Mold Filling with Polyurethane - ACS Symposium Series (ACS

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Mold Filling with Polyurethane H. MÜLLER, W. MROTZEK, and G. MENGES Institut für Kunststoffverarbeitung, RWTH Aachen, Ponstrasse 49, D-5100 Aachen, Federal Republic of Germany

Even today, the production of polyurethane by the impingement mixing principle s t i l l involves high reject rates and/or high finishing costs. This is due mainly to the complexity of the process, which people often failed to recognize in the past. The process must therefore be seen as a linking together of a number of individual processes of varying complexity. Each of these separate processes - particularly achieving an adequate mixing quality and filling the mix into the mold - must be optimized i f a good-quality molding is to be ensured. There has, however, always been a large amount of uncertaintly with regard to the latter operation, filling the mold, as there was no way of describing the processes taking place inside the mold. The consequences were and s t i l l are today - defects in the molding due to the entrapment of air bubbles. In order to examine this subject more closely, the relevant parts of the mold (flow restrictor, gate, mold cavity) were made of transparent plastic (PMMA) so that the injection process and the flow conditions could be observed and recorded with a video camera. With the aid of these tests, it was then possible to evaluate the influence of some flow-restricting and gating systems commonly used in practice with regard to air entrapment. In addition to this, there is a simple method - the filling pattern method - with which the mold designer can, as early as the development stage, obtain a visual idea of the mold- filling process. This method enables major errors in mold design to be eliminated at a relatively early stage. In industry, polyurethanes are frequently referred to as "tailor-made plastics". Despite this very promising description, the proportion of polyurethanes in the total plastics consumption is at present well below 10 %. If we also consider that the majority of this is made into foam slabs for processing in the upholstered furniture 0097-6156/85/0270-O237$06.25/0 © 1985 American Chemical Society

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REACTION INJECTION MOLDING

i n d u s t r y and i n the i n s u l a t i n g s e c t o r , i t becomes apparent that there i s a c o n s i d e r a b l e amount o f r e t i c e n c e towards t h i s m a t e r i a l . R e a l l y , f a r g r e a t e r use ought t o be made of polyurethanes f o r the manufacture o f h i g h - q u a l i t y t e c h n i c a l mouldings s i n c e they have favourable p r o p e r t i e s and allow c o n s i d e r a b l e scope i n p r o d u c t i o n . One o f the major reasons why t h i s i s not the case i s , without doubt, the h i g h l y complex p r o c e s s i n g technique which, even today, i s not always f u l l y understood i n p r a c t i c e . This p r o c e s s i n g technique, which i s c h a r a c t e r i z e d by the exceedingly f a s t formation - o f t e n i n the space o f seconds - o f the a c t u a l "polyurethane" m a t e r i a l i n the mould, must be viewed as a s e r i e s o f i n d i v i d u a l processes that take p l a c e one a f t e r the other o r p a r a l l e l with one another. For the manufacture o f p e r f e c t mouldings, i t i s e s s e n t i a l t o ensure that each one o f these separate processes i s performed e n t i r e l y f r e e o f f a u l t s . T h i s a p p l i e s not o n l y t o a constant metering and homogeneous mixing o f the components needed f o r producing the m a t e r i a l , but a l s o i n p a r t i c u l a r t o the subsequent processes o f feeding the f i n a l mix i n t o the mould and ensuring that the expandi n g m a t e r i a l p r o p e r l y f i l l s out the mould c a v i t i e s . I t i s p r e c i s e l y with these two steps, which have a major i n f l u e n c e on the q u a l i t y o f the subsequent moulding, that a l a r g e number o f problems a r i s e , s i n c e any e r r o r s made d u r i n g these p a r t s o f the process can no longer be c o r r e c t e d i n view o f the enormous speed o f the chemical reaction. F i l l i n g the mix i n t o the mould In impingement i n j e c t i o n mixing, the two or more components needed f o r producing the m a t e r i a l are i n j e c t e d a t high energy i n t o a r e l a t i v e l y small mixing chamber. The idea i s t h a t , through impingement o f the j e t s o f l i q u i d and the complex flow c o n d i t i o n s that subsequently form ( s w i r l i n g ) , a homogeneous mix i s produced w i t h i n extremel y short p e r i o d s o f time. The m a t e r i a l then flows a t very high speed ( > 2 m/s) out o f the mixing head and, with d i r e c t feed i n t o a mould c a v i t y , would r e s u l t i n s e r i o u s d e f e c t s i n the f i n a l moulding, such as entrapped a i r and flow l i n e s . I t i s t h e r e f o r e necessary t o provide an element between the mixing head and the mould c a v i t y i n which the flow o f mix i s slowed down so that i t can subsequently flow homogeneously i n t o the mould without t a k i n g up any a i r . From t h i s o b j e c t i v e , we can d e r i v e a v a r i e t y o f demands made on the design o f the gate o f a polyurethane mould with regard t o the flow behaviour. The most important o f these i s that i n c o n s t a n t o r l a r g e changes i n c r o s s - s e c t i o n should be avoided and that the mix should flow through the gate i n t o the mould a t as low a speed as p o s s i b l e and i n a laminar manner.

