Compression mould: Compression moulds are mainly used for the manufacturing of the thermosetting plastic product such as Cookies handles, electrical switches etc. Compression moulds are made of HCS (High Carbon Steel) steel or hot die sheet. The core and cavity should be heat treated to required hardness for maintain the dimension accuracy of product and also the life of mould. This mould is also used for thermosetting component when are not possible processing method due to processing limitations. It is made of HCHCH steel or hot die steel. The core and cavity should be heat treated to the required hardness for maintaining the dimensional accuracy of product and the job life of mould. PE(LDPE,HDPE,HMHDPE), PP,PVC,PA,PS,PC,PET,EVA,SAN,TPE.
1.) Compression mould is filled by directly pressing core and cavity.
2.) Compression mould material wastage is high as comparing transfer method.
3.) Marching cost is low.
4.) Punch is used for material feeding.
OR
Compression moulding is a process of moulding a material in a confined shape by applying pressure and heat. The mould is closed with a top force or plug. Pressure is applied to force the materials come in contact with all mould areas. Heat and pressure are maintained until the moulding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression moulding is a high-volume, high-pressure method suitable for moulding complex, high-strength fiber glass reinforcements. Advanced composite thermoplastics can also be compression moulded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand.
Operating Process
1. Turn machine on and set to appropriate pressure and temperature.
2. Have mould inserted into the machine.
3. Heat up mould to appropriate temperature required.
4. Check temperature with digital/analog thermometer
5. When desired temperature is reached, extract mould from the machine.
6. Open mould into two halves.
7. Put appropriate amount of resin into one of the halves.
8. Close the mould halves.
9. Place back into the machine.
10. Press the button that will cause the platen to clamp together
11. Pressure is now being applied to the mould.
12. Pressure is now being applied to the mould.
13. The heat and pressure begins to liquefy the resin and fills the mould.
14. When the appropriate amount of time has passed, the platens release pressure from the mould.
15. Wearing gloves, remove the mould from machine.
16. Separate the mould halves.
17. Push the ejector pin to knock finished product out of mould.
18. Repeat these steps to make additional moulds.
19. Temperatures and time setting may vary with different resins or moulds.
20. When done, shut the machine down appropriately and clean work area.
Compression Moulding: is a technique principally for thermoset plastic moulding in which the moulding compound (generally preheated) is placed in the heated open mould cavity, mould is closed under pressure, causing the material to flow and completely fill the cavity, pressure being held until the thermoset material has cured. In this process a measured quantity of a plastic material is shaped or formed by heat and pressure. A known weight or volume of the moulding powder is kept in an open mould cavity. The mould consists of two halves, a male and a female part, and is normally preheated. Mould halves are attached to the upper or lower platens of a press. These mould halves are called the core or plunger and the cavity. Heat is applied either directly to the mould or indirectly through the platens. The moulding compound, generally in powder form, is placed in the mould cavity after which the mould is closed. Heat and pressure is applied causing the plastic material to soften, flow and fill the cavity completely. Depending on the characteristics of the plastic material and the design of the mould, the temperature ranges from 140°C to 250°C & pressure range from 2000 to 10,000 psi. The mould remains heated and closed until the plastic material cures and sets. A molding pressure of 2000 psi on the total projected area of the part is generally recommended. To this must be added approximately 800 psi for every inch of the vertical height of the molded part, to arrive at total moulding pressure required.
After the material has cured sufficiently, the mould is opened and the piece is ejected by knock out pins. The cure time depends on the size and thickness of the part. This may vary from 20 seconds to 10 minutes. The cure time cannot be predicted accurately, but must be determined by experience or by trial.
Compression molding or the matched die molding is the process used exclusively for thermoset molding. In this process a charge of thermoset resin is put into the cavity of a matched heated mold that is in open condition.
The mold is closed by bringing the male and female half together and pressure is applied to squeeze the resin so that it uniformly fills the mold cavity. While under pressure the material is heated which causes it to cross link and to harden. When the material is hard the mold is opened and the part is removed in hot condition. Thermoset compound, usually preformed, is positioned in a heated mould cavity; the mould is closed (heat and pressure are applied) and the material flows and fills the mould cavity. Heat completes polymerization and the part is ejected. The process is sometimes used for thermoplastics, e.g. Vinyl phonograph records. Little material waste is attainable; large, bulky parts can be moulded; process is adaptable to rapid automation. Extremely intricate parts containing undercuts, side draws, small holes, delicate inserts, etc.; very close tolerances are difficult to produce, Time consuming process.
