Color is normal on the surface of products, though black spots or lines occasionally occur.
The dark spots or black lines are fundamentally caused by the thermal degradation of material or carbonization of impurities due to various factors.
The raw material is mixed with impurities (such as impurities falling off when the operator is unripping the package of plastic raw materials or foreign matters accidentally mixed during color mixing etc) and remnant from decomposition of materials exists in the barrel; insufficient plastication results in overhot or partially overhot melt in the barrel (melt temperature is too high and duration at high-temperature section is too long); overfast injection rate and oversmall section of gate result in excessive shearing of plastics; the surface of molding parts is not smooth and the melt is sprained when flowing through; the inadequate exhausting leads to the storing up of gas in the mold, the rising of temperature thereof under compression and the carbonization of plastics around the air mass; the rough surface of molding parts and the oversmall draft result in the plastic powder (which has been scraped from the previous plastic part during ejection) remnant on the surface of molding parts.
Solutions:
1 Reduce the addition amount of recycled plastic materials, reinforce clearing of remnant in the barrel and cavity and avoid interfusion of impurities in the raw materials.
2 Sufficiently dry the materials, reduce content of volatile component in raw materials and improve thermal stability of resins and additives.
3 Optimize screw structure, reduce plastication pressure and shearing rate of screw, shorten the storing time of melt in the plastication room of barrel and properly reduce melt plastication temperature.
4 Reduce screw injection rate, increase section dimension of nozzle and gate to avoid degradation caused by shearing and overfast flow of melt.
5 Improve mold exhausting condition and optimize exhausting scheme. 6 Reduce roughness on the surface of molding parts and increase draft.
Black spots refer to localized charring phenomena that form along the plastic product’s weld lines, back ribs, protrusions, or near the corners at the end of the flow. Black spots can also appear irregularly at various locations. Sometimes, these black spots may even be distributed in lines, forming black lines.
Distribution characteristics of black spots
Black spots and impurities are characterized by small particles that are black or brown, generally non-reflective, and when large, the impurities appear layered, brittle, easily breakable, and porous after breaking. There are two main characteristics of their distribution:
A. Some are distributed irregularly throughout, some are locally irregular, and sometimes only appear occasionally in a specific area.
B. These black spots and impurities sometimes appear only on the surface of the product, and sometimes throughout both the surface and deeper layers. However, internal black spots closer to the surface are lighter in color than those on the surface, and deeper black spots are not visible at all.
How to identify black spots?
Some might wonder: do black spots need to be identified? Aren’t black spots just points that differ in color from the product itself that we all call black spots? If you observe your product under a microscope magnified 200 times, it’s almost impossible to pick out a product that doesn’t have any different-colored points (completely one color). After magnification, you will see many different-colored black spots on the product’s surface… Therefore, there must be a standard for judging any product.
Standards for judging black spots
Where do the standards come from? For injection molding suppliers, it’s essential to be clear about two standards:
A. Black spot control standards provided by the supplier of the raw materials. B. The appearance inspection standards of the product’s corresponding customers (control standards for black spots).
If the raw material’s different-colored spot control standards are lower than the customer’s control standards for the product’s appearance, there can be a significant rate of defects in different-colored spots during production. Thus, injection molding suppliers need to ensure that the material’s control standards for different-colored spots are consistent with the product customers’ acceptance standards (preferably stricter than the customer’s standards).
For customer standards, you can click on “Injection Molded Part Acceptance Standards” to learn more.
Methods to identify black spots
The commonly observed shapes are different; most of what is seen with the naked eye is “circular,” while under magnification, shapes like flaky, multiple points clustered together, cloud-like, strip-shaped, etc., can be seen. So, we generally judge based on the area of the points. Does this mean we need to thoroughly inspect every part of the product surface for black spots? This can be very time-consuming. Typically, product-end customers will have a standard inspection method: under standard lighting, at a standard distance between the product and the eyes, products that do not show (different-colored spots) within a standard time are generally qualified products.