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However, adhering to these c r i t e r i a , which are very necessary from the p o i n t o f view of flow behaviour, tends to c o n f l i c t with a number o f s o l i d economic arguments, so that they are o f t e n almost incompatible. Of these, we are t h i n k i n g above a l l of the necessary low production c o s t s , which permit only a g a t i n g geometry that i s simple to design or simple to manufacture, a small volume t o avoid e x c e s s i v e l y high m a t e r i a l l o s s e s and good d e f o r m a b i l i t y . The compromise between the gate form necessary from the p o i n t of view o f flow behaviour and d e s i r a b l e from economic aspects i s thus a l s o r e f l e c t e d i n the gate concepts most commonly being used at present: the d i r e c t gate and the f i l m gate / l - 3/. Direct gating With the d i r e c t gate (Figure 1), the flow of mix does not have to pass through any changes i n d i r e c t i o n or c r o s s s e c t i o n on i t s way to the mould c a v i t y and, because the m a t e r i a l i s e x p e l l e d from the g a t i n g zone by means o f a c l e a n i n g p i s t o n , a l s o works completely f r e e o f l o s s . From an economical p o i n t of view, t h i s gate v a r i a t i o n has to be regarded as the most i d e a l s o l u t i o n , but more widespread usage i s excluded as i t has c e r t a i n s e r i o u s d i s advantages as regards the flow behaviour. A p p l i c a t i o n of t h i s gate design r e q u i r e s the generation o f a p u r e l y r a d i a l flow i n the mould c a v i t y - something which i t i s only p o s s i b l e t o even c l o s e l y achieve i n the case o f t h i n s e c t i o n mouldings (Figure 2)· At w a l l thicknesses o f more than approx. 6 mm, on the other hand, there i s a r i s k that the flow f r o n t on the w a l l opposite the g a t i n g p o i n t w i l l break up and lead to turbulence and, i n c r i t i c a l cases, to the entrapment o f a i r (Figure 3) /3/. Although the formation o f a p e r f e c t r a d i a l flow i s made e a s i e r by the use o f h i g h e r - v i s c o s i t y systems due to the much higher v i s c o s i t y f o r c e s , the problem i s t h a t the systems being used i n recent years are having s h o r t e r and s h o r t e r cream times, which means that higher metering r a t e s are a l s o being c a l l e d f o r . Consequently, any p o s s i b l e advantage brought about by a higher v i s c o s i t y becomes l o s t again. In a d d i t i o n t o t h i s , the use o f a d i r e c t gate means that a number of other p o i n t s a l s o have to be observed, without which p e r f e c t moulding production i s not p o s s i b l e e i t h e r . In p a r t i c u l a r , a t t e n t i o n has to be p a i d to using a mixing head t h a t works a b s o l u t e l y f r e e o f prefeed and overrun and conveys the mix i n t o the mould c a v i t y completely f r e e of turbulence. And yet i t i s above a l l these problems o f prefeed and overrun which have s t i l l not been very s a t i s f a c t o r i l y solved today, c o n t i n u a l l y r e s u l t i n g i n moulding d e f e c t s under production c o n d i t i o n s .

REACTION INJECTION M O L D I N G

240

Figure

F i g u r e

Figure turbed

2.

1.

D i r e c t - g a t e .

D i r e c t - g a t e

3. D i r e c t - g a t e f l o w - f r o n t .

with

( s o u r c e - f l o w ) .

v o r t e x - f o r m a t i o n

and

d i s -

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241

F i l m gating Because of these problems with d i r e c t g a t i n g , much more frequent use i s made i n p r a c t i c e of the s o - c a l l e d " f i l m gate". This second g a t i n g concept can be d i v i d e d according to the design i n t o the "fan gate" and the "sprue gate" (Figure 4 - 5 ) . A fan gate converts the c i r c u l a r flow channel at the o u t l e t to the mixing head i n t o a r e c t a n g l e with a high length to thickness r a t i o (Figure 4) while maintaining a constant c r o s s - s e c t i o n , with the r e s u l t that the flow of mix leaves the gate as a kind o f f i l m with a maximum thickness of 2 mm. Because the free flow c r o s s - s e c t i o n remains cons t a n t , the fan gate does not cause any change i n the mean flow r a t e and thus has a p u r e l y d i s t r i b u t i n g f u n c t i o n . The requirement that none o f the flow should become separated i n the g a t i n g zone and the requirement that the m a t e r i a l l o s s through the gate should be as small as p o s s i b l e combine, however, to r e s t r i c t the p o s s i b i l i t y o f a very broad d i s t r i b u t i o n o f the flow o f mix i n the mould. This i s because, i n order to prevent turbulence and p a r t s of the flow breaking away, an opening angle o f approx. 2 0 ° must not be exceeded f o r an enlargement o f the c r o s s s e c t i o n , which means t h a t , with a wide g a t i n g s u r f a c e , a correspondingly long and uneconomical design would have to be s e l e c t e d . On the other hand, a narrow gate and a wide mould i n v o l v e the r i s k o f entrapped a i r or that the mix w i l l c o n t a i n a i r when e n t e r i n g the mould, from which the a i r bubbles can no longer escape and consequently lead to d e f e c t s i n the moulding (Figure 6) / 4 / . I f i t i s not p o s s i b l e f o r economic reasons to put the gate across a whole or almost whole side o f a moulding ( long d e s i g n ) , then t h i s gate design must be regarded as h i g h l y problema t i c a l and should be avoided wherever p o s s i b l e . Improved fan gate

