Resin Transfer Molding (RTM) is a low pressure, closed molding process which offers high-quality dimensional and surface finish composite moldings using liquid thermoset polymers reinforced with various forms of fiber reinforcements. Typically polymers of Epoxy, Vinyl Ester, Methyl Methacrylate, Polyester or Phenolic are used with fiberglass reinforcement.
Other reinforcements are offered for more demanding applications such as Arimid, Carbon and Synthetic fibers either individually or in combination with each other. The matrix selection of polymer and reinforcement, dictates both molding material cost, as well as, molding mechanical and surface finish performance. Along with the polymer and reinforcement the addition of mineral fillers may be added to enhance fire retardantcy, flex modulus and surface finish.
Reinforcements are presented in their dry form to the mold in binder-bound chopped mat, random-continuous strand mat, stitched “conformable” mat or woven cloth. The fiber has been either “preformed” to the exact shape of the molding tool in a previous operation or is hand-tailored during the loading process in the molding tool. After the fiber is installed into the mold, a premixed catalyst and resin is injected into the closed mold cavity encapsulating the fiber within. The primary surface of the molding may be gel-coated, a process of spraying the mold surface before installing the fiber. If a gel coat is not required, the exterior finish would be the same from the front to back of the molded part. RTM having the inherent advantage of low-pressure injection usually does not exceed 100 psi of resin injection pressure during the mold-fill process. the most common applications of using the RTM process are molded with a cross-section thickness of 4mm with 30% fiber content by weight of the total laminate.
Evolution of the Process
During the late1960′s through the early 1970′s the composite industry experienced significant growth, this was driven by two primary industries which were the Marine industry as the boats converted fully from the previous wood materials and the Automotive industry to meet both styling needs and weight reduction of the various exterior body panel components.
To meet the rapid market growth, the suppliers of the primary polymer resins and fiber reinforcements developed their products to support the sheet molding compound (SMC) as needed by the automotive industry and the open molding process as needed by the marine manufactures.
It should be noted that generally the primary materials of resins and reinforcement for both the SMC and the open molding, as well as, the RTM process are nearly identical, making then primary differences to be found in the actual molding process methods.
SMC (Sheet Molding Compound)
The SMC process is a high temperature process performed in a matched set of steel molds heated to over 350° (f), the SMC compound is laid in a measured volume in the heated mold with all of the needed, resin, catalyst, mineral fillers and reinforcement premixed into the compound in a previous compounding process, then the mold is forced closed under greater then 2000 psi of pressure squeezing the compound to fill the entire mold cavity rapidly curing within 1 to 3 minutes. Once cured, the mold is opened and the molding is ejected as a finished part needing only minimal surface preparation prior to finish painting. One added key advantage of the SMC process is the ability to mold the part to “net” shape, that is, normally SMC parts are molded to the final net shape of the part which does not then need post mold trimming, this eliminates the need to trim parts after molding and provides the most consistent part to part trim repeatability.
The key to justification for the use of the SMC process is SUSTAINED DAILY PRODUCTION VOLUME needs that allows for the tooling cost to be amortized in a favorable manner across the planned production volume rate and life of the part to be produced. Having the ability to produce a finished molding every 3 to 5 minutes enables typical production volumes found in the automotive industry of typically greater then 20,000 units per year to be met and with that volume need, the cost of the steel molds and related high tonnage presses can be justified.
Open Molding Process
The open mold process is performed in molds made from basically the same resins and fiberglass reinforcements as are the parts produced in the molds. The molds are created over a simple form, may it be a professional CNC cut model or a simple shape hand made of screen covered with bondo body filler, in either case the mold is created by applying a gel-coat over the model, then using sheets of fiberglass wetted with catalyzed resin applied in individual layers built-up by hand until the desired mold thickness is achieved at which time a basic steel or wood frame is fasten to the backside of the mold to provide support and to position in an ergonomic manner for producing parts within.
