DR PAUL NUGENT I wNOp

Do You Really Need?Much has been wr i t ten o f la te about the benef i ts o f process

cont ro l in ro tomold ing. l t s imply makes sense for ro tomolders

to use some form of measurement to improve the qual i ty andef f ic iency of the i r operat ion. So no mat ter what system you

dec ide to use, jus t maxe sure you are us ing one.

The quest ion is jus t how much cont ro l do you need? The

ul t imate form of data wi l l come as Roto log curves taken f rom

ins ide the mold and wi l l g ive you rea l - t ime ins ight in to the

stages of the process tak ing p lace. You may want to measure

temperatures in spec i f ic locat ions on a mold, you may even

want to measure pressure ins ide the mold. However , the

chal lenges in gather ing th is data have been wel l documented,

wi res, thermocouples, connectors , s l ip - r ings, pressure

t ransducers are a l l ava i lab le and can work wel l in a cont ro l led

env i ronment . In a h igh-pressure, fas t moving operat ion,

however , where operators are under pressure to make par ts ,

somet imes the addi t iona l s teps and deta i ls requi red can be a

d is t ract ion and somet imes re l iab i l i ty o f the component par ts o f

the cont ro l system can cause de lays.

The a l ternat ive is 'hands- f ree ' ro tomold ing where the

machine takes over the gather ing and management o f the data.

Non-contact in f rared sensors coupled wi th machine baseo

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ana ly t i ca l so f tware such as the IRT sys tem o f fe red by Fer ry

I n d u s t r i e s , I n c . o f O h i o c a n m e a s u r e t h e s u r f a c e t e m p e r a t u r e

o f a mo ld as i t moves . The da ta ga the red i s no t spec i f i c to

a n y s i n g l e p o i n t b u t i s a c t u a l l y a m a p o f t h e m o l d s u r f a c e a n d

f r a m e .

Th is mapp ing fea tu re o f fe rs some in te res t ing ins igh ts i n to

t h e p r o c e s s . R o t o m o l d i n g u s e s a h i g h - e f f i c i e n c y s c r u b b i n g o v e n

( t yp i ca l l y ) t o t rans fe r hea t to mo lds w i th h igh -speed a i r . Th is

can be ve ry e f fec t i ve when p roper l y des igned bu t even under

t h e b e s t c o n d i t i o n s , t h e d e s i g n o f t h e m o l d , i t s t h i c k n e s s , t h e

f r a m e t h a t i t i s m o u n t e d o n , t h e m a c h i n e a r m a n d r o t a t i o n

s e t t i n g s w i l l c r e a t e c o n t i n u a l l y v a r y i n g s u r f a c e c o n d i t i o n s a n d

h e a t t r a n s f e r r a t e s . T h i s m e a n s t h a t n o t a l l o o i n t s o n t h e m o l d

- , , - t ^ ^ ^ - : ^ ^ ̂ r r L ^r u r r d u s I r J s o L , r e S O l l l e r a t e : S o m e a r e a s w i l l b e h o t t e f a n d

s o m e w i l l l a g b e h i n d . T h e I R T s y s t e m c a n s e e t h i s d i s t r i b u t i o n

o f t e m p e r a t u r e a r o u n d t h e m o l d a n d t r a c k s t h e r e g i o n s w h i c h

w i l l d i c t a t e t h e m o l d i n g c y c l e . D u r i n g t h e h e a t i n g c y c l e , t h e r e

w i l l b e a c o o l a r e a o n t h e m o l d w h i c h w i l l n e e d t o r e a c h

a c e r t a i n t e m o e r a t u r e i n o r d e r t o c u r e t h e m a t e r i a l i n s i d e

p r o p e r l y w h i l e d u r i n g t h e c o o l i n g c y c l e , t h e r e w i l l b e a

h o t s p o t o n t h e m o l d w h i c h w i l l r e t a i n h e a t l o n g e s t . B y f o l l o w i n g

t h e s e e x t r e m e s , t h e m a c h i n e n o w h a s i n f o r m a t i o n w h i c h c a n

b e u s e d t o c o n t r o l t h e c y c l e - w i t h o u t t h e n e e d f o r i n p u t f r o m

the ooera to r .

