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Vac­u­um infu­sion for large-area fiber com­pos­ite com­po­nents — an alter­na­tive to injec­tion molding

23.02.2022   | Aaron Stoffers

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Injec­tion mold­ing process­es are ide­al for high-vol­ume pro­duc­tion of plas­tic com­po­nents. Once the mold has paid for itself, the unit costs are rel­a­tive­ly low. How­ev­er, this pro­duc­tion tech­nique is often unprof­itable for small batch­es, as mold con­struc­tion often rep­re­sents a dis­pro­por­tion­ate share of the man­u­fac­tur­ing costs for small quan­ti­ties. Fiber com­pos­ites, which are usu­al­ly man­u­fac­tured by vac­u­um infu­sion, offer an alter­na­tive for large-area parts.

How does the process work?

Injec­tion mold­ing and vac­u­um infu­sion process­es are based on dif­fer­ent prin­ci­ples. In injec­tion mold­ing, the liq­uid mate­r­i­al is pressed into the mold under high pres­sure, while vac­u­um infu­sion sucks the mate­r­i­al in by neg­a­tive pressure.

In prac­tice, a lam­i­nate mold, which deter­mines the sub­se­quent shape of the com­po­nent, is first coat­ed with a lay­er of gel­coat. Pre­cise­ly cut fiber mats are then insert­ed into the con­tours of the mold, form­ing the core of the com­pos­ite. The com­po­si­tion of these mats depends on the appli­ca­tion. Most com­mon are glass and car­bon fibers, but aramid fibers (e.g. Kevlar) or var­i­ous min­er­al (basalt) or veg­etable (jute, flax, cot­ton) fibers can also be used. For infu­sion, infu­sion tubes are placed at pre­de­fined loca­tions and the mold is sealed all around with a film.

As soon as the prepa­ra­tion is com­plete, the air is sucked out from under the foil so that a vac­u­um is cre­at­ed. Liq­uid resin is now fed into the mold via the hoses, which is “pulled” into all cor­ners and free spaces by the vac­u­um. The com­pound is there­fore not active­ly pressed into the mold, but is pushed into the evac­u­at­ed mold by the ambi­ent pres­sure. To ensure that no dry spots remain, a lit­tle more resin is pumped in than nec­es­sary. The excess mate­r­i­al is col­lect­ed and recy­cled if pos­si­ble. The resin mix­ture then cures under vac­u­um and com­bines with the fiber mats to form a fiber-rein­forced mate­r­i­al.
The fol­low­ing fig­ure illus­trates the prin­ci­ple. At the top, the resin is intro­duced into the lam­i­nate through the two feed lines. It slow­ly migrates through the com­po­nent in a so-called flow front (bound­ary between dark and light yel­low), while at the bot­tom the white suc­tion line con­tin­ues to main­tain the vac­u­um. Final­ly, the flow front reach­es the suc­tion line and the fill­ing process comes to a halt.

Beispiel einer Vakuuminfusion blog/vacuum-infusion-for-large-area-fiber-composite-components-an-alternative-to-injection-molding/

Exam­ple of vac­u­um infusion

Advan­tages and dis­ad­van­tages of fiber composites

Com­po­nents made of fiber com­pos­ites are often sig­nif­i­cant­ly lighter than injec­tion-mold­ed plas­tic com­po­nents for the same per­for­mance, because the fibers give the mate­r­i­al extreme strength in the lon­gi­tu­di­nal direc­tion and thus indi­rect­ly reduce com­po­nent weight, since less mass is required for the same load-bear­ing capac­i­ty. This prop­er­ty makes fiber com­pos­ites attrac­tive for all areas of appli­ca­tion in which weight reduc­tion plays a role, e.g. motor sports, boat build­ing or aerospace.

A spe­cial fea­ture of fiber com­pos­ites is that their stiff­ness and ten­sile strength depend on the ori­en­ta­tion of the fibers with­in the com­pos­ite matrix. This allows their mechan­i­cal prop­er­ties to be flex­i­bly tai­lored to the area of appli­ca­tion. This pays off both in terms of price and weight, as the com­po­nents can be opti­mized specif­i­cal­ly along the stress curves in use.

For exam­ple, fiber-rein­forced com­pos­ites make it pos­si­ble to devel­op lighter ten­sion rods that are enor­mous­ly sta­ble against loads in the direc­tion of ten­sion, but less able to with­stand orthog­o­nal forces (which occur less frequently).