Initially the open mold process was facilitated by brushing a layer of gel-coat into the mold which will create the actual exterior finish of the final molded part. Then sheets of fiberglass mat were wet-out by brushing catalyzed resin over the mat, the wetted resin and fiberglass is rolled tight against the gel-coat and the additional subsequent layers to ensure the air within is rolled out, this is done using a roller similar to a paint roller which is controlled by the operator manually rolling the wetted glass mat to eliminate the entrapped air. Developments in the 1970′s from the equipment suppliers provided equipment to spray the gel-coat and the resin with chopped fiberglass into the mold, which significantly improved the application time, still then the fiber and resin needed to be “rolled” by the operator to eliminate entrapped air and to smooth the backside of the part.
The open mold process is able to cure at room temperature, without added heat or pressure, the actual process will require from 4 to 6 hours from start to finish for each molding produced.
Meeting the Demand from the 1970′s through 2000
The automotive industry was served well with the SMC process since their volume justified the capital cost associated with the tooling and high tonnage presses required. The marine markets could never justify SMC tooling. The varied Marine models, the need to react to nearly constant product changes and the low volume need of each of the moldings makes the SMC process very impractical for the Marine market offering the open molding as a practical method for their needs. If the volume requires the need for more then one molding per shift then additional molds are created, each at a very nominal cost, so the open molding process can be very flexible in meeting the needs of the marine or similar volume markets.
The Orphan Alternative Method – RTM
Through-out the evolution of the open mold and SMC processes, there has been an alternative closed molding process. Like the SMC process in which a matched set of tooling (an upper and lower mold half), the resin transfer molding (RTM) process has been available for use in both the automotive and marine markets, as well as, all others.
The RTM process was not accepted by the automotive markets due to their need for rapid or high volume production and the marine market also looked the other way due to the added cost in tooling since two mold halves were needed and in the open molding process only a cavity or single half mold is needed.
Few RTM Specialists
Through the 1980′s to present only a few have specialized in the RTM process methods and tooling designs. Many in the industry would recognize names such as Applicator – Sweden, Plastech T.T. – United Kingdom, JHM Technologies – USA, Pyramid Composites – USA, as leaders in innovation who supply the industry with the “package” of needed tooling, equipment and training to carry out the production molding of RTM products.
There have also been a limited number of molders who also adopted the advantages of the RTM process as their methods to supply the various OEM needs in the market, many would also recognize Cincinnati Fiberglass, Nero Plastics, ETM, Able Body, ASC, New Boston Composites all within the USA, various other molders can be found in France, England, Australia and Belgium as well.
Within the molder community there are also a number of open molders who have a portion of their production produced one form or another of RTM, this however is generally a smaller portion of their main method which is open molding.
Distinguishing the Differences
The SMC process produces the highest productivity with the highest surface finish quality. Expected tooling life for a steel tool as used in the SMC process is greater then 100,000 moldings.
Open mold, offers the lowest cost tooling, with a exterior finish directly from the mold that meets the needs of a high gloss smooth surface normally well suited for most Marine and general non-automotive surface needs. The lower quality surface as compared to SMC is due to the mold materials (composites versus steel as in SMC). Those in the automotive industry, as well as, others who need to meet a “Class A” exterior surface finish who do not have the volume to justify the SMC process can use the open molding process provided the surface of the molding is sanded, primed and finish painted to meet the finish specifications of the product needs. Additionally, the open mold process by having only a single mold half, the back side of the molding is fully dependant on the skill of the operator to maintain the part thickness and finish smoothness. This then requires many industries to grind by hand the individual parts to fit due to the variance common to the manufacturing process.
RTM can enjoy the benefits of steel tooling to achieve the true “Class A” surface finish standards, yet then the economics would make sense to put the mold in a press and form SMC, so the RTM process is typically performed using composite tooling construction materials such as the open molding process. The advantage of having two mold halves eliminates the variance in part thickness common to the open mold method, making the RTM part very comparable in dimensional accuracy and repeatability of the SMC process.
Summarizing the differences
SMC has enjoyed the high end use in the industry, providing the benchmark for surface finish, production rate and tooling life, carrying the highest capital cost.
