Introduction The injection molding process must be able to operate at low cycles and high yields in order to remain competitive in the market.
Once a process is optimized for the rate and minimum cycle time, the loss of output will reduce the profitability of the plant.
Due to improper shrinkage, short lenses, surface defects known as splay, black spots and colored stripes, the yield can be reduced by the size of the part.
Black spots and colored stripes may be derived from raw resin, plasticizer, non-
Return and runner system.
The focus of this paper is to troubleshoot the black spots and colored stripes from the plasticizer.
The purpose of this paper is to provide common trouble-shooting techniques to determine the root cause of black spots and color stripe contamination in injection molded parts.
These problems are illustrated by three case studies.
Although black spots can be produced from any flow part of the process, the plasticizer is the focus of this article.
Background: technical solutions for the diagnosis and elimination of defects for root causes may be difficult, time-consuming and expensive to identify.
If the system method is used, the time required for the trouble shooting process can be reduced, thus reducing the cost.
This method starts with verifying the operation data, performing simple calculations, and issuing strong assumptions (1-5).
Next, the troubleshoot must develop experiments that validate or invalidate the hypothesis.
Once the root cause is identified, the best technical solution will depend on many factors, including the cost of production lost, the time and cost of implementation, the acceptance of the machine owner and the risks associated with the modification process.
There are several root causes of black spots and color stripe contamination in the molded part.
These root causes include the degradation of resin in the spiral channel, non-
Return and runner system (6-9)
, Degradation materials or pollution entered with raw materials, and poorly dispersed pigments in the Masterbatch (10).
For example, a screw with a very small flying radius when pushing screws and trailing screws can allow the resin to stay here for a long time due to the Moffat Whirlpool (11).
These areas cause degradation of the resin, and the degradation product will eventually pollute the parts with black spots.
The mixing segment usually has stagnant areas that cause resin degradation.
For example, if the outlet and inlet area of the screw dam is improperly designed, it may cause resin degradation (9).
The metering part of the screw must be the rate control step of the plastic refining.
If there is no rate control in the metering part, the metering part will partially fill the resin.
There will be a partial stagnation of the partially filled channel, allowing the degradation of the resin (6,8).
The troubleshoot procedure should always verify that the metering part is running as a rate control step.
The calculation of the metering part can be found elsewhere (12,13).
Black spots in beige parts use single-ton press with 105mm diameter injection molding small car interior parts
Screw plasticizer.
By adding the masterbatch to apolycarbonate, the part is beige-
Acrylic Acid Ding benzene (PCABS)resin.
About 7% of the parts had to be scrapped due to black spots.
Figure 1 shows a photo of the part.
Scrapped parts increase the cost of the plant by reducing production, increasing resin consumption, and carrying out a higher level of quality control inspection on the printing machine.
The plastic rate data is measured and verified, and then compared with the calculated specific speed (drag flow rate).
For this screw, the measurement rate and the calculation rate are 3, respectively. 2 and 2. 7kg/(h rpm).
Since the measured rate is slightly higher than the calculated rate, there is a negative pressure gradient in the metering part of the Screw, so the metering part of the screw is the control step of the process.
That is to say, the screws are operated according to the design.
If the measured rate is significantly less than the calculated rate, the upstream part (
Solid conveying)
It may be a rate control step that results in only partial filling of areas in the metering and transition sections.
Areas filled with these parts may cause resin degradation, resulting in black spots in the parts.
At this point, assume that there is a stagnant area in the screw or non-screwreturn valve.
The only way to determine if an abnormal area occurs is to remove the screws from the barrel and to make an amine of the channel.
The flow of the resin to the hopper is closed and the plastic generator is allowed to rotate until the screw is basically empty and the nozzle stops flowing out.
Next, the transmission pipe is removed and the screw is pushed out of the barrel when hot.
In the threaded part of the Screw, there is a small layer of carbon resin at the push and trailing fly corners deep in the screw.
The flight radius is about 20% of the channel depth; i. e. , R/h = 0.
As shown in figure 2.
The rest of the screws and non-screws
There is basically no degradation resin in the Hui.
Based on these observations, it was concluded that the black spots came from the stagnant area of the crew in the corner of the flight caused by a small flight radius.
For this application, the flight radius should be at least equal to the local depth of the channel (7).
A high-performance screw with a flight radius equal to the depth of the local channel is designed, manufactured and installed.
Using this new screw, the black spots are basically excluded in the process.
Black stripe of gray part a injection molding factory is producing large electronic parts from high
Impact Polystyrene (HIPS)resin.
Molded parts are in batches of natural hip resin and gray concentric plaster in gray color.
Many molded parts have to be scrapped due to black stripes, as shown in figure 3.