The dis­ad­van­tage of fiber-rein­forced plas­tics becomes appar­ent toward the end of their ser­vice life. Unlike injec­tion-mold­ed parts, fiber-rein­forced parts are not read­i­ly recy­clable. The rea­son for this is the chem­i­cal com­po­si­tion of the mate­r­i­al. It is not a homo­ge­neous mass, but a com­pound of poly­mers (often duromers) and rein­forc­ing fibers that stick togeth­er. This bond is almost impos­si­ble to reverse in terms of process tech­nol­o­gy. Com­po­nents man­u­fac­tured in this way can there­fore not sim­ply be melt­ed down and recycled.

On the one hand, this is detri­men­tal to the company’s envi­ron­men­tal bal­ance sheet, since fiber com­pos­ites are not biodegrad­able and require cost­ly dis­pos­al. On the oth­er hand, it is detri­men­tal to prof­itabil­i­ty, as the com­pa­ny has to arrange and finance dis­pos­al — a chal­lenge that is cur­rent­ly occu­py­ing the oper­a­tors of off­shore wind farms, for exam­ple. For this rea­son, fiber com­pos­ites are crit­i­cal for appli­ca­tion areas with a short ser­vice life.

blog/vacuum-infusion-for-large-area-fiber-composite-components-an-alternative-to-injection-molding/

Vac­u­um infu­sion of a rotor blade in detail

Vac­u­um infu­sion vs. injec­tion molding

Some of the advan­tages and dis­ad­van­tages of fiber-rein­forced plas­tics arise not from the mate­r­i­al prop­er­ties, but from the man­u­fac­tur­ing process. Since vac­u­um infu­sion does not require high-pres­sure resis­tant molds, the invest­ment costs are sig­nif­i­cant­ly low­er, which is par­tic­u­lar­ly notice­able for small quan­ti­ties. This cir­cum­stance makes fiber com­pos­ites par­tic­u­lar­ly attrac­tive for small batch­es, where the high costs of tool­mak­ing often can­not be spread over high volumes.

In addi­tion, vac­u­um infu­sion is more suit­able for large-area com­po­nents, since lam­i­nate molds can be pro­duced with less effort. For exam­ple, pro­duc­ing a meter-long boat hull by injec­tion mold­ing would be nei­ther eco­nom­i­cal nor prac­ti­cal. Lin­ing a cor­re­spond­ing mold man­u­al­ly, on the oth­er hand, is fea­si­ble with sig­nif­i­cant­ly less effort.

How­ev­er, the high num­ber of man­u­al steps is also a weak­ness of vac­u­um infu­sion when it comes to large series. The time required scales very poor­ly. As soon as larg­er quan­ti­ties of a com­po­nent are required, this man­u­fac­tur­ing process los­es its eco­nom­ic effi­cien­cy. There­fore, vac­u­um infu­sion is par­tic­u­lar­ly suit­able for small batch­es or indi­vid­ual projects.

Con­clu­sion

Vac­u­um infu­sion is a com­mon process for the pro­duc­tion of fiber com­pos­ite com­po­nents. It is par­tic­u­lar­ly suit­able for small series or man­u­fac­tures, since the invest­ment costs for molds and tools are com­par­a­tive­ly low and the prop­er­ties of the mate­r­i­al can be indi­vid­u­al­ly tai­lored to the appli­ca­tion. Even very large-area com­po­nents are no problem.

How­ev­er, this man­u­fac­tur­ing approach is less suit­able for large-scale pro­duc­tion because it involves a high num­ber of man­u­al work steps. The automa­tion poten­tial is there­fore lim­it­ed and scales poor­ly for large series.

For small batch­es, how­ev­er, vac­u­um infu­sion is very well suit­ed. The process should always be con­sid­ered for cor­re­spond­ing projects.

Please note: 

We have sev­er­al spe­cial­ists for fiber com­pos­ites in our part­ner net­work and have also already com­plet­ed some projects. Con­tact us if you have a need! 

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Whitepa­per:
Secure the Start of Production 

Learn which fac­tors influ­ence your SOP!

blog/vacuum-infusion-for-large-area-fiber-composite-components-an-alternative-to-injection-molding/

Author: Aaron Stoffers

Aaron Stof­fers stud­ied dual mechan­i­cal engi­neer­ing at Hanover Uni­ver­si­ty of Applied Sci­ences and Arts and lat­er com­plet­ed a master’s degree in busi­ness admin­is­tra­tion. Since 2019, he has been work­ing for Jaeger Mare Solu­tions as a project man­ag­er in the field of off­shore wind energy.

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