RTM Light (LRTM) – The Lower Cost Alternative Takes the Lead in the Closed Molding Industry
The Light RTM process (also know as LRTM, ECO, Vacuum Molding, VARTM) for fiber reinforced composites, has recently taken the lead as the most popular closed molding process for low to medium volume applications. This leading technology has now displaced the former “conventional or traditional RTM” (Resin Transfer Molding) process in most typical Marine, Automotive, Industrial and Medical composite molding applications requiring two finished sides and close dimensional tolerances.
The recent surge in the acceptance of the LRTM (Lite RTM) process over the previous traditional RTM process is in part due to the lower tooling cost of LRTM. Yet, it is also favored by the ease of converting from the open molding process, which still remains the prominent molding process in the industry.
The LRTM process is founded on what would be considered minimal tooling structure as compared to the traditional RTM or other closed molding process such as SMC or BMC. The tooling structure needed for the LRTM process is similar to that of the open molding process since there is minimal internal cavity pressure. The typical LRTM mold has one rigid half normally for the “A surface” (typically the female cavity) of the mold, with then a light weight “see through” semi flexible upper mold half. Each mold half is made from a thermoset polyester or vinyl ester tooling gel-coat surface having a fiberglass reinforced thermoset polymer laminate backing that is then normally supported by steel box tubing to retain the tooling shape and provide an ergonomic orientation.
LRTM tooling costs are a fraction of the price of equivalent RTM molds, up until recently they were thought to produce less then half the production rate of their higher pressure RTM comparable process, yet with recent innovations in the tool design and process controls, the gap in productivity is closing between the two processes.
The advantages of closed molding for either true RTM or RTM Light, offers working environments for the molding operators that arefar more comfortable and healthy. They then are willing to apply their skills ofquality and productivity at a consistently higher level each day. Even though it is true that RTM Light will not yet meet the production rates that are enjoyed in traditional RTM, RTM Light will provide a 300 to 400% increase in per square foot productivity over open mold, with significant improvement of bill of material compliance and lower operator employment turnover.
Today the industry direction is towards closed molding and especially that of Light RTM, this is driven by numerous factors such as reduced employee turnover, elimination of VOC emissions and improved bill of material compliance combined with two finished side moldings that can be engineered to fit into assemblies with far less manual post mold rework.
The pending restrictions on VOC emissions now coming to an imminent deadline and the most recent threat by the EPA of listing Styrene, the most prominent VOC in the open molding process, as a suspect Human carcinogen, has all aided in the recent increased demand in converting many open molders to the closed molding process of RTM, Light RTM and Vacuum Infusion processes.
Comparing the RTM and LRTM processes
Individual characteristics of each closed molding process begin to be identified when we consider the fundamental differences in the process methods and related limitations.
With traditional RTM, we are feeding the mold cavity with resin under moderately high flow rate and pressure that is only limited by the structural ability of the molding tool and perimeter clamping or press system to sustain mold closure. Working within these concerns, we then will build RTM tooling and clamping systems with structure great enough to sustain the flexing caused by the highest expected injection pressure during the molding cycle. Thus, we may choose to use tools made of cored composite with steel tube backing structure or either CNC machine cut aluminum or steel.
As with any manufacturing process, cost becomes the true limiting factor. It is generally favored to use tooling (molds) that are made from fiberglass reinforced composite materials to reduce tooling cost as compared to the alternative of using a machine aluminum or steel material. Machine cut steel or aluminum tooling is typically limited to very high volume applications, which can amortize the tooling cost over far more molding cycles.
