Development and Validation of a Test Mold for Thermoplastic Resin Transfer Molding of Reactive PA-6

polymers

Article Development and Validation of a Test Mold for Thermoplastic Resin Transfer Molding of Reactive PA-6

Róbert Boros, Ilya Sibikin, Tatyana Ageyeva and József Gábor Kovács *

Department of Polymer Engineering, Faculty of Mechanical Engineering, Budapest University of Technology and Economics, M˝uegyetemrkp. 3, 1111 Budapest, Hungary; [email protected] (R.B.); [email protected] (I.S.); [email protected] (T.A.) * Correspondence: [email protected]

Received: 28 February 2020; Accepted: 7 April 2020; Published: 22 April 2020

Abstract: Thermoplastic resin transfer molding (T-RTM) is a cutting-edge manufacturing technique for high-volume production of composites with a recyclable thermoplastic matrix. Although a number of reactive thermoplastic matrices as well as industrial manufacturing equipment for T-RTM are commercially available today, the design of a T-RTM mold is still based on the skills and personal experience of the designer. This study summarizes the best knowledge and expertise in mold design and manufacturing and introduces an innovative mold for T-RTM. A concept and basic principles for designing a T-RTM mold are formulated in this study. The mold developed is manufactured and validated.

Keywords: thermoplastic resin transfer molding; mold design; reactive polyamide; thermoplastic composite

1. Introduction Modern automobiles must comply with the rigorous environmental regulations and it is not surprising that weight reduction and recyclability are of paramount importance for automakers. At the same time, car structures must withstand signiﬁcant static and dynamic loads under a great variety of environmental conditions. Fiber-reinforced composites with polymer matrices are materials that can meet these requirements. Basically, there are two kinds of polymer matrices: thermoset and thermoplastic. Until very recently, the upfront position in structural application has been occupied by thermoset matrices, as they possess the viscosity low enough (~1 Pas) to impregnate the reinforcement without the application of considerable temperature and pressure. A number of well-established production techniques exist to manufacture thermoset composites (Figure1). By contrast to thermosets, most thermoplastics have much higher viscosity, thus making the impregnation process rather complex. However, certain advantages of thermoplastic composites, including higher toughness, recyclability and weldability, attract the attention of both producers and customers. The high viscosity of thermoplastics can be bypassed by the use of reactive thermoplastic systems [1,2]. These systems usually consist of a monomer, activator and initiator. Due to their low molecular weight, the components of the reactive mixture exhibit almost water-like viscosity and can easily impregnate the dry reinforcement. Consequently, reactive thermoplastic systems can be used in liquid composite molding (LCM) techniques, especially in thermoplastic resin transfer molding (T-RTM). Although there is a number of reactive thermoplastic systems suitable for T-RTM (polyurethanes, polyamides (PAs), polyesters, polycarbonates, polymethylmethacrylates and others) [1], the most promising polymer for the

Polymers 2020, 12, 976; doi:10.3390/polym12040976 www.mdpi.com/journal/polymers Polymers 2020, 12, 976 2 of 13 Polymers 2018, 10, x FOR PEER REVIEW 2 of 13 automotivegrowing, industryand reached is PA-6 8 Mt [ in2– 4 2015.]. The Almost production 1.5 Mt ofvolumes this volume of PA-6 was are used steadily by the growing, automotive and reachedindustry 8 Mt [5]. in 2015. Almost 1.5 Mt of this volume was used by the automotive industry [5].

FigureFigure 1. Comparison1. Comparison of of various various polymer polymer composite composite production techniques techniques (LP (LP-RTM—Low-RTM—Low Pressure Pressure ResinResin Transfer Transfer Molding; Molding; HP-RTM—High HP-RTM—High Pressure Pressure Resin ResinTransfer Transfer Molding; Molding; LFI—Long LFI— FiberLong Injection; Fiber SMC—SheetInjection; SMC Molding—Sheet Compound; Molding R-RIM—ReinforcedCompound; R-RIM— ResinReinforced Injection Resin Molding; Injection T-RTM—Thermoplastic Molding; T-RTM— ResinThermoplastic Transfer Molding). Resin Transfer Molding).