Molder claims that the color concentrate is not fully mixed into the resin and apre-
Color resin is needed to solve this problem. A pre-
Color resin is a resin that mixes colors together to increase the cost of resin and parts.
Parts are formed using 2500 tons injection molding machine with diameter of 125mm and length of 21to-diameter (L/D)plasticator.
Factory personnel tried and reported several debugging operations and alleither failed or provided an unacceptable solution.
These measures include increasing the return pressure, increasing the load on the concentrated color, and placing a static mixer inside the nozzle.
With the increase of back pressure, black stripes appear when the color concentration increases from about 2 to 5%.
However, the color vortex is relieved with the addition of a static mixer to the nozzle.
This technique is not accepted due to the problem of filling parts and the increase in cycle time.
The static mixer requires that the injection pressure be raised to an unacceptable level to maintain the cycle time.
The screws used for this process are traditional single
A rotating screw with a screw dam located in the measuring part of the screw.
The spiral Dam starts with the entrance of the channel pushing the side metering section.
The dam ends at the end of the measuring section and at the tail of the flight.
The weakening of Damwas 0.
89mm relative to the main flight.
The spiral damis schematic diagram shown in figure 4.
Calculate the specific rotation flow rate of the screw at 6. 4 kg/(h rpm).
Estimate the flight radius size based on the radius cut point on the flight edge.
In this case, the flight radius is about 20% of the depth of the channel.
The flight tradii of this screw is very small and is most likely an area of resin degradation. The 2.
69 kg and the gate are 34 respectively.
6 s at the rpm of 52 rpm and 9.
Screw tip pressure 5 MPa.
The shrinkage of the screw is quite stable, indicating that the plastic increase rate is constant.
For this machine and screw, the measured ratio rate is 5. 4 kg/(h rpm).
Calculate the specific rotation flow rate of the screw at 6. 4 kg/(hrpm)
And there is enough positive pressure gradient to reduce the specific rate to 5. 4 kg/(h rpm).
So this screw is working properly and full of hydraulic pressure.
To determine if the masterbatch is the source of the black stripe, the masterbatch is removed from the raw material and only natural HIPS resin is used.
After about 10 parts, there is no pigment in the house.
However, as shown in figure 5, there are many particles of black material in the part.
Most of the particles are between 1 and 5mm in diameter and are located inside the part; i. e.
, Away from the surface.
If concentrated toner is used, the particles are invisible.
Some small particles occasionally touch the surface of the tool and produce a strong color stripe.
These stripes may be stripes that are concentrated in color observed during the cabinet forming process.
Assuming that the black particles are degradation resin from the screw and non-screw stagnation areasreturn valve.
The next step is to remove the screws and look for areas that are stagnant.
In order to locate the area where the material is degraded in the crew, the flow of particles to the screw is stopped and the rotation of the screw continues until all natural HIPS resin is emptied from the plasticizer.
Next, remove the screws from the barrel and check for black and degraded resin.
There is a relatively large amount of severely degraded resin at the entrance of the Channel collection side spiral Dam, as shown in Figures 4a and 6.
It is well known that this area is a place where it has lived for a long time; i. e.
, The location of resin degradation.
The degraded resin is soft and tough, and the color is black.
At the push and trailing flight radius, there is also a considerable amount of this degradation material, starting from the middle of the transition segment, extending the rest of the length of the screw. Thenon-
There is also a considerable degree of black degradation resin on the surface of the Hui.
These tough black spots are the root cause of some black swirls.
Don\'t know is a screw or non-screw
The return rate is a major contributor to black spots.
These procedures convince factory personnel that it is their craft equipment that makes black stripes, not resin or masterbatch.
The only permanent solution to this problem is to eliminate the area where the polymer is degraded.
Is the existing screws and non
A callback that needs to be re-placed with streamlined equipment; i. e.
, Screws and non
Return nozzlethat in the area where there is no resin stay for a long time. A high-
Performance screws are designed for injection molding machines.
The design of this screw has a higher compression ratio and has a large flight radius in all parts of the screw.
News starts using high-
Performance screws and natural hips are made using 2% light gray concentrated color masterbatch.
For the same 2 for this startupthe.
69 kg parts were produced and the set point temperature per barrel was the same as before.
After stable operation (about 10parts), the 2.
69 kg and the gate are 33 respectively.
Run 5 s at 52 rpm and 9.
Screw tip pressure 5 MPa.
The traction rate of the ironing board is stable, which indicates that the rubber rate is constant.
The specific rate of measurement is 5. 6 kg/(h rpm)
, A specific rate of about 3% higher than the original screw.
The specific rotation flow of the screw is calculated to be 7. 1 kg/(h rpm)
, There is enough positive pressure gradient to reduce the specific rate to 5. 6 kg/(h rpm).
