TOOLING: Flash-Free Molding: Part 1
I have been living and breathing injection molding for 26 years. I’m excited about this column as an opportunity to share lessons learned in that time and to assist the industry with troubleshooting problems. Over the years, I’ve questioned everything I’ve been taught, and I strongly believe in facts versus opinions or assumptions. Thanks to this,
I’ve accomplished many improvements after being told something wouldn’t work or is not achievable. I take a logical approach to every issue or problem. Nothing is allowed off the list of possible root causes unless proven otherwise. No assumptions or opinions should block a potential solution from being investigated.
My main focus is to have a robust process window, and this typically relies heavily on the tooling. I’ve been successful in focusing on the tool to address root causes to help achieve a broad processing window. While I agree that processing is the number-one tool for eliminating molding problems, every time you “process around” a root cause, your overall window becomes smaller. But when a change is made to the tool to address an issue, it provides a more robust solution. There are so many variables in plastics materials, processes, and machinery—all of which can easily change over time—that if your processing window is narrow, you will certainly run into problems.
THREE RULES OF FLASH PREVENTION
In this two-part series, I’ll cover flash. Remember this essential principle: Flash is avoidable if three conditions are met, no matter what process or material is being used. I’ve had many people challenge this statement, but I’ve never been beaten by flash if these three conditions are met:
• Clamp pressure must exceed cavity pressure.
• The tool must be robust enough to prevent deflection both in line with clamp force and perpendicular to clamp force.
• All shutoffs or parting lines must be “true net,” meaning they are tight with no gaps, damage, or anything holding the mold open.
It’s important to understand these concepts. If you have two mating surfaces perfectly matched, the tool is robust enough to prevent any deflection from cavity pressure, and if clamp force exceeds any amount of cavity pressure, absolutely nothing can penetrate the mating surfaces. Venting is often raised as a question here—it’s an important topic that
I’ll cover in the future. For now, I’ll cover the first two rules of flash prevention, and the third rule in Part 2 next month.
HOW MUCH TONNAGE DO YOU NEED?
The industry has standard formulas for clamp tonnage per square inch, based on the particular plastic being used. These are typically good standards to go by, but they don’t even come close to what pressures you can encounter when wall thickness and flow length come into the picture. Flow length and wall thickness can drastically increase the pressure needed to fill the part.
I reviewed a tool design a while back that had the runner length increased to gate at the end of a huge part. It was a glass-filled material and the concern was with warpage. The part had many contours and holes, so the orientation of the glass was going to get mixed up anyway. My first concern was with the runner size and no regrind being allowed. I suggested gating in the middle of the part to drastically reduce the runner scrap. (I know some of you are thinking “hot runner” right now. That’s another topic.) We had a warpage and pressure study performed before deciding on the tool design.
Warpage was found to be the same whether the part was gated on the end or the middle of the part. The pressure results were telling. If gated on the end of the part, the fill pressures exceeded 12,000 psi. If gated in the middle, the pressure was only 5500 psi. The difference in machine size needed to ensure the clamp force exceeded the cavity pressure was a 120-ton machine vs. a 300-ton press.
Wall thickness can have the same impact. I like to bring up these two factors when the question of required clamp force arises, because they throw the standard industry formulas right out the window.
PREVENTING TOOL DEFLECTION
Now let’s look at how parting lines and tool deflection can contribute to flashing. An inadequate tool design can contribute to flash by allowing deflection in line with clamp force and perpendicular to clamp force. Also, improper spotting, timing of components, and fitting of components can contribute to flash.
The tool must be designed to withstand more than the amount of plastic force against it. Over the years, I’ve been amazed at how easily plastic pressure can deflect or move what appears to be a robust mold. One area of focus should be where the plastic is injected into the mold via the runner system and gates. I’m sure everyone reading this article has seen flash in a new tool at the runner or gate area. These areas see extreme amounts of plastics pressure and are typically at the weakest area of the tool, in the center.
Support pillars become a very important component to resolve this issue, and many do not understand the importance of adequate surface area and preload. (I didn’t myself for years.) You can never have too much surface area of support. Two important areas of support to consider are the area between the rails on the ejector (moving half) having adequate support with pillars, and any areas on the stationary half that would need support where steel has been removed for hot runners or mechanics.
