HP-RTM Yesterday, Today And Tomorrow: What Leading Industry Experts Say

 

Decades of development have led to breakthroughs in HP-RTM, but the future requires industry solutions that can adapt to operating costs, complexity and process control requirements.

 

 

Two BMW technicians carry a cured, mostly finished side frame

 

Structural carbon fiber reinforced plastic (CFRP) parts for BMW i3 and i8 models have entered mass production.

 

Some see high-pressure resin transfer molding (HP-RTM) as a new technology.

 

But others see it as a modern version of an earlier RTM process, like the one used to make Dodge Viper parts 25 years ago.

 

So what is the difference between HP-RTM and RTM?

 

Slavko Karas, project manager at Mubea Carbo Tech in Austria, said:

➤ Low-pressure RTM is usually injected at a pressure of 10-20 bars (about 1-2 MPa), with a standard cycle time of 30-60 minutes.

➤ Although the cycle time of low-pressure RTM can be as short as 5 minutes, it is only suitable for the production of very small parts.

 

 

Advanced process control: As composite manufacturers have integrated the HP-RTM process into automated production lines, robotic preform handling equipment (left) plays an important role.

 

Automation is also very important for HP-RTM. But to control the process well, more investment is needed.

 

Equipment suppliers have increased their investment in this regard, with the goal of achieving integrated lightweight structures and achieving cycle times of less than 10 minutes.

 

What is HP-RTM? When was it developed?

 

HP-RTM involves:

★ A fiber preform

★ A closed mold

★ A press

★ A resin injection system

 

Today’s resin injection system is a jet mixing head, like the one first developed by Cannon in Italy for polyurethane (PU) foaming applications in the 1960s.

 

In fact, suppliers of metering, mixing and injection systems for the polyurethane and reaction injection molding (RIM) industries were early HP-RTM developers, such as:

★ Cannon

★ KraussMaffei

★ Hennecke

★ FRIMO

 

Structural RIM (SRIM), though using RIM technology, is more like RTM, with the mixed resin injected into a fiber preform in the mold, but with a lower fiber content, typically 30% by weight.

 

When the fiber weight percentage is as high as 75%, it is considered an HP-RTM part.

 

In its heyday, the development of SRIM involved automated preforming, and the concept of production lines for SRIM and RTM was blurred.

 

By 2005, RTM was widely used in the Dodge Viper.

 

French composites company Sotira also used the technology for a wide range of OEMs, including projects with Ford Motor Company and Aston Martin.

 

HP-RTM has since entered a turning point.

 

At the JEC 2007 show, RocTool introduced induction heated molds for “high-speed RTM”, which, while not necessarily HP-RTM, reduces cycle time by 50%.

 

In 2008, carbon fiber producer Toray included “rapid-cycle RTM” in its report “CFRP: What is needed for mass production of automobiles?”

 

 

High-volume molding of complex components: HP-RTM is already used in the series production of large, integrated CFRP automotive parts, such as the side frame of the BMW i8

 

However, all of the above efforts are already done by the BMW Group:

★ For more than 10 years, BMW has been using CFRP for the roofs of its M3 and M6 models, choosing HP-RTM because of its advantages in terms of cycle time, surface quality and industrial production capabilities.

 

★ During this period, BMW has mastered valuable experience in the series production of CFRP components, which has enabled it to meet the challenge of moving from small-batch production to fully industrialized large-scale production.

 

Dr. Thomas Wolff, Head of CFRP Technology Development at BMW, said:

➤ Reducing costs and shortening cycle times are key issues that BMW needs to solve when facing complex shapes of i3 and i8 body parts.

➤ HP-RTM makes it possible to produce large, complex structural parts, such as side frames.

➤ BMW is now able to mass-produce these parts with extremely high process stability and high quality levels.

➤ Compared with the roof, BMW can reduce the production costs of CFRP body parts by about 50%, assuming the same constraints and process chain.

 

However, as the complexity of the structure and shape increases, other challenges are posed.