How to handle black-spotted injection-molded parts
All injection-molded parts found with black spots, black specks, or black lines are treated as defective products.
How are black spots formed?
The sources of black spots are generally two types: Material Carbonization—material degradation at high temperatures (or over long periods); Contamination—impurities.
Carbonization
Most high polymer materials need to be heated and sheared during processing. During this process, carbonization of the material is unavoidable (100%). For injection molding machines, there are many places that can increase the chance of carbonization:
(1) Wear on the screw surface
Wear on the screw surface can cause the melt to linger there, leading to carbonization over time;
(2) Residual molten plastic
At various connection points of the machine, such as the screw rocket head, nozzle, flange, etc., gaps can easily form where the melt can linger and produce carbonized black spots.
(3) Long production cycles
During production, long cycles, or large machines (small products), can cause the material to stay too long in the barrel, increasing the likelihood of carbonization.
(4) Adhesion
When cooled plastic is remelted, many engineering materials (such as PC, which adheres well to metal surfaces after cooling) can remain on the metal surface even when reheated. Over time (at high temperatures), a carbonized layer forms. This is a source of black spot defects for production. Therefore, it is necessary to insulate during short stoppages (<4H) to prevent the material in the screw from hardening and adhering, while a lower insulation temperature will not lead to carbonization. Thus, the recommended insulation temperature is generally about 20C-30C above the softening point (for example, the softening point of PC is about 147C, with a recommended insulation temperature of about 180C). During shutdowns, it is necessary to expel the material from the barrel and pass a material that is less prone to carbonization through the machine before directly turning off the power.
(5) Screw capacity
The product size and the screw size determine the dwelling time. Small products encountering large machines mean the material itself may stay too long in the barrel. Prolonged time can cause small molecules to degrade and carbonize easily. For most materials, a dwelling time of less than 5 minutes is ideal. The maximum time should not exceed 15 minutes. For longer durations, it is appropriate to lower the temperature at the rear end of the barrel to reduce the time at high temperatures. Dwelling time calculation formula: Maximum injection volume of the machine * melt density * cycle * coefficient (2.0-2.5) / product weight; or Maximum measuring length * cycle * coefficient / (solvent setting position – residual injection material)
(6) Connection dead angles
At the connection point between the screw and the rocket head, flange, nozzle, etc., inside the barrel, some “dead angles” can form where the melt is not easy to flow (stays), causing long dwell times and leading to degradation and discoloration (resulting in some yellow streaks, black spots, etc.). First, during assembly, ensure the completeness of the threads and precise assembly, and the contact end face should not be damaged. If severe, replacement is necessary!
(7) Surface wear
Uneven surfaces are more prone to wear on the screw surface, making it easier for the screw skin material to “linger,” which has a significant impact on black spot defects. Metal barrel screws, depending on the material, wear differently. Generally, PC or PMMA requires a hardened (plated) screw. For filled materials (glass fiber, mineral powder), an alloy screw is needed. And even with an alloy screw, it still needs to be replaced after a certain period!
(8) Chemical corrosion
Corrosive water, CO2, O2, halides of hydrogen—such as hydrochloric acid, acidic residues, electrolytes, and other impurities. During the processing process, corrosion on the surface of the screw or barrel can also cause the surface to “linger” and produce black spots. Similar to wear, for general materials, ordinary nitrided screws can meet the requirements, but for fireproof materials, a screw (barrel) with nitriding + plating level, or even thick plating treatment, is better for preventing chemical corrosion.
Contamination
Production environment: The colors of other substances in the external environment might differ, and their temperature resistance might be lower, making it easy to produce black spots. For light-colored materials, if produced in an open workshop environment using ordinary hot air drying equipment. There are at least two problems here:
A. Long-term drying, the air sucked in from the open environment and heated into the hopper, if it exceeds 16H, ordinary transparent materials can turn into light gray particles… meaning the defect rate can reach 100%. In normal production, the hopper cycle is 4-12H, but the proportion of air intake contamination is still very high.
