Rotational Molding

An Introduction to the Basics

Rotational Molding
Personal Introduction
 R. Dru Laws
 Employed with Mity-Lite Engineering
throughout and after schooling
 Bachelors in Mechanical Engineering
– From BYU
 Masters in Polymer Engineering
– From QUB (in Ireland)
 Engineering Director for Mity Fence
Systems
 Owner/Founder of RotoScientific
Rotational Molding
Example Product
Rotational Molding
Example Product
Rotational Molding
Example Product
Rotational Molding
Example Product
Rotational Molding
Example Product
Rotational Molding
Example Product
Rotational Molding
Presentation Outline

 Introduction
 The Materials
 The Machines
 The Molds
 The Method
 Conclusion
Rotational Molding
Introduction

 Also known as rotomolding or

rotocasting
 Process for manufacturing large,
hollow, one-piece plastic products
 Best know for the manufacture of
tanks and toys (Little-Tykes)
 Suitable for small & complex
shapes
Rotational Molding
Introduction
 Historical Development
– 1850’s: Patent granted for rotomolding
principle
– 1910’s: Method used for chocolate
products
– 1940’s: Method used for vinyl plastisols
– 1950’s: Invention of PE for the process
– 1960’s: Rapid expansion in toy industry
– 1970’s: Gradual expansion for technical
products
– 1990’s: Significant improvement in process
control (at QUB)
Rotational Molding
Introduction

 1500 Rotomolders around the world

Rotational Molding
Introduction

 North American Market Segments

Playground
2%
Industrial
16%

Toys Materials Handling

40% 3%

Containers
9%

Tanks
Household 18%
2% Others
10%
Rotational Molding
Introduction – The Process
Rotational Molding
Introduction

 Strengths
– Small production runs
– Large part capability
– Low tooling costs
– ‘Stress-free” products
– Short product development lead times
Rotational Molding
Introduction

 Weaknesses
– Slow cycle times
– Limited material choices
– Difficult to automate
– Poor image
– Low technology culture
Rotational Molding
Presentation Outline

 Introduction
 The Materials
 The Machines
 The Molds
 The Method
 Conclusion
Rotational Molding
The Materials
Others, 2%
PVC, 13%

LDPE, 2%
MDPE, 3%

XLPE, 8%

HDPE, 11%
LLDPE, 61%
Rotational Molding
The Materials

 Low Zero Shear Viscosity is required

to aid in the flow of the polymer
within mold
 MFI
– Lower MFI resins – high strength
products
– Higher MFI – good surface finish and
surface detail
Rotational Molding
The Materials

 Particle shape and size is crucial to

the flow characteristics as the mold is
rotating
– Standard size is a 35 mesh powder (500
micron, 0.0197 inches)
Rotational Molding
The Materials

 Dry-flow/Bulk-density characteristics
– Dry-flow is the time it takes 100 grams
of resin powder to pass through a funnel
of given dimensions
– Bulk-density is the quantity of
undisturbed powder than can fit inside a
beaker of specified dimensions
– Inversely proportional to each other
Rotational Molding
The Materials
Rotational Molding
The Materials

 Other Material
– Foams
• Post-process
• In-process
– Liquid reactive polymers
– Micro-pellets
– Additives
– Ceramics
Rotational Molding
Presentation Outline

 Introduction
 The Materials
 The Machines
 The Molds
 The Method
 Conclusion
Rotational Molding
The Machines
 Rock-and-Roll
 Clamshell
 Vertical
 Shuttle
 Carousel
– Fixed
– Independent
 Electric & Jacket
Rotational Molding
Rock-and-Roll Machines
Rotational Molding
Rock-and-Roll Machines
Rotational Molding
Clamshell Machines
Rotational Molding
Vertical Machines
Rotational Molding
Shuttle Machines
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Fixed Carousel Machines
Rotational Molding
Independent Carousel Machines
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Electric Machines
Rotational Molding
The Machines

 The “ARM” and “PLATE”

Rotational Molding
Presentation Outline

 Introduction
 The Materials
 The Machines
 The Molds
 The Method
 Conclusion
Rotational Molding
The Molds

