Designing for plunger-type transfer molds.

Transfer molding is a popular process for manufacturing rubber products. For reasons of productivity and economy, processors like to fill mold cavities in the shortest possible time consistent with quality of the finished article. Various ways have been used to shorten fill time which include such techniques as higher press pressure, higher temperatures, lower viscosity of the compound, more sprues per cavity and larger diameter sprues.

Some theoretical studies of rubber flow during filling of plunger-type transfer molds have been based on the assumption that the rate of transverse flow in a transfer pot should equal flow rate through the sprues (refs. 1-3). That is to say, the time required to fill mold cavities will depend upon the number of sprues per cavity and their diameter in addition to the aforementioned factors of viscosity, temperature and pressure. This seems a reasonable assumption based on one tradition practiced by the rubber industry, design and build a transfer mold with small diameter sprues and enlarge them, if necessary, to achieve a shorter fill time.

Is this traditional approach to shorter fill times always correct? Is fill time always limited by flow rate through sprues? The answer is no. Studies of tooling designs show that in some transfer molds pad flow rather than sprue size limits the fill rate. When this happens, a different design strategy is required to shorten or optimize fill time. The strategy in this situation must emphasize the size and shape of the transfer pot more than the number of sprues per cavity and their size.

The following discussion describes development of a way to determine whether pad flow or sprue flow limits cavity fill time for a specific mold design. A strategy factor value (SFV) is determined for a specific tooling design based on a combination of the rubber compound properties, press tonnage, pad and mold temperatures for which it will be used. The numerical value calculated for SFV can be used to determine whether flow is approximately equal between the pad and sprues or limited by design of either the transfer pot or the sprues. SFV can also be used to derive strategies for design modifications.

Rubber flow in transfer molds

A simplified model for a reservoir, plunger and sprue used in rubber transfer molding is portrayed in figure 1. The force (F) of press closure on the plunger results in two different forces, or pressures, in the transfer pad. One is in the transverse direction ([P.sub.T]) and the other ([P.sub.C]) is in the axial or sprue direction. Two types of flow are also depicted in this model, transverse flow in the transfer pad ([Q.sub.T]) and flow through the sprue ([Q.sub.c])- Arrows are used to indicate the directions of pressure and flow. Values of flow rate can be calculated from pot and sprue dimensions, viscosity, pressure and temperatures using Hagen-Poiseuille equations presented in the following, simplified forms:

Capillary flow through the sprue is defined as - [Q.sub.c] = [Pi][PD.sup.4]/(128[Eta]L (1)

where P is the applied pressure, D is the sprue diameter, L) is the sprue length and [Eta] is the absolute viscosity of the rubber compound used, corrected both for shear rate and temperature. The fact that flow rate is directly related to the fourth power of sprue diameter demonstrates why enlarging sprues is a common way to get faster fill times. Interestingly, once a mold design is set, press pressure and temperatures established, the only variable in equation 1 will be viscosity. Since viscosity becomes lower as temperature rises, equation 1 predicts a potential for faster flow rates through a sprue during the later stages of cavity filling.

Transverse flow in a circular pad is defined as - [Q.sub.T] = [Pi]P[Eta.sup.4]128[Eta] (2) where R is the radius and h is the thickness of the circular pad. It can be seen in equation 2 that flow rate will decrease rapidly as pad thickness decreases as it will do during mold filling and, that decreasing viscosity will provide a slight increase in flow rate as the compound heats up. From a design standpoint, it can be seen that flow rate will decrease the longer the distance that the rubber compound has to flow to reach the sprue.

Strategy factor value is defined as the ratio of [Q.sub.c]/[Q.sub.T] and the SF values reported in this discussion are calculated by - SFV -- [kSQ.sub.c]/([CQ.sub.T]) (3) where S is the total number of sprues in the plate, C is the number of cavities in the mold and k is a constant. Using a value of 10 for k produces a SFV in a convenient numerical range.

Testing the strategy factor concept

For the purposes of this discussion, two categories of plunger-type transfer mold designs are defined. They are:

* Category A designs are those in which flow through the sprues will have more effect on mold filling time than flow through the transfer pot. Designs in this category usually include large, multi-cavity molds for small, closely packed, symmetrical parts with the pot area approximately the same as the total area of the cavity blocks. A Category A layout for a 100 cavity tool in a 10 x 10 arrangement is portrayed in figure 2. Each cavity block is for molding a cylinder of 0.85 in. OD by 0.64 in. ID by 2.0 in. length fed by four sprues. Similar designs for 4, 25, 225 and 400 cavity tools are included in this study.

* Category B designs are those in which flow in the transfer pad has more effect on mold filling time than flow through sprues. Designs in this category usually include: (a) molds for large o-rings, seals or tubes where the pot covers the whole are of the part rather than the perimeter area, (b) molds in which the cavity area extends significantly beyond the sprue locations (e.g., strips, elbows) and (c) may include molds purposely designed with an oversized pot in anticipation of adding more cavities at a future date. Two versions of a Category B layout for molding a 0.375 in. thick, circular seal are presented in figure 3. Version A uses a circular-shaped plunger and version B uses a doughnut-shaped plunger.

The mold designs described in figures 2 and 3 were used in the computations for testing the SFV concept.

All computations of flow analyses and SFV were performed using a computer algorithm written by the author. Some pertinent features included in this algorithm are:

* Consideration of flow only in the transfer pad and through sprues. It does not analyze flow in cavities.

