Blog

My name is Randy Kerkstra, and I have been living and breathing injection molding for 26 years. I’m excited about starting this blog to share lessons learned over the years and to assist the industry with trouble-shooting problems they might have. And I’m thankful to Nanoplas for letting me use their website as a avenue to do this. Over the years I’ve questioned everything I’ve ever been taught, and I strongly believe in facts versus opinions or assumptions. Because of 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 that comes my way. Nothing is allowed off the list of possible root causes unless proven otherwise; no assumptions or opinions should take a possible solution off the list.

My main focus is to have a robust process window, and this typically involves the tooling side heavily. I’ve been very successful focusing on the tool to address root causes to help with having a large processing window. The plastics industry uses processing as the main tool for eliminating issues, and while I agree that processing is the number one tool, every time you process around a root cause your overall window becomes smaller. Whereas when a change is made to a tool to address an issue it becomes a more robust solution. There are many variables with the plastic, process, and machinery that can easily change, and if your processing window is small you will run into issues.

Here I’ll cover flash as my first topic (likely a 2 to 3 article series). Flash is typically an easy issue to diagnose. One of my favorite quotes with this issue is, “Flash is avoidable if 3 conditions are met no matter what the process or material being used.” I’ve had many people challenge this statement, but I’ve never been beaten by flash if these 3 conditions are met:

–        Clamp pressure must exceed cavity pressure

–        The tool must be robust enough to prevent deflection in line with clamp force and perpendicular to clamp force.

–        All shut-offs 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 mentally understand this concept. If you have 2 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 always something that’s brought up with this statement, which is a very important topic I’ll cover at some point in the future. For now, I’ll go into cavity pressure versus clamp pressure with this article and will cover the other 2 points in depth with the next article.

The industry has standard formulas for tonnage per square inch based on the plastic being used. These are typically good standards to go by, but they don’t even come close to what pressures you can see when wall stock and flow length come into the picture. Flow lengths and wall stock can drastically increase the pressure needed to fill the part out. I reviewed a tool design a while back that had the runner length increased to gate at the end of a longer part. It was a GF material and the concern was with warpage. But 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 re-grind 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 ended up having a warpage and pressure study done before they decided on the design.Randy Kerkstra

First they found the warpage concern was the same whether it was gated on the end or the middle of the part. The pressure results were amazing. If gated on the end of the part the fill pressures exceeded 12,000 psi. And if gated on the middle of the part the pressures were only 5,500 psi. The difference with machine size to have clamp force exceed the cavity pressure was a 120 ton machine to a 300 ton machine. Wall stock thickness can have the same impact. I like to bring these 2 items up when required clamp force is brought up because they throw the industry standard formulas right out the window.

Next month I’ll get into the parting lines and tool deflection and where they can contribute to flash issues. I’ll also cover details of where the injection molding machine can contribute to flash issues, and, at times, these issues can be tricky to determine whether it’s the machine or the tool.

Please feel free to send questions or issues my way. I will help were I can.