B. During production, a common mold surface, just opened for a short while, will gather a lot of dust (the machine itself is an electric device, although it’s grounded, it can’t be “0 static”), and the production process is also a factor in “different-colored spots.” Therefore, for light-colored products, which require high standards for “different-colored spots,” it is recommended to produce in a clean room.
As mentioned before, material particles are mostly processed by machines, and they themselves have a certain proportion of black spots (hence the need to obtain the manufacturer’s standards). Additionally, the material’s ability to withstand high temperatures and high-temperature oxidation in these formulations, under conditions where the capacity is insufficient, processing can be quite painful, and it can be easy to encounter yellowing (streaks), blackening (streaks, spots), and other issues.
we discuss some short-term insulation and some precautions and procedures during direct shutdowns and cleaning the barrel screw requires attention to the method:
1. Cleaning
(1) Generally, we can use relatively hard materials to clean the screw barrel, using the scraping principle to preliminarily clean out some of the surface residues, such as PMMA material (relatively high hardness)! For example: undried PS, PC (undried and not quickly melted, also a bit harder), etc.
(2) For cleaning-filled material types, PC+GF material can be used for cleaning. Because glass fiber (GF) is solid under normal processing temperature ranges (<=400C), we all understand that glass is very hard. Note, do not use this method on ordinary material screws as it will cause severe wear. Please note, during cleaning, remember to repeatedly raise (20C) and lower (20C) the temperature for cleaning. Principle: A, the change in viscosity; B, the change in internal pressure, both can take away more residues.
(3) In cases of severe black spots, directly disassemble the screw barrel for polishing.
2. Maintenance
During shutdowns: As previously discussed, it’s essential to clean off the production materials (engineering class materials), this step, in my understanding, is more of a management and maintenance process! Cleaning the residual materials in the barrel, here a relatively high-viscosity material is a good choice for cleaning.
Firstly, this kind of modified material needs to be close to the production material in temperature, secondly, it has good viscosity. We recommend using some special washing materials for cleaning, this kind of special cleaning material has very good viscosity. During the cleaning process, its exchangeability is outstanding and this cleaning material has good chemical activity (very good compatibility). This can continue the chemical reaction with some of the “lingering points”, dead-end positions of the materials inside the barrel during a shutdown. In the next startup, This can take away more residues (reduce the chance of black spots).
After cleaning the color (shooting a few times), finally shoot empty (recommended minimum dwell time of 15 minutes, give it some time for reaction compatibility). Of course, changing material production can also improve black spots. This kind of material’s color cleaning ability is much stronger than ordinary materials, this can save a lot of time very effectively. Its feature is that it can stay in the barrel for a relatively long time. At the same time, it is compatible with some of the dead-end materials inside, reducing subsequent black spot production.
The creation of black / brown specks is one of the most common defects you’ll find in injection molding. This defect can often be a nightmare for processors as they can appear and quickly disappear at irregular intervals during the molding process. Black specks tell the processor that there is certainly some level of contamination present within the system.
Black / brown specks can almost always be attributed to a heat or residence time causation. There are times when a post-molded part can come in contact with grease or dirt on the molding surface. But if the defect is within the molded part then a processor can be sure that there is contaminated material present in the system.
Specks can be caused by a few different factors including the following:
Overheating of the processing resin causing degradation
Excessive shearing of the resin caused by back pressure and incorrect screw surface speeds
Too much residence time in the barrel resulting in degradation
Or the presence of a former resin that is heat sensitive degrading within the processing resin
It can be tough to get control of degraded material with processing adjustments alone. A good commercial grade purging compound like Asaclean is the best solution for purging the system of degraded material so the processor can reestablish the process with a clean screw and barrel. It is always a good idea to use a purging compound when changing over resins that will run on the same press. This will ensure that the previous resin that could be more heat sensitive is not lingering in the system to possibly cause problems with the next run.