* Consideration of the fact that transverse pressure creates more pressure at the periphery than at the center of the pot. Therefore, outer cavities fill faster than those in the center of the mold (ref. 2). In the algorithm, fill time is defined as the moment when a center cavity is filled and SFV is based on [Q.sub.c] and [Q.sub.T] at that same moment.

All analyses reported herein are based on a constant pressure of 2,000 psi to simplify interpretation of calculated flow data and SFV. This is typical of the pressure used to fill larger transfer molds. All computations are based on using a rubber compound having a Mooney viscosity of 35 (ML 1+4'), a viscosity index of 62.7 kPas, a viscosity-shear rate slope of -0.83, and a viscosity change of minus 1% for each 1[Degrees]F increase in temperature.

Results of computations

The effect of the number of cavities in a Category A design mold on both fill time and SFV is shown as a log-log plot in figure 4. By definition (equation 3), the SFV of a mold should decrease as the number of cavities increases and this is observed in the plotted values. Computations also demonstrate that fill time increases significantly as the number of cavities increases. This is due to transverse pressure pushing some compound toward the periphery of the pot and filling the outer cavities first. After the outer cavities are filled, compound is now trying to flow toward the center of the pot at the same time that transverse pressure is trying to impede its progress, hence the longer fill times.

The effect of sprue size (diameter) on SFV and fill time of the same Category A design molds is portrayed in figure 5. It can be seen that an increase in sprue diameter modestly increases SFV but dramatically shortens fill time. These calculations affirm the practice of enlarging sprue diameters to shorten fill time in some types of transfer molds.

Figure 6 presents an illustration of the effect of transfer pot dimensions on SFV and fill time for variations in Category B mold design. Some differences are to be noted between these effects and those observed for Category A designs.

* In Category A designs, fill time decreases with increasing SFV. The opposite happens in Category B designs, fill time increases with SFV.

* SFV values computed for Category A designs are in the range from 0.1 to 10. SFV values for Category B designs range approximately from 1 to 100.

* The shortest fill times for either category occurred when the SFV ranged between 1 and 10.

The effect of changing sprue size in the Category B mold designs is presented in figure 7. Note that for the 20 in. diameter circular pot, changing sprue size affected the SFV but had little influence, if any, on fill time. Enlarging sprue size does show some effect on fill time when a doughnut-shaped pot is used, one in which the compound does not have to flow such long distances and at smaller thickness.

Computations were also performed on the Category B mold design to determine the effect of changing the number of sprues feeding the seal cavity from four to eight. Flow analyses predicted the same fill time in both sets despite a two fold difference in SFV. This is additional confirmation that transverse flow is limiting filling in this type of mold.

A summary of these results, plus additional experience gained while computing SFV and fill times on other category A and B mold designs, is presented in figure 8. This summary also includes the effect of pressure on fill time. It can be seen from relationships between fill time and SFV plotted in figure 8 that it might be difficult to ascertain whether a mold design is category A or B when SFV ranges between 1.0 and 10. This suggests that a third category, designated C, might be used to classify those designs that are nearly optimum with regard to fill time.

Strategies for optimizing mold designs

The following strategies presume certain goals on the part of the mold designer:

* A wish to retain the number of cavities in design being evaluated or perhaps add more cavities.

* A wish to achieve a reasonable fill time, not necessarily the minimum fill time, in lieu of major design changes.

Strategies for Category A designs

The dominant factors affecting fill time are:

* Amount of pressure applied;

* total number of cavities or distance from center to edge of the cavities;

* size and number of sprues per cavity.

Strategies for design changes include:

* Increase sprue diameter;

* increase number of sprues per cavity;

* determine whether cavities can be packed closer together(*);

* reduce the number of cavities(*).

Reduce the pot area to match overall cavity area for items marked (*) and remember not to oversize the pot with the intention of adding more cavities in the future.

The first three items may not significantly change SFV but they should reduce fill time. If still faster fill times are needed, the number of cavities must be reduced which, if the pot area is also reduced, increases the effective transfer pressure.

Strategies for Category B designs

The dominant factors affecting fill time are:

* Amount of pressure applied;

* size of the transfer pot and the distance rubber has to flow to reach sprues;

* size and number of sprues per cavity. Suggested strategies for design changes are:

* Reduce pot size and shape (if needed and possible);

* increase sprue diameter;

* increase the number of sprues per cavity;

* reduce the number of cavities.

Reducing pot size significantly reduces both the SFV and fill time. Increasing sprue diameter and/or increasing the number of sprues per cavity creates smaller changes in both fill time and SFV than changes in pot size and shape.

Strategies for Categot), C designs

Few changes, if any, are required for mold designs in this category. However, an opportunity exists to test modifications which reduce sprue diameter or the number of sprues per cavity. Also, there is an opportunity to test designs with more cavities in the mold should future needs require them.

Summary

Calculation of a strategy factor value is shown to be a useful way of determining which category of plunger-type transfer mold design one is working with, assessing whether design changes are needed and, if needed, deciding a good strategy for making these changes.

The algorithm for calculating SFV is incorporated into version 2.0 of a commercial software package called GCMTRAN, developed by George Colbert & Associates, Ltd. This software package was written for predicting fill time in plunger-type transfer mold designs using data readily available to rubber factories.

References

1. J.R. Scott, IRI Trans. 7, 169 (1931).

2. P.J. Lieder and R.B. Bird, Ind. Eng. Chem. Fundam., 13, 336 (1974).

3. L.J. Lee, R.M. Griffith and J. G. Sommer, Polym. Eng. & Sci., 24, 403 (1984).