Interview Dipl.-Ing. Helmut Gries in Kunststoffe International 3/2016
“It's Hard to Believe How Much Can Go Wrong in Practice”
Helmut Gries on the Concept of System in Injection Molding and the Hard Choices when Configuring Thermal Processes
For 18 years, Helmut Gries has been the operating manager in charge of sales and marketing at gwk Gesellschaft Wärme Kältetechnik mbH. Every day he observes how hard it is for processors to keep their own processes in control while working against time. It's an existence somewhere between engineering compromises and the ideal of an efficiently networked total system.
When you travel to gwk, you could say you meet Helmut Gries at home. When the specialist for thermal processes (engineering, components, system solutions) moved from Kierspe to the former Battenfeld plant in Meinerzhagen in 2014, Gries was returning to his roots, so to speak. Nearby he began his career in plastics, and here he is developing a field of business further.
Kunststoffe: Mr Gries, for some time now the heating-cooling specialist gwk has been preaching the expansion of system technology. What does this concept imply for you and your customers?
Helmut Gries: The starting point is always our customer's application – the molded part, he wants to produce in the desired quality in the shortest cycle time possible. From our point of view, the project begins with thermal configuration of the mold. Depending on the complexity of the part and the cycle time required, various questions arise when you're looking for the optimum solution: How many different heating-cooling circuits and what temperatures does each of them require? Is continuous temperature control sufficient? Does first-run temperature with flow control make sense for the individual circuits? Or is a variotherm system the right solution? One with two water circuits, or a single cooling circuit in combination with a ceramic heater? Do we need a mold insert with close-to-cavity temperature control? And can we at all run the heating-cooling channels where they would make the most thermic sense? We don't treat each of these questions separately, but as parts of a whole.
Kunststoffe: What else is involved?
Gries: It's always a question of joining all equipment and circumstances together into an overall systems. For example, it must be ascertained whether the water quality is good enough for the temperature control systems to operate in increased temperature zones up to 200°C. Without water treatment, the high water temperatures at which chemical processes take place will cause valves and pumps to clog up within a few weeks. Even banalities are involved, such as connecting the mold: What diameter couplings are needed? But actually, the first step is choosing the plastic. Is a certain material at all suitable for making an invisible weld? What kind of surface quality can I achieve? With a 35 percent glass fiber-material, it's difficult to achieve a smooth surface by the variotherm method. We do have the capability to substantiate our databank and empirical results by materials testing ...
Kunststoffe: … to take the step from theory to practice.
Gries: Once gwk had moved to Meinerzhagen, we set up a pilot plant where we can verify the system, so to speak, and get the application running for our customer. We optimize the processing parameters and test various ways to injection mold it, such as by foaming with MuCell, and use a thermal imaging camera to analyze the heat balance in the mold indirectly on the molded part. We collect an enormous amount of processing knowledge here that not every supplier of temperature control equipment can offer. If so desired, we accompany installation at the customer's plant until the process is running reliably.
Kunststoffe: That almost sounds like a perfect world.
Gries: Of course, we're talking about an ideal case here, when we are consulted right from the start and don't make any mistakes ourselves. In practice, it often turns out differently. We are called whenever there is a problem with part quality, or when the cycle time is too long. Then we discover, for example, that pressures and flow rates supplied by the cooling system are not adequate. These often include equipment from different suppliers that doesn't work together properly. Sometimes we have to exchange individual system components to make the process run at all. That's what the concept of system technology is all about – taking the entire chain in hand.
Kunststoffe: That would then be freestyle – is the compulsory more often trouble shooting?
Gries: Where existing plants are involved, yes. It's hard to believe what can go wrong in practice. For example, the role played by the distance between the temperature control unit and the injection molding machine is often overlooked. Just recently, a customer of ours complained that our temperature control system couldn't produce the required heating power. When our service technician took a look at it, he started by pulling a 200 meters unnecessary hose out from under the machine. Of course it was impossible to maintain set temperature with such a great heat loss.
Kunststoffe: Why is the view to the obvious so often missing?
Gries: In discussions with our customers, we repeatedly observe that they are under too much daily pressure to optimize their processes, even if they have the required know-how. That's a typical scenario in the automotive supply business. After our man trimmed the hoses: just look, the process is running! But on the machine right next to it: the same old chaos. Only rarely will the customer take time to correct the problems – partly because the temperature control system is “only” part of the periphery. As specialists in system technology, we can handle that. And since we run into pressing needs every day, we also spot great potential savings for our customers.
