Compression Moldings

Compression molding is an example of a simple process which does not require expensive equipment but produces cheaply and in simple shapes. Unlike most other processing methods, compression molding does make materials inexpensive through the use of large quantities of fillers. Compression molding applies unidirectional forces usually perpendicular to the material s surface which may thus affect filler orientation. 

These effects depend on filler concentration and its interaction with rubber. Pressure causes flow in the free rubber which has lower viscosity than rubber which has interacted with carbon black particles. As a result particles may or may not achieve a favorable distribution. If they fail to do so, the resistance is high. 

Compression molding is the method by which massive (700 kg), cylindrical (billet), rectangular, and sheet shapes of PTFE are produced. The blocks and cylinders can be solid or annular and are by far the heaviest objects produced from any fluoropolymer. The height of a cylinder can exceed 1.5 m. These billets are cut (skived) into wide thin films ( 0.5 mm thick) or sheets (7 mm thick). Sheets, blocks, and cylinders are utilized as stock shapes for machining more complex shapes. The same principles are applied to mold any shape. 

Compression molding of a closure one-half of the mold is shown 

Compression molding is a major technique for the construction of fiber-reinforced polymers. It is a high temperature and high-pressure process, which is widely used for all thermosetting polymers (polyester, epoxy, etc.). It is cost effective for high nms. 

In the hand lay-up technique the fibers are placed in a mold and the resin is later applied by rollers. One option is to cure using a vacuum bag. This removes the excess air and the atmospheric pressure exerts pressure to compact the part. The simplicity, low cost of tooling and flexibihty of design are the main advantages of this procedure. On the other hand, the long production time, intensive labor, and low automation potential are some of the disadvantages. 

A compression molding machine is composed of stationary and movable molds. For the preparation of PE composite, the polymer is mixed with other fillers/fibers etc. and is keep between upper and lower mold. Then, the mold is closed and heat and pressure are applied to obtain a homogeneously shaped composite. Slow cooling or rapid cooling (quenching) can be apphed at the end of holding time. 

To characterize polyethylene and its composites, a large number of techniques can be used such as Differential Scanning Calorimeter and 

Thermogravimetric Analysis to study the thermal properties. Scanning Electron Microscope and Transmission Electron Microscopy to analyze the morphology of the materials, Atomic Force Microscopy to carry out a surface analysis and Dynamic Mechanical Analysis (DMA) to evaluate the mechanical properties, etc. Also, it is important for the study of such properties as rheology and X-ray diffraction. 

Polyethylene-based composites can be tested for mechanical characterization such as tensile, flexural, impact and hardness. All tests should be carried out by using international standards, such as ASTM. 

The compression molding process is a manufacturing approach in which precharges of fibers and pol3miers are compressed in a 

TABLE 10.1 Polymer Materials Often Used in Microinjection Molding, Moid Temperature (Micromolding/Conventional Molding), Maximum Aspect Ratios (ARs), 

Minimum Structurai Thickness (Smin), and Typical Applications 

Polymer Mold Temperature Micro/Conventional Mold (°C) Aspect Ratio (AR) Minimum Thickness, 5 min (//m) Application 

PMMA PC 60-140/90-110 20 20 Optical fiber connector Cell container 

Before leaving this section, we should make a few comments on the size and features of //IM machines (see Giboz et al., 2007). Typically the screw ranges from 5 to 16 mm in diameter. Some systems use a screw to plasticate the polymer and a plunger to push the melt into the mold. The clamping force mns from 49 to 147 kN, the injection capacity is from 0.082 to 10 cm, the injection pressure is from 1700 to 3500 X 10 Pa, and the injection speed is from 180 to 1200 mm/s. 

Three high volume technologies were used to produce composite parts compression molding, filament winding, and pultrusion. Furthermore, two compression molding techniques were used, one suitable for continuous fiber-reinforced and another for long chopped fiber-reinforced composites (LFTs). 

In this process, the polyurethane mix is poured into a mold that can be placed into a compression molding press with heated platens. The material is allowed to gel, and a top plate is placed on the material. The molding press is fully closed. The pressure applied is normally on the order of 1.50 to 2.0 MPa (250-300 psi) based on the projected area of the molding e.g., a parts plan area is 300 x 400 mm in size. Its area is 0.300 x 0.400 m2 = 0.12 m2 therefore, a pressure of 1.500 x 0.12 = 0.18 MPa on the mold is required. The molds must be capable of withstanding the direct heat and pressure of the press. Aluminum or steel molds are preferred for this process. 

Air vents are not strictly required in this process, but it may be advantageous to briefly release the pressure and then reapply it to the molding. The exact time to apply the pressure varies from system to system and must be determined by experimentation. 

There are several key points in these methods to allow proper success. The pot, and if possible the sprues, must be mold-released to ease demolding. The sprues must be designed so that the remaining pad in the pot can be cut off. The taper of the sprues must be such that they are wider on the part side than the pot side. It is normally best to trim the sprues and material in the vents straight after demolding to ease the later trimming and cleaning of the part. 

