TMC method as raw stock preparation For FRP

THICK MOULDING COMPOUNDS (TMC):

It is a recent method of preparing SMC. The material is made in sheets similar to SMC, with thickness upto 2”. It also have many characteristic of BMC but have improved physical properties and can be continually processed.

Process:

In TMC machine the impregnation takes place in the nip of the two rapidly counter rotating rolls, whose spacing can be varied to provide controlled, thorough fiber saturation. Though TMC is apparently superior, SMC & BMC are the most widely used materials.

SMC Process as Raw stock preparation in FRP

SHEET MOULDING COMPOUND (SMC):

This class of material, made in sheet form upto ¼” thickness, consists basically 1-2” long, chopped strand glass fiber roving as the reinforcing material, a polyester resin and a number of additives. The principal additives here (besides the catalysts) are the thickeners, fillers, thermoplastic polymers and pigments.

The function of the thickener (MgO) is to cause a controlled increase in viscosity of the resin, after thorough fiber wetting has taken place. The thickening of the resin then insures a uniform flow of reinforcement and resin to allparts of the mould when heat and pressures are applied. This minimizes the problem of resin-rich or resin-starved areas found in B-staged material when heated and pressed.

The function of approximately 30% by weight of powdered thermoplastic resin is to decrease the shrinkage of the polyester resin. Such resin formulation are called low shrinkage or low-profile resin. SMC containing thermoplastic resins (acrylics, PVC, styrene copolymers, PE, etc.) can be compression moulded to extremely smooth, high gloss surfaces. Gel coats are not used, but the moulded parts are usually finished by painting with automotive primers and finished coats.

Process:

Here the SMC fibers are saturated with the resin by two doctor blades which meter the polymer on the PE films. The resin must be very accurately controlled to obtain uniform impregnation and because of the method of saturation, the fiber layer cannot be too thick up to ¼” approximately.

PREMIXES OR DMC OR BMC RAW STOCK PREPARATION METHOD FOR FRP

PREMIXES: (dough moulding compound DMC or bulk moulding compound BMC)

They are the mixtures of polyester resin; a mineral filler (usually CaCO₃), clay, silica clay, etc.; reinforcing fibers (glass, nylon etc.); appropriate catalysts; pigments and lubricants.

Process:

The resin and filler are mixed to produce a thick pourable liquid or a soft putty like substance. To this material fibers are then carefully added and mixed. Improper or excessive mixing may cause breaking of glass fibers which may result in very rapid degradation of the physical properties of the moulding. The length of the glass fiber have a considerable effect on the final properties of the moulding compound. Shorter fiber result in low material strength and longer fiber do not disperse well in the mixing operation, are apt to break up and may interfere with flow in moulding operation.

Usually the premixed material is wrapped in PE film to minimize the loss of the styrene. The material can be stored for 3-4 weeks at room temperature depending on the catalyst used.

Often, the premix are extruded or rolled into long lengths or cords to obtain preforms of desired length, simply by chopping them.

Polyester premix or BMC are among the cheapest plastic moulding compounds. Epoxy resin based BMC have relatively higher cost.

Advantage:

The premixes can be moulded by compression or transfer moulding with fairly low pressure, fast cure cycles and high strengths.

WET LAYUP AND PREPEGS PROCESS IN FRP

WET LAY UP TECHNIQUE:

It is the simplest method, where continuous lengths of mats, fabrics or rovings are passed through a resin bath and through squeeze rolls. This process is used where material are required for immediate use or only in small or moderate quantities. The final product is usually quite tacky and somewhat difficult to handle especially when stored as rolls.

PREPEGS OR B-STAGED MATERIALS:

Here the fibers are drawn through a bath, squeeze rolls and then through one or more heating zones. In the heaters the laminate is stripped off solvents and other volatiles and then the resin is advanced to the B-stage so that relatively dry and only slightly tacky material results. The degree of B-staging achieved is a function of the temperature, speed of travel, resin content, and the resin type.

This process is used for laminates to be cured at high temperature or by UV radiation and usually result in material having relatively long shelf lives and can be easily handled and stored. The final product is known as a pre impregnated fabric or roving.

LIST MATERIAL (RAW STOCK) PREPARATION TECHNIQUES IN FRP

MATERIAL PREPARATION TECHNIQUES (RAW STOCK):

Reinforced plastic consists of two essential components viz.:

The fibrous material and the liquid resin.

The fibers must be thoroughly coated with resin so that all the air in the fiber texture is replaced with resin. Various methods are employed to produce this raw stock which then can be made to conform a particular mould contour.

LIST OF TECHNIQUES:

1.      Wet layup technique

2.      Prepegs or B-staged materials

3.      Premixes

4.      Sheet moulding compound (SMC)

5.      Thick moulding compound (TMC)

MATERIAL HANDLING IN FRP

STORAGE AND HANDLING OF MATERIAL used for FRP:

RESIN:

1.      All the resins used in reinforced plastics should be stored in a cool, dry place.

2.      Epoxy resins are not as sensitive to the effects of storage temperature as are polyesters and phenolics and hence do not require storage below room temperature.