 

Erich Fries, head of KraussMaffei Composites Business Unit, said:

➤ Producing parts with high fiber content leads to high pressure accumulation in the mold, and to fill the cavity, the pressure needs to reach 110 bar (about 11MPa).

➤ Low-pressure machines with gear pumps can run at 40 bar (about 4MPa). KraussMaffei uses HP axial piston pumps, which can run at 200 bar (about 20MPa). Therefore, companies are now using HP mixing and injection heads.

 

Shorten Cycle Time

 

 

 

HP-RTM cycle times depend not only on the speed of the resin’s reaction, but also on rapid fabric cutting, preforming, preform insertion, mold cleaning and final handling.

 

HP-RTM has been the trigger for the development of fast-curing resins.

 

Although this work has been going on since the 1990s, BMW pushed the envelope by using Hexion’s EPIKOTE fast-curing epoxy resin for its original CFRP roof and Huntsman Advanced Materials’ Araldite LY 3585/Hardener XB 3458 epoxy resin system, which is said to cure in 5 minutes at 100°C, for the i3’s life module.

 

By 2012, the 5-minute-curing resin, EPIKOTE Resin 05475/curing agent, EPIKURE Curing Agent 05443, was replaced by the same resin with curing agent EPIKURE Curing Agent 05500 to meet a 1-minute injection processing window and 2-minute cure time at 120°C.

 

Meanwhile, Dow Automotive Systems has improved its VORAFORCE 5300 epoxy resin, reducing cycle time from less than 90 seconds in 2014 to less than 60 seconds in 2015.

 

Erich Fries of KraussMaffei says:

➤ Epoxies that cure in 1 minute exist, but they are only suitable for simple flat parts, not for parts with complex structures.

➤ Depending on the size and complexity of the part, the actual production time is generally between 3 and 7 minutes.

 

Matthias Mayrs of Engel Austria Says:

➤ The cycle time of HP-RTM is not only a function of the curing time, but also of the entire process, starting with the dry fabric.

➤ Starting with the dry fabric, a preform is produced, which is then placed in the mold and the HP-RTM process is started. To speed up the overall cycle time, larger production units and more investment are necessary.

➤ In practice, preforming is a bottleneck. Cutting, configuration and the formation of the fabric layers take longer than the molding cycle.

➤ Cleaning the mold is also a problem. Although mold release agents mixed into the resin can help mold cleaning, the mold cycle numbers touted by suppliers of mold care products during mold preparation can only be achieved under ideal processing conditions and depend more on the mold surface and design.

 

Preforming operation of the BMW i8 side frame

 

Thomas Wolff of BMW:

➤ Future efforts to reduce cycle times lie in automating individual sub-processes, such as mold cleaning, which has great potential.

 

Press and Preforming

 

Preforms are a bottleneck in production: A BMW i8 side frame preform undergoes a visual quality check before being fed into BMW’s HP-RTM processing unit. BMW says the key requirement for productivity is high permeability of the preform during resin injection

 

BMW also cited a need for improved preform permeability.

 

A preform that allows quick and easy resin infiltration requires a shorter wetting time and lower pressure buildup in the mold.

 

Another option is to reduce the viscosity of the resin.

 

Sebastien Taillemite, business manager at Arkema, says:

➤ Technologists recommend a resin viscosity between 50 and 200 cps to achieve good injection speed and impregnation.

➤ Epoxy resins have a higher viscosity and therefore must be heated for HP-RTM applications.

➤ Arkema’s Elium liquid thermoplastic polymer (LTP) has a viscosity of 100 cps at room temperature and can be injected without heating. Also, injection pressures must not be too high, otherwise it may cause the fabric preform to move, resulting in fiber failure.

 

Indeed, the preform must have high permeability to ensure easy flow of resin.

 

If the resin viscosity is high, the relationship between injection and mold pressure must be balanced to successfully produce the part.

 

In the past few years, many studies have explored mold filling simulations and the optimal configuration of temperature, pressure and preform binder to avoid fiber failure while reducing cycle time.