So this screw is properly operated and filled with hydraulic pressure.
For the rest of the trial, black stripes were never observed. The high-
After installation, the performance screws are closely monitored for about a month.
Black stripes were never observed during this period, and factory personnel said the problem had been resolved.
Pigment steak in gray part a injection molding factory is producing most of it with natural polypropylene (PP)resin.
The natural resin is mixed with a 35-to-1 disappointment ratio of a light gray concentrate.
The problem that the mold has black stripes on the surface of the part, resulting in a high scrap rate.
Figure 7 shows a photo of a black stripe.
When the back pressure on the screw increases from 0. 7 MPato 2.
5 MPa, the proportion of black stripe parts decreased from 50% to less than 10%.
The injection molding machine is equipped with plasticizer with a diameter of 140mm, 20L/D.
Press design with pressure strength factor 10.
That is to say, the back pressure setting for 2.
5 MPa the pressure when the screw is discharged during rotation is 25 MPa.
The specific rotational flow of the screw metering segment is calculated at 9: 00. 5 kg/(h rpm).
Due to the short length of the metering section (2 diameters)
During the operation, the specific operating rate of the screw will be highly dependent on the discharge pressure; i. e.
, Back pressure setting.
The screw is able to operate at a maximum speed of 99 rpm.
This press has a new barrel, barrier screw and non
A comprehensive operation inspection of the press was carried out.
In order to achieve the best results, molderhad spent quite a bit of time working with the current process.
They conclude that high temperatures help save time and minimize defects.
The main concern is that the defect comes from the degradation of PP resin, or is related to the color masterbatch pigment.
Some adjustments were made to increase the return pressure and reduce the barrel temperature, resulting in a constant or \"flat\" barrel temperature \".
The decision of the molding machine the temperature curve of this fixed cylinder at235 (degrees)
C, plus 2.
The 5 MPa back pressure makes the operation of the barrel heating area more stable and the recovery time more consistent, while minimizing the level of color stripes.
However, the cycle time increased to an unacceptable 85 s level with the rate limited by the plasticization process.
At the beginning of the trial, the goal of the plant was to run the cycle time of 65 s with a scrap rate of 1% or less due to all defects.
The plasticizer runs at 235 (degrees)
For all four barrel zones, the ascrew speed is 99 rpm and the back pressure is 2. 5 MPa.
The barrelzone located above the screw metering section is measured at 262 (degrees)
C. temperature exceeding 25 (degrees)
The temperature exceeds the temperature point.
In this case, the cooling capacity of the region cannot remove the dissipated energy quickly enough.
The process gives a cycle time of 82 seconds, but the scrap rate remains at a minimum of about 10%.
The plasticization time was 36 s, and the instantaneous specific rate of the process was determined to be 3. 45 kg/(h rpm).
This plastic-increasing rate is much smaller than the calculated rotational flow rate of the screw; i. e.
Calculate the rotation flow rate of the screw at 9 hours. 5 kg/(h rpm).
At present, the molding time of 36 seconds limits this pressure.
The observation shows that the time of adding plastic to the screw is slightly different.
Although these discrepancies are small, the concern is that the screws and plasticizer do not work properly.
The injection temperature is measured by producing an \"air injection\" and then measuring the temperature by hand
Temperature sensor.
Air injection is when the nozzle of the
plastic molding machine is pulled out of the mold, and then the material is discharged onto the cardboard to produce a molten substance that can be observed.
The temperature of the measure dat injection is 259 (degrees)
C, the temperature is considered relatively high for this type of process.
To understand why the plasticizer operates at a low specific rate, several changes have been made to the process.
Reduce the back pressure to 1 first.
The discharge pressure is 4 MPa of 14 MPa.
This reduces the plasticization time to 21 s and increases to 5 than the rate. 9 kg/(h rpm).
Due to black stripes, the parts produced under these conditions have 100% scrap.
The increase in this specific rate and the calculation of the flow confirm that the discharge pressure is the cause of the low specific rate.
Next, the process of concentrating the color is turned off and it is allowed to run out.
Remove the color from the system to determine whether the color stripe is caused by a degraded PP resin or by a poorly composed pigment.
If the stripe is caused by a degradation resin, the stripe should still be present after removing the color concentrate.
Once the part color is completely natural, there is no black stripe in the part.
The screw speed and reverse pressure vary between 50 and 90 rpm and 0. 7 and 2.
5MPa respectively, trying to destroy the process.
The assumption is that if any degradation resin is accumulated on the screw due to the apoor screw design, then the change in screw speed and reverse pressure will cause the degradation resin to exit with the injection, produce stripes in parts.