The matrix can be either a thermoset or thermoplastic. The oldest and still widest used material for compression molded products is phenolic. The thermoset materials used to manufacture fiber reinforced compression molded articles is unsaturated polyester sheet or bulk, reinforced with glass fibers, known as sheet molding compound (SMC) or bulk molding compound (BMC). In SMC, the 25 mm long reinforcing fibers are randomly oriented in the plane of the sheet and make up for 20-30% of the molding compound s volume fraction. 

FIGURE 2.22 Fabrication protocol for silicon-template-embossed devices [1011]. Reprinted with permission from the American Chemical Society. 

For proper embossing, the PMMA was baked at 80°C for 8 h to reduce the absorbed water to 0.1% (from - 0.4%). During embossing, the molding die was heated to 150°C and pressed into the PMMA wafer with a force of 4000 lb for 4 min at 160°C. During demolding, the PMMA wafer was maintained at 85°C [208], 

Besides PMMA, compression molding was also used to fabricate microstructures on PC chips (1 mm thick). High temperature (188°C) and pressure (11 metric ton pressure applied by a hydraulic press) were used. Before bonding, the hydrophobic channel surface was treated with UV irradiation (220 nm) to increase surface charge, which would assist aqueous solution transport. The molded chip was thermally bonded to another PC wafer. During use, the bonded chip did not yield to a liquid pressure up to 150 psi (134°C, 4 metric ton, 10 min) [938], 

Moreover, a Zeonor plastic plate, normally used to manufacture CDs and DVDs, was hot-embossed (130°C, 250 psi) using a Si master. The embossed chip (with microchannels 60 im wide and 20 pm deep) was thermally bonded to another Zeonor plate (85°C, 200 psi, 10-15 min) [808], In another report, a 2-mm-thick cyclo-olefin (Zeonor 1020 R) substrate was embossed using a Si master to create 20-pm-wide and 10-pm-deep channels [788], 

Hot embossing was also used to make polymer chips on PMMA [149,211,215], polystyol [103,210], or PC [211,212], 

During the preform-heating part of the cycle, the main problems to be considered are heat transfer and flow (or elastic deformation) of the compressed particulate matter. Referring to Fig. 13.45, the heat transfer problem can be described with the following form of the energy equation  

The heat transfer problem just discussed can be solved in a fashion similar to the one used in Section 5.3, to yield T(z, t). In principle, once the temperature field is known in the preform at any time before fr, the plunger force can be calculated. The preform can be taken as a solid that slips at the mold surface and has a temperature-dependent compressive modulus. At any time t tf, each layer of the preform will deform by an amount such that (a) the force on every layer of thickness Az is the same (and equal to the unknown quantity), and (b) the sum of the compressive deformations of all the layers equals the deformation imposed on the preform by the plunger at the given time. The force 

Assuming that T(z, tf) = Tw, we can deal with the flow problem alone for tj t tc. Referring to Fig. 13.45, as long as the preform radius is less than Ro, we can treat the problem as an isothermal radial flow of an incompressible fluid flowing between two disks that approach each other at a constant rate h. Recapturing the results for the velocity field, pressure distribution, and plunger force needed to squeeze an isothermal Power Law fluid at a constant slow or moderate squeeze rate, we have 

The plunger force FN can be calculated from the preceding equation to give 

Since the fluid is incompressible and nonreactive at this stage, its volume is constant, implying that 

This equipment has a pair of upper and lower endless belts (5) (6), facing each other, which are tensed by rolls (1) to (4), pressing plates (7) for pressing a material mixture via 

The heating means (11) in the pressing plate may be achieved by first providing a heat insulating portion (13). A preheater (12) is also available. The pressure from a hydraulic cylinder (not shown) is communicated from a hydraulic piston (15) to the pressing plate, and then applied as compression force to the mixture via the rollers and the endless belts.  

Material is supphed from hopper (14) and then caused to drop on the lower endless belt.  

The process temperatures are veiy low in the range of 50 to 140°C. Also, applied pressure forces are very low in the range of 0.1to20kg/cml  

The above shows that plasticizers are less influential on processing results than in technologies in which material must flow in order to fill some shapes in processing equipment. But some technological influences of plasticizers are hsted below with their influences on material properties  

It is assumed for simplicity that the mold plates are circular in the first instance and have a radius Rai. An initial separation Ho is maintained for times t 0, and at r = 0 a constant normal load is iqiplied to the upper plate. Often it is of interest to determine the function hi(t) inificating the separation distance between the two plates at different times of r 0. Such a flow can be analyzed after assuming a quasi-steady-state and lubrication roximation for a power-law non-Newtonian fluid as done by Scott [19] to give the relationship among the force, the flow geometry, and the material flow parameter. Shenoy and Saini [29] used Eq. (8.6) to modify the derivations of Scott [19] and present the results in Ae following modified form. 

