Compression Molding

    Compression molding, also called matched-die molding, is a molding process used almost exclusively for molding thermoset materials. The other processes discussed thus far are used chiefly for thermoplastic materials (extrusion, injection molding, blow molding, thermoforming, and rotational molding) or for both thermoplastics and thermosets (casting and foaming). When thermoplastics are molded using methods similar to compression molding, these methods are usually called by separate names, such as cold forming, sintering, or ram extrusion (discussed later in this chapter). 

18.2.1. Concept

    The principles of compression molding are quite simple. A charge of thermosetting resin is placed in the cavity of a matched mold that is in the open position. The mold is closed by bringing the male and female halves together, and pressure is exerted to squeeze the resin so that it uniformly fills the mold cavity. While under pressure, the material is heated, which causes it to crosslink (cure) and to harden. When the material is hard, the mold is opened and the part is removed. This process can be compared to waffle making. 

    Some products made by compression molding are shown in Photo 18.1. Notice that the parts can be reinforced. For example, the helmet and the automotive body panels—Or might be nonreinforced—the other parts shown. The parts might be large or small; they can be rigid or made of flexible rubber or silicone. Clearly, the compression molding process can be used for a wide variety of products. 

18.2.2. Machines and Molds

    Figure 18.1 is a diagram of a compression molding machine, which shows the most important elements of the compression molding process. A compression molding machine is shown in Photo 18.2. 

    The molding machine itself (sometimes called the compression press) consists of a heavy metal base onto which slide rods and a compression unit that slides up and down on the slide rods are attached. These slide rods guide the movement of the compression assembly from its open to closed positions. The movement of the compression assembly and the force to clamp the assembly against the base are supplied by a hydraulic unit mounted above the compression assembly. This action can be fully manual (with the hydraulic pressure supplied by a manual pump), semiautomatic (where a pressure valve is activated by an operator-controlled switch), or fully automatic (where time is the triggering factor). Note that the unit shown in Figure 18.1 closes downward (called a downstroke machine), but could be reversed so that the compression unit is on the bottom and 467 therefore closes upward (upstroke machine). Compression machines are normally very rugged and massive and often last for several decades. 

Photo 18.2 Large commercial compression molding machine. (Courtesy of Wabash Hydraulic Presses) 

    The physical opening between the base and the compression assembly is called the daylight opening and is an important factor in choosing the proper compression molding machine. The daylight opening should be sufficient to accommodate the platens and the molds and leave space for loading and unloading the mold. This can be a problem in some cases where the part is deep and the vertical height of the molds is great. 

    Compression molds are subjected to very high pressures, perhaps greater than those in any other type of plastic-molding process. Therefore, compression molds tend to be built on rugged, massive plates that can support the mold and withstand the pressures of the mating of the molds. A typical mold set is shown in Figure 18.2. The mold base plates (platens) are attached to the base and the compression unit of the press with large anchoring bolts. These platens are so large that they allow coring for insertion of cartridge heaters or some other convenient heating system such as oil or, in some cases, steam. Care should be taken in choosing a heating system to ensure that the platens' heating capacity is sufficient to heat the molds to the proper temperature and to maintain that temperature during the molding cycle. The molds are attached directly to the platens. Sometimes the platens have cutout zones for the insertion of the upper and lower interchangeable mold cavities, but in other cases the molds are simply bolted onto the surfaces of the platens. 

Figure 18.2 Compression molding mold set. 

    The parts are normally ejected from the mold by using knockout pins or ejector pins. These can be activated by a small hydraulic cylinder or, if the knockout mechanism is placed within the compression assembly, the movement of the compression assembly away from the base can activate the knockout mechanism. To further facilitate part removal, the mold is coated with mold release and then, if sticking occurs, additional mold release agent is sprayed into the mold to assist with part removal. 

    The maximum force required for a particular molding operation influences the choice of a compression press. The total force that can be exerted by the machine is called the press capacity, machine rating, or machine size and is stated in force units: newtons (N) in the SI system or tons in the English system. This machine capacity is a function of the area of the hydraulic ram and of the hydraulic pressure in the line going from the hydraulic pump to the hydraulic cylinder. The relationship between these hydraulic factors and the press capacity is given by equation (18.1). [When English units are used in equation (18.1), the conversion fact or, 2000 (pounds/ton), is used because the size of a machine is typically expressed in tons. When SI units are used, the press capacity is usually expressed in newtons (N) and no conversion factor is needed because pascals (Pa) are equal to N/m2 .] The press capacity is usually specified by the equipment manufacturer, and the operator generally will not be aware of either the area of the ram or the hydraulic pressure. 

Equation 18.1

In metric units:

Equation 18.1 in metric units

    The force required to mold a particular part can be determined and then compared with the press capacity. If the force required is less than the press capacity, then the press can be used for that particular part. When selecting a machine for a particular molding job, a safety factor (approximately 1.3 times the calculated force required) is usually employed. The force required depends on material characteristics and part geometry. Sufficient force must be exerted on the material to consolidate it and move it into all regions of the mold. Low-viscosity materials require less pressure than do those with high viscosities. Because of the complex nature of this material dependency, it is difficult to develop a precise mathematical expression for determining the pressure required to mold a part. Sophisticated models have been created that give approximations of the pressure required as functions of the material parameters (such as viscosity), but these are used mainly for research purposes rather than for production development. 

Figure 18.3 Compression mold for simple box showing projected molding area and depth.

