Product Defect Analysis

It refers to a phenomenon in which resin cannot be supplied to barrel or supply amount is unstable.
There are following modes:
(1) No pre-plasticization at all
(2) Unstable pre-plasticization time
(3) Sometimes insufficient filling occurs
Amount of resin supplied to barrel during plasticization is unstable:
(2) Principle of plasticization
The key to plasticization:
It is difficult for barrel surface and pellets to slide, while screw surface and pellets can slide easily.

(2-1) Improper screw speed
Generally, the higher screw speed, the stronger conveying force of pellets. Therefore, if screw speed is too slow, conveying force of pellets will be weakened, resulting in unstable pellet supply and poor metering. On the contrary, if speed is too fast, pellets will move together with screw and will also not be able to move forward.
(2-2) Too high or too low back pressure
Back pressure has effect of suppressing gas from being entrained in resin and stabilizing amount of injected resin, but it also has effect of weakening conveying force. Therefore, if back pressure is too high, metering will become unstable.
(2-3) Improper barrel setting temperature
Barrel setting temperature affects temperature of pellets in barrel. In other words, since surface state and rigidity of pellets change, it also affects metering. In particular, setting temperature below hopper and its adjacent areas will have a great impact on metering.
Generally speaking, if temperature setting from nozzle to the bottom of hopper is from high to low, and setting temperature below hopper is low, metering will remain stable. This is because when temperature rises, surface of pellets will melt, and friction between pellets will increase, causing them to intertwine with each other or stick to screw or cylinder. Before material is in a viscous flow state, the higher temperature, the greater friction between plastic and metal
(2-4) Use of recycled materials
Recycled materials are usually irregular in shape, so compared with ordinary pellets, friction between pellets is likely to increase, which can easily cause poor metering. In addition, powder will stick to screw after mixing, which will weaken conveying force.
(2-5) Inherent problems of grades
In sliding grades, screw rotation force cannot be well converted into forward conveying force because sliding between metal is too good, which can easily cause poor metering. In addition, in impact-resistant grades (as well as PA, LCP, etc.), friction between pellets is likely to increase, which can also easily cause poor metering.
(2-6) Check valve wear or screw wear
Causes melt reflux, making plasticization unstable
(2-7) Material is not dried

(3-1) Adjust screw speed
First, adjust screw speed. If you want to regularly observe whether there is any poor metering, measure metering time. Perform 50 to 100 consecutive moldings and change speed in several stages. Make a judgment based on whether metering time suddenly becomes longer. If adjusting screw speed alone cannot solve problem, you can change back pressure or barrel temperature at the same time.
(3-2) Reduce back pressure
The lower back pressure, the stronger conveying force of pellets and the more stable metering. However, if back pressure is too low, gas will be involved more and resin amount will be unstable.
(3-3) Reduce temperature below barrel hopper
Specifically, temperature below hopper should be gradually reduced little by little. Excessive reduction will make it difficult for pellets to melt and may even clog barrel, so it should be adjusted gradually (about 10℃ each time).
(3-4) Recycled materials
Try to make recycled pellets and initial pellets same size. At the same time, remove as much powder as possible.
(3-5) Problems inherent in grades
Sliding grades have property of being easy to slide due to addition of oil or lubricant. If problem cannot be solved by adjusting screw speed, back pressure and barrel temperature at the same time, it is necessary to consider changing grade or screw design.
In impact resistance grade, adhesion between pellets is the biggest obstacle to metering. In this case, it is particularly necessary to reduce barrel temperature below hopper or add an anti-slip agent.

Gate marking refers to small flow marks near gate. As shown in figure.

 

 

(1) In terms of process:
a. Low mold temperature
b. Fast injection speed
(2) In terms of mold:
a. Small gate size
b. Impact gate paste
(3) In terms of material: Low material fluidity
(3) Countermeasures for gate markings:
(1) Increase mold temperature
(2) Reduce injection speed (when passing through gate)
(3) Enlarge gate or change its position
(4) Use a grade with better fluidity or add lubricating additives
To reduce speed when passing through gate, it is best to use multi-stage injection.

 

Bubbles refer to a phenomenon in which surface of a molded product bulges or gas is trapped inside.