T h e c o n v e n t i o n a l w i s d o m i s t h a t a n o u t e r s u r f a c e

r e m p e r a r u r e c a n n o t g i v e e n o u g h d e t a i l t o a l l o w f o r p r o c e s s

con t ro l . However , t es ts such as those recen t l y ca r r i ed ou t

b y D r u L a w s o f t h e C h r o m a C o r p o r a t i o n R o t a t i o n a l M o l d i n g

C e n t e r i n M c H e n r y , l l l i n o i s s h o w t h a t i n t e r n a l t e m p e r a t u r e

m u s t f o l l o w t h e o u t e r s u r f a c e t e m o e r a t u r e t h a t d r i v e s i t . S o m e

o f t h e s e n s i t i v i t y t o p o w d e r p h a s e c h a n g e s i s l o s t b u t t h e r e

r e m a i n s a r e l a t i o n s h i p b e t w e e n t h e o u t s i d e t e m p e r a t u r e a n d

i n s i d e w h i c h i s s u f f i c i e n t f o r m o l d e r s t o u s e t o s t a b i l i z e t h e i r

p r o c e s s . T h e i d e a l s c e n a r i o i s f o r a m o l d e r t o u s e a R o t o l o g ( o r

de r i va t i ve ) sys tem to observe the in te rna l mo ld tempera tu re

w h i l e t h e m a c h i n e o b s e r v e s t h e e x t e r n a l s u r f a c e . W h e n t h e

i d e a l c y c l e i s i d e n t i f i e d b y R o t o l o g , t h e m a c h i n e c a p t u r e s t h e

s e t t i n g s f o r p r o d u c t i o n [ w h e r e R o t o l o g i s n o t a v a i l a b l e , m o l d e r s

s i m p l y a d j u s t t h e i r m o l d i n g c y c l e u n t i l t h e y p r o d u c e a p a r t w i t h

t h e r e q u i r e d d e g r e e o f c u r e a n d l o c k i n t h e i r p r o c e s s s e t t i n g s

f rom the re ] . These se t t i ngs a l l ow p roduc t ion s ta f f t o ach ieve

t h e i r m a i n o b j e c t i v e , r e p e a t a b i l i t y , n o t a n a l y s i s .

C o n s i d e r t h e t e m p e r a t u r e p r o f i l e s h o w n i n F i g u r e t . T h i s i s

a n I R T t r a c e s h o w i n g h o w t h e o u t e r m o l d s u r f a c e t e m p e r a t u r e

r i s e s d u r i n g t h e m o l d i n g c y c l e a t a n o v e n s e t - p o i n t o f 5 5 o ' F

(zBS 'C) . The IRT sys tem goes one s tep fu r the r by add ing

a ' T h e r m a l I n d e x ' w h i c h i s a n i n t e g r a l o f t h e m o l d t i m e -

t e m p e r a t u r e p r o f i l e a n d g i v e s a n i n d i c a t i o n ( a l t h o u g h n o t a

s p e c i f i c m e a s u r e m e n t ) o f t h e e n e r g y a p p l i e d t o t h e m o l d b y

t h e m a c h i n e . B y c o m b i n i n g a m i n i m u m r e q u i r e d t e m p e r a t u r e

f o r t h e m o l d s u r f a c e ( r e m e m b e r t h e I R T t r a c k s t h e c o o l e s t

l o c a t i o n o n t h e m o l d ) w i t h a s p e c i f i c T h e r m a l I n d e x , a

r e p e a t a b l e p r o c e s s c o n t r o l p o i n t i s c r e a t e d f o r t h e m a c h i n e .

D u r i n g t h e c o o l i n g c y c l e , w i t h t h e a s s i s t a n c e o f l R T , t h e

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Po lyca rbona te L igh t G lobe (Fo rmed P las t i cs , I nc . )

J A N U A R y - F E B R U A R y 2 o r o . R o r o w o R L D @ | l l

Crossl ink Polyethylene Storage Box (Formed Plast ics, Inc.)

4 2 | n o r o w o R l D @ . J A N U A R v - F E B R U A R v 2 o t o

machine is ab le to turn water on and of f and cont ro l the

f ina l ex i t po in t for the mould based on actua l mold sur face

temperatures as shown by F igure z . By observ ing the enrry

temperature o f the mold in to the coo l ing s ta t ion, the IRT

equ ipped mach ine i s ab le t o ad jus t au toma t i ca l l y f o r de lays i n

the demold ing s ta t ion or mismatches in cyc le t imes between

arms. l t w i l l a lso compensate for changes in ambient condi t ions

in the factory by shor ten ing or extending cool ing cyc les to

ensure that molds are coo led to the same poin t : th is can

be s ign i f icant dur ing seasons wnere morn ing to a f ternoon

temperature swings can be as much as jo-4o"F ( t6-zz"C) .

In essence, the process can be cont ro l led by a temperature

and thermal index in the oven and by three set t ings in the

cooler . By f ine- tun ing these set -po in ts , the molder is ab le

to more t ight ly cont ro l par t cure leve ls and ho ld t ighter

d imens ions . And once t hese a re se t , i t i s t he mach ine wh i ch

gathers the data and senses the changes, not the operator .

There are no wi res or e lect ron ics to worry about around the

molds and i t is th is s impl i f ica t ion o f the cont ro l process which

makes 'hands-free' so attract ive.