Name of compression mould part: Mould plunge, Guide pin, Mould cavity, Mould core, Punch, Guide bush, Guide pillar, Top plate, Bottom plate, Socked screw, Ejector pin, Ejector plate, Ejector back plate, Ejector guide pin, Allen bolt.
Plastic materials: Those Materials which show high melt strength & good stretch properties at the extrusion temperature are suitable for formation of parison & blowing.
Thermosetting resins: Phenol formaldehyde, Urea formaldehyde, Melamine formaldehyde, Polyester, Epoxy.
Special Thermoplastics: Ultra high molecular weight HDPE.
Following Polymers are more often blow mounded:
PE(LDPE,HDPE,HMHDPE), PP,PVC,PA,PS,PC,PET,EVA,SAN,TPE
LDPE us used for more flexible items.
HDPE is used for Rigid Bottles, Chemical Drums, and gasoline tanks.
PP is used because of high stiffness, good chemical resistance, clarity and good glass and good resistance to high temperature.
PVC & PS for general purpose articles requiring transparency at modest cost.
RPVC – Can be bi axially oriented in extrusion stretch blow moulding.
PC is used for containers which show good transparency, excellent impact strength, good heat resistance, good printability.
PET is used in basically oriented stretch blow moulding & commonly used for carbonated beverages, packaging of drinks pharmacy products & water bottles.
EVAL (Ethylene Vinyl Alcohol) is used as barrier layer in Multilayer containers.
Parameters:
Preheating:
1. Preheating is useful to reduce moulding time and allows rapid heating of large pellets or masses of powder.
2. It helps to remove moisture and other volatiles prior to moulding .
The quantity of resin put into the mould:
1. If sufficient resin is not placed in the mould cavity, the part will not be formed completely.
2. If too much material is applied into the mould, it may not melt completely or excess flash may be produced.
Pressure of the moulding process:
1. The fluid plastic is held under pressure, often ranging upwards from 2000 psi, for a sufficient length of time for the material to undergo polymerization or cross linking, which renders it hard and rigid."
2. Different materials require different forces to mould them.
3. High-viscosity materials require more pressure than materials with low-viscosities.
Down stroke or Upstroke of the machine:
1. A down stroke machine consists of a compression press in which the platen moves downward to close the mould.
2. An upstroke machine consists of a compression press where the platen moves upward to close the mould.
Mould Temperature:
1. The mould is subjected to high levels of heat, approximately around 300ºF to 375ºF. Appropriate heating is crucial to allow for the resin to soften it to flow into the mould cavity.
2. If the temperature is too low the plastic material my not melt completely and produce an unfinished part.
3. If the temperature too high, it can cause the part to get burned or warped.
Breathing Time
1. When the mould is closed, several changes takes place
The temperature of the material has been raised almost that of the mould
Chemical reaction starts
Volatile gases have developed
2. If volatile cannot escape through mould vents or between clearances of cavity moulding pressure drops to zero.
3. After breathing, the mould is again pressurized for the duration of the cure.
4. With slower reacting materials or thick sections, degassing can be accomplished more than once, but only in the early phase of the cycle.
5. Usually visual inspection of surface or interior sections indicates the correctness of the degassing sequence.
6. Condensation resins are more troublesome in formation of blisters or porosity.
7. Any compound is likely to contain moisture and all materials and mould cavities contain air, which may make degassing necessary.
8. Since breathing a mould included in the cure cycle, production time is not increased.
Moulding Cycle Time
1. Moulding cycle time can be represented as a function of the force required to close the mould at a constant rate.
2. In the “plastication” stage (t<tf) the force increases rapidly as the polymer feed is compressed and heated.
3. The second stage flow commences, once the yield stress of the elastomer is exceeded. tc is the point at which the mould fills and compression of the melt occurs.
4. Ideally to aid mould filling the majority of chemical reaction should take place after tc.
Types of moulds: Compression moulds are usually constructed of case-hardened tool steel, in order to withstand the high pressures of compression moulding. There are three basic types of Compression moulds.
Flash type mould: The flash type of mould is used to produce a shallow shape. With this type of mould. A slight excess of the moulding powder is loaded into the mould cavity and, on closing the top and bottom force. The excess powder is forced out and 'flash' is formed. This can be removed quite readily, though it tends to make the process somewhat wasteful. However the moulds are relatively cheap and are suitable for building up into tools containing multi-impressions. Another advantage is that, while to some extent it is wasteful as far as raw materials are concerned, very slight labor costs are necessary in weighing out the powder.