 

(2-1) Air inclusion
1. Air inclusion in barrel:
* Screw speed is too fast
* Back pressure is too low
* Too much back suction
2. Air inclusion in mold cavity:
* Injection speed is too fast
* Gate is too small
* Competitive flow
* Main channel slope is too large
(2-2) Resin degradation
1. Barrel temperature is too high
2. Barrel residence time is too long
3. Screw diameter is too large

(2-3) Material is not dried

 

(3) Countermeasures for bubbles
(3-1) Reduce air entrapment
1. Reduce air entrapment in barrel:
- Reduce screw speed
- Increase back pressure
- Do not set too much backsucking volume

1. Reduce air entrapment in cavity during filling:

- Reduce injection speed
- Adjust gate position, size and shape.
- Adjust main channel demolding slope
Key is to grasp flow pattern through filling test, then establish corresponding countermeasures on this basis.
(3-2) Inhibit material degradation
Reduce barrel temperature (within recommended operating temperature range, do not reduce excessively)
Choose a reasonable screw diameter
Thoroughly clean barrel and various auxiliary machines to prevent mixing
(3-3) Improve exhaust conditions
(3-4) Use high-viscosity materials or add additives
High-viscosity materials are generally not prone to voids, so trying to use this material is also a method.
Additives such as glass beads can also be added.

(1) What are voids (appearance)
It refers to a phenomenon in which voids are generated inside a molded product.
Voids generally occur at the thickest part of product wall!
There are two main causes of voids: one is mixing of a large amount of gas, and the other is shrinkage of resin at thick wall. The former is called "gas voids" and the latter is called "vacuum voids" to distinguish them.

 

Figure 1. Voids appearing at thick walls

 

Figure 2. Difference between gas voids and vacuum voids
(2) Causes of vacuum voids (insufficient holding pressure)
Main reasons for insufficient effective holding pressure are as follows:
Process:
(1) Too early pressure change
(2) Low holding pressure setting value
(3) Short holding pressure time
Mold:
(1) Small gate size
(2) Thin runner
Equipment: Wear of screw check valve set
Material: Too much shrinkage of material
When mold temperature is too high, it is easy to form dents, and when mold temperature is too low, it is easy to form voids

 

(3) Countermeasures for vacuum voids
(3-1) Add resin
In process: confirm turning point
Increase holding pressure; extend holding time
In mold: increase gate size
Increase diameter of main channel and branch channel
Gate should be located as close as possible to part where void occurs (thicker part)
In equipment: Check whether screw and three-piece set of check valve are damaged
(3-2) Slow down surface curing
If it is a vacuum void, increasing mold temperature can reduce void. But be aware that this method may induce dents
(3-3) Change material grade or add additives

(1) What are dents (appearance)
“Dents” refer to a phenomenon in which a pit is formed due to shrinkage of resin.
When resin is cooled and solidified, its volume decreases significantly. This is why dents are formed. Since shrinkage ratio (shrinkage rate) is roughly fixed and the thicker product, the greater shrinkage, dents are generally formed in the thicker part of molded product.

 

Figure 1. Dents on test specimen
(2) Causes of sink marks
(2-1) Low effective holding pressure: insufficient resin filling
During holding process, amount of resin that shrinks due to cooling and solidification is filled by holding pressure. If for some reason effective holding pressure is low = resin is difficult to fill, sink marks are likely to form when mold temperature is high, and voids are likely to form when mold temperature is low.

 

Figure 2. Sink marks when holding pressure is low
Main reasons for decrease in effective holding pressure are as follows:
(1) Holding pressure setting value is too low (or gate is unbalanced)
(2) Holding time is too short
(3) Gate size is too small
(4) Runner is too thin
(5) Check valve fails
Gate position is important, so it must be set as close to thick wall as possible.
(2-2) Slow cooling: Too much shrinkage
The thicker thickness, the greater shrinkage during cooling; at the same time, the slower cooling, the greater shrinkage. Therefore, the higher mold temperature, the larger dent.

 

(2-3) Insufficient cooling time
Insufficient cooling time causes insufficient rigidity of solidified layer, resulting in dents
(3) Countermeasures against dents
(3-1) Try to increase holding pressure
It can be considered that applying holding pressure = increasing filling amount of resin. In order to make it easier to apply holding pressure, an effective treatment method can also be adopted, which is to expand size of main channel, runner and gate, and move gate to vicinity of dent.
(3-2) Reduce mold temperature
If mold has a high temperature value, try lowering it gradually. Dents can be reduced by reducing amount of shrinkage.
(3-3)Increase cooling time
(3-4) Check condition of check valve
a. Is it worn? b. Is there any impurity that prevents check valve from closing?
Remove check valve from front end of screw and check each contact surface. If there is burnt melt, remove it with a copper brush; do not burn it with a blowtorch, as this will soften valve metal and cause it to wear faster.
(3-5)Reduce thickness
If possible, it is recommended to reduce thickness as much as possible. If it is a reinforcement, it should reach about 1/3 of thickness of base
(3-6) Change materials or add additives
High-viscosity materials are generally not prone to voids, so trying to use this material is also a method.
Additives such as glass beads can also be added.

