Bound Brook, New Jersey 08805. Several parameters affecting the metering accuracy- in reac- tion injection molding (RIM) arc identified and their effects.
Factors Affecting Metering Accuracy in Reaction Injection Molding OLAGOKE OLABISI and Y. GAU Union Carbide Corporation Research and Development Bound Brook, New Jersey 08805 Several parameters affecting the metering accuracy- in reaction injection molding (RIM) arc identified and their effects evaluated on a laboratory-scale machine (mini-RIM HP-6. Polyurethane Technology of America). The experimental results show that the delay time must lie greater than a critical value i n order for the system pressure to attain the steady-state value; that the matching of the recirculation and pouring pressure is necessary to have consistent amounts ofmaterials at the calibration or mixing head; and that a variable momentum ratio can have an adverse effect on the metering accuracy. Two approaches are nscd in the determination of the optimum process parameters for the mini-RIM machine and an attempt is made to explain the effect of momentum ratio variation on the metering accuracy in RIM.
INTRODUCTION
R
eaction injection molding (RIM) is a promising process which produces shaped and structured polymer products rapidly in t h e mold directly from monomeric and oligomeric reactive liquids. Highpressure impingement mixing is generally used so as to take advantage of the low viscosity of the reactive systems. There are six significant unit operations in RIM, namely, metering, mixing, mold filling, part curing, part ejection, and post curing. However, the technical problems associated with each of these operations cannot always be decoupled. Rapid reactions require rapid mixing, rapid heat transfer, coupled and complex flows. Depending on the extent of reaction, the rheological behavior of the reacting systems becomes increasingly complex and this in turn affects several flow regimes in RIM as well as the metering accuracy, mixing, and mold filling. This paper deals with the analysis, evaluation, and optimization of a bench-scale RIM equipment. The emphasis is on the understanding of the machine and the process variables which affect the amount of materials delivered. Although the work in this study was carried out on a specific piece of laboratory scale equipment, the information presented could be extrapolated to other RIM machines.
EXPERIMENTAL In order to assure good metering and mixing of a given chemical system on any RIM machine, the following two conditions must be satisfied: (a) The mass flow rate, Q , of the two streams must be proportional, (the proportionality constant, a,is a unique property of the chemical system in use). 1 82
QA
= ~ Q B
(1)
If the momentum ratio of the two streams equal unity (this is not a design specification on commercial machines), the additional relation is
V A Q A= VBQB
(2)
Consequently, the cross-sectional areas of the flows are related by
AA/AB = ( P B / P A ) and the fluid velocities are related by
av,
(3)
v,
(4) The Reynolds number (Re) of the streams must exceed -100 (1-3); this being the value above which good mixing was consistently achieved. This value happens to fall within the region where a change in slope occurs in the behavior of the discharge coefficient (see Discussion). That is, =
Re = vDp 2 100
(5) P where V is the fluid velocity; D , the nozzle diameter, p the fluid specific gravity and p, the fluid viscosity. Process Equipment This work employs the Model HP6 (Polyurethane Technology of America), mini-RIM equipment, a schematic diagram of which appears on F i g . 1 . The major components of the machine are: The Metering System. The metering system consists of the hydraulic drive, the ratio control mechanism, the metering cylinders and the valve block. The metering cylinder B is driven (seeFig. 2 ) directly through a clevis joint by the hydraulic cylinder. The POLYMER ENGINEERING AND SCIENCE, FEBRUARY, 1982, Vol. 22, No. 3
Factors Affecting Metering Accurucy in Reuction Injection Molding pose a side load on the metering cylinderA (not in line with the hydraulic drive) and cause it to wear prematurely. This is accomplished by confining the A cylinder to move in a straight line through a guide rail arrangement. The mixing head plunger was modified to improve its sealing capability by using a hardened steel plunger. The rear chamber of the metering cylinder is connected to an overflow tank filled with DOP. The DOP prevents any seize-up that could be caused by small back leakages. The Operating Parameters Fig. 1 . Schematic of mini-R1M
‘
8.89+d
rr
;,d
d’
8.89~1
Fig. 2. Ratio triungle
metering cylinder A is driven by a lever arm through another clevis joint. The stoichiometric ratio between these two metering cylinders is controlled by the position of the movable pin which can be adjusted continuously by a simple lateral translation. Figure 2 is a schematic of the right triangle which permits a continuously variable stoichiometric ratio since the pin at “3” is movable. The valve block has a double role in the fluid recirculation system. It sets the equipment either in the low recycle mode or in the high recycle mode; it also provides a high pressure recycle during the delay time and the injection time. The Mixing Head and Piston Plunger. In order to set the pouring pressure during a shot one adjusts the height of a screw which limits the size of the orifice. This type of tuning does not require any disassembly for the adjustment and is very practical to operate. But when a precise knowledge of the velocity is essential, the use of a preselected orifice with a step increase or decrease in opening is preferable. For a more complete description of the equipment, the reader is referred to ReJ 1 . The mini-RIM used in this study was modified somewhat and is different from that ofRef. 1 in the following respects. The ratio control mechanism was modified to reduce its tendency to imPOLYMER ENGINEERING AND SCIENCE, FEBRUARY, 1982, Vol. 22, No. 3
