VACUUM BAGGING TECHNIQUES

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What is vacuum bagging?

Vacuum bagging (or vacuum bag laminating) is a clamping method that uses atmospheric pressure to hold the adhesive or resin-coated components of a lamination in place until the adhesive cures. (When discussing composites, “resin” generally refers to the resin system— mixed or cured resin and hardener—rather than unmixed epoxy resin.)

Modern room-temperature-cure adhesives have helped to make vacuum bag laminating techniques available to the average builder by eliminating the need for much of the sophisticated and expensive equipment required for laminating in the past. The effectiveness of vacuum bagging permits the laminating of a wide range of materials from traditional wood veneers to synthetic fibers and core materials.

Fig: Sketch of a vacuum bag, the individual elements and a leakage.

SILICONE FOR VACUUM BAG

Silicones for Producing Reusable Vacuum Bags

  • Our silicone rubber is a brushable platinum cure silicone developed especially for making high performance, reusable vacuum bag. It is easy to mix and apply. Rubber can be applied with a brush or spatula to vertical surfaces without sagging.
  • CHEMZEST offers a series of addition cure silicone elastomers for the manufacture of reusable vacuum bags for resin infusion. These silicones formulated as a sprayable material allowing for exceptionally low cost composite manufacture using resin infusion technology.
  • Our silicone solutions enable composite manufacturers to decrease production times, achieve consistent part production and reduce material waste. In addition, the closed mold process enables a much safer working environment since VOC emissions are reduced.

General Attribute

  • 1:1 mix ratio
  • Easy to apply using fully automated mixing and spray equipment
  • Fast curing heat accelerated RTV
  • High tear and tensile strength for repeated uses of the bag
  • Low viscosity

Applications

  • Automotive parts such as bumpers and windscreens
  • Aerospace components
  • Marine & RV components
  • Laminating
  • Composite part building
  • Molded composites

How it works?

  • Vacuum bagging uses atmospheric pressure as a clamp to hold laminate plies together. The laminate is sealed within an airtight envelope.
  • The envelope may be an airtight mold on one side and an airtight bag on the other. When the bag is sealed to the mold, pressure on the outside and inside of this envelope is equal to atmospheric pressure: approximately 29 inches of mercury (Hg), or 14.7 psi.
  • As a vacuum pump evacuates air from the inside of the envelope, air pressure inside of the envelope is reduced while air pressure outside of the envelope remains at 14.7 psi. Atmospheric pressure forces the sides of the envelope and everything within the envelope together, putting equal and even pressure over the surface of the envelope.
  • The pressure differential between the inside and outside of the envelope determines the amount of clamping force on the laminate. Theoretically, the maximum possible pressure that can be exerted on the laminate, if it were possible to achieve a perfect vacuum and remove all of the air from the envelope, is one atmosphere, or 14.7 psi.
Fig: A typical vacuum bagging lay-up before and after vacuum is applied

Vacuum bagging equipment

The vacuum bagging system consists of the airtight clamping envelope and a method for removing air from the envelope until the epoxy adhesive cures. This section discusses the components of this system , which include both specialized equipment and commonly available materials.

Vacuum Bagging Materials

  • Release Fabric
  • Perforated Film
  • Breather material
  • Vacuum bag
  • Mastic sealant
  • The plumbing system
  • Mold release

Our product benefits

  • High mechanical characteristics
  • High dimensional stability
  • Self bonding properties ( a second layer can be cast within 24hrs after casting the first layer)
  • Good chemical resistance against polyester,vinylester
  • Perfect surfaces in both side of the mold
  • Translucent product in order to recognize the filling of the resin

MAJOR APPLICATIONS

CURING TWO PART SILICONE

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  Curing Two Part Silicones for Potting, Encapsulating and Mold Making

  • Two-part silicones are used for potting electronic assemblies, encapsulating electronic components, and making molds for cast urethane rubber.
  • Each part contains functional ingredients, and the two parts are mixed together in a specific ratio.
  • The entire system requires curing so that the silicone hardens and acquires its end-use properties.
  • Curing, the crosslinking of polymer chains, is a chemical reaction that is accelerated using a catalyst. Sometimes, post-curing is also used.

Addition Cure Silicones

Silicone elastomers are designed with either addition (platinum) cure or condensation (tin) cure chemistry. Addition curing systems are 2-part liquid silicones that utilise a platinum catalyst to cure by either room temperature vulcanisation (RTV) or heat to accelerate cure.

Addition cure silicones do not produce by-products during the crosslinking (curing) process. The cure chemistry takes place internally, making them ideal for moulds or parts that are manufactured in a sealed environment. Addition cure silicones have very low shrinkage properties with great dimensional stability when exposed to either humidity or heat. This type of chemistry should be considered if the final mould will be stored for long periods of time between use.

Properties include:

  • Extensive range of durometers/hardnesses
  • Compatible with silicone fluids to increase softness
  • Can be heat cured
  • Casting resin resistance
  • Low shrinkage, below 0.1%
  • High tear and tensile strength
  • Good abrasion resistance
  • Excellent dimensional stability resulting in durable moulds
  • Fine detail reproduction
  • Food grades available

Addition cure silicones can be used with various resins and materials including concrete, stone, brick, fibre glass filled resin, polyurethane, polyester, acrylic resin, and even another addition cure silicone.

Applications: 

General mouldmaking, Prototyping, Dental replication, Food moulding, Architectural replication & sculpting, Composites/fibre glass part production, Tampo pad printing, Special effects, Prosthetics & orthotics

Avoiding inhibition with addition cure silicones

When working with addition cure silicones, it is important to avoid inhibiting the cure process by bringing the uncured material into contact with certain chemical compounds. The poisoning of the cure catalyst stops the crosslinking necessary for the silicone to properly cure. The result of inhibition is tackiness at the pattern interface or a total failure of the silicone to cure.

Materials known to inhibit cure are sulphur-containing modeling clays, natural rubber such as latex and rubber gloves, masking tape, amine or sulphur-containing material, condensation cure (tin-catalysed) silicones, PVC stabilizers, and epoxy resin catalysts.

TIN CATALYST CURING

  • For liquid silicone rubber cured with a tin catalyst, it could be assumed that the tin catalyst does not respond to heat.
  • Only the addition of more catalyst or the crosslinker will increase the speed of the curing.
  • The higher the quantity of reactants used, the higher the amount of heat evolved during the curing process and the faster the curing process will be.
  • The amount limit of the catalyst or the crosslinker will depend on the reaction temperature; if it is too high, it could cause degradation in the silicone rubber, so the final product would have lower mechanical properties than desired.
  • The resultant temperature profile also depends on the thickness of the final product: the thicker the part, the lower the initial temperature of the reaction will be and the slower the temperature will be increased due to the slower thermal conduction but after some time the overall temperature will be higher in comparison with standard thickness products.

PLATINUM CATALYST CURING

  • For liquid silicone rubber cured with a platinum catalyst, the increase of the processing temperature has an effect similar to increasing the amount to be cured.
  • To optimize the speed in this case, in general, it could be said that for every ~10°C, the cure rate could be increased in approximately 20-25%.
  • If the built-up temperature is not well controlled, the probability of degradation will be higher. It is recommended that if the processing temperature is increased, the amount of curing agent should not be increased at the same.
  • First, it is necessary to analyze the effect of specific temperature on the cure speed and then, an adjustment of the curing agent.