For the case when the molding plates are flat strips instead of circular disks the equation suggested by Oliver [30] is used and written in the modified form [29] as 

Shenoy and Saini [32] have provided a simplified approach to the prediction of primary stress difference in polymer melts as discussed in Giapter 5. This is based on the relationship between the unified normal stress difference function and the unified viscosity function curves through a strain-dependent single- 

Using Eq. (6.3), the above equation can be written as Thus, the relaxation time can be estimated as 

it may be concluded that in the compression-molding operation of UHMWPE, one need not be concerned about the effects of elastic forces and, hence, it is possible to describe the entire mechanics througih Eqs. (8.16) or (8.18), which are dependent on geometric parameters and a single-point rheological characteristic value of MFI. 

Flash type Semi-positive type Positive type 

Variation in raw material charge weight results in variation of part thickness and scrap. 

Dimensions in the direction of the mold opening and the product density will tend to vary more than those perpendicular to the mold opening. 

Flash molds do not require that the quantity of material is controlled. 

Tumbling may be required as a finishing process to remove flash. 

FIGURE 14.17 Flash mold open, with preform in place, and closed, with flash forced out between mold halves. (Data from Vaill, E. W., Mod. Plast., 40, 767, September 1962.) 

Compression molding is used predominantly for large flat or curved auto parts such as hoods and fenders and with thermosetting resins. Because compression molding of a thermoplastic resin involves heating and then cooling the mold, most such materials are more efficiently molded in the injection machines described in Section 14.6. 

---

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

Compression-molded specimens property data on extmded wire specimens are similar. At 23°C... 

Sample 250-pm compression-molded films of Teflon PFA 340 from G.E. resonance transformer 2 MeV capacity, at a current of 1 m A. 

PCTFE plastic is available in products that conform to ASTM 1430-89 Type 1 (Grades 1 and 2) and is suitable for processing into parts that meet MIL-P 46036 (Federal Specification LP-385C was canceled 1988). Standards for fabricated forms are available for compression molded heavy sections (AMS-3645 Class C), thin-walled tubing, rod, sheet, and molded shapes (AMS-3650). PCTFE plastics have been approved for use in contact with food by the FDA (55). 

Eoamable compositions in which the pressure within the cells is increased relative to that of the surroundings have generally been called expandable formulations. Both chemical and physical processes are used to stabilize plastic foams from expandable formulations. There is no single name for the group of cellular plastics produced by the decompression processes. The various operations used to make cellular plastics by this principle are extmsion, injection mol ding, and compression molding. Either physical or chemical methods may be used to stabilize products of the decompression process. 

Type of polymer Extmsion Expandable Eroth Compression mold Injection Sintering... 

Manufacture of friction elements includes the impregnation of fabrics and subsequent lamination, the wet-dough process, and the dry-mix process. Elements from the last two are prepared by compression-molding the formulation for up to an hour at 150—175°C. Thick brake elements require a carefully controUed heating-and-cooHng cycle to minimize stresses created by expansion and contraction (see Brake linings and clutch facings Fillers). 

Wet System Compression M olding. Wet system compression molding was the first high volume method for manufacturing reinforced plastic parts, in such appHcations as the Chevrolet Corvette, industrial trays, tote boxes, luggage, refrigerator liners, and other commercial appHcations. 

Hybrid Processes. There are also hybrid processes that have evolved to meet specific product needs. As an example, automotive leaf springs utilize a filament winding system to prepare impregnated fiber bundles that are then compression molded to final configuration. 

Acrylics processed by compression-molding techniques give dentures as satisfactory and dimensionally stable as the estimated 5% of the dentures made with special resins and elaborate processing equipment (208). Promising fiber-reinforced dentures have been reported (209—212). 

Anode electrode assembly, electrode panel between compression molded gaskets. 

Cathode electrode assembly, electrode panel between compression molded gaskets. A range of gasket, electrode materials, and electrode geometries is available... 

Molding and injection Compression ratio Compression molding pressure (lbf/in. ) Compression molding temperature (°C) Injection molding pressure (Ibf/in. ) Injection molding temperature (°C) Molding qualities Mold (linear) shrinkage (in./in.) Specific volume (Ib )... 

Arc Extinguishing Plastics 1964 E Injection or compression molded and extruded Fuse tubing, lightning arrestors, circuit breakers, panel boards... 

A plate used for holding plugs in place in compression molding. 

Sheet molding compound (SMC) consists of randomly oriented chopped fibers in a matrix of resin and filler. SMC is produced in the continuous manner shown in Figure 1-16. Note that the polyethylene film protects the roller system from getting gummed up with the resin-filler paste. The rug-like rolls of SMC are then used in compression molding machines to create large parts such as the sides of cars and trucks. 

---