ReactiveReactive PA-6 PA- is6 is synthesized synthesized from a a cyclic cyclic monomer monomer called called ε-caprolactamε-caprolactam (ε-CL) (ε -CL)via anionic via anionic ring ringopening opening polymerization polymerization (AROP) (AROP) with with the the addition addition of of an an initiator initiator and and activator. activator. Commercially Commercially availableavailable reactive reactive PA-6 PA systems-6 systems ensure ensure a polymerization a polymerization time timein the in range the range of 2–5 of min. 2–5 This min. fact This together fact withtogether the rapid with impregnation the rapid impregnation of the dry reinforcement of the dry reinforcement by the reactive by the mixture reactive allows mixture composites allows withcomposites the reactive with PA-6 the matrixreactive to PA meet-6 matrix the demands to meet the of massdemands production of mass (Figureproduction1). In (Fig recenture 1). years, In greatrecent improvements years, great have improvements been achieved have in been the development achieved in ofthe equipment development for processingof equipment reactive for thermoplasticprocessing reactive composites thermoplastic [2,3,6,7]. composites This fact confirms [2,3,6,7]. anThis overall fact confirms readiness an ofoverall the industry readiness to of adopt the AROPindustry in the to productionadopt AROP ofin composites.the production Manufacturing of composites. Manufacturing equipment for equipment T-RTM is for produced T-RTM is by KraussMaproducedffei by [2 KraussMaffei,8], Engel [7,9 [2,8],], and Engel some [7,9], other and companies. some other However, companies. there However, are very there few are sources very few that reportsources success that inreport designing success T-RTM in designing molds. T-RTM molds. TheThe mold mold determines determines the the geometry geometry andand thethe qualityquality of the part, dictates dictates the the flow flow route route of of the the reactivereactive mixture, mixture, and and bears bears the the loads loads induced induced by by the the moldmold carriercarrier and cavi cavityty pressure. pressure. It It also also provides provides thethe heat heat required required to to polymerize polymerize the the reactive reactive mixture. mixture. TheThe most relevant relevant information information about about the the process process comes from the cavity [10]. By now, considerable experience has been accumulated in the design and comes from the cavity [10]. By now, considerable experience has been accumulated in the design and manufacturing of molds for various well-established LCM techniques. However, the specific nature manufacturing of molds for various well-established LCM techniques. However, the specific nature and behavior of the reactive PA-6 system means that designing and manufacturing T-RTM molds is and behavior of the reactive PA-6 system means that designing and manufacturing T-RTM molds is not not a trivial case [11]. First of all, the viscosity of the reactive mixture that is injected into the T-RTM a trivial case [11]. First of all, the viscosity of the reactive mixture that is injected into the T-RTM mold mold is well below 1 Pas for a certain amount of time. With the progression of the AROP of CL the is wellviscosity below of 1 the Pas reactive for a certain mixture amount starts ofto time.increase With exponentially. the progression However, of the the AROP period of CL of thetime viscosity when of thethe reactiveviscosity mixture is very startslow can to increasebe as long exponentially. as 100 s [12]. However, To avoid thethe periodleakage of of time the whenreactive the mixture viscosity is veryduring low this can period, be as long the T as-RTM 100 s mold [12]. Tomust avoid be properly the leakage sealed. of the Secondly, reactive the mixture AROP during of CL thisoccurs period, at thehigh T-RTM temperature mold must (up be properlyto 200 °C) sealed. and molten Secondly, CL thehas AROP a highly of CL alkaline occurs nature at high [13]. temperature Therefore, (up the to 200materials◦C) and moltenof the T- CLRTM has mold a highly must alkalinewithstand nature this harsh [13]. Therefore,environment. the Thirdly, materials the of AROP the T-RTM reaction mold is mustwater withstand- and oxygen this harsh-sensitive, environment. and therefore Thirdly, we must the AROPequip the reaction mold iswith water- a special and oxygen-sensitive,vacuum system andthat therefore will assure we must that the equip medium the mold in the with mold a special is free vacuumof moisture system and thatoxygen. will Due assure to these thatthe and medium some in the mold is free of moisture and oxygen. Due to these and some other aspects of the AROP of CL, Polymers 2020, 12, 976 3 of 13 the T-RTM mold will have several unique features in its design that differentiate the T-RTM mold from all thePolymers other 2018 LCM, 10, x molds. FOR PEER REVIEW 3 of 13 The present study describes the process of designing a research T-RTM mold used for the productionother aspects of PA-6 of the composite AROP of partsCL, the via T- theRTM AROP mold will of ε -CL.have several The results unique of features the in-mold in its design pressure that sensor measurementsdifferentiate are the alsoT-RTM presented mold from and all explained. the other LCM molds. The present study describes the process of designing a research T-RTM mold used for the 2. Materialsproduction and of MethodsPA-6 composite parts via the AROP of ε-CL. The results of the in-mold pressure sensor measurements are also presented and explained. 2.1. Materials 2. Materials and Methods The reactive system for the production of samples consists of an ε-CL monomer (Brueggemann Chemicals,2.1. Materials Heilbronn, Germany), 3 wt % sodium caprolactamate initiator (Bruggolen®® C10, ®® BrueggemannThe reactive Chemicals, system Heilbronn, for the production Germany) of samples and 3 wt consists % N-carbamoyllactam of an ε-CL monomer activator (Brueggemann (Bruggolen C20P,Chemicals, Brueggemann Heilbronn Chemicals,, Germany), Heilbronn, 3wt % Germany). sodium caprolactamate The components initiator must (Bruggolen be dried in®® anC10, oven at 30 ◦CBrueggemann for 24 h prior Chemicals, to experiments. Heilbronn, Two materialGermany) batches and 3wt (ε-CL %/ initiator N-carbamoyllactam and ε-CL/activator) activator were prepared(Bruggolen to fill®® the C20P, dosing Brueggemann units. The Chemicals, glass fabric Heilbronn, used for trialsGermany). has plain The components weave and must an areal be dried density of 310 gin/m an2. oven at 30 °C for 24 h prior to experiments. Two material batches (ε-CL/initiator and ε- CL/activator) were prepared to fill the dosing units. The glass fabric used for trials has plain weave 2.2. T-RTMand an areal Equipment density of 310 g/m2.