The shortcoming of the composite tooling has proven to be the service life of mold surface. Increased tooling surface life from composite tooling has been revolutionized through the use of tooling with replaceable tooling surface “skins.” Today there are a number of skin tooling system designs in the market, which are compatible with the RTM and LRTM processes. One of the most recent designs is the patented ZIP RTM design as offered exclusively through JHM Technologies, Inc. Having the ability to replace the perishable mold surface through the use of mold surface “skins” has given composite fiber reinforced tooling an effective manner to complete in providing long lasting tooling and high quality moldings. For additional information on the ZIP RTM process contact JHM Technologies, Inc. at 810-629-6515
One remaining limitation of composite tooling has been the ability to control the injection pressure so as to not over pressurize the mold and deform the molding cavity. To support this concern, the JHM Technologies team has offered simple pneumatic pressure controls (VMPC mold protection guard) on the standard line of equipment and an electronic version of the VMPC for PID closed loop electronic system on the JHM Intelliject PRG series of equipment – both allow the use of composite tooling to be utilized to their structural limits but not beyond. While optimizing the productivity of the composite tooling, these systems are providing a very cost efficient refinement to the traditional RTM process on a global scale.
As we look at even lower cost alternatives and refinements, we begin to consider that still the largest portion of our investment in RTM is the tooling and related clamping system. In order to address the need of applications that may only need 3 to 5 molding cycles per mold, per shift, we find that traditional RTM is not cost effective to meet those production volumes. Realizing that it is injection pressure that is adding the cost in the RTM process by having to build tooling of structure great enough to not bend with the injection, as well as, to support the tooling with additional external clamping from a press or perimeter clamps, the direction has been taken to revert back to one of the original methods of molding composites. Historically, the use of vacuum and atmospheric pressure was used to clamp molds and to aid in moving the resin through the fiber pack within the mold cavity.
Different however than the use of a vacuum bag or consumable products common to many “resin infusion” processes, with the LRTM & ZIP RTM processes we have elected to use still an FRP upper mold that is typically 6 to 8mm in cross-sectional thickness having a rigid flange perimeter that circumvents the entire mold cavity. Generally, we do not use any steel tubing to gain structure over the mold cavity as common in the traditional RTM molding tool design. We expect that the upper mold will have a limited degree of flexing and use that flexing as both an advantage, as well as, finding it to be one of the limitations of the process cycle. This design would be typical of a RTM Light mold.
In RTM Light, resin flow rates cannot be accelerated above an optimum level that allows for the greatest flow rate, while maintaining inner cavity mold pressures below the clamping forces of the external atmospheric pressure. Normally, this limits the injection pressure of the RTM Light to less then 1 bar (14 psi) with the cavity held at a constant vacuum level of -.5 bar (15” hg)
As with any composite closed mold production process method, RTM Light demands high quality, accurate crossection molds in order to provide good mold life and consistent production of repeatable parts. This is a key element to successful molding and is a common oversight for those who venture into either RTM Light or RTM on their own. We strongly encourage those who wish to take advantage of the closed molding processes of RTM or RTM Light, to allow us to be a major part of achieving their success. Our complete package of support includes product design, mold building, operator training and supply of the specific equipment needed to efficiently mold their products in production.
The JHM Technologies, Inc. team has been recognized as world specialists in this field for well over 25 years. We offer the complete range of resin injection equipment and tooling accessories, specifically designed for this popular process. Also provided are well proven mold build techniques described in a comprehensive RTM Light technology and mold build manual, which is available to all attendees of our global training courses.
FREQUENTLY ASKED QUESTIONS OF THE LRTM / ZIP RTM PROCESSES
Below is a list of frequently asked questions of the Light RTM and ZIP RTM molding processes that come into the JHM Technical Team:
Q) Why would I choose a closed molding process over the open molding process?
JHM) While the open molding process offers a great amount of freedom in the form of mold changes, low cost tooling and proven performance of products in the market, today we are faced with numerous factors that were not so concerning just a few years ago. Such as, VOC emissions, employee turn-over and training cost, bill of material compliance, two finished sides, and greater through-put per square foot of floor space. Closed molding offers a solution to each of the concerns listed for open molding, especially with those relating to employee turn-over, we see that virtually is eliminated when a molder converts to closed molding, the work environment is so dramatically improved the workforce is far more stable. The VOC’s will drop below 5ppm as a norm. Those two reasons alone are normally enough to get the open molder convinced closed molding the way to go, then when you add the greater control over bill of materials, the two finished sides and the higher productivity from the mold set the benefits then show to outweigh any normal practical reason to not convert to closed molding. While all of the benefits listed are available from each of the various closed molding processes, the recent innovations in the RTM Light process and especially the added benefits of the ZIP RTM technology makes the decision generally very easy to accept closed molding now for both current conversion and new products as they come in.