It’s necessary to understand the importance of support pillars and the surface area of support they create, not just the number of them or the way we’ve designed them for years. Just by going from a 1-in. to a 2-in. support pillar, you will gain over 400% more surface area. And going from a 2-in. to a 3-in. support pillar will gain you another 200% of surface area.
Now you’re wondering about how much preload should be on the support pillars. That’s not a standard spec, as many will tell you. First, I want you to understand why. If you have a solid block of steel, zero preload would be needed. So, the more surface area you have with support, the less preload is needed. Everyone has their own opinion on what is adequate, especially tool makers. The most likely answer would probably be around 0.003-0.005 in. of preload. But I’ve had tools that needed to be preloaded as much as 0.015 in. in the center and gradually reduced until the pillars meet the rails. I call this crowning the preload — heaviest in the center and tapering off toward the edges.
Rail width should be another consideration, along with the distance between the rails and the unsupported area that creates. Here is a mental picture of how the area between the rails affects required support: For the sake of an example, if the support rails are only 1 in. apart (that’s one thin ejector plate!) you would probably need zero support. And if your support rails were 36 in. apart, how much support would you need? I have seen many 120-ton tools with 2-in. rails and 500-ton tools with 2-in. rails, so why wouldn’t the width ratio have changed with the tool size? Just food for thought. Take it into consideration in the mold design that the rails should never be too wide, and the distance between them should never be too small when considering the surface area of support.
How can flash occur perpendicular to clamp force? Three areas come to mind from experience—mold components that sometimes cannot withhold the plastic pressure they see: laminated inserts, inadequate slide locks, and hydraulic cylinders for cores, slides, etc. I’ve been amazed by how much steel can deflect when it sees thousands of pounds of plastic pressure. You really need to focus on the mold design and the components that will see cavity pressure perpendicular to clamp force. This requires knowing the square inches of cavity surface, the material being used, part design, the process, flow lengths, and the possible pressures per square inch that they can create.
I could give many examples where tools were designed with hydraulic cylinders that were not able to withhold the plastic pressures they would experience. The formula to determine this is not complex. First, you need to know the maximum plastic pressure the cylinder would see. You never design at a 1:1 ratio (that is the opposite of robust), so I would recommend at the very least a 1.5:1 ratio, or designing for 150% of the maximum plastic pressure anticipated.
I’ve found that many tool designers underestimate the maximum plastic pressures. Once you know the cavity pressures base on the square inches of cavity surface perpendicular to clamp force, you need to determine the surface area of the bore size of the cylinder (3.14 × radius2) and the hydraulic PSI that will be used. Make sure you understand the operating hydraulic pressure—some machines will use system pressure based on the injection process.
I have seen many cases when inadequate slide locks contributed to flash issues. Again, it comes down to understanding the amount of plastic pressure they see based on the square inches of cavity perpendicular to clamp force. When lock angles are inserted they are weaker than solid steel. Nothing wrong with them, but keep in mind that you need a robust tool if the cavity surface area they are forming is large. They can also be interlocked into the opposite mold half to make them more robust.
Even if the lock angle is in the solid cavity steel (not a bolted insert), that does not mean it is robust enough to withhold cavity pressure. I was amazed some years back when working with a glass-filled nylon part that had a flash issue with the slides on a 650-ton tool. I would have bet everything I owned that it was not deflection and was a preload issue on the lock angle because of how robust it appeared. I was wrong. I ended up putting an indicator on the side of the tool during the injection process, and the side of the tool was deflecting 0.012 in. We ended up having to fasten a thick plate to the side of the tool with large bolts to prevent flash from deflection.
I have also witnessed a tool being deflected 0.060 in. from the surface area of cavity on the slide, and it bent the cavity block from the tool not being designed with enough support on the slide locks to withhold maximum cavity pressures.
Laminated cavity inserts can also be an issue if the cavity block is not robust enough to prevent deflection. This is probably a rare case with mold design, but I had my hands full some years back with the side of a tool deflecting under pressure. We ended up doweling the cavity block to the support plate with many large dowels to help prevent the deflection.
Next month I will follow up with shutoff angles and wear issues causing flash, along with how the machine itself can be the root cause of flash.
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