2.2.A T-RTM T-RTM pilotEquipment production line consists of a mold carrier, two ε-CL dosing units and a research T-RTM mold. The mold carrier and dosing units are supplied by KraussMaﬀei GmbH (Munich, A T-RTM pilot production line consists of a mold carrier, two ε-CL dosing units and a research Germany).T-RTM Themold. mold The carriermold carrier (Figure and2) dosing is a hydraulic units are press supplied with by a bottom KraussMaffei stroke GmbH and a ( tiltingMunich upper, platen.Germany It restricts). The dimensions mold carrier and (Fig theure weight2) is a hydraulic of the mold. press The with machine a bottom also stroke provides and a tilting a communication upper interfaceplaten. with It a restricts pressure dimensions sensor inside and the the mold. weight of the mold. The machine also provides a communicationThe RimStar Thermo interface 8-8with dosing a pressure unit sensor has a inside dynamic the mold. self-cleaning mixing head and is capable of increasingThe RimStar injection Thermo pressure 8-8 dosing to 20 MPa.unit has This a dynamic unit is self used-cleaning to impregnate mixing head a preform and is capable and to of create the matrix.increasing The injection second pressure unit is ato CometPiston 20 MPa. This unit machine is used (KraussMa to impregnateﬀei GmbH,a preform Munich, and to create Germany) the for processingmatrix. ε The-CL second with various unit is solida CometPiston additives. machine (KraussMaffei GmbH, Munich, Germany) for processing ε-CL with various solid additives.

Figure 2. Shuttle mold carrier and a mounted T-RTM mold (KraussMaﬀei) (1—upper frame; 2—upper Figure 2. Shuttle mold carrier and a mounted T-RTM mold (KraussMaffei) (1—upper frame; 2—upper plate;plate; 3—the 3—the upper upper half half of of the the T-RTM T-RTM mold;mold; 4 4—the—the b bottomottom half half of ofthe the T-RTM T-RTM mold; mold; 5—lower 5—lower plate; plate; 6—lower6—lower frame). frame).

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Polymers 2018, 10, x FOR PEER REVIEW 4 of 13 2.3. The Concept of Designing a T-RTM Mold 2.3. The Concept of Designing a T-RTM Mold A T-RTM mold can barely be separated from the clamping and dosing units. The synergy of these units andA the T- moldRTM ismold even can stronger barely thanbe separated in conventional from the polymerclamping processes, and dosing because units. The the synergy T-RTM processof these units and the mold is even stronger than in conventional polymer processes, because the T- has certain similarities with injection molding, reaction injection molding and thermoset Resin Transfer RTM process has certain similarities with injection molding, reaction injection molding and Moldingthermoset (RTM). Resin Consequently, Transfer Molding the design (RTM). process Consequently, of the T-RTM the design mold process could of contain the T-RTM features mold of the designcould process contain of thefeatures mold of of the the desi above-mentionedgn process of the mold techniques, of the above besides-mentioned its own techniques, unique characteristics. besides Figureits3 owndemonstrates unique characteristics. our decision-making Figure 3 demonstrates matrix, which our decision could serve-making as matrix, a road which map incould T-RTM serve mold designas anda road development. map in T-RTM In mold the design decision-making and development. matrix, In wethe decision collected-making the information matrix, we coll aboutected all the standardthe information units of closed about molds all the (forstandard injection units molding, of closed reactivemolds (for injection injection molding, molding, andreactive thermoset injection RTM). We highlightedmolding, and with thermoset colors theRTM). relevance We highlighted of each unitwith incolors diﬀ erentthe relevance kind of molds,of each andunit within different black dots the closestkind of analogues molds, and of with each black unit dots in di theﬀerent closest molds analogues to those of each of the unit T-RTM in different mold. molds to those of the T-RTM mold.

FigureFigure 3. A 3. decision-making A decision-making map map for for T-RTM T-RTM moldmold design (green (green color color—a – a relevant relevant unit unit for this for type this type of mold;of mold; yellow yellow color—partially color – partially relevant relevant unit;unit; red color color—not – not relevant relevant unit; unit; black black dot dot—the– the closest closest analoguesanalogues of each of each unit unit in diinﬀ differenterent molds molds to to those those ofof the T T-RTM-RTM mold mold)) (based (based on on ref. ref. [14 [–1417]–).17 ]).

