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Ashok Kumar Parthipan

Entry No: 2015JPT2542

Polymer Processing Assignment 1 – Manufacturing of Bottle caps

Introduction

Screw type bottle capping are seen in water bottles. These bottle caps are made up of PP in most cases[1] 

and HDPE in some cases. Bottle caps are manufactured by injection molding process.

Bottle cap design

The below bottle cap is taken as reference in making the design of bottle cap

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Justification

S.No Processing Method Best suited Problem in making bottle cap

1 Extrusion For extrusion of profiles of fixed

cross section [2] 

Process uses a die but a mold is

required to make the bottle cap

2 Blow Molding For making hollow objects [3]  Blowing the preform of a bottle

cap may end up with improper

threads also bottle cap is too

small so blowing becomes

difficult

3 Compression /

Transfer Molding

For simple cavities especially

used for thermosetting raw

material [4] 

Mostly PP and in some cases

HDPE is used those are

thermoplastics

4 Rotomolding For making large sized hollow

objects

Possible by making a core and

cavity and raw material is fed

into the mold when the mold is

closed. Ejection is done by

driving the core in anti-clockwisedirection. But process is time

consuming

5 Injection Molding For making small objects Possible and best method to

manufacture bottle caps

Processing

Raw material used is PP in most cases. As seen above injection molding is the best suited processing

method for making bottle caps, injection grade PP is preferred. Commercial isotactic PP has melting point

approximately 165°C[5] and is processed in the range between 190-230°C. The MFI of injection grade PP

varies from 10g/10min to 30g/10min. Mold temperature is maintained at 20-35°C. Injection pressure of

70 – 100kg/cm2 is being maintained [6]. Shown below is a typical injection molding machine [7].

The raw material is fed in the feed zone which is moved to the compression zone and metering zone by

the rotation of the screw. The material is melted by the band heaters programmed at the above

mentioned temperature located on the barrel. The melt is pressurized to the sprue of the mold, this

molten polymer passes through the runners from the sprue to reach the cavities.

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After filling the cavity the material is held in mold for some time for cooling then the component is ejected.

As the core of the cap has threads the product too have threads so ejection of the caps is brought by the

help of gears. Core is connected to gears which is at the back of the core plate. A drive motor is used to

actuate the gears which in turn unscrews the core and ejects the molded caps [8]. The use of drive motor

can be eliminated by the use of rack and pinion mechanism which rotates the core as the core plate moves

back in open stoke.

Core of the bottle cap

Depending on the size of the mold machine for processing is chosen. If the mold is a simple single cavity

mold then it can be processed in a hand operated injection molding machine also.

References

1. http://www.earth911.com/food/the-verdict-is-in-keep-the-bottle-caps-on/

2. https://en.wikipedia.org/wiki/Extrusion

3. https://en.wikipedia.org/wiki/Blow_molding

4. http://www.technologystudent.com/equip1/hypress1.htm

5. https://en.wikipedia.org/wiki/Polypropylene

6. Polypropylene processing guide, INEOS Olefins and Polymers, USA

7. http://kdmold.myweb.hinet.net/MOLDABILITY/zones

8. http://www.moldmakingtechnology.com/articles/molder-only-runs-one-mold-with-fast-servo-motor-

instead-of-two