diploma Compression moulding Notes.

1.What is Compression moulding?

Ans:- Compression moulding is the process for moulding thermo-set polymers like phenolics, melamines, ureas, diallyl phthalates, unsaturated polyesters, silicones, epoxies, alkyds, etc.

Thermo-set plastic materials are one type of polymeric resin that are capable of forming chemical cross linking of the molecules which is called curing of the materials. In compression moulding method, the plastic, in the form of powder, pellet, or disc, is dried by preheating up to a temperature of 70˚C–80˚C to remove the moisture from the material and the preheated thermo-set material is loaded directly into the mould cavity.

•  Further, the temperature of the mould cavity is held at 150˚C–200˚C, depending on the material.

•  The mould is then partially closed, and the plastic, which is liquefied by the heat and the exerted pressure, flows into the recess of the mould.

•  At this stage, the mould is fully closed, and the flow and cure of the plastic are complete.

•  Finally, the mould is opened, and the completely cured moulded part is ejected

•  In compression moulding, a pressure of 158 kgf/cm2 to 211 kgf/cm2 is suitable for phenolic materials.

•  The lower pressure is adequate only for easy-flow materials and a simple uncomplicated shallow moulded shape.

•  For a medium-flow material and where there are average-sized recesses, cores, shapes, and pins in the moulding cavity, a pressure of 3,000 psi (211 kgf/cm2)     or above is required.

•  For moulding urea and melamine materials, pressures of approximately 1 to 1.5 times of phenolic material is necessary.

2.Compression moulding material?

Ans:-

1. Thermoset plastics material are in powder, granulated or nodular form, having bulk factors ranging from 1.2 to 10. Some are used in the form of rope, putty or slabs.

2. Phenol formaldehyde (PF) is a common material used and the moulded part will have good electrical properties. With the filler of glass fibers the moulded part will have good impact strength. A very wide range of properties can be obtained from PF because of their compatibility with a variety of reinforcements and fillers, so the properties are dependent on the fillers used.

3.Application of thermoset material?

Ans:-

1. Melamine formaldehyde - often used in plastic dinnerware.

2. Urea formaldehyde - common in heat resistant handles for kitchen ware, or outlet sockets for household use.

3. Alkyds and polyesters - often used in high voltage insulators in TV sets, or for arc resistance and insulation in circuit breakers and switch gear.

4. Dialkyl epoxy - housings for electronic components.

5. Silicone - high temperature requirements to 600^oF.

 

4.Thermoset moulding?

Ans:-

1. The common fillers used in thermo-set moulding are silica, glass, wood flour, natural or synthetic fibers and combination of these.

2. Thermo-set mouldings are glossy, opaque, dark coloured and inherently flame retardant.

3. They are stiff, hard having low elongation properties and possess good creep resistance.

5.Types of compression moulds?

Ans:-

6. What is Hand compression moulds?

Ans:-

• Hand compression moulds are used for smaller production runs or prototypes, experimental jobs that require lower mould costs and parts having open tolerances and less intricacy.

• Hand compression moulds weigh less than 15 g. for easy manual handling and also the operations are fully manual.

• hand moulds are slow in operation, requires longer cycle time and labour intensive which increases the cost as compared to other type of moulds.

• Moreover, the moulds are more easily damaged by misalignment, mishandling, etc. It is preferred for simple components only. 

7.Semi Automatic compression moulds?

Ans:- The semi-automatic moulds are used for mass production of jobs and complex components. They are classified into the following types:

1. Semi-automatic open flash mould

2. Semi-automatic fully positive and landed      positive mould

3. Semi-automatic semi-positive mould

8. Semi-automatic open flash moulds?

Ans:-

• The flash-type compression mould is used to produce shallow shaped components and in this mould a slight excess of moulding powder is loaded into the mould cavity.

•  On closing the top and bottom platens, the excess material is forced out and flash is formed.

•  Only resins having high melt viscosities can be moulded by this process.

• Moulds are cheap, and very slight labour costs are necessary in weighing out the powder.

•  The moulds are suitable for building up into tools containing multi-impressions.

9.Semi-automatic fully positive & landed positive moulds?

Ans:-

•  The positive type of compression mould in which the plunger telescopes within the cavity, compressing the thermo-set material and the moulding i

formed. 

•  There is very little clearance between the plunger and the cavity wall.

•  In the positive mould, almost all the pressure is exerted on the material and a very little material is allowed to escape as flash.

•  The clearance between the plunger and cavity varies between 0.035 mm to 0.135 mm per side.