Q) What is the per square foot cost of an RTM Light Mold?
JHM) There is not a simple per square foot cost that will work for all shapes and sizes. Factors such as part thickness effects the cost of the sheetwax material needed to calibrate the mold. Flat simple shapes are easier tools to build then complex parts with ribs or multiple shapes, parts requiring Automotive “class a” finishes cost more then industrial surfaces. I can say that if you consider the typical size and shape of a car hood you would expect to pay approximately $140.00 to $210.00 per square foot for the mold not including the master pattern (plug) of the hood design.
Q) What is the skill level needed to operate an RTM process?
JHM) The comparison can be made to that of the open molding process which is one that requires the operator to have the skill to make constant adjustments as needed for glass to resin ratio, part thickness, laminate build rate, uniformity of laminate, and so on. The closed molding processes of RTM, LRTM and vacuum infusion eliminate the need of the operator to make the common adjustments needed. Thus the result is that the closed mold product is far more consistent with dramatically fewer defects as compared to the open molding process. Having a closed mold with a fixed cavity volume which a prescribed amount of fiber has been inserted prior to the resin injection eliminates much of the skill needed as compared to the open mold process. Today, the injection equipment offers controls that are plain and simple to use even with the newest operator in the plant. The combination of all these factors and more, makes the closed molding processes of RTM, LRTM very easy to manage and able to provide consistent product molding results with even limited skilled operators.
Q) What is the per square foot material cost for an RTM or LRTM molding?
JHM) There are numerous factors that affect the cost of materials for any molding process. Normally the RTM and LRTM moldings have a crossectional thickness of 1/8” with fiberglass reinforcement at a weight percentage of 30% and the remaining 70% of the molding weight is resin. Based on the values above the typical cost of materials alone per square foot is $1.70 for either RTM or LRTM moldings. Applications needing high surface quality such as automotive Class A or those that need high mechanical strengths would cost a bit more for the premium materials needed to meet their application.
Q) What is the primary cost factor that influences the final product cost for a RTM or LRTM molding?
JHM) In the case of ALL composite molding processes, the final finish requirements dictate as much as 60% of the final product cost. This can be illustrated by comparing the surface finish of an industrial tank to that of an automotive car door. The industrial tank will accept a surface which has some minor waviness or minor pits, as well as, low gloss. The car door on the other hand, must have a mirror like wave free surface, free of pits, fiber print and smooth finish which can be coated with automotive quality paint and then equal the sheet metal body as common to the automotive industry. In the process of producing a molded composite, the initial factor that influences the molding surface finish the surface of the molding tool. Molds made of Aluminum or Steel retains their quality far longer then any type of composite mold material. Yet, the cost of machined molds of aluminum or steel are normally cost prohibitive, leaving then the only practical choice to be composite molds. The normal surface finish life of a composite mold having to meet the automotive class a specification may only be 500 molding cycles, parts molded beyond the 500 cycles begin to require excessive post mold hand finishing to meet the surface requirements. This is exactly why the replacement skin molds came into existence, today the ZIP RTM process offers the most practical cost efficient method to mold and maintain the surface quality of the moldings by enabling the mold surface to be replaced at a nominal cost, much in the same manner of making a molded part. Molding programs which produce molded parts for non critical surface finish applications will often achieve molding tool life of 8,000 to 10,000 cycles before a mold is retired, while high quality requirements as common to the Automotive or Marine industries may begin adding excessive cost after just 500 molding cycles unless the mold surface can be replaced or refinished at a nominal cost.
Q) What are the VOC emission levels for RTM or LRTM?
JHM) It is known that the alternative molding process such as open mold produces VOC levels greater then 50ppm with even the most advanced technologies of equipment and materials. The closed molding processes of RTM, LRTM and Vacuum Infusion will drop the VOC emissions for the same molded products to below 5 ppm making it fully compliant with the most restrictive VOC emission standards.
Q) What additional factors influence the overall cost impact of closed molding?