(IKV version)

A fundamental improvement to t h i s gate design i s the fan type developed at the IKV on the b a s i s o f experience gained from other p r o c e s s i n g operations (Figure 7), which guarantees unproblematical d i s t r i b u t i o n o f the mix across the w i d t h . The b a s i c idea o f t h i s p r i n c i p l e i s that approximately the same pressure drop should be achieved along each strand o f the flow. The consequence o f t h i s i s that the mould i s f i l l e d simultaneously over i t s e n t i r e width and the flow f r o n t subsequently spreads out almost l i n e a r l y i n the mould. T h i s means that entrapped a i r , as can r e s u l t , f o r example, from a c i r c u l a r movement o f the flow f r o n t ( d i r e c t gate or narrow fan gate. F i g u r e 6 ) , can be avoided. The d e c i s i v e f a c t o r s f o r the pressure drop along a strand o f the flow are simply i t s path length s and i t s v e l o c i t y , i f we assume that the v i s c o s i t y o f the mix remains c o n s t a n t . Tests with t h i s g a t i n g p r i n c i p l e


242

Figure

4.

Fan-gate.

F i g u r e

5.

Sprue-gate.

16.

M U L L E R ET A L .

243


have shown t h a t , as i n other areas with s i m i l a r runners, the v i s c o s i t y can be regarded as being independent o f the shear r a t e (and there i s thus Newtonian b e h a v i o u r ) . The pressure drop along any strand o f the flow can then be given as 4P es = ^PR(0 * JPsd)

(1)

9

I t i s thus made up o f the pressure drop i n the runner i t s e l f and the pressure drop i n the runner s l o t . For the pressure drop i n a runner, we get, with laminar flow (2) and i n a runner s l o t

(3) where

V = volume throughput h 1 = length of the runner R l = length o f the s l o t 7/ L = h a l f width of the s l o t

height o f the s l o t r a d i u s o f the runner v i s c o s i t y o f the mix

R

s

Because o f the symmetry o f the runner, only h a l f of i t i s shown i n F i g u r e 8 and f u r t h e r c a l c u l a t i o n need t h e r e f o r e a l s o only be c a r r i e d out f o r t h i s h a l f . Regarding t h i s i l l u s t r a t i o n we get: f i l m width L = B/2 throughput v£ = VW? s l o t length l = Y (1) g

In view of the requirement t h a t the mean v e l o c i t y i n the gate i s the same at a l l p o i n t s 1 i n the mould, the flow volume i n the runner decreases l i n e a r l y with the l o n g i t u d i n a l coordinate 1.

vii)

= Vo 1

(4)

We can then d e r i v e the f o l l o w i n g equation f o r the pressure drop along a strand o f the flow:

12 v n L h y«>+ 0

0

3

dt

i f TCR(D

u

(5)

0

I f p ( l ) ί f (1), i . e . ρ = c o n s t , f o r any strand o f flow, then the f o l l o w i n g must apply

the (6)

244


Figure

6 .

Fan-gate.

Figure

Figure

7.

8.

M a t e r i a l - f l o w

Fan-gate

Fan-gate

i n t o

the

(IKV-version).

(IKV-version).

mold.

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MULLER ET AL.

From t h i s i t

245

Mold Filling with Polyurethane follows

= Won

A

àl

that

*

P

3

Lh

+ Won

1

àx

Q

,

ο

(7)

LoXRUP

A l l i n v a r i a b l e s can be shortened, so that we oet I 3% dy 2h R(l) 3

(8)

This equation represents the general r e l a t i o n s h i p between the s l o t height h , the l o c a l (island) length y , the width parameter 1 and the runner r a d i u s R (see F i g u r e 8) and can be i n t e g r a t e d when R and h e x i s t as a f u n c t i o n o f y and 1. By i n g e n i o u s l y v a r y i n g the boundary c o n d i t i o n s , we can o b t a i n s o l u t i o n s that are a l s o easy to r e a l i z e from a production p o i n t of view, f o r example by t a k i n g the runner r a d i u s R and the s l o t height h as being constant: = R = const. h ° = const.

(1) From t h i s we get y = y =

ν y

-

2h 3nRé --2àL 3xRé 3

h

3

iidi j £ + c 2 2

(L

(10)

L

2

(L - l )

3xRé °

(9)

(11)

J

T h i s gives us the p r o f i l e o f the " i s l a n d " geometry and o f the runner. A p a r t i c u l a r l y s u r p r i s i n g p o i n t i n t h i s conn e c t i o n i s t h a t t h i s i s a s o l u t i o n which i s very c l o s e to the "Staubalken" gates with f i l m gate that are often used. From the p o i n t o f view o f i n d u s t r i a l p r o d u c t i o n , t h i s s o l u t i o n i s nevertheless s t i l l very complicated, since the runner would have to be made i n an e l l i p s e . I t would be f a r more favourable i f , i n s t e a d o f the e l l i p t i c a l form, i t could be made i n a c i r c u l a r form. With the a i d o f an e r r o r c a l c u l a t i o n , which uses the d i s t a n c e o f the e l l i p t i c a l path from a corresponding c i r c u l a r path as the c r i t e r i o n f o r e v a l u a t i o n , i t can be shown that t h i s i s s a t i s f i e d with s u f f i c i e n t accuracy f o r the i n d u s t r i a l s c a l e i f the L / y r a t i o i s approx. 3 or above (Figure 9) / 5 / . T h i s has §ls8 been demonstrated i n the t e s t s p e r formed with a gate manufactured a c c o r d i n g l y . For t h i s purpose, the gate module and the mould were made o f PMMA, so that the flow processes could be recorded with a video camera and subsequently evaluated at a slower speed. In a l l the t e s t s that were c a r r i e d out i n which the v i s c o s i t y and the volume throughput were v a r i e d , the m a t e r i a l nevert h e l e s s entered the mould c a v i t y simultaneously at a l l


