In-Mold Assembly: All Together Now
The drive to eliminate secondary operations has pushed multi-shot injection molding towards the final frontier: assembling separate components in the tool. Got in-mold assembly in your plant yet?
November 1, 2012 by Mark Stephen, editor
If the manufacturing sector learned one thing from the Great Recession it might be this: get more efficient, fast. In multi-component injection molding, efficiency means eliminating secondary operations by assembling separate components in the tool via snap-fits, welding, and co-molding of incompatible materials. Or in three words: in-mold assembly.
The term “in-mold assembly” (or IMA) is — like a lot of things in life — subject to varying definitions. In this article, we’ll distinguish it from the general field of basic multi-shot molding or hard/soft overmolding. Although IMA generally requires multiple shots, the essence of the concept is putting together inside the mold separate components that would otherwise be assembled outside the mold through snap-fits, welding, or adhesive bonding.
The special technology for IMA is more likely to be found in the tooling than in the molding press. Machine builders such as Engel, KraussMaffei Corporation, and Sumitomo (SHI) Demag maintain that a machine for IMA is not much different than one for other multi-component molding jobs; the distinguishing factor, they add, is the know-how to integrate the automation and program the sequence of operations
In some cases, suppliers of special tooling for IMA have teamed up with particular press builders. An example is the partnership between Ferromatik and Foboha, a German builder of molds with one or more four-face turning “cubes.” Likewise, Denmark’s Gram Technology, a supplier of molds with multi-face turning “spin stacks”, has cooperated in several projects with Sumitomo (SHI) Demag. Engel takes a multi-faceted approach. “We either build the mold ourselves or work with partners in North America,” said Joachim Kragl, Engel’s director of advanced injection molding systems. “Since we also build the machines for multi-component molding and have our own automation capabilities, IMA is a significant area of expertise for us.”
For their part, mold builders have developed a variety of ways to implement IMA. Some approaches use single-face tools with sliding or rotating plates, while others use multiple-parting-line stack molds in traditional linear or newer rotary table designs. In many cases, single-face tools use rotating or sliding plates to change cavity configurations between shots; an older but still useful single-face IMA concept produces hollow parts by molding two separate halves and then bringing them together via sliding plates. Both single-face and stack molds gain added versatility with one or more turning centre turrets, each of which can have four faces to perform different functions. And many applications utilize a robot or similar device to transfer parts between cavities or even between machines.
A GOOD FIT
Okay, so now that we understand what IMA is, why should molders use it — especially since IMA almost invariably means a more complex molding setup? Turns out there are lots of good reasons. “The most important benefit of IMA is that the single parts stay in the mold position before and during assembly,” said Paul Caprio, president of KraussMaffei Corporation. “That means that in every cycle the same cavities will be assembled together with the same conditions of cooling and shrinkage, which reduces tolerances, is easier to adjust, and leads to smaller total tolerances for higher part quality.”
And if you’ve heard that the added complexity of IMA guarantees a more complex process (as opposed to setup) than a conventional molding approach, don’t believe it. “The complexity of the overall manufacturing process is actually reduced with IMA,” said Joachim Kragl. How so? Every step outside the molding machine added to provide the part or assembly with the functionality needed to fulfill the requirements in its final application is essentially — as the Kaizen business model would put it — “muda”: wasteful and non-productive. IMA puts paid to that. Take your average three-component part, for example. “Conventional molding of a three-component part usually involves molding the parts separately and then feeding them to either inline or offline automation equipment for assembly,” Kragl said. “Utilizing IMA, only one machine and process is to be controlled, compared with three individual machines and processes along with perhaps a complex automation line for the conventional approach.”
In addition to eliminating the need for separate presses and molds, the IMA process can really streamline your production. “Compared with dropping the parts or shipping them elsewhere, IMA assembly gives you a much better service assembly,” said Michael Uhrain, a regional manager for Sumitomo (SHI) Demag.
Want a concrete example? At the recent NPE2012 trade show, KraussMaffei integrated a CXW 200-380/160 SpinForm swivel-platen machine with German moldmaker Zahoransky’s TIM Stack (Total Integrated Manufacturing) IMA assembly process for a cap-and-closure application. “The lightweight, three-part plastic closures made of polypropylene came out of the machine ready to assemble onto a bottle,” said Paul Caprio. “The manufacturing cell was designed to save floor space, labor, and other resources.”