The moldThe mold incorporates incorporates two two mixing mixing heads; heads; oneone is dedicated dedicated to to forming forming the the part, part, while while the second the second one isone installed is installed to create to create a class a class A A surface surface onon thethe part. The The T T-RTM-RTM process process requires requires a vacuum a vacuum system, system, a temperaturea temperature and and pressure pressure controller, controller, an an oil oil temperingtempering unit unit and and a ahydraulic hydraulic ejector ejector system. system. In the In the next section,next section, we willwe will discuss discuss these these units units of of the the moldmold in more more detail. detail.

3. Results3. Results and and Discussion Discussion

3.1. Details3.1. Details of the of Designthe Design Process process of of the the T-RTM T-RTM MoldMold

3.1.1.3.1.1. Product Product Design Design

Any mold design process in the polymer industry starts with determining product geometry. From a research point of view, a simple plate would not satisfy our needs, because we would like to Polymers 2020, 12, 976 5 of 13 test more than just the influence of different chemical formulations, textile types, mold temperatures, pressures and dosing speeds on the output of the process. We would like to vary thickness, and create products with various architectures and surface qualities to cover typical automotive applications. We designed a total of three different demonstrators to fulfill our expectations and all these products had to be produced with the same mold. To make the explanation simpler, we divided global mold construction into two parts: the construction of the form plates (Figure4, pos. 1), where we place the preform and produce composite; and the construction of the supporting structure (Figure4, pos. 2), where we place auxiliary features of the mold and where we mechanically bring it into contact with the mold carrier.

3.1.2. Process Requirements As mentioned before, the mold is where we place reinforcement textiles, impregnate them and polymerize the reactive mixture. We need resin inlets to introduce the mixture into the mold. We must ensure that the reinforcement ﬁbers will be impregnated, that the medium in the mold is free of oxygen and moisture, and that the temperature of the mold is adequate to trigger the AROP reaction. We also need to be sure that we can dose the material with an appropriate dosing rate and that the clamping force is enough to keep the mold tightly closed. As the viscosity of molten ε-CL is extremely low, we must provide appropriate sealing.

3.1.3. Determining the Geometry and Structure of the Mold After the part is designed, we calculate the overall dimensions of the mold by taking into account the most relevant processing parameters. The dimensions of the mold were limited by platen size and the geometry of the product. The range of the daylight and the opening stroke of the mold carrier limit the maximum and the minimum heights of the mold. The height of the form plates depends on product geometry and the auxiliary elements required for producing the part. There are two distinct industrial strategies for defining the dimensions of the form plates. The first approach pursues the maximum reduction of weight and leads to a decrease in robustness, while the second approach is the opposite: to create a very robust structure at the cost of sacrificing weight savings. In order to calculate the overall height of the mold, we need to know the height of the form plates and we need to meet three more requirements; the total height of the mold must be more than the minimum daylight and less than the maximum daylight plus the ejection stroke, and it should still be enough to accommodate and position the mixing heads (Figure4, pos. 3, 4) and other units. To fulfill these last requirements, we need a supporting structure that connects the mold to the press and provides necessary space for the mixing heads and their units. Polymers 2018, 10, x FOR PEER REVIEW 6 of 13 Polymers 2020, 12, 976 6 of 13 Polymers 2018, 10, x FOR PEER REVIEW 6 of 13

(a) (b) (a) (b) Figure 4. (a) The research T-RTM mold (1—form plates; 2—supporting structure; 3—first mixing Figure 4. (a) The research T-RTM mold (1—form plates; 2—supporting structure; 3—first mixing head; head;Figure 4 — 4.second (a) The mixing research head; T-RTM 5— sealing; mold (1 —6—formoil temperature plates; 2—supporting controllers; structure; 7—vacuum 3—first system; mixing 8— 4—second mixing head; 5—sealing; 6—oil temperature controllers; 7—vacuum system; 8—hydraulic hydraulichead; 4— second ejector mixingsystem; head; 9—position 5—sealing; sensor 6— ofoil the temperature ejector system; controllers; 10— temperature 7—vacuum system; sensor; 8 11—— ejector system; 9—position sensor of the ejector system; 10—temperature sensor; 11—pressure sensors). pressurehydraulic sensors). ejector system;(b) The 9 cross—position-section sensor of the of formed the ejector part system; (P1—first 10 — inlettemperature point; P2 — sensor;second 11 inlet— (b) The cross-section of the formed part (P1—first inlet point; P2—second inlet point). point)pressure. sensors). (b) The cross-section of the formed part (P1—first inlet point; P2—second inlet Inpoint) order. to be able to vary the thickness of the product, we created a set of six spacers on the In order to be able to vary the thickness of the product, we created a set of six spacers on the bottom part of the mold (Figure5). The thickness range is 2–12 mm, which covers the most typical bottomIn partorder of to the be mold able to(Figure vary the5). Thethickness thickness of the range product, is 2– we12 mm,created which a set covers of six thespacers most on typical the industrial applications, including the possibility of using foam cores. A set of three inserts enables the industrialbottom part applications, of the mold including (Figure 5). the The possibility thickness of range using is foam2–12 mm,cores. which A set covers of three the inserts most typical enables manufacturing of products with different geometries. The first insert is for the monolithic flat part theindustrial manufacturing applications, of products including with the different possibility geometries. of using Thefoam first co res.insert A isset for of thethree monolithic inserts enables flat part (Figure6a), the second one is for the sandwich panel (with a foam or other types of core) (Figure6b) (Figurethe manufacturing 6a), the second of products one is for with the different sandwich geometries. panel (with The a firstfoam insert or other is for types the monolithic of core) (Figure flat part 6b) and the last insert is the one for producing a part with ribs (Figure6c). Parts with ribs can be produced and(Figure the las 6a),t insert the second is the one isfor for producing the sandwich a part panel with (with ribs (Figure a foam 6c).or other Parts types with ribsof core) can (Figurebe produced 6b) either in one shot or in two shots (overmolding). eitherand the in lasonet insert shot oris thein two one shotsfor producing (overmolding). a part with ribs (Figure 6c). Parts with ribs can be produced either in one shot or in two shots (overmolding).

