In all phases of the molding process—first stage, second stage and cooling—the cavities must be treated the same. If not, you run the risk of several different injection molding defects. We can prevent these issues in advance by ensuring six key factors are as they should be.
What is Cavity Balance?
The measure of how evenly a multicavity mold fills is called cavity balance. Multicavity molds aren’t as consistent as single-cavity molds, but since multicavity molds can usually produce parts at a lower cost, it’s essential to make sure the mold is designed for a balanced multicavity. If one cavity fills ahead or behind the others, the plastic in that cavity will process differently, resulting in a different part.
We should emphasize the importance of cavity balance throughout the entire process. During the fill, pack, hold, gate seal, shear and cooling phases, every cavity must be experiencing equal conditions.
Injection Molding Defects Caused by Poor Cavity Balance
If you don’t have proper cavity balance in your mold, you could end up facing:
Short shots
Flash
Sink
Burns
Voids
Dimensional issues
Warp
Sticking
Gloss
This is a pretty big range of issues that can derail your production and potentially your cost savings of using multicavity molds.
Six Factors for Perfect Cavity Balance
In order to ensure cavity balance, the following conditions must be equal in all cavities:
Flow length
From the material inlet to each cavity, the flow length should be designed to be the same for all cavities.
Flow diameter
Similar to the importance of flow length, the flow diameter through the runners has to be equal for each cavity. Varying flow diameters can cause a whole new set of problems.
Shear
Even with identical flow length and diameters, eight-cavity molds will still run with imbalance as the four inner cavities usually fill before the outer cavities. This can be combatted with John Beaumont’s MeltFlipper®, which solves the root cause of imbalance. You can also add a hot-runner system that lets you group four cavities at a time, although you could still find problems with shear imbalance, albeit not as sever.
Cooling
Efficient cooling is required to ensure there are no differential mold temperatures. Start the inlet water at the center of the mold, look for a mold surface temperature variation of 10° F and the difference between water inlet and outlet temperature of any circuit should be a maximum of 4° F.
Venting
Equal venting of all cavities lets them each experience the same conditions. If one cavity is poorly vented, it will probably burn or short.
Clamp pressure
Without equal clamp pressure applied across a mold, you risk non-uniform venting.
Multicavity molds are great for production time and cost savings as long as you ensure these six key factors are consistent every single time.
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When you see a non-uniform fanning out from the gate, which appears as a different color or gloss level to the surrounding plastic, you may be dealing with gate blush.
What is Gate Blush?
Also known simply as blush, or gate shear or halos, gate blush is a dull or discolored portion usually found just inside the gate location of a part. Occasionally, gate blush will show up in areas where there’s a wall-stock transition.
Blush can be confused with jetting, flow lines and flow marks, so you need to be careful when troubleshooting. Since jetting gives a blush-like appearance, you’ll want to confirm you’re dealing with gate blush before troubleshooting.
Troubleshooting Gate Blush
If you’re experiencing blush, it could be due to the molding process, mold itself or the machine. See below for possible causes.
Molding Process
Mold
Machine
Material
Injection velocity
Gate geometry
Nozzle
Nozzle temperature
Hot runner tip temperature
Machine performance
Mold temperature
Cold slug well
Melt temperature
Gate location
Table 18.1 Gate Blush Troubleshooting Chart, found in Injection Molding Advanced Troubleshooting Guide: The 4M Approach (p. 142)
How to Eliminate Gate Blush in Injection Molding
Most often, the culprit is an injection velocity that’s set too high. However, the solution isn’t as simple as lowering the injection velocity. Not at first, anyway. Before you reduce the fill speed, ensure your gate size and design are adequate. If your blush is caused by something with the tooling design and you try to process around it, you risk other defects, particularly short shots.
Pay attention to all temperatures: nozzle, mold and melt. If the nozzle or mold temperatures are too high, it can impact the first plastic in the mold, resulting in blush. A melt temperature that’s too high can hurt the quality of the initial formation of plastic out of the gate.
As we mentioned, check the size and design of the gate before attempting to process around a potential tooling issue. If the gate isn’t flush to the mold surface, that needs to be adjusted before you change any of the settings with the molding process.
If the problem is with the machine itself, your first place to look should be the nozzle. Its length, style, tip type, tip orifice and heaters all need to match the documented process.
Although less common, there could be issues with the material. PC/ABS blends and TPOs tend to be the most prone to gate blush.
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Are you experiencing bubbles during your injection molding processes? Let’s go through the possible causes—and fixes—to get rid of bubbles.
What are Bubbles in Injection Molding?
Bubbles, sometimes known as gas traps or blisters, are cosmetic defects that detract from the looks of the part, which is especially irritating if the part will be seen. When diagnosing bubbles, you need to be careful, as the characteristics are similar to voids. Before you can troubleshoot bubbles (or voids), you need to be sure which one you’re dealing with.
Is it a Bubble or a Void?
