PREMIX MOULDING (BMC MOULDING): (GUNK MOULDING) in FRP

In this process finished part is obtained while in BMC / DMC is the raw stock manufacturing process.

                     1.         It is a form of compression or transfer moulding.

                     2.         It uses dough or putty like moulding like compound called premix.

                     3.         Preparation of premix: The premix can be made by careful mixing of the ingredients in the mixer and in large production, this mixture is directly fed to an extruder which then prepares long lengths of material which is cut to sizes and fed to the press as premix.

                     4.         Compounding: It can be done at high pressure of several tons or at low pressure of several hundred pounds.

                     5.         Trouble shooting in processing premix compound:

Problem: Possibility of non-homogenous compound being prepared and/or a non-isotropic part being prepared.

Solution: Careful attention to mixing procedure and part design details (tending to accurate resin segregation or trap filler during compound flow in the mould)   

                     6.         Advantages: Rapid cycles at lower pressures with low cost materials.

                     7.         Applications:

a.       Mouldings are most competitive in areas where the die and sand castings were formerly used.

They find wide acceptance in electrical and electronics field, business machine housings, home appliances etc. because of good physical and electrical properties; ability to mould delicate inserts to precise tolerances.

MATCHED DIE MOULDING process of FRP

Material:

To facilitate the mass production of reinforced plastic parts, a correct combination of temperature, catalyst concentration, and resin formulation must be used which allows gelation to occur in a time long enough for complete resin flow and short enough for complete resin flow and short enough to be commercially feasible.

Equipment: (the mould)

a.       The metal moulds may be made of Aluminum, Zinc, or Kirksite for short runs and steel for long runs.

b.      The mould should be designed to trap resin and shear the fibers during last 1/16 to 1/8 inch movement. The matching (or shearing) surface should be accurately machined and hardened. There should be a small (0.002 to 0.004 inch) clearance between the shearing surfaces.

c.       The mould is heated usually by hot oil, steam or electrical heaters which are thermostatically controlled. 

d.      Guide pins are used for location and it is well out of the resin area to prevent fouling.

e.       Ejector pins are usually not used in these moulds to prevent fouling. The parts are removed by compressed air or sometimes air passage is included in the mould to eject the part. This passage is closed with tape during moulding operation.

f.       Pinch off are used as a means of cutting fibers. If it were used for location, rapid tool wear will result.

Process:

a.       The moulds are coated with mould release compound.

b.      The male or punch is on the bottom of the mould and the heated preform (or cold form where only the resin exotherm is used for cure) are placed over the punch. This tend to minimize dragging or creasing of the preform when the mould closes.

c.       The preform and resin loading should be consistent to obtain the optimum in reproducible parts. The pressure applied may range from 50 to 300 psi or high up to 3000 psi or higher and the processes are accordingly known as:

                    i.            Low pressure moulding

                  ii.            High pressure moulding.

Ø  Low pressure moulding: It is used when preforms are combined with the resin at the mould just prior to moulding or SMC is used.

The moulding pressure distributes the resin throughout the fibers and trims the preform to the required dimensions.

The press has dual closing rate. (1) Fast: To bring the two parts of the mould together rapidly and (2) Infinitely variable slow rate: To effect the final closing.

Manual control are used but consistent quality can be obtained with presses having automatically controlled closing and automatically monitored temperature, pressure and time.

Ø  High pressure moulding: It uses pre impregnated SMC material or fabric either chopped to small pieces or cut to specific configuration. Moulding at pressure up to 3000 psi is very similar to the normal compression moulding.

Advantages of matched die moulding:

a.       Reproducible parts with relatively high speed.

b.      Good finish on all surfaces.

c.       Very little trimming is required.

d.      Process is independent of the operator’s skill.

e.       Low cost reinforcement can be used.

Ø  Rubber plunger moulding (variation of matched die moulding):

a.       Preform is placed in the heated female cavity.

b.      The rubber male plunger (made of RTV Silicone) is then forced in to the cavity touching first at bottom, then deforming to put pressure all over the part.