There are many factors and conditions that affect the processing of quality plastic parts. Defects In injection molding occur during fill time or pack/hold time. Examples of defects caused in the filling process can be flow lines, jetting, gas burn, weld line, etc. Examples of defects during the pack/hold pressure time can be a sink, voids, flash, wrappage, etc.
Flow lines – a ripple-looking pattern on the front of melt flow caused from filling too slowly. Possible molding process issues – low temperature, too low injection speed, insufficient hold pressure, insufficient cushion. Solutions – increase temperature, increase velocity, increase pack pressure and time. Mold could have cooling issues or be too cold.
Sink/Voids- Area(s) of a plastic part not filled out or sunk in on the surface. Many molding processes to check – too short cure time, too slow velocity, too small shot size, pressure loss, insufficient hold time. Possible mold problems – too hot mold temperature, Gates, runner too small, part dimension too thick. Solutions – Increase pack pressure and time, increase injection speed, allow proper cooling time, more up transfer position.
Black specs / Burn – dark spots, streaks, chunks appearing in and on the surface of the parts. Common molding issues are degraded material, excessive injection speed, and pressure, clogged or not enough vents/ gas burn, and residence time of the material. Mold issues could be inadequate vents or plugged/damaged vents causing trapped gas. Solutions – Purge for carbon build-up, inspect vents for damage or blocked, add more venting if needed (common practice is at least 30% of parting line to be vented), decrease the temperature, decrease velocity, back pressure, make sure material is not residing in the barrel too long (residence time).
Warp / Bend – Parts that twist, bend, warp out of shape and dimension after being ejected from the Mold. Pressure and over-packing are very common causes. Mold problems could be unbalanced or not enough cooling/water, mold temperature too high, gate size, part sticking during ejection. Molding issues include too much pack and hold pressure, too low melt temperature, and Injection speed too slow. Solutions- increase velocity, move transfer up, increase mold and melt temp, decrease hold pressure
With so many different variables, conditions, settings, etc. causing problems within your plastic processing equipment it’s vital to control what we can. One major component and many times overlooked is a quality purge compound. Keeping your equipment purged and cleaned on a regular basis will eliminate time troubleshooting later. Purging your equipment is absolutely one variable you have complete control over.
Dark spots on injection molded products can be caused by a variety of factors. It is important to first identify what the root cause of these spots are in order to correctly address them.
The most common causes of dark spots on injection molded parts are: inadequate mold temperature, incorrect gate size, imbalanced venting pressure, contaminants or foreign material in the raw materials (such as metal) and incorrect filling time. Inadequate mold temperatures can lead to incompletely cured thermoplastic materials that give off an oily residue when exposed to heat and pressure during the manufacturing process; this oily residue accumulates at the surface creating dark discolorations. Incorrect gate size will cause incomplete packing out of the part due to low pressures or excessive flow resulting in poor part cooling with thicker sections showing discoloration more than thinner ones; this will also be exacerbated if there is insufficient venting pressure which prevents air from being displaced properly as plastic flows into the cavity. Contaminants and foreign material from outside sources such as water, dust particles or metal shavings/fragments can become mixed with polymer pellets prior to entering into heated barrel causing dark spots upon hardening; these must be identified and removed prior to production for optimal results. Lastly, incorrect filling times during injection molding will affect part quality thus leading to uneven distributions across different sections due to high plastic shrinkage rates causing some areas having less density than others resulting in unwanted appearances like darkened colorations on certain regions of a given product surface need additional consideration for additives such as lubricants and waxes accordingo improve process reliability , reduce cycle times & cost while meeting performance demands expected by modern customers .
Depending on your specific needs you may choose one approach over another but whatever solution you decide it is important that all steps along production processes takes place under strict monitoring & control systems ensure reproducible outcomes every single time . With proper maintenance tools & diagnostic techniques any problems related irregularities should easily diagnosed before they lead bigger issues later down road !