Open mold, provides a rapid tooling lead time with tools that can easily be changed to meet engineering or marketing needs. The surface finish out of the mold serves many applications with the use of a gel-coat step prior to forming the actual molding laminate, if higher surface finish is needed, then post mold sand, prime and paint can meet the needs of the market. The primary disadvantage is found the operator dependency for part quality and uniformity, as well as, lately VOC emission restrictions.
RTM has many advantages to offer, yet since it has not the ability to produce at the rate of the needs for the automotive industry and has tooling cost perceived to be too costly for the marine market, has historically been restricted to only a few niche applications.
Per pound Cost
||$1.07 to $1.89
||1.1 to 1.3
|Molding Cycle Time
||3 to 5 minutes
||12 to 90 minutes
||>1,000 to 5,000
|* Tool life is determined by the quality of the parts produced to meet the dimensional and surface finish standards of the product molded. Parts having the need to meet a “Class A” automotive finish would have a lower expected mold life while applications requiring an “industrial” grade of general industrial finish would have extended tool life.
All of the processes can produce a similar product as far as strength is concerned. It is rare that a product can not be produced in one process due to a strength issue.
Selecting the Right Process
The selection of the best molding process is first determined by the daily production volume needs. If the daily and long-term production needs can justify the investment cost of SMC tooling and if the molded product surface can be as molded or post mold painted, then the SMC process is the most cost effect method to mold the part.
If however the part needs gel-coat as a finish or the volume does not justify the cost of the steel SMC tooling then the open mold or RTM process methods need to be evaluated.
Open mold serves well for daily production needs of 1 to 3 parts per shift as long as the part produced can tolerate the part thickness variance and the rough part back side finish. This true as well as if the total VOC emissions using the open mold process does not exceed local restrictions.
Current RTM Developments
Three primary factors are having a major impact on the use of the RTM process today.
Background to Current Developments
- The first is the Environmental agencies have recently reduced the allowable styrene (primary volatile organic compound “VOC” contained with the resins used for SMC, Open mold and RTM) level in the work place to levels that are very difficult to meet using the open molding process.
- The second factor is the labor force of today is one that is not interested in working in an open mold environment. Thus the skill level needed to maintain the desired / required product quality and consistency is becoming increasing rare. The result is much higher production cost due to required “re-working” in a post mold operation of the products molded to meet quality standards.
- The third factor is the innovation coming from the suppliers of equipment and tooling who have changed the mold design to incorporate the use of vacuum in balance with the injection pressure enabling lower cost tooling through the reduction in tooling structure and previously needed external clamping hardware.
The RTM process has been faced with competing with the Open mold and SMC processes, while in most low volume applications the cost of Open Mold tooling has made that process the choice for many applications. The high volume applications have chosen the SMC process.
RTM has tried many different tooling methods to both increase tool life and improve surface finish longevity. These efforts have been to use Nickel Shell, Cast Aluminum, then Cut Aluminum. The cut aluminum tooling then led to cut steel and we found ourselves with tooling that equals the cost of SMC tooling so the program may as well go to SMC.
All of this tooling effort or change was in reaction to attempting to improve the surface quality of the molded parts, by improving the damage resistance of the mold surface. The RTM process began with Epoxy tooling surfaces, which led to high temperature Polyester tooling gel-coats to improve release and reduce wear caused by sticking. The high temperature polyester was easily damaged which led to the nickel, the nickel was not easily repaired which led to cast aluminum, the cast aluminum had too much porosity to that led to cut billet aluminum, while that was an excellent surface, for only 20% more we could go to cut steel and then we realized that we were way off course as far as tooling cost to make the process competitive with either SMC or Open Mold.
What is described above represents the evolution of the process over the 1960′s through late 1990′s.