Floor space aside, IMA can open up a world of other savings and benefits as well. Let’s start with money. While it’s true that the initial investment to get into IMA is more than for conventional molding — often significantly more — this outlay has built-in savings over the longer run. “Every step that can be done inside the mold takes tremendous amounts of cost, complexity, risk, and demand for process control out of the manufacturing process,” Joachim Kragl said. “The most obvious savings are in cost for parts or work in progress being stored for subsequent operations — such as assembly, printing, joining, et cetera — and the equipment and personnel required to do these tasks.”
Conventional molding of multi-component parts also requires space for storage before performing post-mold operations, which IMA eliminates — and there’s a cash-saving aspect here, as well, since storage requires increased internal logistics that add to administrative costs.
A possible caveat is that IMA usually requires high part volumes. “Tooling for IMA can cost between 30 to 70 per cent more than for standard molds with the same cavitations,” said John Westbeld, engineering manager for >SAS< Automation LLC. Sources generally agree that jobs with a part volume of around 250,000 are good candidates for IMA.
Molders located in damp or humid climates know the havoc these factors can play with plastics parts by causing shrinkage or affecting dimensional stability. The IMA process delivers here, too, since parts aren’t being stored on the shop floor to shrink, warp, or — literally — gather dust while waiting for a secondary operation.
TAKE IT TO THE LIMIT
As useful as it can be, are there any limitations to IMA? “There are very few technical limitations, as long as the part design allows the assembly in or at the mold cavity or core; a limitation here is if assembly from both sides of the mold is necessary,” said Paul Caprio. “Other factors that might weigh against using the IMA process include a very small production quantity per year or a very short planned production period; usually, neither adds up against the investment sum of the machinery.”
In short, keep the law of diminishing returns in mind. “Typically, a three-component or three-material part is the natural limit of what can be assembled efficiently in the mold, because you usually have a rotary table or index plate to do spinning around, and it gets prohibitively difficult beyond three different materials,” said Joachim Kragl.
As with any cutting edge processing technology, developments in IMA are always coming, and fast. One area to keep an eye on is a process recently introduced by Engel called organomelt; the aim is to replace overmolded sheet-metal assemblies with an all-polymer solution for enhanced recyclability and improved bonding. “The biggest advantage that this technology offers is to produce parts and assemblies with the same mechanical properties as the steel sheet-metal counterparts, but at half the weight or less,” Kragl said. “This is done by using continuous carbon, glass, or aramid fibre sheets that are impregnated with a polymer of choice to best match the material used for overmolding. In the production process, the sheet is preheated outside the molding machine and then inserted into the mold. During the closing movement, the sheet is preformed in the machine and then supporting structures and/or functional elements are attached onto it. With the sheet using the same polymer material as the overmolded features, a superior bond is achieved.” Organomelt can also be combined with gas or water injection technologies, Kragl added, to create hollow structures and tubes within a part, for even higher load-bearing capabilities and improved mechanical properties.
Adding value to automotive parts has been the main driver for new IMA processes for years now, but there’s a growing demand for the technology in one of the fastest growing plastics components segments of all: medical parts molding. How well is IMA stepping up? “There’s a real desire in the medical industry to do assembly molding, but the amount of medical parts currently being manufactured with IMA is nonetheless small,” Kragl said. “The technology is well-suited for mid-sized parts, but small- and micro-sized parts are prohibitively difficult and not cost effective at present.” That seems to be the consensus view. “The IMA medical part applications that >SAS< Automation has worked on have been few and far between because of the incredibly small size of the parts,” said John Westbeld.
Some medical parts aside, then, IMA for multi-shot molding is a snap.
Engel Canada Inc. (Waterloo, Ont.);
KraussMaffei Corporation (Florence, Ky.);
Industries Laferrière (Mascouche, Que.);
>SAS< Automation LLC (Xenia, Ohio);
Verick International Inc. (Vaughan, Ont.);
Sumitomo (SHI) Demag/Plastics Machinery Inc. (Newmarket, Ont.);