Flash Free Molding, Part 2

Injection moulding machine I hope you found the first article on flash free molding interesting. [Read Flash Free Molding Part 1.] I covered cavity pressure versus clamp pressure and will talk tool deflection in this article. Tool design and construction can be a contributor to flash issues. But, if you read the first article and the three conditions are met, flash is not possible. An inadequate tool design can contribute to flash by allowing deflection in line with clamp force and perpendicular to clamp force. Also, on the construction side, improper spotting, timing of components, and fitting of components can be contributors to flash.

The tool must be designed to withstand more than the amount of plastic force against it. I’ve been amazed through the years with how easily plastic pressure can deflect or move what appears to be a robust condition. 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 with a new tool on the runner or gate area. Well, it’s either lack of clamp pressure or, in most cases, tool deflection. 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 pre-load (I didn’t for years). You can never have too much surface area of support. There are a couple of areas to consider when designing a robust tool: 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 going from a 1-inch to a 2-inch support pillar you will gain over 400% of surface area. And going from a 2-inch to a 3-inch support pillar you will gain another 200% of surface area.

Now you’re wondering about how much pre-load 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 pre-load would be needed. So, the more surface area you have with support the less pre-load is needed. Everyone has their own opinion on what’s adequate, especially tool makers. The most likely answer would probably be around .003 – .005 of pre-load. But I’ve had tools that needed to be pre-loaded as high as .015 and gradually reduced until the pillars meet the rails. I call this crowning the pre-load — heaviest in the center and then tapering off.

Rail width should be another consideration along with the distance between the rails and the unsupported area it creates. The following is a mental picture on how the area between the rails affects required support and is not an actual situation: If the support rails are only 1-inch apart (that’s one thin ejector plate) you would probably need zero support. And if your support rails were 36-inches apart, how much support would you need? I’ve seen many 120-ton tools with 2-Inch rails and 500-ton tools with 2-inch 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 when used on components, and at times cannot withhold the plastics pressure they see: laminated inserts, inadequate slide locks, and Hydraulic cylinders. I’ve been amazed by how much steel can deflect when it sees thousands of pounds of plastics 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 see. The formula to determine this is not complex. First, you need to know the maximum plastics pressure the cylinder would see (and you never design at a 1 to 1 ratio) that is the opposite of robust. I would recommend at the very least a 1.5 to 1 ratio. I’ve found many underestimate the maximum plastic pressures. Once you know the cavity pressures base off 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 X Radius X Radius) 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 also have many examples of when inadequate slide locks contributed to flash issue. Again, it comes down to understanding the amount of plastics pressure they see based off 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 half of the tool at times to make them more robust. Even if the lock angle is in the solid, that doesn’t mean it’s robust enough to withhold cavity pressure.

I was amazed some years back with a GF 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 pre-load 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 .012. We had to bolt a thick plate to the side of the tool with large bolts to prevent the flash from deflection. I have also witnessed a tool being deflected .060 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 a tool, and it was a repeat issue with the side of the tool deflecting out under pressure. We ended up doweling the cavity block to the support plate with a bunch of large dowels to help prevent the deflection.

In the next installment, I will follow up with the tool and shut-off angles and wear issues causing flash along with the machine and how it can be the root cause of flash. [Read Flash Free Molding Part 3.]