Black specks, streaks or color mixing errors are the most commonly recurring problems that mar the surface of highly cosmetic injection molded parts. Three likely culprits causing the surface issues are resin degradation, equipment contamination, or process miscues.
Determining the source of the surface problem can help to pinpoint a solution more quickly. Specks can occur due to contamination of the polymer or from resin degrading in a dead spot in of the screw flights carried through by the current resin shot. Streaks are derived from screw contamination like degraded material in a dead spot, or from the wrong choice of screw mixing. Color mixing can be poor due to a problem between the base substrate and the colorant, or because of screw mixing.
Mind Your Material
One of the most practical ideas is to visibly view unprocessed resin under a light to see if there are specks. This is practical when starting with a new resin or when restocking with fresh pellets. Sample the bag, gaylord or hopper at different depths with a sampling wand. Take around two pounds for the inspection, loading it onto a light (white) 12″ x 18″ tray, with good background lighting. If the specks are imbedded in the pellets then contact the resin supplier. If it is on the surface of the pellets then inspect the material handling supply lines. If there are excessive fines, filter them out with a fines separator.
Checking the coloring agent is another preventative step. Is the carrier compatible with the base resin? Be aware that there is no such product as a “universal carrier” for a colorant. The carrier must be compatible with the resin or coloring or physical properties will be compromised. Is the correct letdown ratio being used? Using a higher than quoted letdown ratio in an attempt to get perfectly colored parts is costly. Use of more color is more expensive than resin and it is a sure way to deflate profit margins.
Check that the resin is being dried according to the resin producer’s specifications. Too high a temperature degrades the resin and or causes a color change. Too low a drying temperature does not dry the resin properly and during processing the moisture depolymerizes the resin (hydrolysis). A simple process step is not to leave a machine with resin running idle for any significant period of time. This can encourage a long residence time for the material and degradation.
Another way to check the material is to purge a shot. It is important to check a shot that was developed with appropriate backpressure and melt temperature. Use about 1,000 psi of backpressure plastics pressure (not hydraulic pressure) is critical to completely fill the screw melt channel. Filling the channel minimizes the chance for material being left behind in a void or dead space in the screw. The backpressure limit can be less than 1000 psi if you are using a resin sensitive to degradation.
If the “purge patty” is produced under the correct process conditions, look for specks. If the purging is clean but you are still getting specks when you mold, then look further downstream. If the barrel and screw are not the source then the degradation must be occurring in the hot runner. Hot runner temperature controls are notorious for being out of wack.
If the purging showed an increase amount of specks or streaks, or indeed a “carbon shower” of specks or streaks appeared on the part surface, then that is a good indication that the screw design is to blame.
Evaluating The Equipment
Eliminating dead spots and choosing the right mixing screw design is a major step toward reducing the chance for part surface marring. Use of general-purpose screws and dispersive screw mixing designs are a prime source of black specks, streaks and color mixing problems. This can be especially true when molding of clear materials like acrylic, polycarbonate (PC), or styrene-acrylonitrile (SAN), or sensitive materials like PVC or acetal. With a general-purpose screw this can be unwise. General-purpose screws tend to have dead spots where polymer can hang up and degrade. This material gets picked up during the molding of subsequent materials, forming the black specks or streaks. A general-purpose screw may also not melt resin uniformly, and they are often made from lower steel grades that can accelerate resin degradation.
Performing a screw and barrel cleaning will help reveal the true source of a problem. Molders should run a medium flow polypropylene as a last material before pulling the screw. Check for carbon buildup behind the flights in the metering and transition sections of the screw. Ninety-five percent of general-purpose screws have dead space behind the flights. It is in these dead spaces where material hangs up,accumulates and degrades to carbon. It is not surprising to see a color that was molded days or weeks ago on the backside (or non-push side) of these flights. Both the carbon build up and spotting a week-old color are proof that the new material forwarding is not pushing out all of the previous material in those flights. Rounding the backside of the flights like a farmer’s plow or almost a cone like a hard ice cream cone can eliminate the dead space. Flights need to be radiused in the transition and metering section of the screw.s
A barrier screw or high-intensity mixing device can also be causes of trouble. Barrier screws can yield severe resin degradation and higher black speck production due to the huge dead spots in its design. The screw should prevent unmelted solids from getting through. This can be achieved using a modified barrier design that is not a continuous barrier flight but tapers into a metering flight. Also, the screw should also be highly polished and not have visible sanding or polishing marks. This minimizes the chance for hang up spots.
An interesting discovery to note that is that worn barrels and screws can provide better mixing than equipment at specification. With worn barrels and screws, you get some back flow over the flights, which provide better mixing, and even moving of material out of dead spots.
Misuse of the mixing design in the screw can be a major source of poor part surface appearance. Go for distributive mixing not dispersive mixing. Dispersive mixing is a high shear step that breaks up sand-sized particles into ultra fine micron sized particles. On the other hand, distributive mixing blends the components into a uniform intermix. For color mixing, the distributive mixing design gently tumbles the melt and ends in a slow taper. A good distributive mixing will take a few black granules in a natural resin and generate a uniform gray part color. Poor distributive mixing or dispersive mixing will give you streaks of black in the natural resin. It is best to use a 20:1 L/D screw or better to attain good distributive mixing. If your screw has an L/D ratio of 16:1, you are asking for problems.
