Injection Molding of Thermosets

OVERVIEW

In the mid-l960s, about two years after the introduction of automatic transfer molding machines and screw-preplasticizinghransfer molding machines, the concept of in-line screw injection molding of thermosets (sometimes re- ferred to as direct screw transfer or DST) was developed. It had long been thought that this process, already highly successful with ther- moplastic materials, would be impractical for thermosetting plastics because the critical time- temperature relationship would prove uncon- trollable, and material would set up in the bar- rel.

Development progressed not only on screw and barrel designs but also on stability of ther- moset materials at elevated temperatures, fi- nally leading to successful processing by screw injection molding. (See Fig. 8-1 .)

This technique has had a significant influ- ence on the thermoset molding business by vir- tue of reduced molding cycle time and the po- tential it offers for low-cost, high-volume production of molded thermoset parts.

Today, thermoset injection molding ma- chines are available in all clamp tonnages up to 1200 tons and shot sizes up to 20 lb. (see Figs. 8-2, 8-3, and 8-4).

Thermoset in-line screw assemblies are fitted to horizontal or vertical clamp machines. Most horizontal clamp thermoplastic injection ma-

Reviewed and revised by John L. Hull, Vice Chairman, Hull Corporation, Hatboro, PA.

chines can be converted to injection of ther- moset by changing the screw, barrel, and noz- zle. Because most thermoset barrels are shorter than their thermoplastic counterpart, it may be necessary to reposition the injection assembly closer to the clamp fixed platen.

The thermoset molding materials developed specifically for injection molding have a long shelf life in the barrel at moderate temperatures (approximately 200"F), and react very rapidly when the temperature is brought up to 350 to 400°F as the material is forced through the sprue, runners, and gates, and fills the cavities. This unique development in materials helped to gain acceptance of injection molding as a reli- able production process.

How the Process Operates

A typical arrangement for in-line screw injec- tion molding of thermosets is shown in Fig.

The machine consists of two sections mounted on a common base. One section clamps and holds the mold halves together un- der pressure during the injection of material into the mold. The other section-the plasticizing and injection unit-includes the feed hopper, the hydraulic cylinder which forces the screw forward to inject the material into the mold, a motor to rotate the screw, and the heated barrel which encloses the screw.

Basically, the injection-molding press, whether for thermosets or for thermoplastics, is the same, utilizing the reciprocating screw to

8-5.

239

240 SPI PLASTICS ENGINEERING HANDBOOK

1 1 7

Fig. 8-1. Reciprocating screw injection molding, 600-ton hydraulic clamp. (Courtesy Hull Corporation)

Fig. 8-2. A 3OO-ton clamp, reciprocating screw injection molding machine. (Courtesy H f M Corp.)

Fig. 8-3. A 150-ton toggle clamp BMC injection molding machine, rear side showing stuffing cylinder feeding into injection barrel. (Courresy H f M Corp.)

INJECTION MOLDING OF THERMOSETS 241

e

Fig. 8-4. Close-up of BMC stuffer cylinder, at left, feeding through elbow into injection barrel of plunger-injection molding machine. (Courtesy Hull Corp.)

plasticate the material charge. In most injection molding of thermosets, the material, in granu- lar or pellet form, is fed from the hopper by gravity into the feed throat of the barrel. It then is moved forward by the action of the flights of

the screw. As it passes through the barrel, the plastic picks up conductive heat from the heat- ing element on the barrel and frictional heat from the rotation of the screw.

For thermoset materials, the depth of flights

Movable platen -, Tie F ixed Heating Hopper Clam( bars platen end K$W:?q 7 1- 7 3, cooling-,

)-cylinder piston

nder t o re t rac t Y/////A

Fig. 8-5. Typical arrangement of direct screw transfer molding machine for thermosets. (Courtesy S. Bodner)

242 SPI PLASTICS ENGINEERING HANDBOOK

of the screw at the feed-zone end is normally the same as the depth of flight at the nozzle end; this is a 1 : 1 compression ratio screw. This compression ratio is the major difference be- tween thermoset and thermoplastic molding machines-the latter having compression ratios such that the depth of flight at the feed end is 1; to 5 times that at the nozzle end, giving compression ratios of 3 : 1 to 5 : 1.

