Sound Off: Let's design hot runners into the molding machine

By: 
December 31, 1996


Matt Lofgren
Matt Lofgren, Kendall Healthcare

Editor's Note: Matt Lofgren is a molding supervisor at Kendall Healthcare Products Co. in Ocala, FL. This article is his description of a design for a new type of injection molding machine. Read it over, give us your feedback. Is it old hat? Does it make sense? What are the strengths of his design? What are the weaknesses? Does it have practical applications today? Could it work?

We may be at a crossroads in injection molding machine technology. Since the advent of the reciprocating screw to replace the original ram or plunger design, there have been few technological leaps in the machines with which we process today vs. the machinery molders ran 20 or 30 years ago. We have seen incremental advances in equipment such as microprocessor control, all- electric designs, and tiebarless machines, but the basic design has remained the same.

Hot runner systems compensate for some molding machine deficiencies, and we rely on them to the point where they are nearly an extension of our machines. Unfortunately, this expensive, complex system is useful with only one mold. Want to install another mold that benefits from the features a hot runner system offers? You must design, purchase, and fabricate that next mold around another hot runner unit.

It seems like we are reinventing the molding machine every time we see a new iteration of the latest hot runner system. Granted, hot runner technology has come a long way in the last 10 or 15 years, to the point that many molders swear by it. Hot runners serve many molders quite well; our molding department in particular runs several hot runner molds, for the most part successfully. But here are some of my observations of the drawbacks of hot runners:

  • Hot runner systems are expensive. A typical eight-cavity hot runner manifold system can add $20,000 to $50,000, or more, to the cost of a mold. Even the most rudimentary systems start at more than $10,000.
  • Hot runner tools are quirky. A molder can run into trouble whether starting up, running, shutting down, or cleaning hot runner systems. They are complex systems, particularly valve-gated hot runner molds.
  • Hot runner molds tend to take longer to install and remove. Many shops around the country have gone to or are considering going to a JIT molding environment. While this approach certainly looks good on paper, it puts extra burdens on the molder as far as quick and efficient changeovers go.
  • Hot runners are not suitable for certain materials. I don't know many molders who run 45 percent glass-filled PET through hot runners. I'm sure it's been done, but such systems would have to be fabricated from exotic steels to withstand abrasives in filled materials.

Hot runner systems do a great job if they are properly designed and used for what they were intended. Their inherent advantages make it easy to justify the expense and hassle of these molds. What I'm advocating is to take a new approach: Let's put the money we're spending on hot runner systems into the injection molding machine itself.

Figure 1
Figure 1. The big difference in this machine is six small, but conventional injection units, three high and two wide in a tight pattern in front of the fixed platen.

The Idea: "MultiJect"
I propose a new design for the injection molding machine. The machine is a multi-injection unit with an integrated, standardized mold base that I have dubbed the "MultiJect" injection molding machine.

The "MultiJect" looks like this: Start with a standard 250-ton base injection molding machine, toggle or hydraulic. The machine remains as is from the movable platen on back. The big difference in this machine is six small, but conventional injection units, three high and two wide in a tight pattern in front of the fixed platen (Figure 1). This configuration is variable; it could be a four- or eight-injection unit system or other clamp tonnage of the base machine. The point is, a separate unit injects each cavity of a mold.

The mold itself is also unique. The customary center sprue bushing and runner system are obsolete, replaced by either direct injection of material into each individual part, or a small runner system gating to two (or more) cavities. The result is a molding machine that has all or most of the advantages of a hot runner system built into the machine, with none of the inherent disadvantages.

The individual injection units are independently controlled by their own hydraulic (or electric) circuits to allow for separate control of all normal injection unit functions. Each injection unit is also designed so it swings out independently of the other injection units to allow for maintenance, even while the other units are producing.

The Mold
The mold is unlike anything to which we are accustomed. Instead of a locator ring surrounding the sprue bushing used today, I invision several locator or guide pins to properly align the mold into the fixed platen, automatically squaring up the mold. For the purposes of this description, assume a six-injection unit design. Six small, individual sprue bushings are located to coincide with the tips of the injection units. With extended nozzles we inject directly into the part, or into a simple runner where no sprue would be attached after ejection of the part/runner.

All mold features are still used, including tunnel gating, three-plate mold systems, unscrewing molds, water lines, and others. Just think of it as six individual mold details built into a larger mold base. The mold is clamped to the platens conventionally, or by using Arburg-style bolts through the platens.

This design mimics six small-tonnage injection molding machines running one- or two-cavity tools. It eliminates the expense of six separate presses and operates in the same footprint as a standard 250-ton machine. The biggest benefit is full control of each individual cavity (or two cavities if running two per injection unit). A molder could run a six- or 12-cavity tool with the efficiencies of a hot runner system without the per-tool expense.

