When color isn’t evenly distributed throughout a part, appearing as streaks, swirls or patches of discolored material, you’re probably looking at color swirls. Also known as streaking, color streaks and marbling, color swirls are occasionally misidentified as black and brown streaking, so it’s important to determine which defect you’re dealing with as the first step in your troubleshooting.
If misidentified, many of the solutions to color swirls will actually exacerbate the black and brown streaking, so accurately identifying the problem is a must.
What Causes Color Swirl Problems in Plastic?
When determining the cause—and thus solution—of color swirls, the first thing you need to know is when the problem is occurring. If you’re seeing color swirls after changing from one color to another, the solution won’t be the same as random or always-present swirls.
If the color swirls only appear after changing colors or materials, the culprit probably deals with your purging procedures. Changeovers have to be done correctly or you may be stuck with color swirls for hours.
Color Swirls Troubleshooting Chart
Machine
Material
Molding Process
Screw design
Color concentrate
Low back pressure
Inadequate back pressure
Contamination
Fast screw speed
Hang-up in screw or barrel
Raw component issues
Low melt pressure
Found in Injection Molding Advanced Troubleshooting Guide: The 4M Approach (p. 200).
How to Prevent Color Swirls in Injection Molding
Within the molding process, you need to pay extra attention to back pressure, screw speed and melt temperature. If the back pressure is too low, the material and color concentrate won’t mix properly, leading to color swirls.
Similarly, a fast screw speed can lead to color swirls. The longer the screw rotates, the better the color mix will be, but if screw recovery is too fast, you’ll likely find good results in slowing the speed.
You could be dealing with a melt temperature that’s too low and, since the melt temperature is impacted by so many other factors, it’s important to know exactly what your melt temperature is and that it’s within the recommended range for your material.
If the issue is with the machine, check out the screw design, back pressure and possible hang-up in the screw or barrel, which can prevent it from cleaning out well during color changes. A general-purpose screw design isn’t the best option for color and material mixing, so make sure you’re using a melt-quality screw. If you don’t have enough back pressure, you’ll likely find poor color mixing, which leads to color swirls.
Color swirls emanating from material issues can be due to an incompatible material-to-concentrate relationship, which could be as simple as using a concentrate that’s intended for a different material. You could also be dealing with contamination, which can cause a host of other problems, or a raw component issue.
Sometimes known as dieseling or gas traps, burns in injection molding are literally that: burns. Often appearing as black, smutty deposits on the part and mold surface, these are spots at which the plastic has burned.
Not to be confused with color swirl or brown streaking, burns are actual burns to the part and/or mold and need to be treated as such.
What Causes Burns?
Although usually caused by a venting issue, the culprit can come from one of four areas: molding process, the mold itself, the machine or the material.
When gas gets trapped in the mold, high pressures can cause ignition, which leads to the burns. But why are gases getting trapped? This is what we need to diagnose and solve. If you can avoid it, don’t try to process around a burn issue. Look at tooling issues before you start altering the process.
Burns Troubleshooting Chart
Mold
Machine
Materials
Molding Process
Venting
Buildup on screw, endcap, etc.
Moisture content
Too fast fill velocity
Grease, oil, or other contamination
Screw design
Type
High melt temperature
Barrel heater control problems
Additives
High back pressure
Damage to screw, barrel, tip, etc.
Decompression
Found in Injection Molding Advanced Troubleshooting Guide: The 4M Approach (p. 182).
How to Prevent Burns in Molded Plastics
As we said earlier, improper venting is the most frequent cause of burns in injection molding. When troubleshooting the mold—which is where you should start—make sure there’s proper venting. Improving the venting of a mold will often solve the burn problem. Along with that, make sure the mold is clean. Even if the mold has proper venting, contaminants in the mold can hinder the venting from working properly.
The machine could have buildup on the screw, or the screw design itself could be a problem. There could be damage to the screw (or the barrel or tip or other parts) or you may have control problems with the barrel heater. Any of these things can cause dieseling on your parts.
Next, take a look at the materials. Higher moisture content leads to a greater chance of creating gas as the water gets hot and turns into steam. Some materials, like PVC and acetate, simply burn easier than others, so keep that in mind as you process.
After going through all possible tooling issues, you can get into the process. Again, trying to process around a burn issue isn’t ideal—adjusting fill velocity, melt temperature, back pressure and decompression is fine if you’re confident in your machine and materials (particularly the venting), but trying to change the process before checking out tooling issues will only make things less efficient and, probably, more expensive.
As mentioned earlier, contaminants in the mold can hinder venting. To remove contaminants, try the Nanoplas line of cleaners. Formulated with superior cleaning agents, they get grime and dirt out of the pores of the substrate, cleaning down to the virgin metal.
If there is harder to remove residue, like gas stains, you can use Zap-Ox cream cleanser, which eliminates the toughest gas stains without etching the surface. Simply apply Zap-Ox and move it around until all the residue is gone – no scrubbing required.
Mold Making and Injection Molding: Two Separate Worlds
During the first 14 years of my career, when I was building and designing injection molds, gating and runners were not really a focus. In most cases mold making and Injection Molding are two different worlds, so in my case I had no idea what was going on over in the injection molding side. The gates and runners had no impact on my focus, which was to build robust tools. In 2002 when I made the switch from Mold Making to the Injection Molding arena, gating and runners were an area that jumped out at me right away as not really having any type of standard. (more…)
In some industries, the rise of machines more capable of performing complex tasks is taking the place of human workers. Here’s an article from BetaBoston claiming that in the manufacturing industry things are different…
“It appears the manufacturing industry may be a special case, in which automation and artificial intelligence can actually protect jobs rather than replace them.”
How is this?
The robots are able to accomplish automated tasks that help keep costs down and help American companies win bids against Chinese or other overseas low-cost manufacturing companies. So far, this has worked with the companies who tried it, and none of them have laid off workers, giving these robot manufacturer’s claims more and more validity.
“In today’s world, the green movement is more important than ever. No matter what the industry, it’s a company’s duty to make changes and choices that support the environment.”
Here’s a great article from The Rodon Group about working with injection molders that are environmentally friendly in their practice. The article lists some of the things you should look for in a company including lead manufacturing processes such as waste minimizing technology and landfill-free facilities.
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Mold Making and Injection Molding: Two Separate Worlds