Kunststoffe: The automobile industry is also known for certain spontaneity where final part geometries are involved
Gries: (Laughs.) That's indeed one of the problems we have to deal with. No matter that we were there during conception: part geometry details have been changed twenty times – an ejector or a core puller is required right where we had located a temperature control channel. It also gets tough when the basic mold design has been established; Then you have to do your thermal configuring by making compromises right from the start. Because not every mold maker has a feel for coordinating mold and temperature control technology. In an ideal situation, this dialog begins as soon as the 3D part concept is available.
Kunststoffe: What does thermal configuration aim to do?
Gries: A mold is a heat exchanger. In principle, it is a matter of achieving a uniform surface temperature in the cavity as quickly as possible by arranging the cooling and temperature control channels intelligently. The hard part comes from the fact that the mold design generally doesn't let you maintain the same gap between the temperature control channel and the cavity surface everywhere. In places where most of the heat is applied, at ribs, for example, there is usually an ejector, even if the temperature control channel should lie just there. How do you separate the temperature control circuits in the mold by different pressures and amounts in spite of problems presented by geometry, in order to obtain the most uniform surface temperature possible – solving this problem takes lots of experience.
Kunststoffe: Are there times when you have to ignore the laws of thermodynamics?
Gries: One example: With optical parts, it would be logical to lay a tight net of temperature control channels directly behind the cavity surface to achieve a temperature as uniform as possible. However, we now know that an impression of the temperature control channels would then be visible on the part surface. Contrary to pure thermodynamic calculation, you then move the channels one or two millimeters farther away from the cavity. On the other hand, we find that fundamental errors are repeatedly made in practice: for instance, when a mold at best provides half the heat exchange surface required to dissipate the calculated amount of heat – not to speak of uniform distribution. You would think that injection molding isn't new anymore.
Kunststoffe: New instead is the speed with which close-to-cavity temperature control is being applied in the market. How do you feel about this?
Gries: For many years we were almost alone in preaching this topic and had few allies against the dominant market opinion according to which simple drilled heating-cooling channels were quite enough to control mold temperature adequately. The technology of generative mold insert design has definitely sharpened awareness for close-to-cavity temperature control. A whole series of well-known mold makers and customers who make their own molds are now producing such mold inserts on their own laser sintering machines. So, close-to-cavity temperature control is becoming firmly established in contemporary mold making – it is used specifically for more complicated and thermally critical part zones where temperature cannot be controlled simply by drilling holes; whereas the channels in master mold base design are still bored conventionally.
Kunststoffe: You are now offering what you call the “integrat 4D” process. That means you join bored metal plates by high-temperature vacuum brazing, an alternative to generative construction. Where do you see advantages there for one or the other process?
Gries: Mainly we are talking about insert size. If it is too large, generative production is too expensive; if it's too small, we don't have enough joining surface for vacuum brazing. Moreover, we have to take the temperature control channel diameters, mold insert geometry and the required surface quality into consideration. Both methods differ in this respect and have their own justification.
Kunststoffe: What extra expense accrues to the processor when he purchases this kind of mold insert?
Gries: None at all – if we look at the topic holistically, he saves money with it. Of course, the naked mold costs are higher than with the minimalistic hole-drilling solution. How much higher depends on part complexity and mold size, among other things. On a 50 ton mold, the rather small close-to-cavity mold zone wouldn't make much difference. On small mold, that could present the biggest cost hurdle. But that's not the usual question. If the demands for part quality or economic aspects, such as cutting cooling time in half, require this expense, then that's what's done. The opposite is the case only if we're talking about a small series of a few hundred parts, then it's not worth the trouble. But where it's a matter of millions of parts, as in the packaging industry, the additional expense amortizes itself in a few months.
Kunststoffe: Most users must be aware that temperature control offers them a powerful tool for influencing part properties. Is that also the case for its economics?
Gries: Successful companies recognized a long time ago how much potential it bears. Maybe they just don't talk about it in public, because it's a matter of course for them. On the other hand, there are companies that deny themselves this potential. Recently, we had to explain to a customer that he could save half of his water consumption by segmented heating-cooling with no loss of quality, if he would adapt the flow in each channel to the actual requirement. The amount of water may not be lost, but it of course flows into the cooling system's energy balance.
Kunststoffe: One trend that has established itself in recent years is the use of dynamic temperature control, also known as the variotherm process. How has this development affected you?