When gas forms in the melt stream, you end up with bubbles. Voids are instances of plastic shrink that creates a vacuum in the plastic wall stock.
To determine whether you’re looking at bubbles or voids, slowly heat the area with a torch or heat gun. If it’s a void, the wall stock will collapse and show a sink. If it’s a bubble, the wall stock will swell due to the gas inside expanding.
Troubleshooting Bubbles
Once you know you’re dealing with bubbles, here are the possible causes:
Molding Process
Mold
Machine
Material
High melt temperature
venting
Machine performance
Moisture content
Low back pressure
Hot runner temperatures
Crack in feed throat leaking water
Transparent materials
High decompression
Cracked water line
Screw design
contamination
Venturi effect
unmelts
Table 18.1 Bubbles Troubleshooting Chart, found in Injection Molding Advanced Troubleshooting Guide: The 4M Approach (p. 166)
How to Eliminate Bubbles in Injection Molding
Try running a series of short shots as your first troubleshooting step. Doing so will help you see where the bubble begins to appear, and if it only shows up in specific locations, you can often determine the root cause.
More often than not, the cause of bubbles is excess gas in the part, which can come from high melt temperature, low back pressure or high decompression. Adjusting these settings, again using short shots if you can, could solve the problem.
Otherwise, you’ll need to run through the checklist in the table above and adjust settings as necessary.
As always, using Nanoplas’ innovative line of injection mold coatings such as Heat Cure and Quick Cure, or sprays like Tuff Kote and Dri Kote, will help avoid injection molding problems, particularly when the cause is with the mold, material or machine.
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What Causes Plastic Delamination in Injection Molding?
When you’re able to peel the surface of a molded part, layer by layer, you’re dealing with delamination, an injection-molding defect that is sometimes also known as lamination or layering. This is a bad sign—delamination hurts the strength of the part, thus making it unreliable and potentially dangerous, depending on the part’s intended use.
Common Causes of Delamination
Just like most injection-molding defects, delamination can be caused by one or more of several potential issues. The most common: incompatible materials. When two materials can’t bond together, it results in separation that very quickly becomes delamination.
“Incompatible materials” can be a number of things.
Too much moisture on the material, which is usually due to a failure to completely dry the material before using it. During the molding process, moisture becomes steam, which leads to surface delamination.
Delamination can also be caused by high shear stress, too much injection speed, a high melt temperature or material degradation, so you should look at all potential issues while troubleshooting even though you’ll most frequently find your issue with the incompatibility of materials.
How to Eliminate Plastic Delamination
Preventing delamination first requires you to know the cause, but once you do, you should be able to get rid of the problem.
Make sure you’re using plastics that will properly bond with each other. Always take extra care to ensure every part that goes into the mold has been completely dried. When applying mold-release sprays, do so as intended so as not to overspray. If you find that you need more release spray than you know is wise, then your issue could be with the mold itself and you may want to rethink the design to make ejection easier.
If you find the issue to be with the molding process, such as shear stress or melt temperature, adjust those settings as necessary to eliminate delamination.
Nanoplas Mold Release Products
Our Heat Cure™ and Quick Cure coatings are specifically designed to allow non-stick ejection and prevent delamination. They are semi-permanent coatings applied in-house and can last for thousands of shots.
As opposed to other sprays that go on heavy and can cause buildup in the mold, our Tuff Kote and Dri Kote high-performance sprays go on light and are ideal for avoiding delamination.
Remember: with all mold-release products, make sure you’re applying them properly.
Ejector pin marks, sometimes called pin push, are the glossy or white imprints caused by the ejector pins that show on the class-A surface of the part. These marks can easily crack during the use of the actual products, so you want to prevent ejector pin marks before they happen.
What Causes Ejector Pin Marks?
When the ejector pins try to push the part out of the mold, sometimes the mold sticks, which causes the pin marks to appear on the finished part. However, the issue may not always be with the mold itself, but rather the molding process, machine, or materials.
The most common culprits include:
Over-high injection pressure, over-fast dwell time, overflow mold temperature accompanied by an over-fast or unbalanced cooling speed, which altogether increase the internal stress of products.
Over-small draft, incorrect or insufficient spraying of the mold-release agent or an over-large ejection resistance.
Improper design of the ejector unit and over-fast ejection speeds can result in large ejection stress among the plastic and ejector parts.
Improper design of structure for the products.
How to Eliminate Ejector Pin Marks in Injection Molding
All the above causes create unnecessary internal stress for the molding of products, so we need to reduce the ejection resistance and stress being applied by the ejector pins on the plastic parts.
Avoid excessive injection of melt.
Reduce injection pressure and dwell pressure. Shorten dwell time.
Reduce injection rate, optimize the design of the feed system and realize a sound mold filling.
Properly improve mold temperature, prolong cooling time and reduce cooling speed. Optimize temperature control and product structure to encourage uniform cooling of products in the mold.
Increase draft while ensuring proper retaining mode of plastic parts in the mold.