As the material moves forward in the barrel, it changes from a granular to a semiviscous consistency, and it forces the screw backward in the barrel against a preset hydraulic pres- sure. This back pressure is an important pro- cessing variable. The screw stops turning when the proper amount of material has reached the nozzle end of the barrel, as sensed by a vernier- set limit switch. This material at the nozzle end-the charge-is the exact volume of ma- terial required to fill the sprue, runners, and cavities of the mold.

The heated and plasticated slug of material at the front or nozzle end of the barrel is mod- erately stable for perhaps a minute or more- enough retention time for the mold to complete closing and the machine to be in the high-pres- sure clamp mode.

The screw is then moved forward at rapid speeds (up to 2000 inches per minute) by hy- draulic pressure (up to 20,000 psi) on the plas- tic. The hot plastic melt is forced through the nozzle of the barrel, through the sprue of the mold, and into the runner system, gates, and mold cavities.

The temperature of the material rises from the barrel temperature of about 200°F to mold temperature of 350 to 400”F, and fast cross- linking takes place, curing the part in seconds or a few minutes, depending on the mass of material and the maximum cross-sectional thickness of the part.

When the reciprocating screw has delivered about 95% of the charge, the injection or “boost” pressure of up to 20,000 psi generally is reduced automatically to about half that value for completion of the cavity fill and for holding during cure. This secondary pressure generally is termed the “hold” pressure.

Following cure, the mold opens automati- cally, parts are ejected, and the mold closes.

At a carefully selected time during the cycle, the plasticating process in the barrel is initiated and timed to be complete by the time that the mold is fully closed again. The molding cycle repeats automatically.

In the process, the granular plastic is fed di- rectly into the barrel of the press, and no ex- ternal auxiliary preforming or preheating equipment is required. Despite this, the tem- perature of the material entering the mold is higher and more uniform than that in other thermoset molding techniques because of the homogenizing effect of the screw upon the plastic.

Most barrels are covered with a metal jacket designed to permit heated water to flow across and around the barrel (Fig. 8-6). The water or temperature-controlled fluid not only assists in heating the thermoset material as it moves along the flight of the screw but helps prevent over- heating, as the fluid will withdraw excessive internal heat should it occur because of exces- sive screw rpm or back pressure.

Screw check valves or sleeves, which are standard for thermoplastics (except for heat- sensitive materials such as PVC), are not used with thermosets. They provide restrictions to even flow of high viscosity materials, and eventually cause the material to set up in the valves or sleeves. Some material slippage or back flow along the flights of the screw does occur when the injection pressure approaches maximum. Because of this slippage, the full slug of plasticated material does not reach the cavities. Therefore, one should not attempt to

Fig. 8-6. Close-up of water-heated injection barrel, three temperature zones, on typical thermoset injection molding machine. (Courtesy Hull Curp.)

INJECTION MOLDING OF THERMOSETS 243

use the maximum rated shot capacity of the machine. A rule of thumb for most materials and machines is to use up to 80% of rated ca- pacity.

Nozzles at the end of the barrel usually are water-cooled or temperature-controlled to maintain a proper balance between a hot mold (350-40O0F), and a relatively cool barrel (150- 200 O F ) .

PROCESS AND DESIGN CONSIDERATIONS

Thermoset molding resins require curing by a chemical reaction, or polymerization. Thus, the closer to the curing temperature the material is when it fills the cavity, the shorter will be the in-cavity cycle time. The direct screw injection process substantially reduces cure time from

that of other thermoset processes, particularly for parts having inch wall section or greater (Fig. 8-7). Cycle-time reductions on the order of 20 to 30% are common; even greater reduc- tions have been reported.