3.      Resins which are catalyzed and/or advanced to the B-stage may require a refrigerated storage.

4.      Resins in storage should be kept in tightly sealed containers to prevent the evaporation of volatile monomers and solvents especially for polyesters, phenolics and low viscosity grades of epoxies.

5.      Material, especially polyesters should be stored in containers that exclude light. Common containers are steel drums, tin cans or brown light-proof bottles.

6.      Various monomers and solvents, associated with resins must also be stored under conditions similar to the resin with which they are used and adequate ventilation should be provided to prevent the accumulation of fumes which can otherwise become a fire hazard. Also adequate grounding (earthing) should be provided on all equipment to dissipate any static charges.

7.      When the material which is stored under refrigeration, is used, the container should not be opened until the resin has reached the room temperature to avoid moisture condensation on the resin.

CATALYSTS, HARDENING AGENTS AND ACCELERATORS:

1.      They must be stored in a manner similar to their corresponding resin.

2.      Catalysts for polyesters should be treated with additional precaution. As powerful oxidizing material, they should be stored such that fire can be prevented.

REINFORCEMENTS:

1.      There are no toxic or fire hazards in the storage or handling of glass fibre reinforcements. However organic reinforcement may cause smoke or fume which result in a fire.

2.      In general the reinforcement material must be stored in a dry atmosphere to minimize moisture pickup and to insure high quality mouldings.

CATALYSED RESINS AND B-STAGED FABRICS:

1.      Refrigerated storage is necessary for all catalyzed resins and B-staged fabrics. The lower the temperature, the longer will be the usable life.

2.      The catalyzed resin should be kept in sealed container and B-staged fabrics should be wrapped in several layers of cellophane or Polyethylene (PE) film when in storage.

3.      The material should be allowed to reach the room temperature before being opened or unwrapped to avoid possible moisture condensation on the catalyzed material.

ANCILLARY MATERIALS:

Ancillary materials such as parting agents, sealants, films, fillers, and pigments must be stored under normal room temperature storage conditions.

RELEASE AGENTS for FRP

RELEASE AGENTS: (Parting agents or mold release)

Regardless of the type of reinforcement or the resin used, the parts have a tendency to stick to the mold surface. Hence some type of release agent is always used to facilitate removal of the part from the mold. The release agent functions by forming a barrier film between the mold surface and the part. Various materials used as release agents can be classified as:

1.      External release agents--applied to the mold surfaces.

2.      Internal release agents--combined with the resin.

·         EXTERNAL RELEASE AGENTS:

They can be further classified as--1) films, 2) film formers and 3) wipe-ONs.

FILMS:

It can be obtained in roll or sheet form. The film may be applied as a flat sheet cut to size or as thermoformed sheet made to conform to a 3-D contour e.g. Regenerated cellulose (Cellophane), PVA, Polyester, nylon, and Teflon films are usually used as flat sheets and PE, cellulose acetate, PVC and various fluorocarbon films can be easily thermoformed.

The film will satisfactorily prevent mold sticking and will usually stick to the part after curing so that it can be easily stripped off.

Selection Criteria:

Temperature at which the part is to be cured should be well below the softening temperature of the film material.

FILM FORMERS:

They can be applied either by brush or spry gun on the mold surface. Most common materials are: cellulose acetate lacquers, PVA and nylon solutions, heat cured fluorocarbon coatings and possibly varnishes and/or automobile type paint fillers which can be sanded to a high gloss finish.

These materials are generally used over porous surfaces like wood or plaster of paris. They are also used where the surface are to be filled and smoothed to remove scratches, pits, or the grain pattern of a wooden form. They are frequently used along with a wax, silicone, or fluorocarbon type wipe ON material.

WIPE-ONs:

They are applied with a wiping rag (for certain waxes), a brush (for oily material) or a spray, usually in the form of an aerosol can. In most cases after the initial application the mold is wiped with a rag to smoothen the mold surface. Most common materials are:

WAX: There are number of wax preparations ranging from pure carnauba wax to various soft paste waxes similar to automobile waxes. The pure carnauba wax is generally applied to hot molds.

Advantage:

1) High gloss can be obtained if the material is correctly applied and polished.

2) The mold can be used immediately with no drying or curing period required.

SILICONES:

Various Silicone materials are available in solution form and are wiped or brushed on. Some are air dried and others are heat cured. Silicones are also available in the form of aerosol solutions which are either air drying or heat curing.

FLUOROCARBONS:

The aerosol spray cans of fluorocarbon particles dispersed in a volatile solvent carrier have advantage like-- the excellent release properties and high temperature resistance. This allows them to be used with polymers which are curing at the highest temperature. However, the final gloss may not be as high as that obtained with paste wax.

In general, the use of wrong mold release can result in increased time for part removal or may result in damaged surface or parts due to mold sticking. A mold release should be chosen according to the post molding process being carried out.

·         INTERNAL RELEASE AGENTS:

The chemical compounds used as internal release agents are added to the resin by the manufacturer to minimize the mold sticking after cure. Metallic soaps like zinc or calcium stearate are used for polyester and phenolic resins. Silicone oils and high melting point waxes are used in epoxies. Organo-phosphates and waxes are used in polyester resin. The internal release agents work by being squeezed or exuded on the surface during cure of the resin.