Black spots refer to phenomenon that black spots or streaks appear in molded product.
For some reason, components are estimated to be charred resin or carbonized resin, or dirt.

(2-1) Resin decomposition

 

Figure 2. Gray-black streaks
Material retention
Retention time is too long (material cylinder is too large)
Temperature is too high
Also, pay attention to matching surfaces between flange and barrel, between nozzle and flange. Are there any steps, unevenness, or damage that may cause resin retention?

 

Figure 3. Areas where resin is easily retained
(2-2) Insufficient cleaning
Situation where previously used resin remains in molding machine due to insufficient cleaning is also a cause of black spots. As mentioned in previous item, due to presence of check rings and thread grooves, where resin is easily retained or worn parts of screw
(2-3) Foreign matter mixing (contamination)
Other easily decomposable materials may be mixed in. Check material conveying process and condition of recycled materials
(2-4) Excessive gas
If metering process starts too early, air wrapped in particles in screw feeding area does not overflow feeding port, and air will be squeezed into melt. However, pressure in feeding area is too low to move air to the back. Air squeezed into melt in barrel will cause gray-black spots in product.

 

Figure 3. Causes of black spots

(3-1) First, thoroughly clean until black spots no longer appear
Common cleaning methods:
(1) High-viscosity PE.PP
(2) Chemical cleaning agent
(3) Remove screw and brush with a copper brush
For optical products or those with higher requirements, method of one material per barrel can be used
(3-2) Try to lower resin temperature
It is recommended to measure actual temperature with a resin thermometer. In particular, parts that are prone to stagnation, such as check rings, are most likely to cause black spots, so pay special attention to temperature near them.
(3-3) Shorten stagnation time
Use a molding machine that is suitable for mold size.
(3-4) Whether there is contamination
Occasional mixing of other resins or metals may also cause black spots.
(1) Re-clean thoroughly
(2) Check whether return material is clean
(3) Check whether auxiliary machine and material tube (material conveying process) are clean
(3-5) Expel excess gas

“Slip marks” refer to a phenomenon in which a once-cured surface layer succumbs to subsequent pressure and moves.
Once-cured surface slides laterally under action of holding pressure or injection pressure and is squeezed against mold again, so that pattern appears on the surface of molded product.

 

Figure 1. Slip marks on a sample for appearance evaluation

 

Figure 2. Slip marks near 120 flat plate gate

(2-1) Mold design
Slip marks are basically caused by improper product shape. Although there are also reasons for molding conditions, impact is not significant. There are three shapes that are prone to slip marks:
(1) Corners without rounded transitions
(2) Slightly raised ejector pins.
(3) Sharp edges
When resin solidified layer slides in these parts, marks are very conspicuous, so slip marks are likely to occur.

 

In addition, changing gate position and number of points will change resin flow direction and resin pressure, so occurrence of slip marks will also change.
(2-2) Contains lubricant
At a certain sliding level, a large amount of oil is contained to ensure sliding properties, but this often makes adhesion between melt layers insufficient, which makes it easy to produce sliding marks.
(2-3) Influence of injection speed
When injection speed is low, solidification will accelerate, so pressure will increase accordingly, and sometimes sliding marks will appear under action of this force. On the contrary, even if injection speed is too fast, solidified layer will become easier to move under action of this force.
(2-4) Influence of mold temperature
When mold temperature is low, resin pressure in mold cavity will increase, and sometimes sliding marks will appear under action of this force. On the contrary, when mold temperature is too high, solidified layer will become soft and sometimes easier to move.
& Pay attention to whether gate is balanced

(3-1) Adjust injection speed
Adjust injection speed up and down on existing basis. It is recommended to use multi-stage injection to eliminate slip marks. If this can solve problem, then fine. If not, adjust mold temperature.
(3-2) Adjust mold temperature
Adjust mold temperature up and down on existing basis to eliminate slip marks. If this does not solve problem, then adjust mold shape.
(3-3) Change mold shape
Change shape of part with problems.
Rounded corner transition
Adjust ejector pin height correctly
Reduce sharp edges
Also, changing gate position is effective.
(3-4) Change material grade
When problem cannot be solved and mold shape cannot be changed, changing to a different grade of material is also a countermeasure.

