The latest common plastic injection molding defects and solutions

Common problems in injection molding

Insufficient filling, short shot
Phenomenon-usually occurs at the end of flow or in the area where finished product is thinner
Possible causes-
1. Inappropriate processing injection conditions (injection speed is too low, injection pressure is too low)
2. Excessive pressure loss in gate system (including hot runner)
3. Gate or runner size is too small
4. Excessive pressure loss in shut-off nozzle system
5. Finished product thickness is too small or flow length ratio is too large
6. Filling flow hysteresis

Solution-
1. Use more appropriate injection conditions, increase material temperature, increase mold temperature, increase injection speed, and delay VP switching
2. Reduce gate and runner pressure loss, increase gate or runner size, and check hot runner size
3. Use open nozzles
4. Modify and increase finished product thickness or adjust pouring position
Common basis for judging plastic flow hysteresis and short shot problems
·Flow wavefront diagram
·Wavefront equipotential line
·Gate flow rate curve
·Temperature
·Viscosity
·Solidification layer ratio
·Shear rate
·Speed

Plastic injection molding flow front jet effect

1) First four fluid elements
2) F irst two elements begin to accelerate
3) Accelerated flow element is deformed into a long strip as a result of being stretched by wave front.
4) Direction of deformed fluid elements is consistent with wave front
5) Elements are thrown to mold wall for solidification
Temperature of melt affects viscosity of melt
At the same injection speed, viscosity decreases significantly when temperature increases by 25 degrees.

(1)Flowlength/Thickness
Literature

Amorphous

Analysis of actual case problems

Only by thinking about causes of flow hysteresis can we find right direction

Short Shots
Check and solve
Check cushion
Increase injection pressure
Increase injection speed
Raise melt temperature by
Raising cylinder temperature(s)
Increasing screw speed (unfilled only)
Raising back pressure (unfilled only)
Raise mold temperature
Increase booster time
Use lubricated material
Increase size of sprue/runners/gates
Check cavity vents for blockage
Causes of suture lines

Cavity
-Knit/weld lines

Weld lines

Microstructure of suture area

When observing welding line, it is necessary to pay attention to melt temperature and angle when two wavefronts combine.
Joining line has a darker color
Phenomenon-There is a darker color near joining line on the surface of finished product or near slow or fast flow wavefront, mostly occurring in bright or dark finished products such as white, blue, green, orange, etc.
Possible causes-
1. Improper flow near joining line causes color separation or light refraction effect
2. Overheating of plastic causes discoloration near joining line
3. Poor exhaust causes oxidation degradation

Solution-
1. Adjust injection speed to change wavefront speed
2. Reduce thermal history of plastic
3. Improve exhaust design and change joining line formation position when necessary
Effect of suture bond strength (Nylon66/33%GF)

Item	Unit	Weld line strength	Normal strength
Tensile strength		830	1800
Elongation		1.9	5
Flexural strength	kg/cm2	1310	2600
Flexural mold	kg/cm2	68000	85000
Notch impact strength	kg-cm/cm2	8	65

Note: 1/8" thick specimen
Improper joint line strength
Phenomenon-Finished product is prone to damage near joint line
Possible causes-
1. Poor mold exhaust design
2. Poor melt flow properties
3. Too low injection speed
4. Finished product thickness is too thin and melt is not easy to fill
5. Plastic flow path is too long

Solution-
1. Improve mold exhaust effect
2. Increase injection speed
3. Increase material temperature and mold temperature to improve melt flow characteristics
4. Increase finished product thickness design
5. Redesign gate position to shorten melt flow length ratio
Common bases for judging suture line problems
·Flow wavefront diagram
·Wavefront equipotential lines
·Suture line position
·Suture line angle
·Temperature
·Particle tracking

plastic injection molding defects and solutions

Analysis of actual case problems

Since thickness distribution of lower side of head end area is higher than that of upper side area, plastic emerges from lower side and forms a V-shaped junction at the end.
Seam line
1) Due to product geometry, seam line falls at intersection of circular axis.
2) Based on current filling seconds and melt processing temperature settings, temperature field of seam wavefront bonding area is observed to be about 270℃, which is about 10℃ lower than temperature before wavefront bonding.

Due to difference in product thickness design near gate, flow in area A is stronger than that in area B. In addition, geometric structure of product causes joint line to fall in the area shown in right figure.

