur.
Flash is simple to avoid if your machine can provide sufficient clamping
force. However, the required clamping force can be reduced by reducing
the injection speed and pressure.
Jetting Excessive injection speed can cause the material to jet through the gate
instead of maintaining a cohesive flow front. The turbulence results in
a snake-like pattern on the surface of the part.
Ensure that the Volume of Fluid Algorithm is set to Direct (Fill
Settings -> Advanced -> Options). If jetting occurs, it will be
shown during an animation of the Fill Time plot. Isosurface Mode can be
helpful to see inside the part volume.
Increase the gate size or adjust the melt temperature to allow a proper
flow front to develop. If a small gate is necessary, locate the gate
near an appropriate obstruction such as a rib or boss.
Polymer Degradation High shear heating can degrade the material, lead to discoloration, and
significantly affect the physical properties of the part.
The Temperature Growth at End of Fill plot indicates the amount of
heating due to shear. The Shear Stress and Shear Rate plots can provide
additional insight.
Shear heating effects can be minimized by increasing part, runner, and
gate thicknesses, replacing sharp corners with rounded corners,
increasing mold and melt
temperatures, and decreasing the flow rate.
Short Shot A short shot occurs when the flow front freezes before the cavity has
filled and results in an incomplete part. Flow restrictions, long or
complex flow paths, inadequate venting, low melt or mold temperatures,
and low injection speed or pressure are some common causes.
The Results Adviser indicates if the maximum injection pressure is
exceeded and a short shot is detected.
Depending on the cause of the short shot, a complete fill may be
achieved by increasing part, runner, and gate thicknesses, changing the
gate locations, increasing the number of gate locations, increasing mold
and melt temperatures, or using a higher flowing material.
Sink Marks Sink marks are depressions that develop on the surface of the part as a
result of significant shrinkage. This is common at thicker part
sections, which require more time to cool.
The Sink Marks plot (SOLIDWORKS Plastics Standard) identifies sink mark
locations and estimates the amount of depression. A Warp analysis
(SOLIDWORKS Plastics Premium) can provide a more accurate Sink Mark
Profile plot.
Design parts with uniform wall thickness and use ribs and bosses that
are approximately 30% thinner. Place injection locations at thicker
sections to increase packing pressure in those areas.
Voids Voids form within the part when there is insufficient packing pressure.
Similar to sink marks, they typically occur at thicker areas, but the
shrinkage creates an internal pocket instead of a surface depression.
The Pressure at End of Fill (SOLIDWORKS Plastics Standard) and Pressure
at End of Packing (SOLIDWORKS Plastics Professional) plots can highlight
areas of low pressure.
Design parts with uniform wall thickness and use ribs and bosses that
are approximately 30% thinner. Place injection locations at thicker
sections to increase packing pressure in those areas.
Warping Part warpage occurs as a result of non-uniform shrinkage and molded-in
stress distributions. Common causes include uneven cooling and variable
packing.
The Volumetric Shrinkage at End of Fill (SOLIDWORKS Plastics Standard)
and Volumetric Shrinkage at End of Packing (SOLIDWORKS Plastics
Professional) plots can indicate areas expected to undergo high rates of
shrinkage and cause warpage. A full Warp analysis (SOLIDWORKS Plastics
Premium) can determine the deformed part shape.
Choose injection locations
that result in the uniform filling,
pressure, and temperature distributions. Aim to minimize volumetric
shrinkage,
but more importantly, try to achieve uniform volumetric shrinkage.
Weld Lines Weld lines develop where flow fronts merge together. They often result
in cosmetic defects and the strength of the part in the area is reduced.
Select the Weld Lines checkbox in the Flow Results. The position of the weld lines can be changed by moving the injection
locations or through a design change. Increased material temperature can
improve bonding between the flow fronts.

Don’t forget to come see us at the
Pacific Design & Manufacturing Expo
(Booth 3994) in Anaheim February 7-9, 2017. For more information, check out
our
YouTube channel
or contact us at
Hawk Ridge Systems
today. Thanks for reading!

Picture of Terence Woo

Terence Woo

Terence Woo is a product manager at our Hawk Ridge Systems office in Vancouver, British Columbia. He has extensive experience with SOLIDWORKS and SIMULIA analysis tools and helped hundreds of customers solve problems across many industries. When he isnโ€™t working on simulation models, he spends his free time skiing, golfing, rock climbing and playing hockey.
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