control rotomoulding warpage and achieve high speed cooling
DESCRIPTION
Internal water cooling during rotational moulding process can give symmetrical temperature gradients and this results in little or no warpage, no matter how fast the cooling rate. Water cooling enables you to speed up cooling cycle as fast it is possible.TRANSCRIPT
1
shrinkage
warpage
Internal
water
cooling
release
Control of warpage and advantages of high speed cooling
Prepared by Ian Hansen at RotoConsult [email protected]
21st Century technologyPutting It All Together
2
Distortion where the surfaces of the moulded part do not follow the intended shape of the design
Prepared by Ian Hansen at RotoConsult [email protected]
What is warpage?
3
Rotational moulders typically use unsymmetrical cooling (cooling from the outside only), which often leads to distortion (warpage) of the moulded part• The rate of cooling is controlled by using varying degrees of still air, fan forced air, water mist and water spray, normally from the outside only• If a molten plastic is subjected to fast cooling on one side and slow cooling on the other side, there will be a difference in the structure across the thickness of the plastic when it becomes solid.• The side that cools slowly will have a tendency to be more crystalline. This means that the molecular chains pack together more closely and the plastic will have a higher density in this area.•The side that cools more quickly will be less crystalline because the material did not have time for the molecular chains to align themselves into a crystalline structure, and will end up with a lower density in this area.
Prepared by Ian Hansen at RotoConsult [email protected]
20th Century moulding processes
4Prepared by Ian Hansen at RotoConsult [email protected]
Causes of warpage1 . R e s i d u a l S t r e s sIf the shrinkage is prevented from occurring, then residual stress will be set up in the plastic part.
Insert/Pin
Insert/Pin
Residual stresses
Underground or horizontal tanks typically have substantial ribs which prevent shrinkage occurring, often creating residual stress
5Prepared by Ian Hansen at RotoConsult
Causes of warpage
Mould wall
SHRINKAGE THRU WALL
STRESSES PULL MATERIAL TOWARDS CRYSTALLINE AREA
Exte
rnal
Col
d Ai
r
Inte
rnal
Hot
Air
CRYSTALLINE AREA
WARPAGE OCCURS
FASTERCOOL
LOW CRYSTALLINITY
SLOWERCOOL
HIGH CRYSTALLINITY
2 .Te m p e r a t u r e g r a d i e n t d u r i n g e x t e r n a l c o o l i n g
Plastic wall
6Prepared by Ian Hansen at RotoConsult [email protected]
Mould wall
Shrinkage thru Plastic
wall
Inner Air Space
Cooling
Plastic part
I n t e r n a l c o o l i n g c a n g i v e s y m m e t r i c a l t e m p e r a t u r e g r a d i e n t s a n d t h i s r e s u l t s i n l i tt l e o r n o w a r p a g e , n o m a tt e r h o w f a s t t h e c o o l i n g r a t e .
Control of warpage
S u p e r f a s t i n t e r n a l w a t e r c o o l i n g i s p o s s i b l e u s i n g S u p a c o o l o r R o t o c o o l e r.
Prepared by Ian Hansen at RotoConsult [email protected]
7
20th Century troubleshooting strategies
Design• Mould material• Mould surface finish• Wall thickness• Avoid sharp corners• Effective use of inserts and fixtures• Pigment select
Processing• Clean moulds• Decrease amount of mould release• Decrease oven temperature• Remove water from external cooling cycle• Use jigs after moulding
HO
WE
VE
R; T
hese
strate
gie
s do n
ot
ad
dre
ss the p
rob
lem
cau
sed
by
coolin
g o
nly
on
on
e sid
e o
f the p
lastic
part
8Prepared by Ian Hansen at RotoConsult
21st Century processing strategies
Processing – Internal water cooling
• Symmetrical, uniform cooling from both the inside and outside of the moulding produces a much more homogeneous crystalline structure across the thickness of the part.
• By spraying water into the centre of the moulding to internally cool the molten plastic during the cooling phase, permits water cooling to be used on the outside of the mould early in the cooling cycle.
• Fastest cooling occurs when internal and external water cooling is used and you achieve little or no warpage.
Supacool or Rotocooler
9
Supacool unit mounted on 5,000L mould
Prepared by Ian Hansen at RotoConsult [email protected]
Supacool unit
Airline
Quick connect coupling
Standard vent
10Prepared by Ian Hansen at RotoConsult [email protected]
•injects air + water mist •greatly reduces cooling times •reduces warpage
Rotocooler unit mounted on 6,200L mouldRotocooler unit
Airline
11Prepared by Ian Hansen at RotoConsult [email protected]
Internal water cooling
Internal water cooling
12Prepared by Ian Hansen at RotoConsult [email protected]
21st Century processing strategiesSupacool or Rotocooler
http://www.laplastecnica.co.nzhttp://www.maus-gmbh.de