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the d i f f e r e n t p o i n t s and produced a p e r f e c t l y s t r a i g h t flow f r o n t . Thus one o f the e s s e n t i a l c o n d i t i o n s has been f u l f i l l e d f o r a v o i d i n g d e f e c t s such as entrapped a i r i n the mould, namely the p o s s i b i l i t y o f p r e d i c t i n g how the flow f r o n t i s formed i n the mould. T h i s p o i n t w i l l be d e a l t with again i n more d e t a i l l a t e r . T h i s g a t i n g p r i n c i p l e , notwithstanding i t s outstanding advantages, i s f r e quently s a i d t o have economic disadvantages, s i n c e the g a t i n g module has t o be r e l a t i v e l y l a r g e and thus r e s u l t s i n a very l a r g e l o s s o f m a t e r i a l . However, with the high r e j e c t r a t e s t h a t polyurethane p r o c e s s i n g i s a t present having t o contend with and which are l a r g e l y a t t r i b u t a b l e to an i n c o r r e c t design o f the gate, t h i s i s an argument which needs thorough examination. On the c o n t r a r y i t would seem t h a t such assessments are made a l l too prematurely and t h a t savings are being made a t the wrong p l a c e . By doing t h i s , the manufacturer i s l a n d i n g h i m s e l f with unp r e d i c t a b l e f i n i s h i n g c o s t s o r a high r e j e c t r a t e . Sprue gate One example o f a p r i n c i p l e which i s nowadays i n widespread use, but i s nevertheless regarded very c r i t i c a l l y form the p o i n t o f view o f flow behaviour, i s the s o - c a l l e d "sprue gate". With t h i s v a r i a t i o n (Figure 10), the flow of mix i s f i r s t conveyed i n a type o f runner along one side o f the mould c a v i t y and then enters the c a v i t y v i a a f i l m gate whose length may vary. T h i s , however, produces a number o f problems t h a t can cause major d e f e c t s i n the moulding. These problems c o n s i s t e s s e n t i a l l y i n the f a c t that the mix, because o f i t s impulse content, flows i n t o the mould c a v i t y i n two major areas (Figure 11), which means f i r s t l y that there i s no p r e d i c t a b l e formation o f flow f r o n t and t h a t secondly a i r becomes entrapped i n v a r i o u s s e c t i o n s o f the mould (see F i g u r e 11) /4/. I t i s t h i s entrapped a i r i n p a r t i c u l a r , because i t i s i n the lower p a r t o f the mould c a v i t y and can no longer be exp e l l e d v i a a vent g e n e r a l l y s i t u a t e d i n the upper area, that causes a l a r g e amount o f f i n i s h i n g work. T h i s g a t i n g v a r i a t i o n should t h e r e f o r e be regarded as h i g h l y c r i t i c a l and avoided where p o s s i b l e . After-mixers The gate module i s a necessary element f o r conveying the mix produced i n the mixing head i n t o the mould c a v i t y under c o n t r o l l e d c o n d i t i o n s . In p r a c t i c e , however, other elements are a l s o i n s e r t e d between the mixing head and the a c t u a l gate (runner), which are s a i d i n p a r t i c u l a r t o have the job o f " a f t e r - m i x i n g " . A t y p i c a l element such as t h i s i s the s o - c a l l e d " f i r - t r e e mixer" shown i n F i g u r e 12. I t i s , however, a f a l l a c y t h a t t h i s element could make a mixture t h a t had p r e v i o u s l y not been s u f f i c i e n t l y homogenized i n t o one that^was adequate f o r good-quality