 

Figure 4. Shape countermeasure example

Silver streaks refer to phenomenon that white spots or silver streaks appear on the surface of a molded product.

 

(2-1) Gas generated by resin decomposition
Resins are chemical substances and therefore gradually decompose as temperature increases. The higher resin temperature or the longer it stays, the more it decomposes and the more likely silver streaks will appear.
(2-2) Air inclusion
(1) If screw speed is too fast and back pressure is too low, amount of air involved in plasticizing resin will increase. As a result, streaky bubbles will appear on the surface of molded product and silver streaks will easily form.
(2) Runner silver streaks: Taper of main runner is too large, exceeding 10 degrees. Generally, it is 4-6 degrees.
(2-3) Moisture (opening direction of water streak is along material flow direction. In places where product is not completely filled, front end of fluid is very rough)
(1) Insufficient drying of material
(2) Water leakage in mold cavity
(2-4) Small exhaust port

 

Figure 4. Dried ABS and undried ABS
When gas is not completely exhausted, bubbles will remain on the surface of molded product, making silver streaks more likely to appear.
(2-5) Mixing of different materials
If material produced last time or material used to clean barrel is mixed in due to insufficient cleaning, and temperature of resin is low, gas may be generated and silver streaks may be induced.
In addition, materials with dyeing aid white oil, lubricant silicone oil, plasticizer dibutyl ester, stabilizer, and antistatic agent are prone to surface peeling and silver streaks.

(3-1) Check metering status
First, check metering status. If screw speed is too fast or back pressure is insufficient, silver streaks are likely to appear. Please adjust gradually and observe whether there are any changes. If there are any, please adjust to optimal value.
(3-2) Check resin temperature: resin decomposition
Each resin has its recommended operating temperature (indicated on product bag and product catalog). Check whether actual temperature is within range. If it is out of range, it must be adjusted to within range. At the same time, check whether screw size is reasonable
(3-3) Strengthen drying; at the same time, check whether mold is leaking
Confirm drying temperature and drying time
(3-4) Whether there are contaminants
(1) Re-clean. Because retained resin is constantly decomposing.
(2) Check whether return material is clean
(3) Check whether auxiliary machine and material pipe are clean
(3-5) Check exhaust port
Occasionally, silver streaks will occur due to poor exhaust, so check whether exhaust port is large enough.
(3-6) Check taper of main channel

(1) What is color difference (appearance)
Color unevenness means that color of product surface is different. It can appear near and far from material head, and occasionally in material flow area with sharp edges.

 

(2) Causes
-Uneven pigment mixing
-Material degradation
Reasons related to process parameters and improvement measures:
1. Material is not evenly mixed. Reduce screw speed; increase barrel temperature and increase screw back pressure
2. Melt temperature is too low. Increase barrel temperature and increase screw back pressure
3. Screw back pressure is too low. Increase screw back pressure
4. Screw speed is too high. Reduce screw speed
Reasons related to design and improvement measures are shown in the table below:
1. Screw stroke is too long. Use a barrel with a larger diameter or a larger aspect ratio
2. Melt stays in the barrel for a short time. Use a barrel with a larger diameter or a larger aspect ratio
3. Screw L:D is too low. Use a barrel with a larger aspect ratio
4. Screw compression ratio is low. Use a high compression ratio screw
5. There is no shearing section and mixing section. Provide a shearing section and (or) mixing section

Fusing occurs where resins merge. As shown in figure below, there will be welding in part with openings.

 

(2-1) Low resin temperature
When two resin flows meet, they fuse. At this time, the lower temperature of the two, the more obvious fusion. Since two resin flows at fusion point do not mix with each other (because they are semi-solidified while moving forward in jet), if temperature is low, surface layer will become thicker, texture will be obvious, and strength will also decrease. This is because adhesion between the two becomes weaker. On the contrary, if temperature of two resin flows is high, adhesion will be stronger and appearance will become less obvious. Conditions for lowering resin temperature are:
Low mold temperature
Low barrel (especially nozzle) setting temperature
Slow injection speed
Low material fluidity

 

(2-2) Low pressure
At fusion point, two molten resins are squeezed, and adhesion here depends on pressure applied there. The lower holding pressure, the more obvious fusion and the lower strength. As solidification proceeds, pressure transmission will become more difficult. In addition, if gate runner size becomes smaller and gate position becomes worse, appearance and strength of fusion will deteriorate.