The process parameters include: The velocity of the drive cylinder which should be controlled in such a manner that it is constant at all loads. The delay time which represents the period of time between the start of the forward movement of the drive cylinder and the opening of the piston plunger in the mix head. The shot time which corresponds to the duration of time duringwhich the mix head piston plunger is open. The recirculation pressure of fluidsA and B which is measured during the high pressure recirculation. The reactants flow through the pressure relief valves by proper switching of valves. The pouring pressure of fluids A and B which is recorded during an injection period. The ratio control distance. This control is peculiar to the mini-RIM equipment. The calculation of the theoretical distance is shown inFig. 2 . Some other types of ratio control mechanisms are: coupling of metering devices by gear mechanism, and individually driven metering equipments. The temperature of fluids A and B . The material parameters are: the viscosity, the specific gravity, other pertinent rheological quantities. The shot weight and the calibration weight of fluids A and B are the dependent variables. Causes of the Metering E r r o r One of the causes of the metering error is the slip leakage which occurs in the metering cylinders. The leakage amount is a function of width, length, and shape of clearance; it also depends on the viscosities and the pressures of fluids A and B , For a lance-type cylinder, with the assumption that the clearance between the cylinder and the piston is small so that the approximation of the plane couette flow holds, the velocity of the fluid through the clearance, toward the back of the cylinder can be approximated by the following expression (4):
U
=
AP
(y’ - yH)
+ VyIH
(6)
where y is the perpendicular distance from the piston (y = 0 ) toward the cylinder (y = H); H, the gap clearance; I , the piston length; V, the piston velocity; /.L, the fluid viscosity; AP, the pressure difference. In addition to the slip leakage, the other factors which contribute to the metering errors are: 183
Olagoke Olabisi and Y. Gau The amount of entrained air (5). Air can be introduced into the reservoir tanks, either from a faulty design of the return line or from the feeding line if it is connected to the top part of the tank whereby air is entrained as the fluid falls through space into the tank. The air could then be compressed by the high-pressure pumps and could expand after passing through the high-pressure orifice d u e to the pressure drop. The presence of air could result in erratically foamed product and cause the metering ratio to shift; it also reduces the amount of material pumped out of a given length of time. The elasticity of the hose (6). In cases where the hose pressure changes rapidly, particularly during the start of an injection, the contraction and expansion of' the hose can be enough to cause errors in the ratio of fluid delivered to the mix head. Temperature. The specific gravity and viscosity are strongly temperature dependent. A change in temperature of a few degrees can translate into a variation of the fluid properties resulting in a change of friction factor in hoses and a change of pressure drop across the small orifices. That is why a good control of temperature is so important to the metering accuracy. It should be noted too, that a constant fluid recirculation through a small orifice with a large pressure drop (-1000 psi or larger) can generate a large amount of heat by viscous dissipation. Momentum Ratio. It is easy to achieve a reproducible and accurate metering when the exit ports of both streams are open to the atmosphere, but it is usually difficult to determine the flow ratio under actual operating conditions (7). The problem arises because the pressure and velocity at the impingement point as well as the pressure drop of the two jets are difficult to d e t e r m i n e experimentally and analytically. T h e analysis of the collision in still air of two diametrically opposed plane turbulent wall jets (8) reveals that the location of the collision is given by XIi/Xzi = ],/I2,where Xi denotes the distance from the virtual origin to the position of collision and J the jet momentum flux. The virtual origin is the position at which a slot of infinitesimal thickness emits the same momentum flux as the real slot. A similar relation may hold in the impingement mixing chamber. With X denoting the distance from the orifice, the impingement location for various ratios of momentum can be determined. However, the amount of material delivered remains unknown. Lee, Ottino, Ranz and Macosko (2) performed a flow visualization study in the mixing chamber and observed that with a momentum ratio of 2.5, the higher velocity stream p e n e t r a t e s the c h a m b e r f u r t h e r a n d moves t h e impingement point over. The pressure drop was also found to be larger on the low velocity side d u e to this back-up.
should be greater than a critical value to have consistent results at the calibration ports or mixing head. For a velocity of the drive cylinder of 3.05 cm/s (at lowpressure recirculation), the critical delay time is 0.8 s. The delay time minimizes lead-lag and reactant crossover problems resulting from the disparity of the pressures in the two fluid circuits. It allows the system to accommodate the lag of the cylinder which moves at a slower speed (A side) and takes longer to reach the injection pressure. An example of this start-up transient is shown in Fig. 3 which represents the pressure traces from pressure transducers located close to the mixing head for sides A and B . The delay time of 0.8 s was determined from such a plot.