Advantages:

a.       Low tool cost because only one machined tool is required.

b.      Slight undercuts can be produced, depending on the part.

Disadvantages:

a.       Has only one good surface i.e. the side contacting the punch and it is similar to a pressure bag surface.

Limit exists for the size of parts that can be produced, because large rubber punches are impractical.

SPRAY UP TECHNIQUE IN FRP

SPRAY UP TECHNIQUE:

Equipment: spray-up machine basically consists of:

1.      Roving cutter: May be mounted remotely (far away, not closely) or directly on the spray head.

2.      Two component resin catalyst delivery system: It has taken number of forms:

a.       Pressure pot system

b.      Air less system

c.       Double nozzle guns which mix catalyst and resin in the air

d.      Single mixing head guns.

3.      Glass delivery system: It also varies with the remote system:

a.       The chopped roving (A form of fibrous glass in which spun strands are woven into a tubular rope. The number of strands is variable but 60 is usual) is fed to the gun by hose instead of chopping at the point of delivery.

b.      Some guns do not chop at all but deliver the roving as a continuous strand which is laid in a swirl pattern.

Process:

a.       The unit (regardless of configuration) combines a catalyzed resin with an accelerated resin.

b.      The glass fibers are wetted with this mixture which is sprayed on the mould surface.

c.       Some spray guns are used to spray gel coats and some used to spray the resin, catalyst and a powered filler simultaneously.

Advantages:

Any large reinforced plastic structures can be build up simply and very rapidly.

Disadvantages:

The sprayed composite must be rolled by hand to remove the air and obtain sufficient density. Hence labour costs are equivalent to the hand lay-up techniques used to produce structures with good physical properties. The process will not yield consistently high properties.

Applications:

The process was initially used to make studio sets. Now it is greatly accepted in the areas where physical properties are not of primary importance like large outdoor display signs, tank linings, shower and tub enclosures, roofing etc. Gel coating and spray-up are very compatible and their combination produces moulding with very fine single surface finishes.

OTHER HAND LAYUP TECHNIQUES IN FRP

1.      VACUUM BAG MOULDING:

a.       It is the simplest method of pressurizing the lay-up and always result in a higher quality hand lay-up laminate than that made by contact moulding.

b.      A form-fitting bag is used on the outer surface of the part. The bag material must be heat resistant and should not affect or be affected by the resin.

c.       Film material suitable for bag manufacturing include Cellophane, PP, Nylon, and Fluorocarbons. Neoprene, Silicone, Butyl rubbers are also used.

d.      The bag is tailored to the part to minimize the wrinkles.

e.       Vacuum is drawn to pressurize the lay-up up to atmospheric pressure. Hence higher reinforcement loading is obtained in the finished part with higher density and increased physical properties.

f.       A better surface is obtained on the side away from the mould.

g.      Heat curing is more readily done because the resin content is lower and the pressure inhibits the flow of resin.

h.      A bleeder (breather) sheet is incorporated in the lay-up so as to rub out the part and eliminate air bubbles and excess resin.

i.        A vacuum pump; associated resin traps to prevent pump damage; special bags; breather sheet and vacuum lines are the equipment required, in the process, in addition to that required for contact moulding.

2.      PRESSURE BAG MOULDING:

a.       Uses a tailored rubber bag as the contact and vacuum moulding.

b.      Tooling is considerably stronger and is made to resist pressures from 5-50 psi or higher.

c.       Parts are considerably more dense and contains higher reinforcement content.

d.      The process is mainly applicable to female moulds, because bags used on male moulds tend to pucker and developed folds.

e.       The technique is combinely used with vacuum so as to remove most of the entrapped air.

3.      AUTOCLAVE MOULDING:

It is very similar to the pressure bag moulding.