You can check your screw for mixing capability by adding just a few color granules at the feed throat. Count how many shots it takes before the color shows in parts so you will know residence time. When the color does appear look for streaks. If the color comes out as a pastel, uniformly blended you have good mixing. If it comes out as streaks, you need a better screw design providing backpressure, let down ratio and color carrier compatibility is correct.
The front of the screw also has several components needing close inspection. Check the non-return valve, nozzle-end cap, and nozzle tip for abrupt changes in flow path. These dead spots lead to polymer hang-up and subsequent degradation. The flow path should be smooth and coned shaped all the way through the nozzle tip. Standard nozzle tips have a straight bore and significant dead space. Of course, if there are broken or chipped flights or nicks in the non-return (check) valve or screw tips these are sources of black streaks and must be replaced.
For clear resins, it is especially important that the screw has better chemical resistance than standard original-equipment. Also, make sure you are using at least 25% of the barrel capacity. Long residence time can cause degradation, the cause of most black specks. The nozzle body and screw tip are notorious for causing degradation. Both their design and temperature control can be suspect. Keep the nozzle body as short as possible with a free flow (cone) design. Temperature should be PID controlled, not with variac or percent methods! Instead of controlling to set temperature, the variac or percentage load method just sends current to the heater, providing a temperature that is almost assured to be incorrect.
Placement of the thermocouple on the nozzle body is okay in the hex (where the wrench grabs the hex of the nut) if the nozzle and tip are 3 inches or less in length. For longer nozzles place the thermocouple 1/3 the distance of the nozzle body back from the tip. A butter fly type covered with 2-3 layers of glass tape is recommended. Thermocouple placement on the hex of a long nozzle or on the screw holding the heater band onto the nozzle is not acceptable due to the potential of an inaccurate reading- too far away. It will bring about incorrect temperature control of the nozzle body and tip. Inspect the nozzle body and tip for blockages. Vented barrels should be avoided, as often the screw and vent design are prone to dead spots resulting in resin degradation to carbon.
After the resin, screw and barrel are checked, move on to the hot runner system if the mold has a hot half. Many hot runner systems have dead spots due to cross gun drilling and are prone to degrade resin to carbon in the dead-space corners. Check for hang-up areas or dead spots in the flow path. Check for proper temperature control; burnt-out heating elements, open thermocouples and correct placement of thermocouples. Check watt density and location of manifold and hot drop heaters.
Be sure to check the depth of the hot tip relative to gate surface, account for thermal expansion. Temperature should be PID controlled, repair and replace any controllers that are in % or on variacs. Redesign the hot runner if necessary.
Temperature control of the barrel is also important. Look for hot spots or zones overriding temperature set points with the heater bands constantly on or constantly off. If a barrel zone is constantly on, raise adjust the temperature of the barrel zone preceding it. Raising the temperature puts more heat into the resin just before the following zone. If a barrel zone is constantly off, raise the temperature of the zone before it, while also reducing backpressure and lowering screw rpm. Raising the temperature of the zone preceding it will lower the amount of shear heat that is being put into the resin and the resin processes with less of a hot spots in the barrel. Both lowering the backpressure and the screw rpm will put less shear heat into the resin. This means less temperature override which is causing the hot spot or zone to be constantly off. Check the actual melt temperature using the hot probe technique or an appropriate IR device.
There are two times related to running the machine, which can minimize part surface errors. Plasticating (screw recovery) for the next shot is also a place to prevent part surface problems. With normal cycling parts, take all of the cooling time available except about two seconds for plasticating, and for fast cycles, take a few tenths of a second. This will help stabilize and provide consistent cycle times. Another process tip is to run the screw for as long as you can during cooling, as slower rpm provides better mixing and minimizes hot spots.
About the Author
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|John Bozzelli is a graduate of Marietta College (BS) and Ohio University (MS). His studies were interrupted for a stint in Vietnam (US Army, Purple Heart; Silver Star). Twenty years in Dow Plastics provided extensive experience in polymer synthesis, development, production, and processing. John has been a seminar leader with RJG Associates, Injection Molding Magazine, University of Wisconsin Milwaukee, General Polymers and John Klees. Competent in resin characterization and analysis, his specialty is practical, hands-on injection molding training with both small and large machines. National recognition has come through ten patents, over 60 papers covering plastics, processing, machine specifications, and over 12 years on the national seminar circuit. Feature articles such as the “Productivity”; by Plastics World and ”Scientific Molding” by Injection Molding Magazine October, November and December 1997, have highlighted a couple of exemplar case histories. Check out the August 2001 issue for applications of The Universal Set Up Sheet.John is the initiator of Scientific Injection Molding and teaches the plastic’s point of view for design and processing with a passion you will remember. Take some of your valuable time to learn practical molding techniques that improve your profits tomorrow while eliminating the state of ”fire fighting“ currently found in many molding facilities. Let us keep plastic manufacturing strong in North America.|
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