Molding a thermoset material on an injection molding machine involves approximately the same cost factors as molding a thermoplastic material. Machinery costs, plant space require- ments, and labor costs are about the same. Therefore, the costs that makes the difference are the basic material price and the cycle time required.

Materials

Most materials are slightly modified from con- ventional thermoset compounds. These modi- fications are required to provide the working

'.*

Fig. 8-7. A 35-ton vertical injection molding machine highlighting BMC stuffer. (Courresy Gluco, Inc.)

244 SPI PLASTICS ENGINEERING HANDBOOK

timejtemperature relationship needed for screw plasticating . Additionally, material formula- tions may be altered according to the geometry of the part being molded. The new compounds are priced approximately the same as conven- tional materials.

Although all thermosetting materials have been molded by this technique, the most com- monly used materials are the phenolics. These materials were the first to be injection-molded, and today it is estimated that approximately 400 million pounds are being processed by injec- tion molding annually. Other thermoset mate- rials being molded by the process include melamine, urea, polyester, alkyd, diallyl phthalate (DAP), polyurethane, and alloys such as phenolic melamine and phenolic epoxy.

A wide variety of fillers can be used to achieve the properties required in the finished product. Except in a few special cases, the choice of filler is of greater significance in end- product properties than in the method by which the product is molded.

The introduction of dust-free phenolic in pel- let form brought additional advantages to injec- tion molding. The dust-free material eliminated the former need for separate molding areas and equipment for phenolics. Now these materials can be molded side by side with a thermoplas- tic operation, with minimum danger of contam- ination. In fact, with a change of the barrel and screw, the same machine often can be used for both materials.

Applications particularly suited for the pro- cess are automotive power brake, transmission, and electrical parts; wet/dry applications such as in steam irons, washer pumps, and moisture vaporizers; and communications and electrical distribution parts that require dimensional sta- bility and electrical insulation. (See Figs. 8-8, 8-9, and 8-10.)

Process Effects on Design

Proper design of injection-molded thermoset parts requires an understanding of the flow characteristics of material within the mold. These characteristics affect some properties of the finished parts, and they vary somewhat with the different materials. In that they have a flow

Fig. 8-8. Shot from eight-cavity mold for double outlet wiring device, injection-molded with urea formaldehyde. (Courresy Hull Corp.)

I

Fig. 8-9. (Courresy Wesringhouse Electric)

Steam iron handle screw molded from phenolic.

pattern, such parts are more similar to transfer- molded parts than to compression-molded parts.

Shrinkage, in most parts, occurs slightly more in the direction of flow than in the trans- verse direction. This difference is caused by the orientation of the filler (almost all thermoset compounds contain a filler) as the compound flows through the gate and into the cavity. Such shrinkage is particularly evident in edge-gated parts. To minimize the flow effect, center gat- ing, which distributes the material more evenly, is recommended where optimum shrinkage control is required, such as in molding a round piece within dimensional tolerance. Technol- ogy is now well advanced in the use of warm-

INJECTION MOLDING OF THERMOSETS 245

Fig. 8-10. Two-cavity shot from conventional injection mold, large electrical switchgear housing, BMC polyester material. (Courtesy GE Plastics)

runner molds (equivalent to hot-runner molds as used with thermoplastics), which, when used with center gating, provide a substantial reduc- tion in material consumed in the sprue and run- ner-a cost saving over conventional thermoset molding.

The flatness required in the finished molded part is another design parameter affected by process capabilities. Long, narrow parts that have to be flat may be difficult to mold because of the variation in shrinkage. Similarly, a non- uniform wall section may warp as a result of nonuniform shrinkage. The choice of gate lo- cation sometimes can compensate for such con- ditions; close cooperation with the mold de- signer is recommended at this stage.

The surface finish on molded parts also dif- fers from that of traditionally molded thermo- sets. The greater the distance of material flow required, the more likely it is that flow marks will occur. Avoiding large, flat, polished areas helps reduce rejects and keeps molding costs down. Here, too, gate location is a factor. If large, flat areas are necessary, such areas can be textured or patterned to mask slight irregu- larities. A rough surface texture generally is not desirable, however, because it may cause stick- ing in the mold cavity.

