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COMPRESSION MOLDING

Ian Rahmat Widi Perdana

1. OBJECTVE

1.1 To develop practical skills in compression molding technique for polymer

materials.

1.2 To investigate the effect of cure time and cure temperature to the compressed

polymer product.

1.3 To differentiate a pure thermoplastic sample and a composite sample

2. INTRODUCTION

Compression molding is one of the oldest technologies for processing polymers and

it relatively low cost compared to other process [1]. The plastics materials is placed in a

mold cavity along with certain heat and pressure to get a desired shape. Compression

moldings are also called ad matched die molding which its process can be run by almost

main classes of polymers [1].

The principle of compression molding consist by five steps [2]. First is charging

which defined as a precise amount of polymer compound to fulfill the cavity of the mold.

Secondly is closing process, the mold platens compress the charge and forcing it to flow

and fully filling the shape of cavity. Next step is heating which is allow the sample

completely fulfill the mold. Fourth steps is cooling, the aim of this process is to stabilize

the products. Eventually the last step is opening, the mold is released from the platens and

remove the part of product from the cavity. However, pre heating also is compulsory to

make an uniformity softened polymers that flow smoothly when the mold closes [2].

According to [1-3], pre-heating provides some advantages for the product, such as better

surface finish, minimize flow marks, it helps the compound cooperate with thick sections

in the cavity with less difficulties. This process approximately takes 6 minutes before the

compression process [3].

On figure 2.1, it can be seen that the molding machine consist of heavy metal base

onto which slide rods are attached and a compression unit that slides up and down on the

slide rods [4]. This slide rods lead to the compression process occur either with

automatically or manually. When these slide active, it generates the clamping process to

the mold.

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Nevertheless, compression molding also have some major problems during the

process that may reduce the products quality. According to [3], the problems that may occur

in compression molding are cracks around insert, blistering, short and porous molding,

burned marks, flow marks, thick flash, and so on.

Figure 2.1: common Compression Molding

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3. COMPONENTS AND EQUIPMENT

3.1 Two platens hot press (figure 2.1)

3.2 Rectangular mold (13.6cm x 16cm x 0.1cm)

3.3 Polymer samples

3.4 Electronic weight balance

3.5 Anti-heat gloves

3.6 OHP films

4. PROCEDURE

Pressure, time and temperature of two platens hot press were set. The temperature was

set on 180oC with tolerance ±5oC (depends on material used)

The rectangular mold and the HDPE sample underwent pre-heat at lower platens of hot

press until it achieves temperature of hot press temperature. The pre-heat process took 6

minutes to the sample become molten.

Mold

OHP films

Figure 3.1: the mold and OHP films

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5. RESULTS AND DISCUSSION

From the experiment that we have run, we can see that both of the sample pure HDPE

and PLA+stash are having flash on the compression mold. Both of the sample are using the

same treatments which are temperature is set at 180oC with tolerance around ±5oC and

same timing which are 6 minutes for preheating, 4 minutes for hot press and 2 minutes for

cold press.

However, from this experiment we can also obtained that there are particular

variables that we may consider [4]. First variable is the type of the polymer’s sample that

we want to produce. This variable may lead to how the compression molding will be run.

By knowing its properties, we could know the characteristic of the sample. Second variable

is the weight of the product. A precise amount of the weight of the product will give a

perfect product. To know a precise value of the weight we should consider the density of

the product that we used which can be used to determine the weight needed after the volume

After the pre heat process, the start button on machine was pressed to move up lower

platens until the mold nearly touch upper platens. The pressing process took 4 minutes in

order to shaped the sample on mold (depends on material that been used)

Once it done, down button was pressed to allow lower platen returns to its original

position. The sample is harden enough and it released from the hot press.

Then, this sample was moved into cold press. This process took 2 minutes. It necessary

because we need to make it totally harden and allow complete solidification on the

thermoplastic sample

After it underwent 2 minutes, the sample was release from the cold press.

Then, trim process was run in order to release the flash and to separate it from the mold

The procedures were repeated for PLA (polylactic acid) sample

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of the mold is measured. When the density is

measured by: 𝜌 =𝑚

𝑣, whereas m is weight of the

sample and v is the volume of the mold. Third

variable is the timing of the process, every

polymer have their own characteristic in their

process. Thus, since precise timing needed in this

process, we should run further experiments in

order to find out which timing formula gives better

formula. Lastly is the temperature, temperature

that used also depends on the material being

processed. The tolerance of the temperature also

take part for a product.

On the other hand, samples that we used are thermoplastics. Polyethylene (PE) is a

clear to whitish and translucent thermoplastics [5]. Whereas, pure HDPE is a PE which has

a straight-chains structure (Fig 5.1). It has very little branching on the main chains, hence

it affects to the increasing of its crystallinity and its strength because the chains are closely-

packed together [5]. Moreover, it main properties are excellent for corrosion resistance,

low water transition, excellent insulating properties [6]. However, HDPE is very common

used in electrical insulator, containers, housewares and etc, due to HDPE I one of low cost

thermoplastic and has many industrially important properties, such as good flexibility and

toughness in room temperature.