The major difference and advantage of the process are:

a.       Cure is carried out in the autoclave hence the tooling required is not very pressure resistant. Although pressures as high as 100 psi can be used.

b.      A simple Kirksite or Aluminum tool is ideal because it is only in compression.

c.       Parts are dense and exhibits excellent physical properties.

The major disadvantages of the process are: 

a.       Cost of installing an autoclave.

b.      Limitations imposed by the size of the autoclave on part sizes.

Improvement brought about by the additional pressure do not undo the poor resin distribution or bad placement of the fibrous reinforcement etc.

LIST OF HAND LAYUP TECHNIQUE AND SHORT NOTE ON CONTACT MOLDING IN FRP

v  HAND LAY UP TECHNIQUES: They are classified as:

1.      Contact moulding

2.      Vacuum bag moulding

3.      Pressure bag moulding

4.      Autoclave moulding

1.      CONTACT MOULDING: (open mould fabrication)

It is one of the oldest forming techniques and simplest of the reinforced plastic processes.

Material selection:

It depends on the requirements of the part and the contours to be met with. However general characteristic of the material are:

Resin:

a.       The resin must be thin enough so that it is easily impregnated into the reinforcement.

b.      It should also be thixotropic enough to allow drainage on vertical sections before gelation takes place.

c.       It should be suitable for the contours of the part.

d.      The catalyst system should allow sufficient time for fabrication of the part.

Catalyst:

a.       Heat activated catalyst: Here the part can be worked on all day and oven cured at night.

b.      Do it yourself: Using a room temperature cure catalyst: Small parts can be made at atime.

c.       Sun shine cure; UV catalyst: Here the part can be worked on all day in the shade and then placed in the Sun for cure.

Equipment:

It includes following:

a.       Resin mixers: It includes special stirrer to introduce a minimum of air.

b.      Scale and graduated cylinder for adding catalyst into the resin.

c.       Containers for mixing resin preferably expendable card board.

d.      Squeegees and rollers: Implement with rubber blade or roller for scrapping, squeezing away the moisture or air. This is used to rub out the resin.

e.       Brushes to apply resin.

f.       Table and scissors for cutting fabric and mat.

g.      Viscometer to check the resin viscosity.

h.      Hot bath and gel timer.

i.        Barcol

The last three equipment are highly desirable but are not essential (compulsory)

Process:

a.       Apply a mould release (parting agent) to the mould.

b.      Gel coat application: Apply a coat of resin heavily filled with mineral fillers. This pigmented gel coat insures an optimum surface.

c.       Fiber glass mat application: After the gel coat becomes tacky, the reinforcement layer is laid on the gel coat. This layer may be (i) A sheet of surfacing mat when very fine surface is required. Or (ii) A layer of dry or impregnated fabric or mat on to a resin layer. It is preferred because it results in least air entrapment.

d.      Resin application on mat: Catalyzed resin is either brushed or roller coated. Impregnation of the resin is assisted by the use of a roller or squeegee, which promotes wetting and pushes out air bubbles. Complete wetting is allowed and then more resin is added for the next layer.

e.       Part removal: The part is then allowed to remain un disturbed until gelation takes place. After gelation, the assembly may be heated to accelerate the curing process. The exposed surface may be covered with cellophane or PVA film to prevent the evaporation of any volatile monomer and to minimize the possibility of blocking of air for some polyester resins. The finished part has a smooth inner surface (i.e. surface contacting the mould) due to the gel coat application.

Limitation:

a.       Only the surface contacting the mould has a good, smooth surface. Thus the choice of whether a male or a female mould can be employed depend on the surface which must be smooth (i.e. inner or outer surface) and not on the fabrication condition or part contour.

b.      The process is very slow.

c.       The quality of the part depend on the skill of the operator.

d.      Physical properties of the parts is lower than those made by almost all other processes.

Application:

a.       Well suited for fabrication of very large articles and/or small production runs.

Used for production of prototypes regardless of the eventual high production process.