(a) (b)(b) Figure 5. a b FigureFigure 5. 5. Bottom BottomBottom ( ((aa))) and andand upper upperupper ( ()b halves) halveshalves of ofof the thethe T-RTM TT--RTMRTM mold mold mold (1—ejector (1 (1——ejectorejector pin; pin; pin; 2—spacer; 2 —2—spacer;spacer; 3—textile 3 —3—textiletextile fixer; 4—pressurefixer;fixer; 44——pressurepressure sensor; sensor;sensor; 5—first 5— injectionfirst injection point; point; 6—flowpoint; 66—— leader;flowflow leader; leader; 7—insert; 7 7——insert; 8—vacuuminsert; 8 —8—vacuumvacuum valve; valve; 9—secondvalve; 9— 9— injectionsecondsecond injection injection point; 10 point;point; – oil temperature1010 – oil temperature controllers). controllers).controllers).

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Figure 6. Diﬀerent T-RTM parts (a ,b ,c ) and inserts designed (a ) for a ﬂat part; (b ) for a part with a Figure 6. Different T-RTM parts (a11, b1 1, 1c1) and inserts designed (a22) for a flat part; (b22) for a part with c foama foam core; core; ( 2 ()c for2) for a parta part with with overmolded overmolded ribs. ribs.

WeWe also also used used an an innovative innovative approach approach in in the the sealing sealing of of two two mold mold halves. halves. Traditionally, Traditionally, in in RTM RTM and reactionand reaction injection injection molding molding molds, molds the sealing, the sealing contour contour is parallel is parallel to the to product, the product, which which limits limits the ability the toability vary thickness. to vary thickness. In our mold, In our we used mold, a double we used seal a (Figure double 4 seal, pos. (Figure 5) in the 4, plane pos. 5) perpendicular in the plane to theperpendicular cavity, creating to the a kindcavity, of acreating core–cavity a kind connection. of a core–cavity Both sealsconnection. slide inside Both seals the bottom slide inside half of the the mold,bottom making half of it the possible mold, tomaking vary partit possible thickness. to vary part thickness. ToTo establish establish a a fully fully homogeneoushomogeneous temperature in in the the mold, mold, we we designed designed a aconventional conventional cooling cooling circuitcircuit for for oil oil heaters heaters (Figure(Figure4 4,, pos. pos. 6). 6). TheThe AROP AROP reaction reaction is is water- water- and and oxygen-sensitive, oxygen-sensitive, therefore therefore we we had had to to assure assure that thatthe the mediummedium in thein moldthe mold would would be free be offree moisture of moisture and oxygen. and oxygen. For that, For we that, decided we decided to apply to a apply vacuum a vacuum and equipped and theequipped mold with the amold special with vacuum a special system vacuum (Figure system7). (Figure Our vacuum 7). Our system vacuum in thesystem mold in ditheff ersmold from differs those wefrom can those use inwe standard can use in injection standard molds. injection Its molds. distinct Its feature distinct is feature a cone-shaped is a cone valve-shaped head valve (Figure head7 , pos.(Figure 8), which 7, pos. opens 8), which and opens closes and the ventcloses channel the vent with channel a hydraulically with a hydraulically operated mechanism.operated mechanism. It makes it possibleIt makes to it create possible a vacuum to create in a the vacuum cavity in and the to cavity shutit and off beforeto shut the it off flow before reaches the theflow vacuum reaches outlet. the Thevacuum valve outlet. position The is indicatedvalve position by the is two indicated position by sensors the two (Figure position7, pos. sensors 2). The (Figure valve 7, head pos. (Figure2). The7 , pos.valve 8) head is also (Figure sealed. 7, Wepos. selected 8) is also the sealed. hydraulic We selected system tothe operate hydraulic a tightly system sealed to operate valve a because tightly it createssealed suvalvefficient because opening it creates and closingsufficient forces. opening and closing forces.