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b.c. nan

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p r o d u c t i o n . The e f f e c t of t h i s component i s more to t h r o t t l e the flow of mix and thus i n c r e a s e the mixing e f f e c t i v e n e s s of the mixing head by making turbulences produced i n the mixing head recede more slowly due to the higher l e v e l of pressure /6/. In a d d i t i o n , the g a t i n g volume, i . e . the amount o f scrap i s i n c r e a s e d . A s i m i l a r pressure e f f e c t can a l s o be achieved without disadvantages by reducing the o u t l e t pipe diameter o f the mixing head. The mixing e f f e c t i v e n e s s i s i n t h i s case i n c r e a s e d even f u r t h e r by the f a c t t h a t the i n j e c t i o n nozzles come c l o s e r together and thus a higher energy i s a v a i l a b l e at the p o i n t of impact f o r the mixing. However, when reducing the diameter o f the o u t l e t p i p e , we very soon reach a l i m i t , s i n c e i t c o n s i d e r a b l e i n c r e a s e s the speed at which the mix emerges and i t would be very complicated to slow i t down again s u f f i c i e n t l y f o r i t to enter the mould c a v i t y without t a k i n g up any a i r . I t i s t h e r e f o r e a good idea to provide a l o c a l t h r o t t l i n g arrangement, the e f f e c t of which i s g r e a t e r the c l o s e r i t i s p l a c e d to the a c t u a l mixing chamber. The MQ mixing head from the f i r m Hennecke, St. Augustin, f o r example, i s a very good example o f t h i s . Using an a d j u s t a b l e t h r o t t l e s l i d e v a l v e , i t enables the flow o f mix to be r e s t r i c e d r e l a t i v e l y soon a f t e r the i n j e c t i o n p o i n t . With most of the other high-pressure mixing heads a t present on the market, there i s no p r o v i s i o n f o r t h r o t t l i n g the flow i n t h i s way, which means t h a t , i n these cases, a t h r o t t l i n g device must be provided i n the mould area. As has already been s a i d , the " f i r - t r e e mixer" shown i n F i g u r e 12 i s the most unfavourable s o l u t i o n conceivable f o r t h i s purpose. I t i s true t h a t i t does i t s job of t h r o t t l i n g the flow of mix and thus ensures b e t t e r mixing of the i n d i v i d u a l components, but i t i n v o l v e s a number o f disadvantages i n view of the f a c t that a b s o l u t e l y no a t t e n t i o n i s p a i d to the flow behavi o u r . T h i s r e s u l t s f i r s t of a l l i n l a r g e dead spots (Figure 12), which are l i k e l y to have an e f f e c t p a r t i c u l a r l y i f the m a t e r i a l being used has a very short cream time, as, f o r example, with f l e x i b l e polyurethane foams. The danger here i s that the mix i n these dead spots begins to r e a c t and expand before the f i l l i n g process has f i n ished /4,7/. As a r e s u l t , the flow channel becomes narrower and, due to the very high build-up of pressure i n t h i s area, there i s a r i s k t h a t the mould w i l l open i n the g a t i n g zone and lead to the formation o f f l a s h . The socond disadvantage of t h i s element i s t h a t very l a r g e a i r bubbles are introduced i n t o the mix, f i r s t l y at the beginning of a shot when t h i s element i s f i l l e d - though these bubbles can g e n e r a l l y s t i l l escape from the mix and secondly, something which i s f a r more c r i t i c a l , that a i r bubbles remain s t a t i o n a r y i n t h i s element and are only r e l e a s e d when the shot i s over, when they are d r i v e n i n t o the mould c a v i t y through the movement o f the c l e a n i n g p i s t o n (Figure 12). G e n e r a l l y speaking, these bubbles can no longer get out of the m a t e r i a l .

16.

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_ J

Figure

Figure

11.

Streaming

Figure

10.

Sprue

gate.

relationship

12.

in a

"After-mixer".

sprue-gate.

250


Spreading out of the mix i n the mould c a v i t y Alongside the sources of defects through the choice o f an unfavourable gate or an u n s u i t a b l e t h r o t t l i n g element, the problems of "mould design" i n polyurethane p r o c e s s i n g have only been p a r t l y d e a l t w i t h . In the same way as problems a r i s e i n the s e c t i o n we could term the "runner system", most of which i n v o l v e the flow behaviour, so s i m i l a r problems a l s o occur i n the mould c a v i t y . A f i r s t source of d e f e c t s can be underflow o c c u r r i n g i n the gate s e c t i o n . This phenomenon i s e s s e n t i a l l y due to the mix being i n j e c t e d against the d i r e c t i o n of g r a v i t y (Figure 13) and even i f the gate has been designed to permit favourable flow behaviour, i t can s t i l l lead to considerable a i r entrapment. A r e a c t i o n mix e n t e r i n g a mould that i s enc l i n e d upwards against the d i r e c t i o n o f g r a v i t y i s slowed down by the e f f e c t s of g r a v i t y and f r i c t i o n at a c e r t a i n d i s t a n c e from the g a t e , depending on the flow v e l o c i t y . It then endeavours to flow back to the lowest p o i n t i n the mould c a v i t y . The r e a c t i o n mix s t i l l coming out of the gate nevertheless prevents any backflow on the lower mould w a l l , with the r e s u l t t h a t , on the flow f r o n t , the thickness of the f l u i d f i l m f i r s t increases before any flowing back o c c u r s . The underflow that occurs can, howe v e r , cause a considerable amount o f a i r to become entrapped i n the m a t e r i a l . Where the mix enters the mould i n the d i r e c t i o n of g r a v i t y , the m a t e r i a l c o l l e c t s at the lowest p o i n t o f the mould c a v i t y and pushes the a i r ahead of i t d u r i n g f i l l i n g o f the mould so that i t can be r e leases from the l a s t f i l l e d areas v i a the mould p a r t i n g l i n e or through s p e c i a l v e n t i n g elements. These vents do not n e c e s s a r i l y have to be s i t u a t e d at the highest p o i n t of a mould; t h i s w i l l be d e a l t with l a t e r i n more d e t a i l . In order to be able to e f f i c i e n t l y dispose o f the a i r i n the mould c a v i t y , i t would nevertheless be d e s i r a b l e i f , at the mould design stage, know-how were a v a i l a b l e on the expected formation of the flow f r o n t i n the mould c a v i t y . S i n c e , however, an a n a l y t i c a l d e s c r i p t i o n o f the mould f i l l i n g process appears almost impossible i n the f o r e seeable f u t u r e , there are a number o f s i m p l i f i e d methods for e s t i m a t i n g the spreading out of the flow f r o n t i n the mould c a v i t y , and these o f f e r the only p o s s i b i l i t y of a c h i e v i n g e f f i c i e n t mould venting d u r i n g PUR p r o c e s s i n g . F i l l i n g p a t t e r n method One method which, according to experience gained so f a r , i s able to p r e d i c t with s u f f i c i e n t accuracy the movement of the flow f r o n t d u r i n g the production of l a r g e - a r e a polyurethane mouldings i s the " f i l l i n g p a t t e r n method", which has already been s u c c e s s f u l l y used f o r d e s c r i b i n g the m o u l d - f i l l i n g process i n the i n j e c t i o n moulding o f thermoplastics / 8 , 9 / . For the processing of p o l y u r e thanes, t o o , t h i s method enables a simple graph to be