JHM) Beyond the dramatic improvement in bill of material compliance, VOC reductions, Stability of the Workforce, are the values of post mold assembly operations that can now fit the component assemblies together with engineered two finished sides of the moldings. With the reduced VOC emissions of RTM or LRTM, there is a lower utility cost in not having to exhaust as much ventilation air from within the plants, which reduces the electrical and heating cost. There is also the added customer perception of quality that comes from two finished sides, recent the JD Powers report for the Marine industry stated the customers were considering the Marine products without two finished sides on their hatches as low quality.
Q) What size parts can be molded using RTM or LRTM?
JHM) It is common to mold parts as small as baseball cap or as large as a 25 foot boat hull. This is not to mean these are the limiting sizes, parts smaller then the baseball cap may become challenging to load fiber into and parts larger then a 25 foot boat hull become challenging to manipulate the mold halves, yet the actual molding process truly has the ability to mold much smaller then the baseball cap and larger then the boat hull each application outside those sizes requires careful evaluation to determine the practicality.
Q) How many parts can be molded per shift?
JHM) Based on a 8 hour shift, it is common to expect the RTM process to produce an average of 10 to 12 moldings per shift and the LRTM process is expected to produce 4 to 6 per shift. This is based on typical molding applications with gel-coated finish surfaces using non-heated molds. The RTM process when using a hydraulic press, heated molds with 5 exchangeable mold skins (each in process simultaneously), molding a part of 34 square feet have produced greater then 40 moldings per shift on a sustained production basis. Heated molds used in the LRTM process with all of the benefits added with the ZIP RTM controls and exchangeable mold skins, could produce up to 20 moldings of the same part, without the need for the hydraulic press and far less then half of the tooling cost of the RTM process.
Q) Can parts be molded with “die lock” shapes?
JHM) Yes, However the closed molding processes of RTM or LRTM requires both an upper and lower mold half which can fit one into the other. In order for the mold to close, the part shape must allow for the mold halves to open and close without “die lock” in the line of draw. In order to mold parts with die locked shapes when using the RTM or LRTM processes requires the molds to have loose inserts to be added which can be removed after the molding cycle yet allow for the mold to be closed and opened. Each application needing inserts are unique and we encourage those who have die locked shapes to contact our technical staff or attend one of our tool build training courses.
Q) Since the Upper mold half is allowed to flex do I need an accurate cavity calibration?
JHM) Yes, it is a myth to believe that even though the upper mold flexes, it will not “conform” to the reinforcements or part shape to overcome inaccuracy in the mold building process. This is one of the primary oversights of those who attempt to build tooling without the proper training. The result of using molds which are not accurate are very inconsistent resin flow path control which shows with random dry spots, air voids in radii, as well as, resin rich & resin rich parts. It is critical that molders who are implementing a closed molding program get the proper training on the key attributes of how to build accurate molds; JHM Technologies offers several training programs each year covering those critical points.
Q) Can Cores be molded into RTM and LRTM moldings?
JHM) Yes, it is very common to mold wood or foam cores into a part. The RTM process, having higher injection pressures limits the low density foam materials to typically a minimum of 5 pounds per cubic foot. The advantage of the low cavity pressure assisted by vacuum in the LRTM and ZIP RTM molding process allows for core density as low as 2.3 pounds per cubic foot to be used. It should be noted however, in each case that a core is used; the accuracy of that core to fit into the molding tool is a critical factor in the consistency of the molding process and product quality repeatability.
Q) Can Preforms be used in either RTM or LRTM?
JHM) Yes, many higher volume applications can benefit from the use of preformed reinforcements. History has proven that low to moderate volume applications especially those with several different moldings are best suited to use “cold formable” materials such as Saint-Gobain “Multicore” or Chomarat “Rovicore” both materials are easily formed in the actual molding tool. Large size parts and those with high volume may find the RIMFIRE preforming process a cost effective alternative to cold formable roll goods. The JHM applications team can be very helpful in pointing the molder towards the most cost effective practical manner of loading fiber into their molding applications.
Q) What is the expected repeatable dimensional accuracy of the RTM or LRTM process?
