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I am working on an air pressure expulsion mechanism for plastic injection molds, to avoid the use of ejector pins and eliminate or reduce the ejector pins marks. I would like to know other possible solutions to reduce the marks when you eject the parts from the cavity. I will appreciate your comments or ideas!
I assumed that it is plain tap water and serves to heat and possibly cool the mold. I introduced the possibility of using an automotive coolant and here it would be a heating fluid. Certainly pH would be important and I assume should be kept as near to pH7 as possible since aluminum oxides can exhibit amphoteric properties. There are various inhibitors that can be used in these situations, but misuse or lack of monitoring can even accelerate the corrosion.
I don't know how many molds you have, but you might want to try all three types. Depending on various factors, the IAT may cause deposits and affect the overall performance, so I would be more inclined to try the OAT or the HOAT coolant. The use of such coolants in your circulating system may be an inexpensive way to fix your problem.
Erucamide is traditionally used as a slip in polymer films and the methodology is that the best slips are those least compatible with the polymer, so they will migrate out to the surface and act as a slip agent. I recommend you contact your additive supplier and let them know the materials and your needs and they can advise you what would work best as a mold release agent for your application. If you desire less blooming, either choose a non-blooming product or a poorer performer for your base resin, which would be one that blooms less. Your additive supplier is the best source for choosing the correct product for your application.
Blooming effects with fatty acid amids are common (more with oleamides though). The 1500 - 2000 ppm level you are using is quite high and defined to provide a very good slip effect. The functionality is to migrate with this additive class. The additive is soluble in the amorphous melt and literally squeezed out upon cooling of the melt. The less crystalline the morphology is, the faster the migration. The additive continues to migrate (through the amorphous domains) and this effect may cause sub-sequent blooming problems.
POM is sensitive to chemical attack by both strong and weak acids, also bases. There are many household products that are either acidic or basic. Coupled with manufacturing defects, if the part is used in this type of environment, it would certainly compound the problem. POM because of its very high crystallinity, undergoes a high coefficient of thermal expansion and high mold shrinkage. Once again the high thermal expansion in the presence of frozen-in stresses would certainly contribute towards part failure. The breakage occurred only in some markets and not in others. It would be very educational, if you can investigate in which markets it did not fail - usage conditions etc.
I have processed many materials at these temperatures. You must make sure the plastic itself is at the right injection temperature and the time sequence is not too slow for opening and closing the injection mold between cycles. For the injection mold I would use plastic processing equipment controls for cartridge heaters and thermocouples. These are much more efficient than using oil heaters. Make sure the heaters are within a .25 inches from the injection mold surface and space the thermocouples between the heaters for accurate readings. To cool it faster use a water jacket system with a closed loop to conserve water. This can be designed with common off the shelf plumbing supplies. You can even create an outside retention pond that will add cooling value and looks really awesome outside your facility.
Electric heaters with on-off controllers will be a problem unless the injection molding cycle matches the heating cycle (unlikely), but you can use a variable-resistance (like a dimmer switch) to keep constant voltage (value set by you when you see what works).
That is, as the mold opens, a long thin "string" of plastic is pulled from the nozzle attached to the sprue. When such phenomenon happens it's not necessary related to the molded material but to the process and set up of both injection molding machine and the mold.
Also check to make sure the nozzle and sprue interface is correct (radius) and that there is no misalignment. Even with nozzle contact always forward, typically after injection-hold finish the carriage forward pressure will be reduced. Misaligned or poor contact can cause the nozzle tip not to freeze off or break. Make sure the front nozzle zone heater is not too far forward, every application may be different, but 10mm distance is a good reference. If you have the ability, reduce the final last zone screw position back pressure during recovery and balance suck back.
This is a new product, has not completed sign off yet. The mold has not produced consistently good product, and this is the first time we are using this particular material.
The part presents with silver type streaks that emanate from the injection area they sometimes radiate all the way around like a bicycles spokes or are only in one area but more pronounced. If I run the part and keep the cycle consistent I can almost eliminate the issue however as soon as an operator takes over the problem will get worse within a couple of minutes.
There are only four primary variables in injection molding process that the plastic knows. They are: cavity pressure, melt temperature, flow rate and cooling rate. It is these variations in these four basic plastic conditions that can cause or solve part problems. For the warpage problem it is likely that the suggestion to run the core and cavity half of the mold at a different temperature was the most effective. This is because cooling rate differences on the core and cavity affect crystal size for this HDPE and the retained molecular orientation. Any differences in either of these cause shrinkage variations resulting in warpage. Since there is usually more plastic to be cooled on one side of the part than on the other, that side requires more cooling. Usually the core half must be run, at a lower temperature because cores are harder to cool and often have more plastic to be cooled. So, setting cooling temperature differences is probably the best option though getting completely rid of warpage particularly on HDPE can be difficult. Usually more than one of the four primary variables can affect any part problem.
Problems resulted from a non-uniform wall thickness of a plastic injection molded part are more than imagined. This article tries to analyze typical defects resulting from the part design using non-uniform wall thickness. Hopefully, a common sense of the most important principle of part design is uniform part thickness can be established.
Sink marks and shrinkage occurs most frequently in bulge area (including ribs, snap, etc.), the wall thickness equivalent in these area is greater than the thickness of the ribs and the panel (see Figure below).
When the injection mold start to cool down, the green area solidify first, the red area continue to cool done, it will suck in the other area and it becomes to be sink mark or sink hollow.
If you haven't already considered pack pressure, pack/hold time and cushion size, you might want to take a look at it. Most non-filled, semi-crystalline materials will have warp problems if they are over-packed. Of course, part geometry, wall thickness, etc. will play a part in warping of the part. But I would start with checking the fundamentals of the setup; the pack/hold dynamics would be a good place to start.