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FigureFigure 7.7. TheThe vacuum vacuum system system of of the the T- T-RTMRTM mo moldld (1— (1—hydraulichydraulic cylinder; cylinder; 2— 2—positioningpositioning sensor; sensor; 3— 3—coupling;coupling; 4— 4—rod;rod; 5— 5—valvevalve shaft; shaft; 6, 7, 6, 9 7,— 9—sealing;sealing; 8— 8—valvevalve head; head; 10— 10—vacuumvacuum connector). connector).

WeWecan can createcreate anan additionaladditional coatingcoating onon thethe surfacesurface ofof thethe partpart byby meansmeans ofof openingopening thethe moldmold aa littlelittle bit bit and and creating creating a a space space for for the the second second shot. shot. We We therefore therefore created created two two independent independent inlet inlet points points in thein the mold. mold. The The first first inlet inlet point point (Figure (Figure4,P 4,1) P is1) located is located in thein the bottom bottom half half of theof the mold mold and and connected connected to theto the mixing mixing head head (Figure (Figure4, pos. 4, pos. 3) that 3) dosesthat doses the material the material for forming for forming the part. the Itpart. is placed It is placed in the in flow the leaderflow leader groove groove (Figure (Figure8, pos. 1),8, pos. which 1), helps which make helps the make flow the front flow more front uniform more in uniform the flow in direction. the flow Thedirection. flow leader The flow can leader be filled can quickly be filled and quickly from and there from the there melt startsthe melt to distributestarts to distribute crosswise crosswise in both directions.in both direc Thetions. second The second inlet point inlet (Figure point (Figure4,P 2) is4, inP2) the is in upper the upper mold mold half andhalf connectedand connected with with the mixingthe mixing head head (Figure (Figure4, pos. 4, pos. 4) that 4) that creates creates the the coating. coating. The The second second inlet inlet point point is is located located in in a a cone cone thatthat hashas aa counterpartcounterpart inin thethe stationarystationary moldmold half.half. TheThe heightheight ofof thethe counterpartcounterpart conecone cancan bebe adjustedadjusted (Figure(Figure5 ).5). The The latter latter feature feature is is necessary necessary to to maintain maintain a a constant constant space space between between the the mold mold halves halves after after thethe thicknessthickness is is changed. changed. To To form form a coatinga coating a fewa few tenths tenths of aof millimeter a millimeter thick, thick, we needwe need a gap a ofgap 1–2 of mm 1–2 andmm theand second the second shot, shot, as well as aswell additional as additional external external pressure, pressure, which which forms forms the upper the upper layer. layer.

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(a) (b) Figure 8. a Figure 8. Layout of the mold cavitycavity withwith thethe reinforcementreinforcement insideinside ((a)) (P(P11—first—first inlet inlet point; 1 1—flow—flow leaderleader groove; groove; 2 2—flow—flow front distribution in in the cavity; 3 3—textile—textile fixer; fixer; 4 4—textile;—textile; (b) (b) cross cross-section-section of the textile fixer fixer ( (5—the5—the upper half of the T-RTMT-RTM mold;mold; 6—the6—the bottombottom halfhalf ofof thethe T-RTMT-RTM mold).mold).

In thermoset RTM, there is a well-known issue caused by imperfections on textile preform edges. In thermoset RTM, there is a well-known issue caused by imperfections on textile preform edges. Some textiles tend to fray, which leads to a lower density at the edges and makes it easier for the melt Some textiles tend to fray, which leads to a lower density at the edges and makes it easier for the melt or resin to go first “around” the preform. This phenomenon is called “race tracking” and it results in or resin to go first “around” the preform. This phenomenon is called “race tracking” and it results in dry spots in a product. To avoid such defects, we placed slightly bigger plies, and are able to hold the dry spots in a product. To avoid such defects, we placed slightly bigger plies, and are able to hold the preform edges with special silicone textile fixers (Figure8, pos. 3). This helps to avoid the race tracking preform edges with special silicone textile fixers (Figure 8, pos. 3). This helps to avoid the race effect by guiding the flow to the center of the cavity. The fixers create a higher resistance for the melt tracking effect by guiding the flow to the center of the cavity. The fixers create a higher resistance for flow, forcing it to go first through the preform and impregnate the edges only at the end. We placed the melt flow, forcing it to go first through the preform and impregnate the edges only at the end. We these fixers perpendicular to the flow leader. placed these fixers perpendicular to the flow leader. To measure cavity pressure, we selected a piezoelectric sensor (6161AA 2, Kistler Instrumente To measure cavity pressure, we selected a piezoelectric sensor (6161AA 2, Kistler Instrumente GmbH, Sindelfingen, Germany). This type is used in thermoset RTM. The pressure sensor (Figure4, GmbH, Sindelfingen, Germany). This type is used in thermoset RTM. The pressure sensor (Figure 4, pos. 11) is located as close as possible to the first inlet point. This location allows feedback almost pos. 11) is located as close as possible to the first inlet point. This location allows feedback almost immediately after dosing starts, and allows to observe the pressure drop during the entire filling immediately after dosing starts, and allows to observe the pressure drop during the entire filling and and polymerization stage. The temperature of the mold is controlled with the help of a conventional polymerization stage. The temperature of the mold is controlled with the help of a conventional PT- PT-100-type thermocouple (Figure4, pos. 10), embedded in the upper mold half close to the surface of 100-type thermocouple (Figure 4, pos. 10), embedded in the upper mold half close to the surface of the cavity. the cavity. The last step in T-RTM is the demolding of the product. To do it in an automated way, we need The last step in T-RTM is the demolding of the product. To do it in an automated way, we need an ejector system (Figure4, pos. 8; Figure9). Normally, ejectors are embedded between two plates an ejector system (Figure 4, pos. 8; Figure 9). Normally, ejectors are embedded between two plates operated by the machine. However, the built-in mixing heads with their tubes limit the available free operated by the machine. However, the built-in mixing heads with their tubes limit the available free space for the ejector unit. Therefore, in our case, each ejector is operated by a separated hydraulic space for the ejector unit. Therefore, in our case, each ejector is operated by a separated hydraulic cylinder and each cylinder is fed by the same oil circuit to ensure their parallel movement. cylinder and each cylinder is fed by the same oil circuit to ensure their parallel movement.

