polymer processing

5/28/2018 polymer processing

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SCHOOL OF MATERIAL ENGINEERING

SEM 2 2013/2014

EBT 329/3 : POLYMER PROCESSING

ASSIGNMENT

NAME : RABIATULADAWIAH BT MAHMOD

MATRIC NO. : 111041620

COURSE : MATERIAL ENGINEERING

LECTURERS NAME: DR MOHD FIRDAUS BIN OMAR


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Assignment : Compression Molding

1. Describe the processing of compression molding.

Figure 1 : The technique of compression moulding processing

Figure 2 : The cross-section of compression moulding technique


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2. Give 5 advantages and 5 disadvantages of compression molding.Table 1 : The advantages and disadvantage of compression molding

Advantages Disadvantages-Great for large parts - Uneven parting lines

-Minimal internal stress - High initial investment

-Dimensional accuracy - Mold depth limited

-Simple process - Product consistency

-Fast cycle (1-5 minutes) - Large dimensional tolerance

3. Describe Compression MoldingCompression molding is a processing technique that combines forming the product along

with simultaneous curing of the resin by applying pressure and usually heat.

Figure 3 : Compression molding machine


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Principle of compression molding

1) Charging is a process in determining a precise amount of molding compound into thebottom half of a heated mold.

2) Closing is a process where the mold halves together to compress the charge, forcing itto flow and conform to the shape of the cavity.

3)

Heating is the process where the compound by means of the hot mold and let it flowcompletely fulfill the mold.4) Cooling is the process to stabilize the products.5) Opening is where the mold halves and removing the part from the cavity.Types of compression mold

a) Flash type

Figure 4 : The diagram of flash type

Flash type is a mold in which the mold faces are perpendicular (normal) to the clamping

action of the press, so that the higher the clamping force, the tighter the mold seam. It is

simplified version of semi-positive mold. The flash forms the seal as it spews and rapidlycures, at the mold parting line. It is the least expensive compression mold. The greatest

disadvantage to a flash mold is that there is no restriction to the flow of material and no

backpressure is built up in the molded part. The flash is free to flow horizontally with norestriction. The parts will lack complete density, therefore may be inferior in strength andfinishes. The percentage of flash loss may be extremely high, thus costly if considered on

per part basis. It is also difficult to maintain matching centerline between the force (upper

mold half) and cavity (lower mold half). If the guide pins are worn out, the force and thecavity become mismatched.


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b) Positive type

Figure 5 : The diagram of positive type

Positive type is a mold designed to trap all the molding material when it closed. Positivemold takes all the applied pressure, there is insufficient clearance to allow material to

escape. Any variation in feed weight will give variation in thickness and density of

molding. In general, positive mold is not favored. It is generally used if the material is of

high bulk or if the filler fibers are quite long. It also gives vertical flash. It is difficult toproduce parts with equal density if mold is multi-cavity. For this mold type, generous

flash escapement area (or excessive clearance between force and cavity) is required.

c) Semi-positve type

Figure 6 : The diagram of semi-positive type


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Semi-positive type is a mold whose principle is as the two halves of the mold begin to

close, the mold acts much like a flash mold, as the excess material is allowed to escaped

around the loose fitting plunger and cavity. As the plunger goes in further into the cavity,the mold becomes positive mold with very little clearance and full pressure is exerted on

the material, producing a part of maximum density. This type of mold takes advantage of

the free flow of material in a flash mold and the quality of producing dense part in thepositive mold. The majority of compression mold is semi-positive horizontal flash mold.The space for loading the molding compound is large to minimize flash. The flash

escapement area is located on the force side for easy removal. This type of mold produces

parts having maximum physical strength. Another variation is semi positive vertical flashmold. The flash is vertical, which may be removed more easily. This type of mold is

quite expensive since there are two areas between the force and the cavity that must fit

perfectly. This type of mold will not have mismatch problem.

Typical products of compression molding

1)

Automative parts such as hoods, bumpers, fenders, spoilers.2) Medical equipment such as caps and plugs for blood separation machines andultrasound equipment.

3) Aerospace such as electrical connectors to guided missiles.

Figure 7 : The example of compression molded parts

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