Gates used for thermoset injection molding usually are smaller than those needed for trans- fer molding. This difference provides several

advantages. A smaller gate gives a cleaner gate break that requires little or no hand finishing. A gate can be placed in an area where previ- ously the appearance of a larger gate mark would have been objectionable. A small gate also contributes to faster cure cycles because it increases frictional heat within the molding compound during filling. However, the impact properties of compounds containing long-fiber reinforcement degrade when the material passes through a small gate. When such materials are specified, close cooperation between the ma- terial supplier and the molder is necessary. Other physical properties such as tensile and flexural strength can also be controlled by gate geometry.

Injection-Compression Molding

To minimize fiber orientation, and thereby im- prove the impact strength of injection-molded fibrous compounds, the conventional injection molding process is often modified to allow the mold to remain slightly open-perhaps $ inch- during cavity fill. When the cavity is essen- tially full, the mold is closed and held under full clamp pressure until cure is completed. In this process, the final mold closing acts like compression molding, permitting the fibers to shift from a longitudinal flow orientation to a more random orientation. The design and con-

246 SPI PLASTICS ENGINEERING HANDBOOK

struction of the mold must be such that it pre- vents escape of the injected material to mold parting surfaces during the cavity filling step. Frontal flanges are incorporated to effectively close off the cavity when the mold is partially closed. The injection-compression process often reduces the clamping force required, as compared to conventional injection molding.

Part-Design Considerations

As in most other molding processes, wall-sec- tion uniformity is important. Molding cycles, and therefore costs, depend upon the cure time of the thickest section. Therefore, cross sec- tions should be as uniform as design parame- ters allow, with a minimum wall thickness of E inch. A good working average for wall thick- ness is ; to & inch. Nevertheless, as part re- quirements dictate, heavy walls favor the use of thermosets because the cure rate of thermo- setting materials is considerably faster than the cooling rate of thermoplastics. A rule of thumb for estimating cycle times for a $ inch wall sec- tion is 45 seconds for thermoplastics and 30 seconds for thermosets, when injection mold- ing is used.

Generous radii and fillets are recommended, as in other plastic processes, for maximum strength. Most plastic materials are somewhat notch-sensitive, so avoiding sharp comers is important. Generous fillets are less likely to cause sink marks in the low-shrinkage ther- moset materials than they are in some of the thermoplastics, so more freedom is available in the use of reinforcing fillets.

Draft to allow the release of the molded part from the cavity and force plug (core) should be at least io per side; greater draft is preferred if possible. Because the injection method is au- tomatic, ease of mold release is essential for rapid ejection with minimum distortion of the part. Provision should be made in the part for the placement of ejector pins for this purpose. The use of knockout pins as large as possible (at least ; inch in diameter) consistent with part design and aesthetic considerations will pro- vide trouble-free processing, which ultimately is reflected in lower costs.

I

Venting of gases from the cavity must be done in the short time available during the fill- ing of the cavity. The most effective vent is the parting line of the mold; so try to visualize the material flow from the gate to the most distant point of the part where the gases will collect. If this point is on a mold parting line, then the part is well designed for venting. Avoid long, dead-end comdors of flow and trapped wall sections that prevent venting. Vacuum venting, as described in Chapter 9, is readily adapted to injection molding when complex shapes cannot be adequately vented from the patting line.

Molded-in inserts commonly are used with thermosetting materials. However, because the injection process is automatic, the use of post- assembled inserts rather than molded-in inserts is recommended. Molded-in inserts require holding the mold open each cycle to place the inserts. A delay in manual placement destroys the advantage of uniformity and consistency of the automatic cycle, affecting both production and quality.

Tolerances of parts molded by the injection method are comparable to those produced by compression and transfer methods. Tolerances have been held as low as rtO.001 in./in., but ordinarily, tolerances of k0.003 to 0.005 in. /in. are economically practical for produc- tion. See Chapter 28 for recommendations.