16.

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251


drawn up o f the spreading out o f the flow f r o n t i n the mould, so t h a t the designer can o b t a i n a p i c t u r e o f the m a t e r i a l flow i n the mould back a t the planning stage. The f i l l i n g p a t t e r n method i s based on the assumption that the flow f r o n t i n the mould advances i n l i n e with the Huyghens' wave propagation theory, according t o which each p o i n t on a wave f r o n t i s regarded as the s t a r t i n g p o i n t f o r a new elementary wave. The enveloping l i n e f o r a l l the elementary waves again provides a wave f r o n t , which, i n the f i l l i n g p a t t e r n method, corresponds t o the flow f r o n t p r o f i l e a t a l a t e r p o i n t i n time. The change i n pressure Δ ρ o c c u r i n g with a flow f r o n t movement o f the d i s t a n c e Δ1 i s , according t o the Hagen- P o i s s e u i l l e p r i n c i p l e f o r the w a l l - s t i c k i n g laminar t u r b u l a r flow

J

32 Zip = 41 (

n

2

- çgsina)

(12)

Applying t h i s p r i n c i p l e t o any d e s i r e d flow c r o s s - s e c t i o n n e c e s s i t a t e s , i n s t e a d o f the pipe diameter d, the use o f the h y d r a u l i c diameter d h

(13) which, due t o the laminar form o f flow, a l s o has t o be m u l t i p l i e d by a geometric c o e f f i c i e n t φ . With l a r g e surface area mouldings, the width b c o n s i d e r a b l y exceeds by d e f i n i t i o n the height h (b»h) , which means that under these c o n d i t i o n s the h y d r a u l i c diameter becomes s i m p l i f i e d as d

=2h

h

(14)

and φ assumes the constant value o f 1.5. Since the changes i n pressure o c c u r r i n g during the spreading out o f the flow f r o n t i n the mould c a v i t y must furthermore be independent o f d i r e c t i o n , Ap± = Ap ' 9 ' neglecting the i n f l u e n c e o f g r a v i t y w

e

e t

o

n

2

(15) Assuming constant f l u i d v i s c o s i t i e s on the flow f r o n t during f i l l i n g o f the mould leads f i n a l l y , together with the d e f i n i t i o n f o r the flow f r o n t v e l o c i t i e s t o a simple relationship h

1

'

(16)

h

2

252


According to t h i s e q u a t i o n , the r a t i o of flow length to mould c a v i t y height i s constant at every moment of the m o u l d - f i l l i n g p r o c e s s , which means that the flow f r o n t p r o f i l e i n l a r g e - a r e a mouldings can only be determined from t h e i r geometrical dimensions. Assuming equal f l u i d v i s c o s i t i e s i s , however, only p e r m i s s i b l e to a l i m i t e d extent i n the case o f polyurethanes, s i n c e , with major d i f f e r e n c e s i n cross s e c t i o n , the v i s c o s i t y a l s o changes with the i n f l u e n c e of the w a l l on the c u r i n g r e a c t i o n . In a l a r g e numer o f c a s e s , the m o u l d - f i l l i n g process i n p o l y urethane p r o c e s s i n g can nevertheless be followed with s u f f i c i e n t accuracy using the f i l l i n g p a t t e r n method. F i g u r e 14 shows the flow f r o n t p r o f i l e s f o r a c e n t r a l l y gated c y l i n d r i c a l mould (360 mm d i a . , h = 10 mm), which were determined by experiment and constructed i n a c c o r d ance with t h i s s p e c i f i c a t i o n . The d i f f e r i n g amount to which the m a t e r i a l spreads out w i t h i n one time- step r e s u l t s , i n c e r t a i n areas o f the mould, i n a meeting up of two or more i n d i v i d u a l flow f r o n t s , something which always i n v o l v e s the r i s k o f entrapped a i r i n the mouldings / 1 0 / . T h i s i s a l s o c l e a r l y i l l u s t r a t e d with the example of a RIM moulding f o r e x t e r i o r a p p l i c a t i o n i n automotive c o n s t r u c t i o n (Figure 15), which contains a metal s t r i p i n the middle f o r f a s t e n i n g purposes. The moulding has an a i r bubble entrapped near the top and has been s h o r t moulded at another p o i n t . Both of these d e f e c t s are caused by inadequate venting o f the mould c a v i t y , as i s shown by the flow f r o n t p r o f i l e i n the mould produced by the f i l l i n g p a t t e r n method and i g n o r i n g the moulding r i b s (Figure 16). The flow f r o n t , which enters the lower area of the mould v i a a sprue gate across the e n t i r e width o f the mould, i s slowed down s l i g h t l y on reaching the i n s e r t on the l e f t - h a n d side o f the mould c a v i t y because of the lower height o f the flow channel, whereas i t moves forward at a constant speed i n the r i g h t - h a n d h a l f . The r e a c t i o n mix t h e r e f o r e flows over the upper end o f the i n s e r t on one s i d e and j o i n s up at some d i s t a n c e from the edge o f the mould with the slower r i s i n g flow f r o n t . However, t h i s contact between the two flow f r o n t s prevents the a i r i n s i d e the mould from escaping v i a the p a r t i n g l i n e and i t becomes f i x e d i n the moulding with the subsequent g e l a t i o n o f the r e a c t i o n mix. The d i f f e r e n c e s between the f i l l i n g p a t t e r n method and the a c t u a l m o u l d - f i l l i n g p r o c ess are due to the f a c t that the i n f l u e n c e o f g r a v i t y on the spreading out o f the flow f r o n t i s i g n o r e d . On r e a c h ing the upper edge of the i n s e r t , the r e a c t i o n mix r u n s , under the i n f l u e n c e o f g r a v i t y , i n t o the t h i n n e r , as yet u n f i l l e d area of the mould c a v i t y and t h e r e f o r e meets the r i s i n g flow f r o n t at a lower p o i n t than was to be expected by the f i l l i n g p a t t e r n method. In a d d i t i o n , when c a l c u l a t ing the m o u l d - f i l l i n g p r o c e s s , no account was taken of the moulding r i b s - another f a c t o r which can e x p l a i n c e r t a i n d i f f e r e n c e s from the true flow f r o n t movement. On the other hand, t h i s s i m p l i f i e d a p p l i c a t i o n of the f i l l i n g