LIST OF PROCESSING TECHNIQUES IN FRP

PROCESSING TECHNIQUES USED FOR REINFORCED PLASTICS:

1.      HAND LAY UP TECHNIQUES

2.      SPRAY UP TECHNIQUE

3.      MATCHED DIE MOULDING

4.      PREMIX MOULDING (BMC MOULDING)

5.      INJECTION MOULDING

6.      CONTINUOUS LAMINATION

7.      PULTRUSION

8.      RESIN TRANSFER MOULDING (RTM)

9.      FILAMENT WINDING

10.  SANDWITCH STRUCTURE PROCESSING

11.  FOAM (ELASTIC) RESERVOIR MOULDING (FRM OR ERM)

MATERIAL PREPARATION TECHNIQUES USED IN FRP

1.         PREFORMING:

Definition: (in RP industry)

A technique used to arrange chopped glass fiber in the exact shape of a part to be moulded by the matched die moulding methods.

Process:

The chopped fiber (1.5” long usually) are deposited uniformly over the screen surface having the shape of the final moulded part. Then a small amount of binder resin is applied. Deposition proceeds until sufficient preform thickness is obtained to provide the optimum glass: resin ratio in the moulded part. Then the preform is heated, on the screen, until the binder resin cures, so that the preform is sufficiently sound and can be stripped from the screen and transported to the press for final moulding.

SMC preforms:

The preform is also applied to the use of fully impregnated SMC material which is precut or cut and stapled to fit a mould prior press curing.

Non SMC prefroms:

There are two methods for producing the non SMC preforms:

a.       Directed fiber preforming (or open preforming)

b.      Plenum chamber preforming

1.      Directed fiber preforming (or open preforming):

 Here the cut fibers are sprayed manually on a rotating screen through which air is being drawn to create a pressure drop at the screen interface. This suction holds the fiber in place on the screen while the operator directs the airborne screen of fiber to all areas of the screen. The operator also intermittently sprays the proper amount of an aqueous emulsion having 5% concentration of polyester or acrylic liquid binder. When the fiber build up is complete, the preform is oven cured and removed from the screen. The percentage of binder varies between 2-10% of the finished weight.

Machine:

Several type of machines are available depending on the size and desired production rate. On average size preforms, a common type is a four station rotary mount for the screens where:

1^st screen is being sprayed

2^nd screen is in the oven curing position

3^rd screen is being stripped  

4^th screen in in the ready position

Extremely large preform machine have only one screen.

After preforming, a hood lowers over the screen to form an oven, and the air is redirected through a heat exchanger to provide curing temperature.

Advantages:

Preforms with varying wall thickness, buildup areas can be produced because a wide range of machine sizes are available.

Disadvantages:

The uniformity of product depends almost entirely on the operator’s skill.

2.      Plenum chamber preforming:

It is similar to the directed fiber method and only differs in the method of applying the fiber to the screen. It is an adaptation of the old felt-hat forming machine.

Here the rotating screen is positioned in a plenum chamber and air is drawn through the screen. The roving cutter is mounted at the top of the plenum and the cut fiber falls on a rotating spinner which distributes the fiber uniformly in the plenum chamber. The fibers then “snow” down into the screen. The liquid resin binder is usually applied by spray gun, but in some machines, powder binder is sifted on from a vibrator feeder.

Advantage:

The product is uniform after the initial set up. The process can be completely automated, thus reducing the labour cost.

Disadvantage:

Restriction to uniform wall thickness, difficulty of initial set up on some types of parts and the compounding of the fiber distribution problem as size increases.

2.         FABRIC AND MATERIAL TAILORING:

When only small number of parts are required, tailored sections can be cut from impregnated raw stock by the use of hand shears or knives. For production of large numbers of parts inexpensive steel rule dies can be used or the material may be piled up in number of layers and cut, from a pattern, using a special textile cutting machine. If very simple shapes are required e.g. discs, then commercially available dies may be procured at normal costs from machine shop.