Effect of Recirculation Pressure and Pouring Pressure on Metering Accuracy A set of designed experiments was performed on each orifice of the impingement mixer separately to determine the effect of recirculation and pouring pressure. The results shown in Table 1 indicate that the matching of the recirculation pressure and pouring pressure is necessary to have reproducible amounts of materials at the calibration or at the mixing head. This balancing of the pressure has been explained in the open literature (6) in terms of the difference in volume change in the hose when the shot is started. We believe that the model of a flow of a high velocity fluid through an adjustable orifice provides the right conceptual explanation. The velocity or mass flow rate in any type of flow configuration, can be related to the pressure drop by the following expression:
AP
pressure drop
=
K1Q2
inertia effect
+
K2Q
viscous effect Loss
+
(7)
K , depends on the flow geometry and specific gravity,
t
A
SIDE
RESULTS Effect of Delay Time on Metering Accuracy To study the effect of delay time, baseline experiments with the polyurethane system, RIM 125 (Union Carbide Corporation) showed that the delay time 184
PRESSURE
1740 p x m t
Fig. 3 . Pre.witrc trcice.? POLYMER ENGINEERING AND SCIENCE, FEBRUARY, 1982, Yo/. 22, No. 3
Factors Affecting Metering Accuracy in Reuction Injection Molding Table 1. Effect of Recirculation and Pouring Pressure' ~~
P Recirculation (Psi)
P Pouring (Psi)
Calibration Weight (9)
Shot Weight (9)
1.000 1,800 1,000 1,800 1,400
1,000 1,000 1,800 1,800 1,400
55 49.5 54.5 51 53.5
55 57 46 49 53
* Cyllnder B: Delay Tlme = 1s. Shot Time = I s , Temperature = 4WC, Material urad =
UCC Polyol Niax D-830.
K z on the flow geometry and viscosity, and Q is the mass flow rate. The viscous dissipation and the loss d u e to liquid expansion or contraction contribute to the loss term. Although the loss d u e to viscous dissipation can be determined analytically by solving the energy and momentum equations with appropriate initial and boundary conditions, that d u e to contraction or expansion can only be determined experimentally. The value of this loss (due to contraction or expansion) during calibration will differ from the value during actual injection unless the orifice sizes are equal, and the flow configurations are identical. If the orifice sizes are unequal, there will be a pressure imbalance in the system whose magnitude during calibration will differ from tis value during actual injection. An imbalance in pressure implies an imbalance in the rates of the two reactive streams; consequently, the loss d u e to contraction or expansion during calibration and during actual injection will be different. Inasmuch as viscous dissipation effects could be quite pronounced for fluids of high viscosity, we chose to use low viscosity fluids so as to minimize the effects of pressure loss d u e to viscous effect. An example of the variation of one pouring pressure as a function of t h e other when the recirculation pressures are left at their set values ((a)step), is shown in Table 2. In most of the cases, when the pouring pressure in one fluid circuit is increased, the other pouring pressure shows a decrease in value (Case 11 c, 111 c,d). Case I b can be explained in terms of a higher speed of
the hydraulic drive cylinder. Case I1 b and 111 b are associated with the peculiarity of the mini-RIM equipment and it is presumed to be related to the design of the drive mechanism.
Effect of Injection Time on Metering Accuracy The most common type of pumping systems for precise delivery of fluid in RIM is the positive displacement metering unit and, in particular, the displacement cylinder. In such a cylinder (9), the amount of material pumped is determined by the displacement of the piston. The loss in theoretical capacity which occurs because of back-leakage during metering tends to be constant for a given chemical system. Hence, it is desirable to determine the volumetric efficiency for each metering pump on the machine for each chemical system. The volumetric efficiency of the pumping system is defined as the actual volume divided by the theoretical volume. Figure 4 illustrates the effect of injection time on weight of output. It is in essence, a calibration curve. In preparing this curve, the experiment was performed on cylinder A with poly(dimethy1 siloxane) fluid having a m2/s and a specific gravity of 0.97 viscosity of 50 g/cm3 at room temperature. The delay time was set at 0.9 s. Cylinder B was left empty. The pressure dependence of the calibration curve can be explained in terms of: A larger leak (through the gap clearance) toward the back of the piston with a larger pressure gradient (Eq. 6). A decrease in forward velocity of the hydraulic cylinder d u e to a larger resistant force offered by the higher pressure in the fluid lines.
1.5
v)
Y
Table 2. Variation of One Pouring Pressure as a Function of the Other Side A pPo"rlnb!