Gries: You could say, a hype, which is not surprising because this topic is being presented in numerous seminar talks and articles in the press. Consequently, we get a lot of inquiries from users who, unfortunately, haven't familiarized themselves with the topic in advance. Dynamic temperature control methods offer excellent approaches for producing high-quality parts economically, but they are linked to a complex chain of constraints in order to fulfill expectations. For one thing, energy consumption can end up being higher than expected if the mold has not been designed well thermally, and the temperature control system incorrectly allocated. Some users would then need a small power plant to generate the heating power required for a rapid cycle, because the thermal mass of their systems is much too great. Hardly has this problem been solved, but we get the call, “My parts are worse than before, that is not at all acceptable.” Well, that's what can happen, if I heat up the mold wall without adjusting the injection parameters, or without taking the altered flow behavior of the plastic into consideration.
Kunststoffe: So variotherm temperature control presumes a lot of processing know-how and makes strong demands on the processor to understand the overall system.
Gries: Fundamentally speaking, the approach has to fit the requirement profile. What good is a variotherm system, if the temperature control channel layout doesn't consider where the weld lies, for example? Moreover, the variotherm tempered zone will have to be thermally separated from the rest of the mold by suitable insulation measures, in order to minimize the mass that has to be heated up and cooled down per cycle. The overall picture also includes knowing how to deal with water quality, still a woefully neglected factor in many operations. That brings us back to the topic of system technology.
Kunststoffe: To get back to segmented temperature control. How do you arrive at the best solution?
Gries: It takes a lot of experience to decide, based on a 3D part drawing, in how many circuits to segregate the thin-wall zones and the thick-wall ones. Knowing what temperatures, or what flow rate you have to work with to obtain a uniform surface temperature in the cavity are things you often don't know until you've molded a few parts. We're not looking to sell a complicated, expensive solution, but one as simple as possible. For example, when “integrat direct” automatically regulates the process via the amount of water that could be a solution with a single temperature control unit and one flow temperature per mold half – the customer can cope with that. Packaging is a very rewarding application for flow temperature regulation. By contrast, plastic engineering parts often cannot do without different temperatures in each circuit.
Kunststoffe: How can such complexity be managed reliably?
Gries: Communication between mold and temperature control is sure to gain more importance in the future. Then the optimum temperature profile could be set again quickly following a change of molds. Currently, we have the situation where temperature is often measured outside the mold in the temperature control medium. There are specialists for sensors inside the mold. But it is debatable whether the sensors should be used more for monitoring, or more for regulation.
Kunststoffe: Networking and communication between plant components points in the direction of “Industry 4.0”. What developments do you see coming at you?
Gries: When we talk about communication and system interfaces nowadays, we are assuming a relatively even balance between desired and actual values, flow rate, or a collective fault – that's all that the temperature control unit communicates to the injection molding machine. “Industry 4.0” presumes an expanded framework. Then we're talking about a temperature control reporting via the cloud, whether it is correctly connected and if it is at all suited for the task, based on its performance data, and if it's permanently available, or if its maintenance cycle is due. And by that I don't mean maintenance done according to the number of operating hours, as is commonly programmed at present. I need to detect the actual states and process them intelligently in the overall system. The unit still has to communicate with all the other system components that influence melt treatment, cooling, and the energy efficiency of the thermal process, and then react to changes. Here we are faced with the task of supplying the corresponding data base and the intelligence independent of the machine manufacturer. Just how we go about engineering that is an entirely different question.
Kunststoffe: Out of the future, back to the present, where selecting the appropriate temperature control system is often anything but trivial. This would be a good time for a witty conclusion.
Gries: There never is just one solution. The system that is finally installed has to fulfill the requirements of the mold and the part and be affordable for and manageable by the customer. There may be cases where a combination of laser sintering, boring and vacuum brazing in concert with multi-circuit temperature control, or variotherm temperature control appears to be the ideal solution, but really: that would be overdoing it a bit.
Interview: Dr. Clemens Doriat, Editorial Staff
Dipl.-Ing. Helmut Gries (60) studied mechanical engineering in Aachen, Germany, where he majored in aerospace technology. In 1981 he became project engineer for procedural apparatus at L&C Steinmüller GmbH. In 1984 he joined the sales department of the former Battenfeld Berges Duroplasttechnik GmbH in Marienheide. He continued as manager of sales for thermoset machines when the company merged with Battenfeld Spritzgießtechnik GmbH in Meinerzhagen. Beginning 1990, he headed European sales of injection molding machines with Cincinnati Milacron in Offenbach. Following a interim with Ferromatik Milacron in Malterdingen, he switched 1994 to gwk Gesellschaft Wärme Kältetechnik mbH, where he has been the operating director of sales and marketing since 1998.
Gries is an active member of various networks in the plastics industry, as well as a board member of auxiliary associations of several plastics centers. In his free time, he has made a name for himself as a landscape and wildlife photographer in Southern Africa.