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Vacuum assisted resin transfer moulding (VARTM)/resin infusion (VARI) - A290

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Vacuum assisted resin transfer moulding (VARTM)/resin infusion (VARI)
Document Type Article
Document Identifier 290
Prerequisites

Introduction[edit | edit source]

Vacuum assisted resin transfer moulding (VARTM)/resin infusion (VARI), is a common liquid composite moulding process that relies solely on vacuum to infuse resin into the dry fibre pre-form. As a variation of the resin transfer moulding (RTM), VARTM uses a single sided tool with a flexible vacuum bag. This leads to more economical tooling comparing to other liquid composite moulding processes such as resin transfer moulding where matched die tools are required.

Significance[edit | edit source]

Capital cost is very low and parts can be made inexpensively with this process [1][2]. It is especially cost effective when making large structures at low volume, such as boat hulls [3].

Scope[edit | edit source]

This page provides an overview of the vacuum assisted resin transfer moulding/infusion process. The process is explained from the MSTE perspective to include all the important variables when implementing the process. Key considerations from Thermal and cure/crystallization management (TM), Materials deposition and consolidation management (MDCM) and Residual stress and dimensional control management (RSDM) are also discussed.

Process Demonstration[edit | edit source]

Three videos are presented below to help give more insight into the process. The first video introduces the process and provides some of the theory behind it. The second video demonstrates an activity users can perform at their own facilities to measure the permeability of a fibre bed and better understand how their combination of fibre and resin will perform. The third video provides an overview of the theory and analysis that can be done with data from the activity. A data set is provided for users to work through the activity.

A 'lab activity' exercise is incorporated into the second and third videos. To complete the lab activity, watch the videos and follow the instructions to analyze the data using the accompanying CSV files.

Process overview[edit | edit source]

The video below provides an introduction and overview of the vacuum infusion process.

Activity - process setup and permeability measurement[edit | edit source]

The video below demonstrates how to setup a simple vacuum infusion process that can be used to measure the permeability of a fibre bed.

Theory and analysis[edit | edit source]

The video below demonstrates how to process the flow position vs. time data collected in the activity shown in the video above. CSV files of data collected while performing this process, and the experiment variables are provided below:

Infusion Dataset Process Specs Dataset
CSV Icon-d63js8rms73hd5.svg
CSV Icon-d63js8rms73hd5.svg


Process description[edit | edit source]

In vacuum assisted resin transfer moulding/infusion, dry fibre pre-form is typically manually deposited into a mould with precision and consolidated under a vacuum bag before introducing the resin. Resin is often degassed under vacuum prior to the infusion to remove volatiles such as air and moisture[4]. To initiate the process, resin is allowed to flow through the inlet(s) through the fibre pre-form using vacuum at the outlet(s) as the driving force. For complex geometries, resin inlet(s) and vacuum outlet(s) can be placed and pinched off strategically to ensure the full impregnation of the dry pre-form [2]. When the pre-form is fully wetted, the inlet(s) are pinched and the part is cured under vacuum. Once the part is cured, it is taken off the mould and the consumables are disposed. Post cure at elevated temperature might be required depending on the resin system and cure conditions.

Gel coat are sometimes used to achieve glossy surface finish.[5]

Practice for developing a thermal transformation process stepPractice for developing a thermal transformation process stepPractice for developing a thermal transformation process stepVA-RTM and Light RTM Factory Workflow-8Xgd4kPBNzPm.svg

Micro and Macroscopic flow[edit | edit source]

Microscopic flow occurs when resin flows in between fibres while macroscopic flow occurs when resin flows in between tows.

Microscopic vs macroscopic flow [6]

Micro and macroscopic flow scales should progress at the same rate otherwise one will over take the other, potentially resulting in porosity If flow is too fast, macroscopic flow will overtake, resulting in dry spots within the tow. If flow is too slow, microscopic flow will overtake, resulting in dry spots between the tows. Tow size, fibre architecture and resin viscosity can all potentially affect the flow scales.

Darcy's law and fill time[edit | edit source]

As the resin flows from the inlet(s) towards regions under lower pressure, the resin velocity decreases according to Darcy's law. Darcy’s law governs flow through porous medium\[Q = -\frac{KA}{\mu}\frac{\Delta P}{x}\]

Where,

\(Q = \) Volumetric flow rate

\(K = \) Preform permeability

\(A = \) Preform cross-sectional area

\(\mu = \) Resin viscosity

\(\Delta P = \) Pressure differential across preform

\(x = \) In-plane flow distance of pressure differential

The time taken for resin to flow a certain distance from the inlet can be derived from Darcy's law. Please see How to assess the resin filling time in LCM processes for more detail. When manufacturing large components, multiple resin inlets and vacuum outlets at different locations are typically used. The inlets and outlets can be controlled to open either simultaneously or sequentially. In practice, it is common to open and close the inlets and outlets to manipulate flow fronts in order to fully impregnate the preform. The sequence however, is largely empirically and can even vary from part to part.