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Figure 9. Ejector system (1—sealing; 2—guide bush; 3—ﬁxing plate; 4—ejector pin; 5—coupling; 6—hydraulicFigure 9. Ejector cylinder; system 7—position (1—sealing; sensor). 2—guide bush; 3—fixing plate; 4—ejector pin; 5—coupling; 6— 3.1.4.hydraulic Mold Materials cylinder; Selection 7—position sensor).

3.1.4.We Mold selected Materials stainless Selection steel as the main material for the form plates, as it can withstand the corrosive environment under the high temperature and pressure loads in T-RTM molds. Inserts for different We selected stainless steel as the main material for the form plates, as it can withstand the architectures are made from aluminum. Thermal insulation of the mold is provided by the insulation corrosive environment under the high temperature and pressure loads in T-RTM molds. Inserts for plates made from BRAGLA®® N (Brandenburger Isoliertechnik GmbH. & Co., Landau in der Pfalz, different architectures are made from aluminum. Thermal insulation of the mold is provided by the Germany). This insulation material has a low thermal conductivity (0.22 W/(m K)), while its operation insulation plates made from BRAGLA®® N (Brandenburger Isoliertechnik GmbH.· & Co., Landau in temperature can be as high as 350 C. Moreover, all the seals in the T-RTM mold must also withstand a der Pfalz, Germany). This insulation◦ material has a low thermal conductivity (0.22 W/(m∙K)), while maximum temperature of 200 C, a maximum pressure of 20 MPa and an aggressive alkaline medium. its operation temperature can ◦be as high as 350 °C. Moreover, all the seals in the T-RTM mold must We selected the VitonTM fluoroelastomer (DuPont, Wilmington, DE, USA) to seal the T-RTM mold, also withstand a maximum temperature of 200 °C, a maximum pressure of 20 MPa and an aggressive because this material fulfills all the above-mentioned requirements. alkaline medium. We selected the VitonTM fluoroelastomer (DuPont, Wilmington, DE, USA) to seal 3.2.the T Testing-RTM ofmold, the T-RTM because Mold this material fulfills all the above-mentioned requirements.

3.2. TestingWe produced of the T- bothRTM neat Mold and fiber-reinforced samples with reactive PA-6 at various dosing speeds (20, 30 and 40 g/s). The temperature of the mold was kept at 150 ◦C during the tests. The dosing volumeWe was produced 280 g for both reinforced neat and samples fiber-reinforced and 365 g samples for the unreinforced with reactive samples. PA-6 at We various used dosinghalf of thespeeds mold (20, volume 30 and as 40 a bug/s).ffer The to keeptemperature the nitrogen of the bubbles mold was outside kept of at the 150 reinforced °C during zone the (Figuretests. The8). Wedosing produced volume three was samples280 g for for reinforced each material samples combination and 365 g andfor the with unreinforce each dosingd samples. rate to checkWe used the stabilityhalf of the of mold the process. volume as a buffer to keep the nitrogen bubbles outside of the reinforced zone (Figure 8). WeThe produced in-mold three pressure samples measurement for each resultsmaterial for combination neat PA-6 are and presented with each in Figuredosing 10 rate. After to check reaching the thestability peak of value, the process. the in-mold pressure starts to decline because the volume of the reactive mixture decreasesThe in in- themold mold, pressure which measurement indicates polymerization results for and neat crystallization PA-6 are presented in the mold. in Figure Moreover, 10. After with allreaching three dosing the peak rates, value, these the parts in-mold of the pressure curves are starts very to close decline to each because other andthe seemvolume to beof independentthe reactive ofmixture dosing decreases speed. This in thefact mold, shows which that the indicates rate of polymerization polymerization is and not crystallizaffected significantlyation in the by mold. the dosingMoreover, rate. with Further all three investigations dosing rates, at variousthese parts mold of temperaturesthe curves are can very extend close thisto each knowledge. other and seem to bePolymerization independent of and dosing crystallization speed. This behavior fact shows might that be the aff ectedrate of by polymerization the fabric placed is not in the affected mold, assignificantly the in-mold by pressure the dosing curves rate. changed Further afterinvestigations reaching theat various maximum mold pressure temperatures (Figure can 11). extend this knowledge. Polymerization and crystallization behavior might be affected by the fabric placed in the mold, as the in-mold pressure curves changed after reaching the maximum pressure (Figure 11).