 

Typical of composites, the richness of material and process options makes optimization parameters more difficult.

 

While not insurmountable, the analysis required may seem daunting to manufacturers new to the HP-RTM game.

 

However, BMW has long since mastered this optimization approach and has therefore begun to look for viable options for reducing cycle times using advanced process control.

 

This could be more flexible and faster temperature changes provided by tooling and machines, or influencing the resin system by dynamically adjusting the curing agent during the injection process to give it better flow or faster cure time.

 

This applies to both epoxies and polyurethanes, although polyurethanes posed some challenges for the fabric system developed by BMW.

 

Erich Fries of KraussMaffei Points Out:

➤ With increasing production, the demand becomes greater, so the process must be closely controlled.

➤ This was not necessary 20 years ago, because the cycle was slow and only 1,000 to 4,000 parts were produced per year.

➤ But now, 12,000 to 50,000 parts can be produced per year, so it is necessary to have an interface between the tooling, metering equipment and injection system.

➤ For this, you can use KraussMaffei’s integrated pressure sensors: If you see pressures above 100 to 110 bar (about 10 to 11 MPa), you know there is a high risk of failure. These sensors allow this to be managed with vacuum or by reducing the flow.

 

Matthias Mayrs of Engel Austria Says:

➤ Customized pressure sensors, such as those offered by Kistler, and dielectric sensing systems, such as those offered by Netzsch, are increasingly becoming the standard for monitoring the curing process.

 

In addition, the latest trend in addressing preform permeability issues and thus reducing injection pressures is in favor of mechanically forcing the resin to flow through the mold cavity.

 

High-pressure compression RTM (HP-CRTM), also known as gap injection or wet compression, involves injecting the resin into a partially closed mold.

 

The resin flows through the dry preform (rather than through its length), and then as the mold closes, the resin is mechanically forced through the short distance of the preform thickness.

 

This method requires less clamping force and press tonnage, thereby reducing capital investment.

 

KraussMaffei’s Erich Fries Said:

➤ This gap does not cause the fibers to slip, because various fiber clamping methods are used, such as vacuum.

 

Dimitrije Milovich, president of Radius Engineering, a supplier of RTM tools, injection equipment and presses, pointed out:

➤ We call it “Variable Cavity Geometry” and use an adjustable seal to maintain the vacuum when opening and closing the mold.

➤ In the trial production of the radome, this method was faster and achieved high-quality surfaces.

 

 

Structural components are the specialty of HP-RTM, but suppliers are also looking for Class A surface solutions. For this, KraussMaffei’s surface RTM process takes advantage of gap impregnation and overmolding processes

 

“Surface RTM” is KraussMaffei’s name for “gap impregnation”:

➤ In the first step, a polyurethane matrix material is applied;

➤ In the second step, polyurethane is applied in the same mold for overmolding, i.e. the gap is filled with a thin layer of polyurethane, resulting in a Class A surface sprayable carbon fiber reinforced exterior panel directly from the mold without additional surface treatment.

 

Two other resin-based innovations also address the problems of “fiber misalignment” and “preform permeability” to a certain extent.

 

★ The first is RTM of thermoplastics (abbreviated as “TP-RTM” or “T-RTM”), or what Engel calls “in-situ polymerization”:

➤ The preform is placed in the mold and the mold is closed;

➤ Caprolactam monomer is injected into the mold together with a catalyst and an activator. In about 30 seconds, the resin mixture penetrates the preform;

➤ With a viscosity of 3-5 cps, which is similar to water, and polymerization in the cavity at 150°C, it can become a solid polyamide 6 (PA6) composite in 2-5 minutes.

➤ The relatively low viscosity makes it possible to achieve perfect resin-fiber distribution and up to 65% oriented fiber content.

➤ However, this also leads to leakage problems in the mold, which requires more considerations in the mold design.

 

Since 2009, Engel has been working on this technology together with Fraunhofer ICT and has developed its prototype e-victory 120 machine into a second-generation production system for this purpose.