 

(2-3) Weak exhaust at exhaust port
Fusion is confluence point of resin and may also be end of flow. At this time, if an exhaust port is not set well at this position to exhaust gas, appearance and strength of fusion will deteriorate.

 

Figure 5. Exhaust of fusion part should be sufficient
(1) Quality of fusion line of plastics without reinforcement materials is significantly higher than that of plastics with reinforcement materials.
(2) Quality of fusion line area is closely related to type and content of fillers and reinforcement materials. Additives such as processing aids and flame retardants have an adverse effect on quality of fusion line.
(3) Containing fiber reinforcement materials, arrangement direction of fibers in fusion line area is perpendicular to flow direction. This will significantly reduce mechanical properties of part at this point

 

(3-1) Increase resin temperature of welded part
Try to gradually increase mold temperature and barrel temperature. This is expected to achieve maximum effect, and holding pressure effect will also be improved. In the sense that welded part is formed before resin temperature drops, it is also effective to increase injection speed and form welded part quickly.
(3-2) Try to increase effective holding pressure
Although it is possible to simply increase holding pressure setting, it is recommended to use following conditions that make it easier to apply holding pressure:
1. Increase resin temperature 2. Increase mold temperature 3. Increase injection speed or multi-stage injection 4. Expand gate 1.5. Adjust thickness 6. Use materials with good fluidity 7. Check wear of check valve and screw
(3-3) Check exhaust port
Even if weld is at the end of flow, it is necessary to check exhaust port. At the same time, reduce clamping force to minimum value.
Check whether thickness and size of exhaust port ensures smooth exhaust and whether it is contaminated by mold scale. If exhaust is not sufficient, it will cause gas burning and cause other failures.
(3-4) Check gate and runner
If gate and runner are not big enough, even if temperature and pressure are increased, it may not be effective. In this case, if gate and runner are increased in size, resin will flow more smoothly, and it will be easier to apply pressure, which will improve fusion.
In addition, changing gate position and increasing number of points is also an effective method. By changing gate position, location where fusion line occurs can be controlled
(3-5) Add cold well

 

Note: Due to influence of mold structure, it is impossible to completely eliminate welding mark, so when debugging, do not limit yourself to removing welding mark, but control adverse phenomena caused by welding mark to minimum, which is more important.

(1) Causes of deformation:
a. Uneven temperature
b. Uneven pressure
c. Molecular orientation
(2) Countermeasures for deformation
a. Gate design
(1) Unify flow direction of resin
Figure 4-1-1 Gate design for a slender flat plate

 

(2) Increase gate size (to reduce pressure loss and molecular orientation)
If there is no problem with quality, gate size is sometimes set smaller due to factors such as shortening molding cycle and trimming gate. However, too small a gate size is not good for deformation.
Number or position of gate is inappropriate, which will make flow length too long and flow resistance too large. Corresponding injection pressure must also be increased. Plastic molecules are pulled and squeezed, and mechanical stress is forced into them. Residual stress is large and it is easy to warp.
Pressure near gate is high, and melt volume shrinkage is small; pressure at last filling is low, and volume shrinkage is large; when flow length is too long, upstream and downstream melt volume shrinkage differences are large, residual stress is large, and it is easy to warp.
Decision of gate position should follow principle of balanced filling, that is, time for each melt wave front to reach end of cavity and form a weld mark is basically same, filling is thick first and then thin, flat first and then curved.
b. Mold temperature adjustment
1. Make cooling balanced

 

2. Strengthen cooling of punch and wall thickness

 

3. Mold material
For places where it is difficult to set cooling holes or where temperature may rise, use materials with good thermal conductivity in Table 4-2-2 to achieve good cooling effect. However, due to factors such as cost or strength, it is necessary to select carefully.
Table 4-2-2 Thermal conductivity of mold materials (unit: kcal/m·h·℃)

c. Molding condition factors
Injection and holding time: in principle, it is set to gate closing time.
(1) If injection and holding time are shorter than gate closing time, holding process of fully transmitting and maintaining pressure in mold cavity will be insufficient, and sometimes deformation will occur.
(2) If holding pressure is too high, residual shear stress and compressive stress of melt entering holding pressure will be large, which is easy to cause warping. If holding pressure is too low, melt will flow back near gate, which will cause residual shear stress. Since volume shrinkage of product is large [due to low pressure], shrinkage of surrounding area is small, difference in volume shrinkage inside and outside is generated, residual tensile and compressive stress is generated, which is easy to cause warping.
So: general idea is high mold temperature, low injection speed, and low pressure.