 Materials
– Steel
• Mild
• Stainless
– Aluminum
• Cast
• Machined
• Plate
Rotational Molding
The Molds
 Anatomy
– Frame and Spider
– Two or more sections (with draft)
– Parting line with alignment features
– Clamping hardware and pry-points
– Vent (breather) tubes
– Texture and other features
– Release agents
Rotational Molding
The Molds – Frame and Spider
Rotational Molding
The Molds - Mold Sections
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The Molds - Parting Line
Rotational Molding
The Molds - Parting Lines
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The Molds - Parting Lines
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The Molds - Clamping Hardware
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The Molds – Pry Points
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The Molds - Vents
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The Molds - Vents
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The Molds – Kiss-Offs
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The Molds - Threads
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The Molds - Inserts
Rotational Molding
Presentation Outline

 Introduction
 The Materials
 The Machines
 The Molds
 The Method
 Conclusion
Rotational Molding
The Method

1. Loading
2. Heating
3. Cooling
4. Unloading
Rotational Molding
Rotational Molding
The Method - Loading

 A predetermined amount of resin is

loaded into the mold
 The mold is closed after the flanges
have been cleared of resin particles
 The mold is secured shut
 Rotate the mold to ensure no tools
have been left on the machine
Rotational Molding
The Method - Heating
 Rotate the mold in a high-temperature
environment
 The resin will always sit at the lowest point
of the cavity (powder pool)
 It will slowly coat the mold interior until all
the resin has melted
 The resin will then continue to densify as
the gap between the particles disappears
 Remove the mold from the oven when the
powder has completely melted and
densified
Rotational Molding
The Method - Cooling

 Rotate the mold in a room

temperature environment
 This allows the layer of resin to cool
and solidify
 Water can also be used, but care
should be exercised
Rotational Molding
The Method - Unloading

 Remove the clamping system

 “Pry” open the mold
 Lift the completed, hollow, one-piece
part from the mold.
 Repeat the cycle
Rotational Molding
The Method
 The concern
– How do you really know when it is time
to do the next step?
• When is the resin melted?
• When is it cooled?
– In the early 90’s, QUB invented an
instrument that revolutionized process
comprehension and control
• The Rotolog™
Rotational Molding
The Method – Rotolog™

 Temperature sensors (4 type-K

Thermocouples)
 RF Transmitter
 RF Receiver and Signal Conditioner
 Computer Software
– Real time monitoring of the temperatures
Rotational Molding
The Method – Rotolog™
Rotational Molding
The Method – Rotolog™
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The Method – Temperature Profiles
Rotational Molding
The Method – IAT Profile
Rotational Molding
The Method – IAT Profile
Rotational Molding
The Method – IAT Profile
Rotational Molding
The Method – IAT Profile
Rotational Molding
The Method – IAT Profile
Rotational Molding
The Method – IAT Profile
Rotational Molding
The Method – IAT Profile
We can thus observe:
C
B. When powder first
sticks to the mold
C. When the powder has
been consumed B D
A

D. When optimal melt

sintering has occurred E

E. When solidification is
occurring
F. When the plastic
separates from the mold
Rotational Molding
The Method – IAT Profile

• The Peak Internal C

Air Temperature
(PIAT) is the key
process control
parameter and is
independent of all
other process
variables (point C)
Rotational Molding
The Method

 Other important (QUB) developments

– Pressurization (Dynamic Venting)
– Permanent temperature feedback
(K-Kontrol)
– RotoSim computer simulation
– Worldwide training seminars and
consulting
– PPRC – Rotomolding division
Rotational Molding
Presentation Outline

 Introduction
 The Materials
 The Machines
 The Molds
 The Method
 Conclusion
Rotational Molding
Conclusion

 Though in a state of infancy, Rotomolding

offers unique advantages over other
plastics processes
 There are endless opportunities for
research and development
 Rotomolding has a very promising future,
more so than many competing processes
Rotational Molding
Acknowledgements

 Thank-you to Dr. Brent Strong for

providing me with this opportunity

 Thank-you for your attention

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Rotational Molding