Weld lines
Increase injection pressure
Increase injection hold time
Increase injection speed
Raise mold temperature
Raise material temperature
Vent cavity in weld area
Provide an overflow welladiacent to weld area
Change gate location to after flow pattern
Encapsulation, scorching, and blackening
Phenomenon-Charred streaks, burn marks, periodic discolouration, dark spots are formed at local position of finished product flow end
Possible causes-
1. Exhaust effect of finished product mold at a local position is poor
2. Gas encapsulation causes compressed gas to generate high temperature and form scorching

Solution-
1. Proper mold design requires good exhaust design, especially at the end of flow or glue bonding position
2. Proper mold design, by adjusting cross-sectional thickness of finished product, flow wave front can be filled smoothly
3. Proper exhaust groove depth design
4. Reduce clamping force
5. Use a cavity vacuum device for thin products
Common basis for judging encapsulation and burning problems
·Flow wavefront diagram
·Wavefront equipotential lines
·Encapsulation position
·Temperature curve
·Pressure curve

Air Trap
How does it form?
- Converging flow fronts surround and trap an air bubble
Benefit of air trap prediction
- Air trap prediction to prevent potential surface blemish such as burn mark on surface
- Provide venting location suggestions

Air Trap (cont'd)
Incomplete filling and packing
Often cause surface blemish in final part

Thermal Decomposition Temperature Measurement Thermal Gravimetric Analyzer (TGA)

Burn marks
Decrease injection speed
Decrease booster time
Shearing heating
Improve venting in mold cavity
Alter position/increase gate size
Jetting phenomenon (jetting)
Phenomenon-usually, you can see melt jetting patterns similar to snake patterns on the surface of finished product
Usually occurs near gate or near compressed area
Usually occurs in non-crystalline materials due to their high viscosity and low elongation
Possible causes-
1. Poor design of gate or finished product thickness, unable to form laminar flow
2. No blocking wall at gate position, plastic directly enters mold cavity
3. Gate size is too small or thin-walled area size is too small
4. Too fast injection speed

Solution-
1. Redesign gate size (enlarge) and position (add blocking)
2. Increase gate cross-sectional size
3. Redesign finished product thickness change
4. Adjust injection speed conditions to first enter mold cavity slowly and then speed up filling
Jet marks formed by jets: accordion effect

Mold core offset problem

Common problems
Abnormal warpage and precision control
Impact of embedded parts on products and processes
Uneven residual stress problems
Insert softening and ink flushing problems
Others: embedded part offset,….

Common bases for judging mold core deviation
·Flow wavefront diagram
·Wavefront equipotential lines
·Pressure distribution
·Pressure curve
·Stress distribution
·Mold core deviation

Laparoscopic electrocautery knife
-During on-site forming, it was found that if initial melt enters product mold cavity too quickly, mold is prone to break fixed terminal and deflect downward, resulting in overflow, which will also affect functionality of saline delivery. At the same time, due to offset of fixed terminal, bottom Tip is also prone to upward offset and exposure. On the contrary, when filling speed is slowed down, problem of terminal offset is improved.

Due to geometric design of product, plastic fills SN6 area fastest. It can also be seen from figure that original design SN6 area acts on terminal fastest, force is greater than that of SN21 and SN23 areas, which may cause terminal to deviate under action of external force.
Cause analysis of product quality problems

Cause analysis: This product is a complex rubber pipe for a car. Inner diameter of pipe is 4mm, outer diameter is 8mm, wall thickness tolerance is 0.1mm, and length is 160mm. Mold uses eight cavities, as shown in figure above. Since pipe has two turns and an irregular structure, design of its pouring system is difficult. Original pouring system has a product thickness deviation of up to 1mm in actual production.
Voids
Phenomenon-Circular or elongated bubbles are usually found inside transparent or semi-transparent finished products

Possible causes-
1. Volume shrinkage caused by cooling
2. Insufficient pressure holding efficiency
3. Poor injection gate position
4. Poor finished product thickness design
Solution-
1. Increase pressure holding and holding time, check injection buffer
2. Increase nozzle outlet size or gate cross-section size
3. Design gate position in thick area
4. Modify finished product thickness design to avoid drastic changes
Volume shrinkage of injection molding
In injection molding processes, polymers are cooled from high temperature to normal temperature and solidified inside cavity.
- higher packing pressure and a longer packing time could lessen shrinkage of molded product.
1: Start to fill
2: Completion of cavity filling
3: Cavity pressure arrives at maximum
4: Packing / holding stage begin.
5: Completion of packing/holding stage.
6: Cavity pressure drops to normal
7: Mold tooling and plastic parts demolding.
8:Plastic parts achieve thermal equilibrium.