    However, there are approximation factors, developed experimentally, that permit some simple calculations to relate the force required for molding and the size of the part. These factors indicate the approximate pressures required to move a particular material laterally across the area of a mold cavity. The typical range of pressures for consolidation is 1.5 ksi (1 ksi = 1000 psi) to 8 ksi (10 MPa to 55 MPa) for most common plastic resins. Parts that are more than 1.5-inch (4 cm) deep require an additional amount of pressure to push the material into regions that are deep and narrow. This additional pressure factor is approximately 3 to 4 MPa per additional 3 cm of depth. (If English units are used, the added pressure requirement for depths beyond the first 1 inch is approximately 500 to 750 psi per additional inch of depth.) 

    As the size of the part increases, the force required to mold that part also increases. The part size is defined in terms of the projected area, the area of the cavity using its maximum width and length. If the mold has more than one cavity, then the total projected area is the sum of the projected areas of all the cavities. For shallow parts [usually less than about 1.5 inch (4 cm) in depth], the part size is simply the total projected area of all the cavities. The method for determining the projected area is illustrated in Figure 18.3. The relationship between the force required and the part size for a given material is represented by equation (18.2), 

insert equation 18.2.

where F is the force required to mold the part, A is the projected area of the part, PA is the required cavity pressure for a particular material (PA is usually developed experimentally), ρ is an additional pressure factor (developed experimentally) that accounts for the effect of depth, and de is the depth in excess of the minimum amount. Calculations for determining the size of an actual machine to be used for molding a part are illustrated in Problem 18.1. 

Problem 18.1

18.2.3. Mold Closure Types

    Three types of mold closures are typically used for compression molding: the flash type, the positive type, and the semi-positive type. These mold closures differ in the way the two mold halves mate, illustrated in Figure 18.4. 

    The flash-type mold closure is the simplest of the three types and the least expensive to build. In this closure, the cavity is simply overfilled slightly and the excess material is squeezed out into the gap between the male and female mold halves. When cooled, this excess material is called flash and must be removed from the part. If insufficient material is charged into the mold, the part will be too small. The flash-type closure works best for shallow parts and parts where dimensional control and property control are not critical. 

    The positive-type mold closure differs from the flash type because of its long shearing surface between the male and female halves. This long shearing surface is made when the walls of the plunger and the walls of the cavity are very close together for a considerable distance. The gap between them is kept small so that material will not leak out. Hence, no flash is created. The amount of material charged into the cavity must be carefully controlled so that the mold is filled, but not to excess. If excess material is charged into the mold, the mold will not close. If too little material is charged, a short shot will result. 

    The semipositive-type mold closure conveniently combines the features of the other closure types. In the semipositive system, a horizontal land creates a gap between the plunger and the cavity, a gap considerably larger than that in a positive closure mold but still quite small. If any excess material is in the cavity, this excess moves along the horizontal land and up the vertical land, which also has a greater gap than exists in the positive mold. The result is a pushing of the excess material past the gap regions into a horizontal area where it can easily be removed from the mold. Thus, the part is essentially flash-free. 

    Compression molds are most commonly made of tool steels and stainless steels. Because of the high temperatures normally used, H13 steel is very common. Also common are the hardenable steels because hobbing continues to be an important method for making cavities and is best done in a steel that later can be hardened. But although hobbing is still used for making the cavities, machining continues to be the most important method. EDM is gaining in popularity, but is not as common for compression molds as it is for injection molds, perhaps because of the often larger size of compression-molded parts. 

    Chrome plating is commonly used in compression molds to give parts a mirror finish. This also reduces the cavity abrasion that results from the reinforcements commonly found in thermoset materials molded in compression molding. 

18.2.4. Comparisons of Compression

Molding with Other Molding Processes 

    Compression molding is most obviously comparable to injection molding. Both processes produce discrete parts of widely differing geometries. Compression molding uses thermoset resins as starting materials, whereas injection molding uses thermoplastics. This difference dictates very different mold conditions. In compression molding, the mold is heated so that the thermoset material will cure. In injection molding, the mold is cold so that the melted polymer will freeze. Since curing usually takes longer than freezing, the molding cycle for compression molding is longer than for injection molding. Small compression-molded parts might have a molding cycle from 1 to 2 minutes, whereas injection molding cycles for the same-size part might be from 20 to 60 seconds. The injection molding process can be highly automated, virtually working without the need of operator intervention. Although compression molding cannot reach this same level of automation, it can be semi-automated, with many of the steps carried out with only minor operator involvement. 

    Another consequence of making thermoset plastic parts by compression molding is that reject parts cannot be reprocessed. Once thermosetting has occurred, the part cannot be remelted. Recycling of these parts usually involves grinding the part and using the regrind as a filler. 

    The complexity of the parts that can be molded also differs. Compression molding is limited to shapes that can be made by charging into an open mold. Parts that have hollow sections and other features that require cores and slides are therefore very difficult to make. Undercuts are also difficult to make by compression molding but they can be done with sliders. However, the simplicity of the normal compression molds leads to much lower mold fabrication costs. Extensive analysis and machining of sprue, runner, and gate designs are nil.

    Charging into an open mold does have some advantages. There are no sprues or runners as in injection molding. The part can be used directly with, perhaps, only a flash removal step. Another advantage of open molds is that the distance the resin moves during the molding cycle is much shorter than in closed molds. This shorter path means less orientation of the resin molecules and, therefore, fewer problems with changes in physical and mechanical properties because of the molding process. The short path also allows high filler and reinforcement content and longer reinforcements than could be used in a closed mold process like injection molding. These comparisons are summarized in Table 18.1. 

Table 18.1 Comparison of Compression Molding, Transfer Molding, and Injection Molding