Whereas, the other sample is Polylacticacid (PLA) polymer. PLA itself belongs to

family of aliphatic polyesters commonly made from α-hydroxy acids and it is common

biodegradable thermoplastics [7]. Lactic acid is a compound that plays a key role in several

biochemical processes. It can be obtained either by common chemical synthesis or

carbohydrate fermentations. On this experiment, PLA sample is mixed up with starch and

citric acid. Addition the starch in this compound is to reduce the cost of fabrication of the

PLA itself, as the main aim of adding fillers is to reduce the cost of production [5-7]. On

Figure 5.2: The chain structures of ethylene and when it transformed to HDPE

Figure 5.1: HDPE Product after cold

press

Flash

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Figure 5.3: Polylactic Acid (PLA)

chain structure

Figure 5.4: Citric acid chain

structure

the other hand, according to [8], the addition of citric acid on PLA blend will improve the

barrier properties of the based materials. This additives can also avoid mechanical

properties degradation of the PLA [9]. Citric acid also plays role as a compatibilizer

(esterification, hydrogen bonding), as a depolymerization agent in PLA blends and can also

act as a starch plasticizer [9].

Lab questions

1. Define the method of compression molding

Compression molding is a method of molding process which allows the molding

material generally through pre-heated [2, 3]. At first, the compounded materials is

placed into the mold cavity with precise weight which can be fulfill the mold. Then,

it placed into the hot press which already set with certain temperature and pressure.

Preheated is occurred before the compression occurs to allow the charge completely

melted and completely fill the cavity of the mold. After that, the charge is

compressed when upper and lower platen are met with certain constant pressure. On

this process, it allows the charge become completely mixed up and filling the cavity

of the mold. In thermoset plastic, it allows curing process of the thermoset plastic.

However, for thermoplastic we need to run the cold press, to allows the plastic

completely solidified with certain constant pressure control which avoid warpage

defects [2].

2. Determine the four primary factors in a successful compression molding process.

According to [2], there are 4 main factors which we must consider in order to get a

successful compression process.

The selection of the proper and precise amount of material which will filling

the cavity when the mold are closed. By knowing the mold volume and

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density of material being used is the simplest way to find how much amount

needed in the compression process

Determination of the minimum time required to heat the charge. Actually this

is very important, because polymer properties is very dependent to the heat,

so heating techniques is become very necessary

The prediction of the force required to fill out the mold. This factor is

determined how perfect the product will be, whether it has marks due to much

force applied or not.

Temperature of the mold decreased rapidly during curing. It may effect on

the product properties.

3. What are the pros and cons of compression molding?

According to [2], the advantages of compression molding are

The equipment and process is cheaper than others

Great for large parts

Minimum waves with minimum internal stress and dimensional accuracy

Simple process and adaptable to automatic process (reduce labor cost)

High volume output which lead to cost saving

High quality surface

Can produce complex parts and fast cycle

On the other hand, there are also some disadvantages or contras for compression

molding such as:

High initial investment

Uneven parting lines

No delicate pieces

There will be secondary investment such as trimming

Product consistency

Large dimensional tolerance

Mold depth is limited

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6. CONCLUSION

In brief, on this experiment we have done, we can conclude that compression molding

is one of the simplest method to process polymer product. It basically use for producing great

and large parts product since it has a large tolerance. However, to overcome the problem and

defect during the process of compression molding, there are 4 important factors that play

important role on the process. Those are the precise and proper amount of material which will

fill the mold, the time and heating techniques, the force given in the compression process and

last but not least is the control of temperature.

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7. REFERENCES

[1] S. G. Advani & E. M. Sozer, Process Modeling in Composites Manufaacturing. US:

Marcel Dekker, Inc, 2003.

[2] Dr Mohd Firdaus Bin Omar, Compression Molding, Universiti Malaysia Perlis, Perlis,

2015. [Lectures]. Available: UniMAP Portal, http://portal.unimap.edu.my [accessed on

21 March. 2015].

[3] E. Lokensgard, Industrial Plactics Theory and Application, 5th ed. Canada: Cengage

Learning, Inc, 2009.

[4] D. G. Baird & D. I. Collias. Polymer Processing, 2nded. US: John Wiley & Sons, Inc,

2014.

[5] W. F. Smith and J. Hashemi. Foundation of Materials Science and Engineering, 5th.

ed. New York: The McGraw-Hill Companies, Inc, 2011.

[6] W. D. Callister and D. G. Rethwisch. Materials Science and Engineering, 8th, ed. Asia:

Jonh Wiley & Sons Pte Ltd, 2011.

[7] M. N. Belgacem & A. Gandini. Monomers, Polymers and Composites from Renewable

Resources. Amsterdam: Elsevier, 2008.

[8] H. Abdillahi, E. Chabrat, A.Rouilly and L. Rigal. Influence of citric acid on

thermoplastic wheat flour/poly(lactic acid) blends II: Barrier properties and water

vapor sorption isotherms. 2013.

[9] H. Abdillahi, E. Chabrat, A.Rouilly and L. Rigal. Influence of citric acid on

thermoplastic wheat flour/poly(lactic acid) blends I: Barrier properties and water

vapor sorption isotherms. 2013.