Common basis for judging void and bubble problems
·Temperature
·Pressure
·Volume shrinkage
·Solidification layer ratio

Sinks/oids
Increase injection pressure
Increase injection hold time
Use booster and maximum injection speed (Sinks)
Raise mold temperature (Voids)
Lower mold temperature (Sinks)
Decrease injection speed (Volds)
Decrease cushion
Increase size of sprue/runner/gates
Relocate gates nearer heavysections
Influence of product structure on shrinkage

(2)Sink Mark
Rib width 60%; 80% of Max. filling pressure

Discussion on causes of stress marks (gloss difference)

Stress marks:
It means that after product is formed, different gloss marks are easily formed on the surface of local area of product.
·Area where gloss marks occur mostly appears on female mold side, especially when female mold side is a plane and male mold side has different thickness designs, there is no proper transition.
·Area where gloss marks occur is mostly along above-mentioned thickness change intersection area, they are all reflected on female mold plane, and black parts are the most obvious.
·This problem is most common in PC among 3C products, but PC+ABS can also occur under high shear conditions
Common basis for judging stress mark problems
· Shear rate
· Shear stress
· Speed
· Temperature
· Volume shrinkage
· Density
· Residual stress

Analysis of actual case problems
There are many different opinions on causes of stress marks, but reasons can be classified into two categories:
·Flow stress theory: When plastic flows through thick-thin junction area, molecular chain is rapidly pulled by shear rate, causing stress concentration generated by flow at thick-thin junction, thus affecting surface gloss.

·Shrinkage stress theory: At thick-thin junction, thickness difference at both ends is large. Therefore, when plastic gradually cools and begins to shrink, molecules at junction are affected by different shrinkage effects at both ends, causing internal stress when internal shrinkage occurs, thus affecting surface gloss.

·Geometric variability will affect speed of plastic flow, then affect shear rate of molecules, resulting in differences in shear stress formed in different areas.
·From analysis results, it can be seen that when molecules enter small section from large section, flow speed and shear rate increase rapidly due to Shearflow effect of plastic, and speed at the junction changes rapidly, which easily leads to formation of high shear stress. At the same time, when stress is formed, it continues until plastic gradually cools down and cannot be relaxed and released, and junction is prone to residual stress, which in turn affects appearance gloss.
·When molecules enter large section from small section, plastic is affected by Shear free flow effect, flow rate and shear rate slow down, so that shear stress formed at junction is lower. At the same time, after molecules enter large section, it also helps molecules to continue relaxation and release time.
·Based on above analysis results, when plastic molecules enter small section from large section, junction is more likely to form stress marks that affect appearance.

·Geometric variability will affect speed of plastic flow, then affect shear rate of molecules, resulting in differences in shear stress formed in different areas.
·From analysis results, it can be seen that when molecule enters B section from A section, if thickness difference between the two is large, shear rate will increase rapidly due to rapid increase in plastic flow speed, which will lead to formation of high shear stress. If stress cannot be released, it will cause formation of residual stress.
·As far as analysis results are concerned, when thickness enters 1mm area from 3mm area, plastic flow speed increases rapidly from 150cm/s to 390cm/s, and speed difference is nearly 2.5 times. When thickness enters 2mm area from 3mm area, plastic flow speed increases from 150cm/s to 250cm/s, and speed difference is about 1.5 times. It can be proved that thickness change ratio of thick and thin junction area of product will affect size of molecular shear stress.

·When molecules enter small cross section from large cross section, when thickness difference between the two is relatively large, it is easy to increase plastic flow rate sharply and increase shear rate of molecules, thereby forming high shear stress.
·When above thickness ratio cannot be changed, adding chamfers or R angles to junction area can help alleviate rapid increase in flow rate and buffer generation of high shear stress.
·When buffer area at junction is insufficient, for example, chamfer and R angle are too small or transition zone distance is insufficient, higher shear stress may still be formed at junction.
·Based on above analysis results, appropriate buffer area design at junction can help buffer plastic flow rate and make stress distribution more continuous and uniform.

· When thickness distribution of product is different, in addition to affecting fluidity of plastic due to flow resistance and viscosity, thickness difference will also cause plastic to shrink when it cools, and difference in cooling rate will lead to different shrinkage.
· When thickness difference of junction area is high, molecules in junction area are easily pulled again by slow shrinkage of thick side area after orientation, forming internal stress. Therefore, when molecules cool and shrink uniformly, it helps to form shrinkage internal stress.

