Most molders are still using screw designs that haven't changed much in 30 years. But they don't need it. #scientificmolding #processingtips #screws

In November, as part of the Tech Days webinar series, Plastics Technology held a conference on "Establishing and Maintaining a Robust Molding Process" and I attended the conference. During this webinar, a brief discussion on the new type of injection molding machine led to a discussion on screw design and melt uniformity, which is one of my favorite topics in the molding industry.

Figure 1 Melt model of general injection screw.

During this webinar, Mike Durina made a compelling comment. He is a respected injection molding innovator who runs his own company MD Plastics. Mike pointed out that most molders still rely on the "universal" screw design 30 years ago. With all the technical improvements made to the machine during that time, no measures were taken to improve melt uniformity. We still use "general purpose" (GP) screws, which the famous screw designer Bob Dray wisely calls "useless" screws. That is, they will not melt plastic uniformly. The use of uniformly melted plastic for processing seems to be a top priority, but few people pay attention. This needs to change.

Why is melt uniformity important? Well, it will be difficult for you to name a common molding/part problem that does not cause or exacerbate melt inhomogeneities. Just to name a few:

 1. Weak welding line;

 2. Part failure/performance problem;

 5. Marble pattern (color swirl/spot);

 6. Resin degradation and black spots;

 7. Long time for color change;

 9. Clean the screws frequently;

Figure 2 The degraded polymer at the joint between the thread and the root diameter of a general-purpose screw indicates a dead space.

So how can we obtain melt uniformity? Start with the melt model of the GP screw shown in Figure 1. As shown in the figure, all particles usually do not melt, and the resulting solid bed breakage will lead to poor mixing and degradation (and cause the above problems). In addition, in the area where the thread and root diameter match, the plastic stagnates and degrades Is very common. Figure 2 shows the typical polymer degradation that is often seen when a screw is pulled for cleaning or repair. Although I don't have actual statistics, my experience is that at least 50% to 80% of all machines running today will have this problem. Even if it is only 25%, it is still a major problem in our molding industry. Why is the industry so tolerant? Do you have to endure this?

Figure 3 In this barrier screw, note that the green polymer is still in the barrier thread, even after cleaning. This represents dead space. (Photo: Joe Cascarano)

The fact is that the industry does not have to use GP screws. There are solutions, but be careful who you talk to. When someone started asking machine suppliers and most screw manufacturers these questions, the subconscious answer was: "Try our barrier screws." Proceed with caution. Figure 3 shows a typical barrier screw after cleaning. Please note that the green polymer is still flying on the barrier. Since this is after purging, you can understand that this material will stay there during the production process and will degrade over time. You will get more degradation and black spots. So, if barrier screws are not the answer, what is it?

As early as the late 1980s, I was fortunate enough to participate in a company-sponsored research project to solve the problem of mixing color masterbatches into natural resins. After spending a year and a half and more than $200,000 on about 10 different barrier screw designs that didn't work, we developed a melt homogenizing screw (Figure 4). This is not a barrier screw. This "melt uniformity" screw passed the strict instrumental requirements for color distribution and uniformity. In addition, we tried six months on seven different production machines to manufacture complex parts. The results are impressive (see attached table).

The data speaks for itself. Replace your universal screws with melted uniform screws. You will save time, make your life in the workshop easier, and make more money.

Not convinced? The well-known materials expert and plastics technology columnist Mike Sepe actually purchased one of the screws while working for a mold maker. He wrote about his experience in a column in April 2012. See what Sepe has to say about it: “When we installed the new screw, the melt temperature was reduced by 60º F and the back pressure was reduced from 300 psi. 75 psi hydraulic pressure. The problem of color incorporation and unmelting disappeared; cycle time was shortened; and regular screw cleaning was stopped. The screw paid for itself within seven months and started a revolution in operation. The factory replaced the general-purpose screw with a hybrid screw. The number of'material problems' and'processing problems' disappeared, which is a revelation."

There is only one main problem: Today, I cannot find a suitable store for this design. Believe it or not, I took a picture at a screw shop in Michigan, and their answer was, "No, we won't succeed... A high school student can make a better design." Seriously, if Someone knows a good shop that makes screws, please let me know. I need to replace what I have.

About the author: John Bozzelli is the founder of Injection Molding Solutions/Scientific Molding in Midland, Michigan, which provides training and consulting services for injection molding companies (including LIMS) and other professions. Contact john@scientificmolding.com. 

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