Positive type: The positive type of mould produces a vertical flash in the direction of moulding pressure. In this mould, the upper part of the mould (the force) fits closely into the lower part (the cavity). This type of mould is also easy to manufacture, but no allowance is made for placing excess powder in the cavity. Should this occur in error, the mould will not close. If however, the correct charge added, only a small amount of the flash results and it can be easily removed. A disadvantage with such a mould is that the gas liberated during the chemical curing action will be trapped, thus resulting in blisters. This type of mould is suitable for moulding high bulk factor material and is used only on a small scale for molding thermo set. It is however, used for moulding laminated plastics and certain rubber components. Some of the reasons for discarding this type of mould are the necessity for weighing or measuring an exact charge, excessive wear on sliding fit surface of the top and bottom forces and difficulty of ejecting the moulding.
Landed positive mould: A modification of positive mould is known as a landed as a landed positive mould. It is essential that the width of the land should not exceed 3/8 inch. If it does, the applied pressure will be results in thick flash are fine, and the piece will not be filled out properly. The force or plunger is cut back or relived about 1/16 inch, thus producing a shoulder of the width on its lower end.
Semi positive – flash type: The semi-positive flash type of compression mould combines the features of both the flash and positive type. The moulding powder is forced into a complicated mould, at the same time, making allowance for excess powder and flash. This type of mould is expensive to manufacture and maintain and is therefore used where long runs are required. With such a mould, it is also possible both horizontal and vertical flash. The area of the faces of the mould which come in contact with each other when the mould is closed is generally referred to as the 'land'.
Advantages: Lowest cost molds, Little "throw away" material provides advantage on expensive compounds, Often better for large parts, Lower labor costs, Minimum amount of wasted material & Improved material efficiency, Internal stress and warping are minimized, Dimensional accuracy & stability is excellent, Shrinkage is minimized and closely reproducible, Thick sections and large parts are practically possible, Lower molding pressures allow molding of large parts on presses of lower tonnage.
Sheet Moulding Compound (SMC)
With this method the material is purchased in sheet form. Typically the material is a rubber or rubber like material. The material is cut to a shape which is slightly larger than the mould area and a predetermined sheet thickness based on mass needed for finished part. This cut shape is placed into the bottom mould cavity. Both top and bottom cavities are kept at an elevated temperature. The press is then activated and the material is pressed into the cavities under high compression. The heated cavities activate the curing of the material. The part is then removed and sometimes post cured in a post cure oven.
Bulk Moulding Compound (BMC) or Dough Moulding Compound (DMC)With this method a bulk material is used. Often this material is moulded in to a preformed slug that would meet the material mass requirements for the finished part. This preform is placed in the lower mould cavity and the press is then activated in the same manner as the SMC process. Again, the cavities are heated to cause the material to start its curing process.
Liquid Moulding Compound (LMC)
With this method a 2 component thermoset liquid is used. The LMC material is kept at an elevated temperature as are the upper an lower moulding cavities. During the moulding process the 2 components are mixed and poured into both halves of the heated mould. When the proper gel occurs, the mould is closed and the press is activated. These parts are demoulded and typically post cured to achieve full cure of product.
Advantages
Lowest cost moulds
Little "throw away" material provides advantage on expensive compounds
Often better for large parts
Lower labour costs
Minimum amount of wasted material & Improved material efficiency
Internal stress and warping are minimized.
Dimensional accuracy & stability is excellent.
Shrinkage is minimized and closely reproducible.
Thick sections and large parts are practical.
Lower moulding pressures allow moulding of large parts on presses of lower tonnage.
Disadvantages
Offers least product consistency
Not suitable for fragile mould features, or small holds
Uneven parting lines present a mould design problem
High impact composites make flash control & removal difficult.
The depth of the moulded holds is limited to 2 or 3 times their diameter
Shot weight must be tightly controlled
Dimension across the parting line may be difficult to hold but good accuracy may be obtained through tight process control.
Application
Automotive exterior panels especially for commercial vehicles, Radio & appliance knobs, ash trays & electrical parts.
Flash design: It is thin film of plastic which comes out from the want area and around ejector pin due to high injection pressure in mould.
Two type of flash design: Vertical flash, inclined flash.
Ejector plate: It is a steel plate which is incorporated in the ejector assembly for the purpose of transmitting. The ejection force to the moulding through the ejector element. The size of ejector element plate depends on the bison of the ejector element. The plate should be surfficentty thick in order to present deflection.
Assembly: It is that part of the mould in which the ejector element is fitted. It normally consist of an ejector lot ate refraining plate and ejector rod.