·When thickness difference of product cannot be changed, it is easy to cause differences in product density and other properties and generation of molecular internal stress due to different shrinkage between different thicknesses, which will cause junction area to form traces that affect appearance gloss after product is formed.
·By adding chamfers or R angles to junction area, temperature distribution change can be buffered and continuous, which helps molecules to shrink more evenly and reduce formation of internal stress.
·When buffer area at junction is insufficient, such as chamfer and R angle are too small or transition zone distance is insufficient, there may still be a high volume shrinkage fault difference at the junction.
·Based on above analysis results, appropriate buffer area design at junction helps to continue uniform shrinkage change of molecules.

Flow mark application

What is residual stress?
·Residual stress - After finished plastic material has been manufactured or formed, object still maintains stress in absence of external forces or temperature gradients
·Formation of flow stress - When molten plastic is quickly filled under high pressure, high shearing effect causes molecular chains to be highly oriented, which is main cause of residual stress
·Formation of thermal stress - When finished product temperature is quickly cooled to below Tg of plastic, stress between molecular chains caused by cooling shrinkage cannot be completely released
·Residual stress formation factors - Orientation of molecular chains in internal structure
Differences in cooling rates of temperature gradients inside finished product
Interaction of molecular chains during injection extrusion
Non-uniform directional crystallization
Stress distribution state of injection molded parts thickness section

Because tensile stress area is much larger than compressive stress area, plastic parts tend to shrink in all directions after cooling and solidification.
Q1. Discussion on reasons for product's stress fracture 2. (Volume shrinkage)
After pressure holding is completed, temperature of product neck is still high due to geometric factors of product. At this time, gate has solidified and can no longer provide effective pressure holding. Product has a higher volume shrinkage of more than 15% in neck area. In absence of sufficient melt replenishment, vacuum bubbles and depressions are easily generated inside product.

01. Discussion on reasons for product fracture under stress 3. (Residual stress Von Mises)
Figure shows residual stress distribution inside product after three stages of filling, pressure holding and cooling. Except for end of filling, Von Mises stress value of this area is less than 0.5MPa, and stress distribution of the overall thickness section is also uniform. Compared with other factors, influence of neck stress on fracture is relatively low.

Blue arrow is gate

Plastic used: PBT+GF30%
Causes:
*Joint line position
*PBT material cools too fast
*Local thickness is too thin
*Shrinkage rate difference between aluminum and plastic parts is too large
Problem point: Red arrow is broken at local deep hole position (fixed area, high defect rate)
Analysis of actual case problems
Volume shrinkage distribution shows percentage of volume change when cooling down to normal temperature and pressure after filling or holding at high temperature and high pressure
Positive values represent volume shrinkage and negative values represent volume expansion due to excessive holding. Uneven volume shrinkage distribution can cause warping of plastic parts and mold release deformation.

From simulation, it was found that volume shrinkage rate in original design was about 7%, and volume shrinkage rate in this area was about 9.3~11% higher than that in other areas.
Analysis of actual case problems

Plastic injection molding molecular orientation degree

Plastic injection molding flow front jet effect

1. First four fluid elements
2. First two elements begin to accelerate.
3. Accelerated flow element is deformed into a long strip as a result of being stretched by wave front.
4. Direction of deformed fluid elements is consistent with wave front.
5. Elements are thrown to the mold wall and solidified.
Flow mark application

Finished product is not strong enough and is easily damaged
Phenomenon-Finished product is easily damaged when subjected to stress
Possible causes-
1. Excessive internal residual stress or external mechanical stress
2. Abnormal drying or overheating causes thermal degradation of plastic
3. Improper position of glue line
4. Poor ejection design or ejection conditions

Solution-
1. Stress analysis to check whether design is suitable for mechanical external force it is subjected to.
2. Residual stress can be reduced by increasing material temperature and mold temperature, reducing pressure in mold
3. Check plastic material temperature, residence time and drying conditions in material tube
4. Move glue line away from stressed area
5. Avoid damaging finished product during ejection
Analysis of reasons for cracking after product use

Reason	Improvement methods
Reason	Product design	Material selection	Molding processing
Material cracking
Structural strength
Residual stress
Weld line
Fiber orientation
Stress concentration
Insert parts
Shrinkage holes
Ejection

Gud Mould Industry Co., Ltd

The latest common plastic injection molding defects and solutions

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