JHM) The overall width and length accuracy is effected by the molding resin and process controls effecting cure. The crossection accuracy is effected by the process controls of flow rate and injection pressure. It is expected that the RTM process, when the mold has been built to the needed standards to prevent flexing and the mold is clamped in a press or has proper perimeter clamping, will provide a repeatable part thickness of + .010”. The LRTM process is commonly utilizing tooling that has inherent flexing, while the LRTM process can match the accuracy of the RTM process especially when controlled with the features of the ZIP RTM process methods, yet typically it is found that the nominal variance is + .020” with an occasional isolated area of + .030”
Q) Are all parts molded in the RTM or LRTM processes Gel-Coated?
JHM) No, there are applications that have minimum surface finish requirements, that are not exposed to the weather or UV, that are not gel-coat a common example is a Fan Shroud, in that case the resin is pigmented to provide a through color and the molding is produced without any gel-coat or post mold paint. Normally however, most applications require post mold paint or the final finish out of the mold to be impervious to the elements. This is certainly the vast majority of products in the market. The application of Gel-Coat is one of the aggressive areas of development on going the closed molding applications. This is due to the fact that the VOC’s associated with Gel-Coating are still posing a concern for many high volume molders even though they have eliminated the majority of their VOC’s with the conversion to closed molding. One of the most promising on going development areas for the elimination of gel-coat is to use a vacuum formed thermoplastic sheet into the mold cavity, followed with fiber loading and then closing the mold and injecting the resin forming a composite of thermoplastic outer finish bonded to a fiber reinforced thermoset laminate. The JHM Technical staff can provide up to the minute developments in this area and can be of assistance in determining the best method currently available for each application.
Q) What are the common process standards for the LRTM and ZIP RTM processes?
JHM) The perimeter flange vacuum channel is used for clamping the mold closed and it always drawn down to the lowest vacuum level possible, typically the process standard is an absolute minimum of 23” Hg of flange vacuum. The cavity is normally drawn down to a controlled and regulated vacuum level of 15” Hg. The resin injection flow rate will vary from application to application depending on multiple factors such as resin viscosity, part size, fiber type & loading, yet it is common or typical that the resin is injected at a controlled rate between 1.3 to 2 liters per minute.
Q) What is the difference between LRTM and ZIP RTM?
JHM) The primary differences in the ZIP RTM process in comparison of the LRTM process is found in tighter process controls, more robust tooling resulting in greater tooling life, as well as, the ZIP RTM process can be configured to use multiple tooling skins. When the ZIP RTM process is fully employed, the injection of the resin is controlled with both precise pressure control, as well as, the actual part cavity thickness is controlled in real-time through the use of the CTM (Cavity Thickness Minder) sensor these controls make certain the mold cavity remain accurate during the injection process and the injection does not force the mold open. Then with multiple cavity skins, the lower “bolster” or holding base for the skins can be heated allowing for the mold cavity temperature to be maintained during the injection process. In the ZIP RTM process methods, having multiple skins allows for the steps of Gel-coating to be preformed outside the mold set at ambient temperatures which reduces the potential for porosity and pre-release in radii as it common to spraying gel-coat on heated mold surfaces. The loading of fiber can then be carried out once the gel-coat has cured, both the gel-coating and fiber loading can be while another skin has already passed through each of those steps and is in the heated mold bolster and upper tool set being injected. This then allows through having multiple cavity skins the ability to perform each of the mold steps simultaneously dramatically increasing the through-put of the molding operation. Each of the mold ZIP skins are made off line with nearly the same ease and cost as making a part of the same size. Further details of the ZIP RTM technology are available from JHM Technologies, Inc.
Please note that no one process meets the needs for every molding application. The choices today allow the molders to choose between high volume molding processes such as SMC, BMC DMC, or the medium volume alternatives such as Cold Press molding or conventional RTM, as well as, the LRTM and Vacuum Infusion processes. It is important to realize that each process has an importance and offers unique advantages and disadvantages. The ultimate choice for any molder will be based on many factors including molding volume & speed, part accuracy, resin formulation, fiber requirements and surface finish, as well as, tooling budgets and overall production cost.