Material of VARTM/VARI[edit | edit source]

Reinforcement types[edit | edit source]

Most fibre types can be used in vacuum assisted resin transfer moulding/infusion. No difficulties are know with any specific fibre type. A quick compatibility test can be performed by placing a small quantity of resin onto the reinforcement surface; if the resin is absorbed quickly into the reinforcement, compatibility should be sufficient. [7]


Resin type[edit | edit source]

A wide variety of polyester, vinyl ester and epoxy resin can be used for VARTM, however, resin viscosity needs to be relatively low [7][5], lower than the viscosity of those used for conventional RTM processes. Resin viscosity less than 100 cps are desirable for the resin to impregnate the dry preform under only vacuum pressure [4]. Resin gel time needs to be higher than the degassing plus deposition (impregnation) time[5]

Consumables[edit | edit source]

Spiral tubing[edit | edit source]

Spiral tubing (aka. spiral wrap) is extruded polyethylene or polypropylene in coiled tubular shape, commonly used to direct/disperse resin flow. The tubing is flexible to be bend around the perimeter of the part and can resist collapsing under vacuum pressure. The spiral tubing is typically connected to the resin inlet(s) and vacuum outlet(s). When the inlet opens, resin rapidly flows through the spiral tubing before seeping out and reaching the part, making the spiral tubing an extension of the inlet. This is advantageous because resin flows into the part from a line instead of a single point. When used with vacuum outlet(s), same principle applies which turns the vacuum outlet into a line section instead of a single point.[8][9][10][11][12]

Peel ply[edit | edit source]

Peel ply in vacuum assisted resin transfer moulding typically serves two purposes: separating/releasing the consumables from the part and slowing down resin flow near the outlet(s) (resin brakes). When using any flow medium, peel ply is always placed below the flow medium and above the laminate to separate the two when the part is cured. It is also common practice to wrap peel ply around the spiral tubing for proper release, especially when the spiral tubing is used on top the part in a sequential inlet case.

When used as a resin brake, the peel ply layer is extender beyond the laminate for a certain distance near the vacuum outlet(s). The extended distance is determined empirically and usually changes with part size. When resin has impregnated the laminate and reached the single layer of peel ply, the low permeability can slow down the resin flow significantly to provide operators time to inspect the impregnation and close the inlets. [13][14][15][16]

Flow medium[edit | edit source]

Flow medium (or porous carrier) allows the resin to travel rapidly in the in-plane direction and simultaneously give time for resin to impregnate in the through-thickness direction. The flow medium is typically placed right underneath the vacuum bag and on top of the part. However, flow medium can also be used in the middle of the laminate or on the tool side of the part. Proprietary processes, such as Seemann composites resin infusion moulding process (SCRIMP®)[17] and fast remotely actuated channeling (FASTRAC) utilize specialized resin distribution flow medium or channels to decrease resin distribution time and reduce manufacturing cycle time [18].

Shape of VARTM/VARI[edit | edit source]

VARTM can adopt a wide range of size and shape complexities. In theory, a shape is achievable if the reinforcement can be deposited and resin flow can reach. In practice, sharp/tight turns and undercuts are difficult to fully infuse. Additional resin inlets or vacuum outlets placed at strategic locations might be required to steer the resin flow front in order to fully impregnate those features. Sharp edges in the mould or in consumables may puncture the vacuum bag [5].

Preform[edit | edit source]

See preform for liquid composite moulding

Tool of VARTM/VARI[edit | edit source]

VARI tooling material[edit | edit source]

Because of the lower pressures used in vacuum infusion, the mechanical properties of the tooling are lower than with other liquid composite moulding processes. A typical setup involves a one sided tool with a flexible vacuum bag [2]. A wide range of materials (such as tooling foam, GFRP, CFRP, aluminum or steel etc) can be used to make VARTM tooling. The selection of the tooling material should be based on the processing temperature, air tightness, number of parts to be produced (durability) and cost.