All mold techniques and variations such as cams and movable die sections used in other forms of molding are adaptable to thermoset in- jection molding. Therefore, parts can include such features as molded-in side holes, threads, and undercuts. These refinements increase mold cost, of course, so the decision of how many refinements to build into a mold is usually an economics problem rather than a technical one.

COLD RUNNER MOLDING

Standard injection molds for thermoset mate- rials are very similar to standard mold designs for thermoplastics. Because one cannot grind and reuse the thermoset runner system, there is always interest in reducing the amount of ma- terial in a runner system required to produce parts on a production basis.

INJECTION MOLDING OF THERMOSETS 247

SPRUE BUSHING

l v HEATER

CAVITY

CAVITY

Fig. 8-11. Exploded view of a cold runner mani

Toward this end, molds may be designed to maintain thermoset material in a plastic state in the runner without ejecting it from the mold. This cold runner technique is not too different from “hot runner” thermoplastic mold de- signs, except that heated water is used to main- tain a “runner” temperature of between 150 and 210”F, and cartridge heaters are used to maintain proper cavity and force temperatures for curing. This type of mold is known as a “cold runner” thermoset mold, or sometimes a “warm runner” mold (see Fig. 8-11). Parts may be separated from the runners right at the part surface; or short subrunners may be ejected with the parts, leaving most of the runner ma- terial in the warm manifold, to be used in the next shot.

A multicavity standard injection mold for small thermoset parts may have 50 to 150% runner or waste material. Using a cold runner mold, this waste can be reduced to as little as 10% of the shot.

MANIFOLD

MANIFOLD RETAINER

,fold. (Courtesy Stokes Div., Pennwult Corp.)

INJECTION MOLDING REINFORCED POLYESTERS

A popular material increasingly used in ther- moset injection molding is polyester reinforced with glass or other fibers. This material gen- erally is referred to as bulk molding compound (BMC) in the United States and as dough mold- ing compound (DMC) in Europe. Because it is doughlike in consistency at room temperature, it does not feed from a hopper but requires a special stiiffing mechanism on the injection ma- chine for automatic molding.

Although BMC often is molded by using a conventional reciprocating screw, many BMC injection machines use a simple plunger to force the material from the barrel to the mold. The plunger is used to lessen shear degradation of the fibers as well as to prevent back slippage of material during the injection and hold part of the cycle.

The stuffer mechanism includes a loading

248 SPI PLASTICS ENGINEERING HANDBOOK

chamber, which is generally manually loaded with up to several hundred pounds of com- pound. The stuffer chamber has a large-diam- eter piston, air- or hydraulically driven, to force the room-temperature material into the injec- tion barrel upstream of the reciprocating screw or downstream of the plunger, depending on which injection method is used.

Because the BMC is already plasticized at room temperature, it needs a minimum of heat- ing in the barrel and relatively low pressures, 6000 to 10,000 psi, for rapid injection. Also, because BMC reaches the molder from the sup- plier in a fully homogenized state, no mixing in the barrel is necessary.

Injection Molding Presses

There are a number of injection molding presses specifically designed to handle thermoset polyester operations. All require stuffing cyl- inders because of the physical characteristics of the material. Most FRP is puttylike (BMC) or a fiberlike coated material, neither of which flows freely through normal hopper systems (Fig. 8-12).

On some machines the material or compound is forced from the stuffer cylinder from the top or side into the rear of a conventional screw injection cylinder. The screw acts only as a conveyor that moves the material to the front of the cylinder instead of providing a plasticat- ing function. Then, the screw acts as a plunger. It does not turn as it pushes the material into the mold.

On other machines a plunger instead of a screw pushes the material into the mold. This type densifies material as it pushes it into the mold cavities.

Screws and plungers can be interchanged within the same machine frame, requiring only changes in electricals.