The Benefits

  • Single-cavity control of injection pressures, velocities, and times. This design injects fresh melt into the cavity each time, not melt that's been degrading in hot runners. It also eliminates problems of flash or shorting.
  • Standardized tooling. There is no investment in individual hot runner systems for each mold; we invest in the machine up front and use standard tools to fit the machine. Smaller tools, e.g., less than six cavities in this case, could also be used as long as the tooling is designed for this type of press. Though a single-cavity tool may not be cost-effective using this design, bolsters such as those used to support the molding plates during high clamping pressures could be installed across the face of the platen to prevent tiebar strain should this configuration be required.
  • The ability to purge or perform screw and barrel maintenance on individual barrels while still running other cavities. How much more efficient would your operation be if you only had to lose one or two cavities while changing out a defective heater band while your press kept running?
  • Faster cycle times. As with a hot runner system, we have little if any runner to cool.
  • The capability to shut off single cavities, but with no ensuing balancing problems; nor would a molder have to adjust the shot size after dropping a cavity.
  • The ability to run different color materials in the same mold, on the same shot cycle. This design can mold six different colors simultaneously, eliminating the need to run separate colors on separate machines and mix the product before packaging.
  • Family molds would be much more practical. It's difficult to get good fill on two different sized cavities on the same tool, as most of the family molds are today. With the "MultiJect" system, simply program in separate shot sizes and process parameters for each individual injection unit or cavity combination.
  • No material limitations. Unlike most hot runner systems, any material compatible with today's machines can be molded. Each injection unit could even run a different material if needed.
  • Component parts for the injection units themselves are less expensive than the cost of a comparable, conventional full-sized injection unit. Components—screw, tip, barrel—are standard off-the-shelf parts for smaller conventional presses.
  • Better control of the heat profile. Material runs in a small barrel to feed a single cavity, with a small runner.
  • Balanced pressure distribution. The mold itself is more stable as pressures are evenly divided throughout the tool. The "MultiJect" system practically eliminates unequal flexing of the tool, easing the strain on tiebars at the same time.

Shortcomings
The purchase price of the machine would be higher; I estimate about twice what we pay for a machine today. This would be a one-time purchase though. Our controller technology is up to the task of individually controlling each injection unit and displaying those results on a CRT. The hydraulic circuits would be complex, but most of the circuits would be a duplication of what exists in most machines today.

The tooling would be unique to the machine. The mold base industry and mold designers would have to be sold on the concept. Unfortunately, conventional tooling cannot run

on this type of machine, unless adapters are designed to accommodate them. The tool in my design is offset from the centerline of the platen, and there are limits imposed by the small volume of the injection unit itself. Given the large base of existing tooling, this is the biggest stumbling block (aside from designing and building the machine itself).

Where to Now?
One can see that my mythical molding machine has numerous advantages over the traditional molding machine and its hot runner counterpart. There are disadvantages, but to me, they do not seem insurmountable.

Is this a concept that could catch on in the molding community? Would there be enough support for this concept to have a company take the risks involved to see the "MultiJect" system to its conclusion? I don't have the answers to these questions, but I would like to have some feedback (pro and con) from anyone who has kept an open mind to this point.

Has it been done already?
Matt Lofgren proposes a design for a new type of injection molding machine, but it's not really new. Battenfeld produced a four-barrel injection molding machine in the early '70s. Battenfeld, according to one of the machine's owners, apparently only ever made four such presses before it stopped producing them. According to Battenfeld records, the machine was given the model number PFG 400/4X 3000.

At least one of those presses is still in operation today. The machine started life with Playskool in the early '70s before it was purchased in the mid '70s by now-defunct Keolyn Plastics in Chicago. Jack Glatt, formerly with Keolyn, says the 440-ton press molded structural foam in four-cavity and single-cavity molds with shots up to 60 lb.

In the early '80s Keolyn sold the press to FM Plastics in Rogers, AR. Dave Shallenberg, manager of manufacturing services at FM Plastics, says the four-barrel machine is one of his most reliable presses. "It's become a real workhorse," he says. FM Plastics molds products for computer, business, and medical industry applications using polystyrene, Noryl, and PP. Shallenberg says he uses some single-cavity molds on the machine, but most of the work is done using multiple cavities with shots in the 4- to 7-lb range.

Shallenberg says he's rebuilt much of the machine, adding a controller for each barrel, replacing the original pair of controllers that monitored two barrels at a time.

One of the advantages Lofgren describes in his design idea is the ability to perform maintenance on one barrel or screw while the others continue to run. Shallenberg at FM Plastics says the compact design of the four barrels on the Battenfeld, combined with mold use, prevents him from performing such maintenance.

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