16.

MULLER ET AL.

Figure

13.

F i g u r e 14. filling.

253


Possibilities

Experimental

of the m i x t u r e - i n l e t .

(1) and

theoretical

(2) m o l d


F i g u r e 15. O r i g i n a l fender i n s u f f i c i e n t molded a r e a s .

Figure

16.

Theoretical

o f an

automobile

constructed

flow

showing

front.

16.

MULLER ET AL.


255

p a t t e r n method c l e a r l y i l l u s t r a t e s i n advance which areas of the mould are at r i s k from a i r bubbles and thus enables the mould designer to take e f f e k t i v e measures to prevent i t . Determining the m o u l d - f i l l i n g o p e r a t i o n i n p o l y u r e thane foam moulding n e c e s s i t a t e s a s l i g h t l y modified app l i c a t i o n o f the f i l l i n g p a t t e r n method, depending on the p o s i t i o n o f the mould. In the foam moulding o f p o l y u r e thane, f i l l i n g o f the mould c a v i t y can be subdivided i n t o three i n d i v i d u a l stages: i n j e c t i n g the flow o f mix i n t o the mould, calming o f the m a t e r i a l and expansion o f the m a t e r i a l i n the mould, as i s shown i n F i g u r e 17 f o r a b o t t o m - f i l l e d , angled mould o f constant geometry. The compression and hardening phase f o l l o w i n g the expansion stage i s , however, o f no s i g n i f i c a n c e f o r f i l l i n g the mould, s i n c e compression o f the m a t e r i a l merely b r i n g s about thé f i n a l formation o f the d e n s i t y p r o f i l e t h a t i s then f i x e d i n the hardening phase. The flow f r o n t p r o f i l e forming i n the mould c a v i t y d u r i n g i n j e c t i o n o f the mix can b a s i c a l l y be determined f o r polyurethane foam moulding i n j u s t the same way as i n the production o f s o l i d p o l y urethane mouldings (Figure 18). However, when the mould i s i n c l i n e d at an angle, there i s , d u r i n g the calming phase a f t e r the i n j e c t i o n phase has f i n i s h e d , an e q u a l i z a t i o n o f the flow f r o n t s due to g r a v i t y as a r e s u l t o f the mould c a v i t y being only p a r t l y v o l u m e t r i c a l l y f i l l e d . T h i s e q u a l i z a t i o n i n v o l v e s the r e a c t i o n mix, which has already moved on i n t o the t h i c k e r areas o f the c a v i t y , flowing back, whereas a t the same time the flow f r o n t i n the other areas i s c o n t i n u i n g to advance u n t i l the surface o f the l i q u i d becomes smooth. T h i s l e v e l i n g out process i s not, however, covered by the f i l l i n g p a t t e r n method, s i n c e i t i s c o n t r o l l e d p u r e l y by g r a v i t y . For d e s c r i b i n g the m o u l d - f i l l i n g process d u r i n g the expansion stage, the f i l l i n g p a t t e r n method should t h e r e f o r e be based on the flow f r o n t p r o f i l e at the beginning o f the expansion. As long as there are no geometric c o n d i t i o n s i n the mould c a v i t y below the surface o f the l i q u i d which might encourage entrapped a i r , a p p l i c a t i o n o f the f i l l i n g p a t t e r n method i n polyurethane foam moulding i n a mould i n c l i n e d at an angle i s r e s t r i c t e d merely to the a c t u a l expansion process. On the other hand, i f the mould i s i n a l e v e l p o s i t i o n as i s g e n e r a l l y the case f o r c e n t r a l l y gated polyurethane mouldings - there i s no e q u a l i z a t i o n o f the flow f r o n t s even i f the m a t e r i a l expands d u r i n g the calming phase, which means that the f i l l i n g p a t t e r n method can always be a p p l i e d from the gate onwards (Figure 14). As shown by a comparison between the flow f r o n t p r o f i l e determined by experiment and one c a l c u l a t e d by theory based on t h i s c y l i n d r i c a l mould o f 360 mm diameter, the f i l l i n g p a t t e r n method i s n e v e r t h e l e s s a l s o able to c o r r e c t l y d e s c r i b e the areas a t r i s k from a i r bubbles and the p o s i t i o n o f the weld l i n e s f o r a c e n t r a l l y gated moulding with l o c a l