In 1997 an effort between JHM Technologies, Inc. and Plastech T.T of United Kingdom developed the Multiple Insert Tooling (MIT) method of building RTM molds. The purpose was to replace the mold surface as needed at a very nominal cost. This was done by making the mold with the mold surface removable as a separate “skin” of tooling gel-coat and laminate that would be held in the mold base (the Bolster) by vacuum locking the skin tightly into the Bolster. The actual MIT molding process is injected exactly as the RTM process had been carried out for the last 30+ years, it is only now that we have the ability to address the major cost factor, tooling surface finish life.
A second advantage came of the MIT tooling design that is the ability to have multiple mold (MIT skins), having multiple skins allowed for many of the process steps to be carried out simultaneously, this has allowed the actual through-put of the mold set to be several times faster, actually about 300% increase in production rate.
In the last two years, further developments in the RTM process have born a new innovation originally introduced as “RTM Light”. The RTM Light process has eliminated a major portion of the tooling cost by reducing the backside tooling structure as was needed in the conventional RTM process. The RTM Light process has proved to be the most revolutionary change to the industry in the last 20 years. While MIT has made a major step forward the RTM Light method, by lowering the tooling cost has open the door to the RTM Light process actually competing with Open Mold for total cost, yet yielding a far better molding then open mold having two finish sides and far more repeatable quality.
JHM Technologies has taken the RTM Light process and added controls in place that protect the mold from over pressurizing during the injection process which maximizes the molding process. We then took the example of the MIT technology and produced mold cavities as “skins” much in the way as we did with the MIT technology for RTM. The ZIP (Zero Injection Pressure) RTM process as then born. Zero injection pressure is not to mean that there is zero injection pressure within the mold, it is to mean that the internal mold cavity pressure created from the injecting resin is kept BELOW the surrounding atmosphere which is clamping it closed. Unlike the RTM or RTM MIT process where the internal mold cavity pressure can be from 25 to 90 psi, the ZIP RTM process is typically less then 9 psi and with having drawn over ½ of an atmosphere (-.5 bar) from within the mold, the exterior the mold is being clamped with equal or greater force then the injecting resin pressure. The goal and controls of the ZIP RTM process are to maintain less then zero gain in relation to the exterior atmosphere when measuring the internal mold pressure.
Virtual Example Illustration of ZIP
It is a fact that employing the ZIP RTM principles, one could have just two matching mold skins representing the upper and lower mold halves, those skins could be suspended in mid air, clamp under vacuum and as long as the injection pressure was kept below the exterior atmospheric pressure the mold skins would remain closed and the part could be easily molded within.
The only disadvantages to the ZIP RTM process in comparison to the conventional RTM or MIT RTM process is that it takes longer to inject the mold since the pressure is much lower. The advantages are many with the low cost tooling and ability to handle the upper mold half in most cases manually without the need for a press or mold manipulator system.
Part for Comparison-
Typical Truck Roof having 1.8 meter width, 1.2 meter length and .3 meter height
Materials to be used-
Roof to be gel-coated, fiberglass reinforced with 25% by weight fiber, polyester resin filled 23% with calcium carbonate filler.
Mold cost: The following comparison is based on single cavity impression mold with internal water lines on both the upper and lower mold halves. The conventional RTM and MIT RTM molds are designed to operate as a “stand alone” tool, or can be clamped in a hydraulic press to enhance clamping force and allowing for reduced injection time. The ZIP RTM mold is designed to be a stand alone mold only.
|1) Conventional RTM
Expected production rate per 9 hour shift: 12 to 14 in press 8 to 12 stand alone
|2) MIT RTM
Each added cavity skin -$ 960.00
Expected production rate per 9 hour shift * : 37 to 45 in press 20 to 33 stand aloneMIT RTM License Fee Cost:
5 Year MIT License fee cost 10,000 British Pound Sterling
|3) ZIP RTM
Each added cavity skin $ 960.00
Expected production rate per 9 hour shift * : 11 to 18 stand alone
* Both the RTM MIT and ZIP RTM process methods are listed above with 5 cavity skins to achieve the production rates indicated.
It is expected that a ZIP or MIT skin will have a service life between 500 to 1500 molding cycles depending on surface finish needs of the final molding.