Another type is the coaxial plunger machine. Material is dropped into a stuffer cylinder in- line with a smaller cylinder that pushes the ma- terial into the mold. Advantages claimed for this structure include the short distance that the material has to flow. Angles and comers around which the material must move have been elim- inated.

PLUNGER T Fig. 8-12. Sketch of material stuffer for BMC materials. The stuffer usually consists of a cylinder with a plunger that maintains pressure against the polyester material while the screw is rotating. When the stuffer cylinder is nearly empty, the plunger is withdrawn and additional material is deposited into the stuffer. (Courtesy Stokes Div. , fennwult Corp.)

Regardless of their basic design, machines should be capable of accurately controlling temperatures and pressures throughout the molding cycling with as little glass degradation as possible.

Temperature Control

The object of controlling temperatures is to warm the material in the barrel enough to per- mit free flow to preheat it as it goes through the nozzle and runners, and to cure it as fast as possible after it has stopped flowing in the cav- ity. Because of the temperature sensitivity of thermoset materials, control is very critical. Typical settings are: rear zone of barrel, 125"F, front zone, 150°F; cold runner, 175°F; and both halves of the mold, 335°F. As a practical limit, material coming out of the nozzle should not be more than 200°F. The mold tempera-

INJECTION MOLDING OF THERMOSETS 249

Fig. 8-13. Schematic of sprueless mold for BMC polyester injection molding. (Courtesy Hull Corp.)

tures for most materials are between 300 and 400°F.

Experience shows that the cure time of a i- inch section should be within 15 to 45 seconds, depending on part size.

Other typical press conditions are screw rpm, 20 to 75; back pressure up to 100 psi; injection time 1 to 5 seconds; injection pressure, 5000 to 20,000 psi during injection, and as little as half that during cure; and clamp pressure, 3 tons/sq. in . max (based on cavity projected area).

Besides presses specifically designed for thermoset molding, conversion kits also are available to change over machines originally designed for thermoplastic operations.

Tooling requirements are similar to those for thermoplastic injection molding. The major dif- ference is that adequate provision must be made for heating the molds. Molds should be fully hardened, polished, and chrome-plated. Tool steels such as AIS1 H13 are recommended.

Another significant difference between ther- moplastic and thermoset injection molds is the need for harder runner, gate, and cavity sur- faces because of the extreme abrasiveness of most thermoset materials. Typical hardnesses are 56 to 58 Rockwell C, and such surfaces are often hard-chrome-plated for further resistance to wear.

The low-shrinkage characteristic of BMC makes ejection of pieces more difficult than with thermoplastics. Liberal draft, where pos- sible, and provisions for positive ejection on both sides of the mold generally are required.

Properly hardened molds should wear well, but gate areas are susceptible to wear. Design- ing them as replaceable inserts makes their re- pair and replacement less costly.

Gates should be located so as to minimize effects of knit lines that form after material has flowed around an obstruction in the mold. Also, multiple gates should be avoided to reduce the number of knit lines that form where materials

250 SPI PLASTICS ENGINEERING HANDBOOK

from gates join. Eliminating knit lines reduces chances of rejects. Large-area and short-length gates and runners give the strongest parts.

The mold should permit the material to flow so that it pushes air ahead of it into places where it can be vented out at parting lines or knockout pins. Vacuum extraction of air is another useful technique for venting.

Sharp comers, restricted orifices, and gates that cause glass degradation should be avoided if possible.

Because low-shrink BMC can stick in con- ventional sprue bushings after curing, cold run- ners and sprues can be used. These elements can be oriented in different ways, but in all

cases they are water-cooled to somewhat below mold temperatures. A typical temperature for them is about 175°F. Thus the material ac- tually gets a useful amount of preheating in the “cold runner.” Runners should have relatively large diameters to provide for easier material flow and less degradation. Center-gated, three- plate molds should be considered for parts re- quiring maximum impact strength if cold run- ners are not used.

Problems of flashing common to compres- sion molding to not occur with properly de- signed and maintained injection molds. The lit- tle flash that occurs is paper-thin and easily removed.