 

To feed caprolactam into the HP-RTM process, Hennecke and Mahr Metering Systems of Germany developed a metering/mixing system.

 

Together with BASF and Volkswagen, KraussMaffei demonstrated the process by producing a fiber-reinforced B-pillar in a cycle time of 5 min.

 

★ The second innovation is Arkema’s Elium liquid acrylic/peroxide-initiated thermoplastic polymer, whose viscosity is carefully tuned for HP-RTM:

➤ To manufacture structural parts at production volumes of 30,000 or even up to 200,000 per year, the acrylic resin and initiator do not need to be heated, but the mold needs to be heated to about 100°C.

➤ Elium is insensitive to moisture during injection (which is said to interrupt the polymerization reaction of caprolactam).

➤ The resin is designed for the production of aesthetic composite parts (modulus ≈10-15GPa) and structural parts, which can achieve a modulus of 20-45GPa with glass fiber and 125GPa with carbon fiber.

➤ Elium is about 50% tougher than epoxy resins and can absorb twice the impact energy of polyester.

➤ Structural parts with Elium matrix materials have better aging resistance than those with PA6.

 

To produce automotive parts using Elium, Arkema built an HP-RTM production line together with several partners and conducted trials with five global OEMs, including testing the Class A surface, mechanical properties, chemical and fatigue resistance, and impact testing of Elium parts.

 

HP-RTM: Thermoset or Thermoplastic?

 

Erich Fries of KraussMaffei:

➤ The automotive industry is in demand for thermoplastics due to their recyclability.

➤ It is difficult to produce more than 200,000 HP-RTM parts per year, so RTM with thermosets is not suitable.

➤ If a cycle time of 1 min. is feasible, then the choice is thermoplastics.

➤ Thermoplastics can be injection molded without trimming and post-molding operations, and the basic material cost is cheaper than epoxies and polyurethanes.

➤ Reinforcements can be molded with RTM and then overmolded with ribs, bushings and inserts as accessories.

➤ RTM will always be a solution, but not for every car. So, a hybrid design must be considered, with high-performance parts being RTM and lower-performance parts being molded with organosheets.

 

The so-called organic board molding is a customized blank using PA6 or similar thermoplastic prepreg in an integrated compression molding and back injection (overmolding) process.

 

Matthias Mayrs of Engel Austria:

➤ Thermosets require adhesives and/or fasteners to form an assembly, while thermoplastics have the advantage of being able to be overmolded or heat welded in a second operation.

➤ Injection molding is simpler but not very suitable for structural parts, so this is where Engel is currently focusing on HP-RTM.

➤ Injection molding has limited part size. To make large parts with a nice surface like a roof, a lot of clamping force is needed, so the machine is very large.

➤ Injection molding is not necessarily a better choice for parts with Class A surfaces.

 

Still a long way to go

 

Everyone agrees that HP-RTM still has a long way to go.

 

Sebastien Taillemite of Arkema:

➤ In the automotive industry, a new technology needs to go through a trial process of several years before it can be effective.

➤ In order to be ready for a project in five years, Arkema has started this test.

 

Matthias Mayrs, Engel Austria:

➤ It is not easy to find moldmakers that are competent for HP-RTM and in-situ processes, as molds suitable for HP-RTM and in-situ processes are more complex and require different sealing concepts around the needs of the cavity.

➤ In addition, one must adapt how to insert the preform and ensure that it can be fully wetted.

➤ Engel comes from the field of injection molding, so Engel’s point of view is large-scale production and helping customers realize automation and many advantages of the injection molding process.

 

Erich Fries, KraussMaffei:

➤ The design of the component is also important. In the automotive industry, most people do not know how to avoid designing a “ferrous” component or how to design a component that provides high performance and is producible in terms of preforming and impregnation.

 

Thomas Wolff, BMW:

➤ Many requirements are new and there are no state-of-the-art solutions yet, so a strong partnership with suppliers is very important to get the best and most economical solution.

➤ The scope of partnerships between automotive OEMs and different suppliers will be expanded to develop new solutions.