Molders in a wide range of indust r ies have seen the benef i ts

of t ransfer r ing cont ro l to the machine. The benef i ts can be

seen on the oven cyc le s ide in terms of cure cont ro l , on the

cool ing cyc le s ide in terms of par t s ize and throughput and in

a combinat ion o f both for par ts wi th cha l lenges both in the

process ing window and d imensions.

Contro l l ing the oven cyc le can he lp wi th improved

cons is tency of cure. Th is can be a great he lp when work ing

wi th mater ia ls such as po lycarbonate and aceta l which have

narrower process ing windows than typ ica l po lyethy lenes.

The po lycarbonate l ight g lobe shown on the prev ious page,

produced by Formed Plast ics , Inc . o f Car le P lace, NY is a

polycarbonate l igh i g lobe which benef i ted in terms of in terna l

sur face f in ish and overa l l cons is tency of cure by us ing fu l l - t ime

IRT cont ro ls in the oven. The company a lso works wi th t ight

to lerance aerospace par ts and recent ly was ab le to rap id ly

d ia l in and mainta in mold ing cyc les for new aceta l components

us ing the fu l l - t ime cont ro l system.

Cooling Cycle Control

A drum component produced by Tr i logy P last ics , Inc . o f

A l l iance, Ohio had two chal leng ing to lerances to ho ld , the

opening at the center between the r ibs and the outer d iameter .

One was f ree shr ink and the other was rest r ic ted which maoe

the demold ing temperature cr i t ica l . In the past there was lo ts

of rework as the par ts were o f ten de l iberate ly made overs ize

and then reheated to shr ink them back to s ize on a f ix ture. As

Product ion Manager Bob Moses commented, 'The IRT system

did a fantast ic job o f d ia l ing in par t s ize ' by automat ica l ly

ad just ing the cool ing set t ings and t ighten ing the range of

d imensional var ia t ion. Rework has been reduced dramat ica l ly . "

A washer tank produced by Formed Plast ics , Inc . had

t4 inser ts which had very t ight to lerances on the d is tances

between the inser ts . Once again , by cont ro l l ing the cool ing

Drum Component (Tri logy Plastics, Inc.)

Washer Tank wi th Inserts (Formed Plast ics, Inc.)

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...fupfl wmw$ffiffi ffi w$wreWffiw fumwreNw*$www ffiWpwffiffiffifu.**pwwR WffimffiffiWp ffiffiffi fuw ffimruWffi.ffiwwN ffiwmmmWffiwmffiffiWmmffi ffiffimR$ffiffiffiffiwffi5r wffiWfuwmwK mN#ffimg Kw RfuwewffiwkWwm# w$ Rfuw ffiffiffiwmKwmcyc le and demold ing po in t w i th the IRT system, par t var ia t ion

was dramat ica l ly reduced and the to lerances he ld to the i r

spec i f ied ranges. Scrap ra tes for par ts have dropped f rom

2.8"/o lo t .Solo with increases in throughput also reported by the

system being ab le to compensate for de lays in the pre-cool

s ta t ion.

Shr inkage is obv ious ly an even b igger prob lem when making

very large par ts that in ter lock. Cont ro l l ing par t s ize and shape

for f loat ing docks, seat ing for boats or p layground systems is

a benef i t that grows wi th sca le s ince ro tomold ing to lerances

are measured inch-per- inch (cm-per-cm). Molders who have

implemented cont inuous process cont ro l dur ing the cool ing

cyc le have repor ted much improved cons is tency and qual i ty .

Combined Oven and Cool ing Cont ro l

The cross- l inked po lyethy lene s torage box shown on the

prev ious page requi res carefu l cont ro l o f the heat ing cyc le to

ensure that the chemica l react ion has taken p lace. Produced by

Formed Plast ics , Inc . , i t a lso had t ight to lerance requi rements

to a l low i t to f i t in to a ho ld ing f ix ture for secondary machin ing

on a CNC router . By combin ing cure cont ro l on the oven wi th

cool ing and demold ing temperature cont ro ls , no reheat ing was

requi red for the secondary process.

As a l l the par ts descr ibed demonst ra te , by us ing a s imple

hands- f ree approach that a l lows the machine to gather

temperature data and then cont ro l the process by compensat ing

for changes in ambient condi t ions and product ion de lays, par t

qual i ty can be improved dramat ica l ly and cont inuous ly wi thout

adding to the work load of the operator . l f you are making a

decision about how much process control to add to your system,

lose the wi res and cons ider a 'hands- f ree ' approach - i t i s by far

the s implest and safest approach!

For more deta i ls contact Ann Rowland at Ferry Indust r ies , lnc .

a row I a nd gfe rryi n d u stri es.co m (szo) qzo 92oo.

J A N U A R y - F E B R U A R y 2 o t o . R o r o w o R L D @ | a S