Polymers 2020, 12, 976 11 of 13 Polymers 2018, 10, x FOR PEER REVIEW 11 of 13 Polymers 2018, 10, x FOR PEER REVIEW 11 of 13

FigureFigure 10. 10.The The pressure pressure curvescurves of of the the neat neat PA PA-6-6 during during thethe T-RTM T-RTM process process (DU1 (DU1 and DU2 and are DU2 dosing are dosing Figureunit 1 10.and The dosing pressure unit 2, curves respectively). of the neat PA-6 during the T-RTM process (DU1 and DU2 are dosing unitunit 1 1 and and dosing dosing unit unit 2,2, respectively).respectively).

Figure 11. The pressure curves of the reinforced PA-6 during the T-RTM process. Figure 11. The pressure curves of the reinforced PA-6 during the T-RTM process. We observed significant differences between the in-mold pressure curves of reinforced and We observedFigure significant 11. The pressure differences curves between of the reinforced the in-mold PA -pressure6 during curvesthe T-RTM of reinforced process. and neat neatsamples. samples. To To explain explain this this phenomenon, phenomenon, further further investigation investigation of the of T- theRTM T-RTM process process at various at various temperatures is required. With the current mold configuration, we can only identify the temperature We observed significant differences between the in-mold pressure curves of reinforced and neat andsamples. pressure To at explain one point this in phenomenon, the cavity. As further such, in-mold investigation process of control the T- performedRTM process through at various in-mold pressure feedback would be preferable, as many physical and chemical processes take place in the

Polymers 2020, 12, 976 12 of 13 mold simultaneously. Furthermore, the number of sensors should be increased so that more important data can be collected, and the process can be controlled and monitored more precisely.

4. Conclusions In this study we proposed a design concept and developed a prototype mold for the T-RTM process. The mold has been designed to manufacture parts with various features and thicknesses. The range of these thicknesses can vary between 2 and 12 mm, which covers the most typical automotive applications. The mold developed enables the production of neat polymer parts, fiber-reinforced composites, and sandwich structures with various cores. We can add ribs to the part, and these ribs can be formed either together with the part in one shot or by overmolding on the base part in a second shot. Additionally, we can apply in-mold coating through a second inlet, by which we can form an extra layer of material via compression molding. A number of engineering problems that appear in the T-RTM process were solved. We proposed special designs for the vacuum system and the ejector unit of the mold. We also developed an appropriate sealing to prevent leakage of the low-viscosity mixture, and introduced pressure and temperature sensors to control the whole process. We also indicated that the preform placed into the mold has a significant effect on the T-RTM process. The preform changed the pressure profile during the cycle and the reason behind this is either the influence of the fabric on the AROP reaction, or the influence of the fabric on the measurement of pressure itself. To separate these effects, further investigations are required. To sum up, in the first round of development, we aimed to introduce a concept of T-RTM mold design as well as demonstrate the operability of the developed mold. The mold can be used for further investigation and a deeper analysis of the T-RTM process, and that is the next step of our research.

Author Contributions: Conceptualization, J.G.K., R.B. and I.S.; methodology, R.B. and I.S.; software, R.B.; validation, I.S. and T.A.; formal analysis, I.S. and R.B.; investigation, I.S. and R.B.; resources, J.G.K.; data curation, J.G.K.; writing—original draft preparation, I.S. and T.A.; writing—review and editing, J.G.K. and T.A., visualization, R.B.; supervision, J.G.K.; project administration, J.G.K.; funding acquisition, J.G.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Research, Development and Innovation Office, Hungary (2018-1.3.1-VKE-2018-00001, 2017-2.3.7-TÉT-IN-2017-00049, TUDFO/51757/2019-ITM, Thematic Excellence Program). Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

AROP anionic ring opening polymerization HP high pressure LCM liquid composite molding LP low pressure PA polyamide RIM reactive injection molding T-RTM thermoplastic resin transfer molding ε-CL ε-caprolactam

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