Analyze the ejection resistance and distribution, then use a suitable ejection mold with properly arranged ejector pins, trying to achieve a balanced ejection. If you see large ejection resistance, increase the number of ejector pins in those areas and expand the section dimensions of the ejector pin. Increase wall thickness where the ejector mark can easily occur in an effort to strengthen the products’ resistance to stress.
Increase smoothness for the cavity surface. Polish along the ejection direction of plastic parts in the final stage of polishing. If you’re polishing a mold that already has ejector pin marks, apply polish corresponding to the ejector mark.
Properly spray the mold-release agent.
Nanoplas Mold Release Products
The best way to solve ejector pin marks is with our Heat Cure™ and Quick Cure coatings. These injection mold coatings are applied either in your tool room or on the press and give exceptional release characteristics to the part in the mold, allowing it to eject without sticking and thereby eliminating any pin push marks.
If the only solution is to use a spray, try our Tuff-Kote and Dri-Kote high-performance mold-release sprays. The sprays go on light and last for many shots, as opposed to other sprays that go on heavier and can cause buildup in the mold, which can lead to later release issues or downtime to clean the molds.
Try our industry leading mold release agents for yourself, request a sample today!
There are three types of injection mold release coatings within our Nano Mold Coating family and two ways to apply them, depending on the type of coating you’re using. Our HC and HCF (the food-grade alternative to HC) coatings are applied in the tool room at room temperature and all QC coatings (QC, QCRU and QCSI) are applied in the press in a hot mold.
These coatings have been scientifically formulated with the use of nanotechnology to create a semi-permanent barrier on the surface of molds which facilitates extraordinary plastic or rubber part release. The cured injection mold coating is a non-toxic, and colorless hardened polymer film that is only 100-200 nanometers in thickness so it has no affect on finished part dimensions.
Before we can talk about the actual injection mold coating application, we need to talk about cleaning the substrate, which is the most critical part of the application process. Regardless of what type of coating you’re using, the cleaning process is the same.
First, pre-clean the surfaces with Nanoplas Mold Brite or Power Clean. Remove all debris, oil, lubricants and rust preventatives from the entire mold, including the pores and crevices.
Use the white microfiber cloth included with the kit until you’ve removed all oil and debris. The white cloth makes it easier to see when you’ve gotten everything. Don’t use shop rags as they are often contaminated with lubricants or detergents and we’re trying to get rid of that—not add it.
Next, wet a clean, white cloth with ethanol, alcohol, acetone or MEK solvent to get rid of any remaining degreaser or oils. Before you begin the application process, make sure the surface is completely dry.
Injection Mold Coating Application – HC and HCF
The application method for HC (Heat Cure) and HCF (food-grade Heat Cure) injection mold coatings is the same, done at room temperature in the tool room. You can use a microfiber swab or cloth, depending on the actual mold configuration.
Apply a small amount of coating to the swab or cloth and apply to the surface in one direction. When this is done correctly, the surface should look wet as if you rubbed an alcohol wipe over the surface. If the swab or cloth dries out, reapply coating.
After you apply the coating in one direction, apply a second coating in a perpendicular direction over the same area. The reason for the second coat and change in direction is to make sure the entire surface is coated well, as the coating can be hard to see during application. Make sure there are no streaks or pooling, as that can lead to a sticky residue once cured.
Next, with a standard heat gun, begin the curing process. Set the gun to 550-600 degrees Fahrenheit. With the gun 4-6 inches from the surface, apply heat in a slow, sweeping motion, back and forth, for at least 10 minutes over the entire coated area.
The HC and HCF coatings each have a UV dye that will show up under a black light, allowing you to confirm the entire area has been coated.
You can apply a second coating for a longer coating life if you’d like. One satisfied with your coating, cover the mold with a clean cloth to keep dust from settling into the coating. Let it rest for at least three hours to allow the coating to completely harden.
With any of our Quick Cure family of coatings (Quick Cure, Quick Cure Rubber and Quick Cure Silicone), you’ll apply the coating in the press using the heat control of a thermolator.
After cleaning the mold thoroughly as described above, heat the mold to 120 degrees Fahrenheit. Shake the QC bottle well before using and during any reapplications.
Apply a mist to the surface of the mold or the microfiber cloth included in the kit. Wipe lightly, in one direction, making sure you create a thin, even layer. Remove any excess pooling or streaks immediately. After you’ve covered the entire area, apply the coating again, this time wiping in a perpendicular direction to make sure the area is properly coated.
Once you’ve applied the coating, heat the mold to 240 degrees Fahrenheit and allow it to cure for 15 minutes before production. If your press can’t get to 240 degrees, increase the length of time of the cure proportionally to the reduction in temperature. Once the mold is cured, it’s ready for use.
General Mold Maintenance
When you need to clean your coated molds, we recommend Nano Mold Cleaner, which was designed not to remove the coating. Other injection mold cleaners can gradually remove the coating and reduce mold performance over time.
For a demonstration on how to apply our coatings, watch our video on how to apply injection mold coating.
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