256

R E A C T I O N INJECTION M O L D I N G

Figure 17. technology.

Schematical

Figure

18.

mold

Change

f i l l i n g

of

the

in

flow

the

PUR-foaming

f r o n t .

16.

MULLER ET AL.


257

change i n c r o s s - s e c t i o n , u n l i k e the i n j e c t i o n moulding of t h e r m o p l a s t i c s , weld seams i n polyurethane p r o c e s s i n g do not n e c e s s a r i l y form zones of reduced strength i n the m a t e r i a l , b u t , above a l l , they impair the o p t i c a l q u a l i t y of the moulding and g e n e r a l l y represent an aggregation o f entrapped a i r bubbles. A knowledge of the m o u l d - f i l l i n g process i n polyurethane p r o c e s s i n g thus makes i t p o s s i b l e to recognize back at the mould design stage areas of the mould that are at r i s k from a i r bubbles and to do something about i t by - a l t e r i n g the p o s i t i o n o f the g a t e , - v a r y i n g the thickness of the moulding and - p r o v i d i n g s p e c i a l venting elements, and thus l a r g e l y avoid subsequent m o d i f i c a t i o n s to the mould. Successful a p p l i c a t i o n o f the f i l l i n g p a t t e r n method nevertheless r e q u i r e s not only that the mould i s gated at i t s lowest p o i n t , but a l s o that j e t t i n g i s avoided under a l l circumstances at the beginning o f the m o u l d - f i l l i n g p r o c e s s , otherwise the a i r bubbles that get i n t o the r e a c t i o n mix i n the g a t i n g area cannot be r e moved even by c a r e f u l venting of the mould. Thus, designing the gate to s u i t the moulding a l s o gains a d d i t i o n a l s i g n i f i c a n c e , since i n t h i s case the formation of the flow f r o n t i s v i r t u a l l y given i n advance. A combin a t i o n o f these two elements - gate design and f i l l i n g p a t t e r n method - s h o u l d , however help i n many a p p l i c a t i o n s to e l i m i n a t e back at the mould design stage, serious d e f e c t s i n moulding p r o d u c t i o n .

Literature Cited 1. Mrotzek, W.; Auslegung von Formwerkzeugen für die PUR-Verarbeitung; Promotionsvortrag, 1982; IKV-ArchivNr. P-10-1982 2. Becker, E . ; Reaction Injection Molding; Van Nostrand Reinhold Company, New York 1979 3. Schneider, W.; PUR-Anguβsysteme In: Wirtschaftliches Herstellen von PUR-Formteilen; VDI-Verlag, Düsseldorf 1981 4. Kuchenmüller, K.; Untersuchungen zur Optimierung der Formteilfertigung beim RIM-Verfahren; Diplomarbeit am IKV, Aachen; IKV-Archiv-Nr. D8347; Betreuer: H. Müller 5. Döring, Ε.; Hiemenz, C.; Mathematische Beschreibung sowie Versuche mit den aufgrund der Ergebnisse gefertigten Anguβsysteme; Nichtveröffentlichte Arbeit am IKV, 1979; IKV-Archiv-Nr. B7931

258

6. 7. 8. 9. 10.

REACTION INJECTION M O L D I N G

Mrotzek, W.; Gegenstrominjektionsvermischung niedrigviskoser Reaktionsharze; Dissertation an der RWTH Aachen, 1982; IKV-Archiv-Nr. DS 8210 Jung, D.; Einfluβder Anguβgestaltung auf die Qualität von PUR-Formteilen; Diplomarbeit am IKV, Aachen; IKV-Archiv-Nr. D 8301; Betreuer: H. Müller Menges, G.; Lichius, U.; Bangert, H.; Eine einfache Methode zur Vorausbestimmung des Flieβfrontverlaufs beim Spritzgieβen von Thermoplasten; Plastverarbeiter 11 (1980) 671-676 Napp-Zinn, G.; Werkzeugfüllung bei der PUR-Verschäumung; Studienarbeit am IKV, 1982; Archiv-Nr. S 8206; Betreuer: W. Mrotzek Niemöller, Β.; Methoden zur Vorherbestimmung der Werkzeugfüllung bei der PUR-Verschäumung; Diplomarbeit am IKV; IKV-Archiv-Nr. D 8238; Betreuer: W. Mrotzek

RECEIVED October 5,

1984

Mold Filling with Polyurethane - ACS Symposium Series (ACS

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