Equipment of VARTM/VARI[edit | edit source]

Advantages[edit | edit source]

  • Class A surface finish possible on one side
  • Gel coat can be used to achieve better surface finish
  • Manufacturing cycle time can be very short
  • Possible to make large components (such as boat hulls or wind turbine blades)
  • Inserts, fittings, cores, or ribs can be moulded-in
  • Low capital (equipment) cost due to only a vacuum pump is required
  • Tooling cost is much lower than conventional RTM
  • Near net moulded parts
  • High design flexibility in terms of reinforcement, matrix and core material selection
  • Very low volatile emission

Disadvantages[edit | edit source]

  • One side of the part has good surface
  • Thickness (and fibre volume fraction) variation from the resin inlet(s) to vacuum outlet(s) [1]
  • Lower Fibre volume fraction (50% - 55%)[4] comparing to pre-preg processes
  • Setup can be potentially complicated for large or complexed parts. Easy to make mistakes such as air-leaks which can ruin the part

Application[edit | edit source]

  • Energy
  • Marine industry
  • Automotive industry (customized parts/small batch production)
  • Aerospace industry
  • Sport equipment

Troubleshooting[edit | edit source]



References

  1. ↑ 1.0 1.1 [Ref] Astrom, B.T. Manufacturing of Polymer Composites. ISBN 9780748770762.CS1 maint: uses authors parameter (link)
  2. ↑ 2.0 2.1 2.2 [Ref] Strong, A Brent (2008). 16 Resin Infusion Technologies. Society of Manufacturing Engineers. ISBN 0-87263-854-5, 978-0-87263-854-9 Check |isbn= value: invalid character (help).CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  3. ↑ [Ref] Mazumdar, Sanjay K. (2002). Composites Manufacturing materials, product and processing engineering. ISBN 0-8493-0585-3.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  4. ↑ 4.0 4.1 4.2 [Ref] Campbell, F.C. (2004). Manufacturing Processes for Advanced Composites. Elsevier. doi:10.1016/B978-1-85617-415-2.X5000-X. ISBN 9781856174152.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  5. ↑ 5.0 5.1 5.2 5.3 [Ref] Ghosh, Anup K.; Dwivedi, Mayank (2020). Processability of Polymeric Composites. Springer Nature India Private Limited 2020, corrected publication 2020. doi:10.1007/978-81-322-3933-8. ISBN 9788132239314.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  6. ↑ [Ref] Leclerc, Jean Sébastien; Ruiz, Edu (2008). "Porosity reduction using optimized flow velocity in Resin Transfer Molding". 39 (12). Elsevier Ltd. doi:10.1016/j.compositesa.2008.09.008. ISSN 1359-835X. Cite journal requires |journal= (help)CS1 maint: uses authors parameter (link)
  7. ↑ 7.0 7.1 [Ref] Potter, Kevin (1997). Resin Transfer Moulding. Chapman & Hall, 2-6 Boundary Row, Loudon SEt 8HN, UK. doi:10.1007/978-94-009-0021-9 4 Check |doi= value (help). ISBN 9789401064972.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  8. ↑ [Ref] Limited, Vacmobiles.com (2020). "Polyethylene vacuum tubing, spiral wrap & consumables for infusion". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  9. ↑ [Ref] Composite Envisions LLC (2021). "Spiral Wrap For Vacuum Infusion 1_2_ OD x 3_8_ ID". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  10. ↑ [Ref] Fibre Glast Developments Corp. (2021). "Spiral Tubing 1_2 ID for vacuum infusion in stock _ Fibre Glast". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  11. ↑ [Ref] ACP Composites, Inc (2020). "Spiral Tubing for Resin Infusion _ ACP Composites". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  12. ↑ [Ref] LIMITED, CA COMPOSITES (2021). "Infusion Sprial Tube_ CA Composites". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  13. ↑ [Ref] Fibre Glast Developments Corp. (2021). "Vacuum Infusion Equipment and Methods - Part One - Fibre Glast". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  14. ↑ [Ref] Composites, ILL street. "Vacuum bagging supplies & Vacuum Infusion Supplies _ Peel Ply, PTFE and Release Film". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link)
  15. ↑ [Ref] LLC, Composite Envisions (2021). "Peel Ply Explained and When to Use Each Style - Part 2 - Composite Envisions". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  16. ↑ [Ref] Limited, Vacmobiles.com (2020). "Enhancing resin infusion with peel ply resin breaks and segmented perimeter vacuum lines". Retrieved 29 November 2021.CS1 maint: uses authors parameter (link) CS1 maint: date and year (link)
  17. ↑ [Ref] TPI Technology, Inc. (2001). "An Overview of the SCRIMP ™ Technology" (PDF). Cite journal requires |journal= (help)CS1 maint: uses authors parameter (link)
  18. ↑ [Ref] Allende, Melquiades; Mohan, Ram V. (2003). "Characterization and analysis of flow behavior in the FASTRAC process for the manufacture of sandwich and core composite structures". 48 I. ISSN 0891-0138. Cite journal requires |journal= (help)CS1 maint: uses authors parameter (link)



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For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Vacuum assisted resin transfer moulding (VARTM) - also known as vacuum assisted resin infusion (VARI), vacuum infusion process (VIP) or often just resin infusion. VARTM is a liquid composite moulding (LCM) closed mould process with a single side tool and vacuum bag where the resin is drawn through the preform using vacuum.