In all cases, the part does not contain stripes, which indicates that the screw works properly using natural PPresin.
The process data show that the molder can operate the screw in the case of 99 rpm and back pressure 0.
7 MPa high production-
High quality parts with cycle time of 65 s or less.
The molding condition is restored to the ascrew speed of 99 rpm and the original setting of the reverse pressure of 2.
5 MPa, then add the star batch pigment back to the process in a disappointment ratio of 35 to 1.
Once the pigment is observed in the part, the black stripe reappearsappeared.
According to the data here, the root cause of the black stripe is the color concentration.
Although it is not fully evaluated, during the compound operation, it is possible to produce a reunion of pigments in the mother material.
The use of this barrier screw cannot effectively disperse these reunions.
When the back pressure increases to 2.
5 MPa, the mixing capacity of the screws is slightly improved, and the level of stripes in the parts has decreased.
The obvious goal is to obtain a concentrated color that does not contain reunion pigment particles, allowing the press to use a 0 back pressure operation.
7 MPa and minimum cycle time.
Flow rate of melt (MFR)
For PP resin and masterbatchcolorant, measurements were made to see if the mother material meets the standards defined by Benkreira and Britton (10).
The mixing experiments of Benkreira andBritton show that the viscosity ratio of natural resin should be as high as possible under the processing conditions.
However, the masterbatch with very low viscosity may be difficult to produce because the stress during the compound operation may not be enough to disperse the pigment.
In general, as a compromise, the viscosity of the asterbatch should be about half of the natural resin under processing conditions.
Mfr is 20 and 116 dg/min, respectively (230(degrees)C, 2. 16kg)
It is used for natural PP resin and masterbatch pigment respectively.
Apparently, according to the guidelines developed by benkreira and Britton, the masterbatch does not match well with the natural PP resin.
That is to say, the carrier resin used to make the viscosity of the masterbatch low, allows in the double-
Screw composite process.
However, if the pigment can be dispersed in this carrier resin, the color mother can be accepted.
Compared with mfr, it is preferred to compare the shear viscosity of natural resin and asterbatch.
For these materials, the shear viscosity of natural PP resin and color masterbatch is 160 and 40 Pa respectively under processing conditions.
As expected, the ratio of the shear viscosity of the natural resin to the shear viscosity of the concentrate is about 4, greater than the guiding value 2.
Better Masterbatch can be made with a more viscous carrier PPresin, so that additional stress can be applied in the compound step to help disperse the pigment reunion.
Due to the preset specifications, changing the color concentration is an unacceptable technical solution for the factory.
On the contrary, the pigment reunion in the color masterbatch needs to be dispersed in the plasticizer of the injection molding machine.
That is, a new screw needs to be designed and manufactured, which can disperse the reunion while satisfying the cycle time of 65 s. Ahigh-
A performance screw with a screw mixer was selected for the application.
This screw is specially equipped with multiple scattered dams.
The performance section provides the gap between the peak of the dam and the 1 barrel wall. 1 mm.
The gap between the mixed flight of the spiral mixer and the barrel wall is 0. 89 mm.
The dispersion capacity of this screw is much higher than that of the original screw.
The original screw has an obstacle flight weakening gap of about 1.
5mm and the roots of the screw mixer are cut 4. 8 mm. The high-
Performance screws were installed and tested.
High screen immediately
High-quality parts without black stripes, cycle time is 65 seconds.
In the case of a specific rate of 8, the molding time at the screw speed of 99 rpm is 15 s. 3 kg/(h rpm).
This particular rate is slightly lower than the specific rotation rate of the calculated 8. 9 kg/(hrpm).
The back pressure used for this plastic increase is 0. 7 MPa.
The results show that high-
The performance screw is small enough to provide a sufficiently high level of stress to disperse pigment reunions in the masterbatch.
For many resins, a long stay in the flow path can cause the resin to degrade into dark materials.
If the degraded re-falls off the metal surface forming them, they flow downstream and cause black spots or colored stripes.
These black dots and colored stripes are easily observed and diagnosed in non-black or gray parts.
In almost all cases, the screws must be removed from the barrel in order to determine the source.
Common sources include sources with stagnant flow, such as metering channels for partial filling, small flight radius, and unstreamlined mixers.
For the gray part, black spots may stem from the screw design issues discussed above, or they may be poorly dispersed pigments in the masterbatch.
Taking the masterbatch out of the raw material is the best way to determine if the black spot is from the pigment.
Conclusion The trouble shooting and elimination of black spots and color stripes of injection molding parts are introduced.
Three case studies were presented, demonstrating the root cause and technical solutions for eliminating the problem.
This work is mainly focused on the plasticizer, although the downstream part of the process can cause the resin to degrade and form black spots. References (1. )J. R.
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