MIM/CIM Hot runners refine metal and ceramic injection moulding
Jörg Essinger, head of application technology at Günther Heisskanaltechnik, explains how hot runners can improve the process and cut waste when turning harder materials embedded in plastics into finished products.
MIM (Metal Injection Moulding) and CIM (Ceramic Injection Moulding) technologies take advantage of the freedom offered by the injection moulding process for the production of metal and ceramic components. With these methods, three-dimensional components can be cast in only one working step without the necessity of subsequent machining. Well adapted hot runners are indispensable for manufacturing process safety.
Metal and ceramic powder filled plastics are used in many different fields. Examples of use of MIM are components for consumer products like ballpoint pens or for products for medical use. Products manufactured by CIM are in demand where ceramics are required for insulation, often in conjunction with high temperatures. Examples of use can be found in the lighting industry or in the manufacturing of spark plugs.
Both of the processes are relatively easy to use and make it possible to produce components without subsequent machining. This type of manufacturing can produce complexly formed components that would not be possible at all with machining, or only with considerably more effort.
Using plastics to create metal and ceramic parts
With the MIM and CIM processes, a metal or ceramic powder – in MIM, e.g. consisting of stainless steel or a titanium alloy – is mixed with a binding agent, often polyethylene (PE) or polyoxymethylene (POM), and a special wax, and then granulated. This mixture, called a "feedstock", can be processed with injection moulding like any conventional plastic material. The plastic is then removed from the moulding, the so-called "green part", by heating. After debinding, this component has a porous structure because of the removal of the plastic material and is called the "brown part". By sintering this brown part, the metals or ceramics are baked together to form a component with a homogeneous structure whose density and resistance does not differ from a conventionally manufactured steel or ceramic component. Thus, the advantages of, for example, a metal component, like high mechanical resistance and high conductivity, are combined with a relatively simple way of manufacturing.
Basically, conventional injection moulding machines can be used for CIM and MIM. But if manufacturers wish to employ these processes for a longer period, they should consider installing wear-resistant cylinders, screws and non-return valves because these components are submitted to increased abrasion from metals and, to an even higher extent, from ceramic powders.
Extremely low heat loss with two-stage shaft
Talks with feedstock and plastic parts manufacturers have shown that 80 - 90% of parts produced with MIM and CIM are manufactured by means of a cold runner with a sprue rod. While a large fraction of the sprue rod can be recycled, it would be promising to eliminate this step through the use of hot runner systems.
Both MIM and CIM require a very homogeneous temperature control in the hot runner since the materials have a very small processing window. Variations of the temperature lead to a segregation of the binder and the powder that cause shrinkage differences and the formation of cracks in the component during sintering.
For this application, Günther Heisskanaltechnik recommends its hot runner nozzles developed to meet such higher requirements. The two-stage nozzle shaft guarantees excellent isolation at the forward section of the shaft, which ensures extremely low heat loss between the hot runner nozzle and cavity and very homogeneous heat distribution within the nozzle. In addition, the frozen plastic material around the nozzle forms a "cap", thus providing a thermal separation between hot runner nozzle and cavity
However this feature produces quite the opposite effect for metal filled plastic materials because of the metal powder's conductivity. In this case, the mixture of plastic material and metal powder would draw the heat off the nozzle. That’s why Günther's nozzles used for MIM have special insulating caps made of a highly heat resistant plastics like polyetheretherketone (PEEK) or polyimide (PI) to provide thermal separation. A tubular titanium shaft located around the hot runner nozzle additionally improves the insulating effect.
This is a unique feature of the products from Günther, since most of hot runner nozzles have no two-stage shaft and achieve the sealing by direct metallic contact with the material tube in the mould insert. This leads to very high heat loss which has to be compensated by a higher temperature in the hot runner nozzle. This, in turn, causes and excessive rise of the temperature and temperature variations, so the materials cannot be processed in a reliable way.
Having a large gate point plays an important role
With the MIM technology, the parts are often gated directly, and not via a sub runner. Nevertheless, a relatively large gating point must be used in this case to obtain the necessary throughput and to transfer sufficient heat into the gate point. This is necessary because the metal-filled material transfers some of the heat into the cavity, and the melt freezes quickly due to the high filler content. However, with the CIM technology, a sub-runner is often used. Here, too, a large gate point is of importance in order to cause as little shear stress as possible and to transfer the molten material as quickly as possible into the cavity.
With regard to wear resistance, the hot runner technology used has to meet severe requirements for both MIM and CIM. In products from Günther, nozzle tips made of hard alloys have been shown to provide the best possible wear protection. For this reason, long lifetimes are possible without needing to change some components of the nozzle.
A further quality increase can be achieved with the company's hot runner nozzles and manifolds, which are heated by a fluid. It is well known that particularly for the processing of CIM-feedstock, a very homogeneous control of temperature is necessary to avoid segregation and inhomogeneities of the molten material. This could cause the formation of voids in the component. Because of the inertia of the fluid, a nozzle heated by a liquid even provides more constant temperature behaviour than an electrically heated hot runner nozzle.