Process operation:
Pelleting: permits accurate measurement of the charge to the mould reduces contamination and facilitates preheating. It does not, however, always fit in very well with more automated compression systems.
Preheating: Preheating is useful in that it reduces expensive moulding time, allows rapid heating of large pellets or masses of powder. Helps to remove moisture and other volatiles prior to moulding and because it advances the cure has been claimed to reduce moulding shrinkage.
The moulding stage: The powder material (specified) is placed inside the heated mould cavity and mould is closed, predefined pressure is continuously applied on to the moulding. Since the material will start to cure as soon as it comes into contact with the heated mould. It is important that flow and shaping be completed before the gel point is reached (i.e. the material shows first signs of being cross linked). The optimum cures (i.e. cross-linking) for one property is not necessarily the best cure for another. It is therefore necessary to establish what properties are important in the finished moulding and use cure times and temperatures that give a good compromise to the various requirements, including that of cost. It is particularly important that flow and cross-linking process are controlled and are in the correct sequence. While improvements in the operation can often be made by such refinement as breathing (venting) they will be no value if the basic process is not correct.
Finishing operation: The part finishing is done depending upon the product comes out after curing. Sometimes it is observed that additional material is coming out from the moulding as a flash which can be removed after curing. To make more attractive some other finishing operations can be performed on to the finished product.
Moulding sequence: The moulding sequence in compression moulding is as follows. Mould open, Cavity cleaned, Pellet loaded, Mould closing, Mould closed - Curing stage, Mould open, Moulding ejected.
Moulding cycle: The compression moulding cycle, in many instances the dwell time will comprise a larger fraction of the moulding cycle than indicated in fig.
Machinery & equipments (Compression press): Simplicity is the keynote of the compression moulding press. It basically consists of two platens that close together, applying heat and pressure to mould some material into the desired shape. Most compression presses are hydraulically operated though some are also operated pneumatically. The platens move up and down on four corner posts under pressures that typically can range from 20 to 1000 tons, depending upon press size. Platen themselves can range in size from 8 in. square to 5 ft. square. Various degrees of automation are available in most modern hydraulic presses to feed material and eject the part after cure. Older, simpler systems had temperature, pressure, dwell and time controls. Today’s modern equipment has more sophisticated microprocessor controllers. Presses used for moulding thermoset are available in many different shapes and design and can be classified as either hand, mechanical or hydraulic types. Most compression moulding presses are of the upstroke type as shown in the figure.
Hydraulic system (Upstroke type): In this type, a hydraulic ram moves the bed or bolster of the press upward to close the mould. The strain rods or tie rods hold the upper and lower parts of the press in accurate alignment. So that the two parts of the mould do not mismatch. Adjusting collars on the strain rods allow the daylight opening between platens to be adjusted to the requirements of the mould. The daylight opening is the maximum distance between upper and lower platen. This opening must be large enough so that the compression moulding can be removed from the die. The two parts of the mould are bolted to the upper and lower platens of the press. These platens have heating channels, which are backed with asbestos board insulation. During setting-up, the two platens of the press must be checked for parallelism.
Upstroke type: The disadvantage of the simple upstroke type compression moulding press is that the return is slow, relying entirely on gravity, although pressure can be applied fairly quickly. This limitation can be sorted out by incorporating a pushback ram in the machine.
Down stroke type: A down stroke press has a fixed lower bolster and a moving upper bolster. The down stroke machines are better suited to the moulding of unusually large components, which require a longer stroke. The down stroke type press overcomes the disadvantages of the up stroke press.
Heating and cooling of moulds: Heating is generally done by cartridge type electrical heaters even though hot oil system can also be used. The mould cooling is done by circulation of cold water through the channels.
Types of machines: Presses used for moulding thermoset are available in many different shapes and designs and can be classified as either hand, mechanical or hydraulic type.
Disadvantage: Compression moulding would be slower transfer or injection because in transfer the plastic is roughly heater and is recompressed almost to its final density period to entering the mould.
Limitations: Offers least product consistency, Not suitable for fragile mold features, or small holds, Uneven parting lines present a mold design problem, High impact composites make flash control & removal difficult, The depth of the molded holds is limited to 2 or 3 times their diameter, Shot weight must be tightly controlled, Dimension across the parting line may be difficult to hold but good accuracy may be obtained through tight process control.
Applications: Radio & appliance knobs, Ash trays & Electrical parts. (Electrical switch and fuse boxes), Domestic electrical equipment microwave containers and tableware. Business machine housings. Gas and electricity meter housing, Dish aerials etc.
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