Vacuum assisted resin infusion (VARI) - also known as vacuum assisted resin transfer moulding (VARTM), vacuum infusion process (VIP) or often just resin infusion. VARI is a liquid composite moulding (LCM) closed mould process with a single side tool and vacuum bag where the resin is drawn through the preform using vacuum.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

Permeability refers to the resistance to fluid flow through a porous material.

  • Resin flow through fibre
  • Gas flow through prepreg

Vacuum Infusion Process (VIP) involves placing a preform on a one sided tool, sealing a vacuum bag on top of it, and drawing resin through the preform with vacuum.

Vacuum Infusion Process (VIP) - also known as vacuum assisted resin infusion (VARI), vacuum assisted resin transfer moulding (VARTM) or often just resin infusion. VIP is a liquid composite moulding (LCM) closed mould process with a single side tool and vacuum bag where the resin is drawn through the preform using vacuum.

Resin flow in between fibre tows.

A tow is a bundle or yarn of individual fibres. The tow size is inherent to the fibre manufacturing process (i.e. a tow is manufactured in one process, rather than each fibre individually then bundled together after).

Typically, smaller tows are better because they result in a more homogeneous material.

The larger the tow:

  • The faster it is to deposit material
  • The easier is it for resin to flow between tows
  • Harder for resin to saturate


Typical tow sizes:

  • 1k (thousand)
  • 3k
  • 6k
  • 12k
  • 24k
  • 50k

Resin flow in between fibres.

Fibre architecture refers to how the fibre is assembled.

Most common architectures:

  • Unidirectional
  • Chopped strand mat (CSM) and chopped fibre (chopper gun)
  • Continuous filament mat (CFM)
  • Woven fabric
  • Non crimp fabric (NCF)
  • Braided
  • Prepreg

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

‘Preform’ is the term for the fibre reinforcement. This is the stage between the raw material form after it is processed into an architecture (fabric, mat, etc.) and becoming a composite.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

Resin transfer moulding (RTM) involves loading a preform into a two (or more) piece, matched tool, closing it, and injecting resin under pressure (~15-100 psi, or ~1-7 bar).

Well suited to small to medium sized parts, limited to large sizes due to injection pressure loads and tool cost.

Flow front refers to the leading edge of the resin as it is flowing into and through the fibre preform.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

  • Moisture/chemical resistance
  • Low density
  • Processability

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

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The Knowledge in Practice Centre (KPC) is centered around a structured method of thinking about composite material manufacturing. From the top down, the heirarchy consists of:

The way that the material, shape, tooling & consumables and equipment (abbreviated as MSTE) interact with each other during a process step is critical to the outcome of the manufacturing step, and ultimately critical to the quality of the finished part. The interactions between MSTE during a process step can be numerous and complex, but the Knowledge in Practice Centre aims to make you aware of these interactions, understand how one parameter affects another, and understand how to analyze the problem using a systems based approach. Using this approach, the factory can then be developed with a complete understanding and control of all interactions.

The relationship between material, shape, tooling & consumables and equipment during a process step


Interrelationship of Function, Shape, Material & Process

Design for manufacturing is critical to ensuring the producibility of a part. Trouble arises when it is considered too late or not at all in the design process. Conversely, process design (controlling the interactions between shape, material, tooling & consumables and equipment to achieve a desired outcome) must always consider the shape and material of the part. Ashby has developed and popularized the approach linking design (function) to the choice of material and shape, which influence the process selected and vice versa, as shown below:

The relationship between function, material, shape and process


Within the Knowledge in Practice Centre the same methodology is applied but the process is more fully defined by also explicitly calling out the equipment and tooling & consumables. Note that in common usage, a process which consists of many steps can be arbitrarily defined by just one step, e.g. "spray-up". Though convenient, this can be misleading.

The relationship between function, material, shape and process consisting of Equipment and Tooling and consumables


Workflows

The KPC's Practice and Case Study volumes consist of three types of workflows:

  • Development - Analyzing the interactions between MSTE in the process steps to make decisions on processing parameters and understanding how the process steps and factory cells fit within the factory.
  • Troubleshooting - Guiding you to possible causes of processing issues affecting either cost, rate or quality and directing you to the most appropriate development workflow to improve the process
  • Optimization - An expansion on the development workflows where a larger number of options are considered to achieve the best mixture of cost, rate & quality for your application.
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