•  The flash is formed vertically.

•  The disadvantage of the positive type of mould is that after frequent operation the cavity walls become scored and ejection of piece parts is difficult.

•  Flash is formed on every piece part moulded by the compression method.

•  The thickness and position of this flash depends on the design of the mould, type of material being moulded, and accuracy of the mould.

•  The positive mould is used primarily with material containing coarse fillers.

•  The amount of material placed into the mould cavity must be measured accurately as there is very limited means for the excess material to escape.

 

Semi-automatic fully positive moulds

                          

                                Semi-automatic landed positive moulds

10.Semi-automatic semi positive moulds?

Ans:-

•  Semi-positive mould is a combination of the features of open flash and fully positive moulds.

•  It allows for excess powder and flash.

•  It is also possible to get both horizontal and vertical flash.

•  Semi-positive moulds are more expensive to manufacture and maintain than the other type compression moulds.

•  Semi-positive moulds are suitable for producing components to close tolerance and better surface finish.

•  A clearance of 0.025 mm per side, for a diameter of 25 mm between the plunger (top force) and the cavity is maintained for satisfactory operation of mould.

•  The mould is given a 2° taper on each side of the cavity for allowing the flash to flow on and the entrapped gases to escape along with it, thereby producing a clean, blemish-free mould component.

 

11.Semi-automatic semi positive moulds – Vertical Flash type?

Ans:-

·   Semi automatic semi positive vertical flash type moulds are suitable for moulding components of high-density and critical dimensions as related to cavity and top force.

·   Easy removal of flash on large parts and leaves no flash line scars on the side of the parts.

 

12.Semi-automatic semi positive moulds – Horizontal Flash type?

Ans:-

• These types of moulds are recommended for processing of components having close dimensional tolerance and higher accuracy.

• The component produced out of the mould assures minimum flash finish

 

13.what is Bulk Factor?

Ans:-

• It is the ratio of the volume of the loose plastic powder to the volume of the moulding.

• The bulk factor varies for different thermo-set plastic materials

• The inclusion of different fillers, affect the bulk factor of the material.

• The bulk factor for general purpose material and most mineral fillers varies between 2 and 3.

• In the case of light fabric filler, the bulk factor will increase between 8 and 15.

• For heavy fabric or string filler, the bulk factor to between 12 and 24.

 

14.Loading chamber / powder well design?

Ans:-

• In general, the dimension of the powder well in a compression mould should be kept to a minimum.

• The ‘land’ dimension, i.e., the distance between the actual cavity dimension and the vertical wall of the powder well should be between 4 mm to 9 mm on normal sized mouldings.

• On large compression mouldings, such as large table model radio cabinets, containers, etc., a land dimension of 9 mm to 16 mm can be used.

 

Two advantages are obtained by keeping the powder well area as small as possible:

1. Moulding pressure is increased.

2. Less obstruction is put in the way of the escaping excess moulding material (flash)

• Plastic powder fed to the mould need more space in a loose state than in the pressed moulding.

• The mould cavity design must allow for sufficient space for the loose powder.

• Calculation of loading chamber depth:

                D = (V_T - V_C) / A

Where;

D - Depth of loading space from top of cavity to pinch-off land

V_C -Volume of actual cavity space (cm3)

V_T -Total volume of loose powder (cm3)

A -Projected area of the loading chamber (cm2)

• The loading chamber has to be designed with a flare of 1/3–1°.

• Generally the start of the flare should lie only 4–6 mm above the compacted moulding.

• The upper edges should be rounded off with a radius R=2–5 mm

15.Flash Thickness ?

Ans:-

• The excess material flow out from the cavity and form a thin layer of plastic film called flash over land portion of the mould before solidifying the material.

• Allowances for flash thickness in compression moulds, using thermosetting compounds are:

Rag-filled high impact compound - 0.25 mm

Cotton – flock compounds in large moulds - 0.2 mm

Wood –flour compounds in small moulds - 0.1 mm

Other moulds and compounds allow - 0.13 mm

 

16.Clamp force & Number of impression ?

Ans:-

• Clamping force (kgf) = Projected area of the moulding (cm2) x Compression pressure of the plastic material.

• Actual projected area = 1.2 × Projected Area of the component.

• No. of impression = Clamping force available on the machine / Clamping force required for an impression.

 

17.Important factors considered in thermo-set moulding ?

Ans:-

     1.Temperature

        2. Pressure

        3. Cure time

 

18.What is Cure time ?

Ans:- The period required to harden thermosetting material to partial or complete polymerisation is called the curetime.