Compression molding has been used to make plastic parts since Leo Baekeland introduced phenolic resins in 1910. It is the oldest process used to mold plastic parts.A phenolic is a thermoset resin—when introduced to heat and pressure, it cures to become a plastic part. Thermoset parts have an irreversible chemical crosslinking and cannot be remelted. Thermoplastic materials are compression molded and commonly used in the orthopaedics industry. A thermoplastic material becomes a liquid when heated and freezes to a hardened state when sufficiently cooled.

 

Compression molding requires a vertical press that is usually a hydraulically actuated and an integrated temperature controlled mold. The mold usually consists of two halves—an upper force half and a lower cavity half, or vice versa. It is precisely machined from tool steel to form the desired geometry of the molded part. The press can be up-acting or down-acting. With an up-acting press, the hydraulic cylinder is attached to the lower press platen and moves upward, guided by four identically machined tie bars to close the press in a parallel condition (See Image 1).  In a down-acting press, the hydraulic cylinder is attached to the upper press platen that moves downward to close the press.  In many cases, a press with two daylights is used to double the production.

 

During operation, a preweighed amount of molding material is placed directly into the lower cavity of the temperature-controlled mold. The material can be in the form of powder, granules, pellets, putty-like mass, or preformed blanks. In the case of thermoset materials, the mold is heated from approximately 300º to 350ºF. The heat from the mold softens the material to a viscosity low enough so that the material can flow when pressure is introduced. The press closes at a high speed until just before the point at which both mold halves make contact with the charge of material to be molded. The final closing speed is slower to prevent damage to the mold and to profile the velocity of the close as the charge of material deforms and fills the cavity to achieve the optimum material flow. Once the press is fully closed, the molding material takes the shape of the cavity (see Image 2). With the heat of the mold and the pressure of the press (typically 1–2 tn/sq in. of molded surface area at the parting line of the mold), there is an irreversible chemical crosslinking. The material hardens and in effect becomes a plastic part. With thermoplastic materials, the mold is cooled to allow the part to be hardened. Cycle times can range from 60 seconds to several hours depending on the material used and the thickest wall section of the molded part. Upon cycle completion, the press is opened. In many cases, ejector pins in the mold are actuated either mechanically or hydraulically to lift the molded part away from the cavity, so an operator or a robot can easily remove it from the mold.

 

Some typical compression-molded plastic parts include circuit breakers, insulators, closures, cookware handles and knobs, fuel cell plates, appliance parts, dinnerware, automotive hoods, fenders and valve covers, and interior aircraft parts. Materials used for compression molding include epoxies, urea, melamine, phenolic, polyester, polyimide, polyamide-imide, PEEK, and various fiber reinforced thermoplastics. Many rubber parts are also compression molded. In the orthopaedics industry, various knee, hip, and shoulder replacement joint parts are compression molded out of medical grade ultrahigh molecular weight polyethylene (UHMWPE). UHMWPE is a thermoplastic polyethylene that was first introduced as an orthopaedic implant material in 1962.² The odorless and nontoxic material has high impact strength and many desirable features that make it the choice for molding replacement joint parts. It has low moisture absorption, a low coefficient of friction (comparable with that of Teflon), is self-lubricating, and is resistant to abrasion. It exhibits good wear, an important factor in determining the life span of orthopaedic implants.² 

 

As one of the lowest-cost methods of molding, compression molding wastes little material and has good surface finish. With joint replacement parts, a typical part surface finish might be equivalent to an SPI B-1 finish. Fewer knit lines are produced, and the process can yield better impact and flexural strength in the molded part than can be achieved with injection molding. Normally, any high-volume part would be a candidate for injection molding. However, the high molecular weight of UHMWPE affects the melt viscosity to the point that conventional thermoplastic material processing equipment such as injection molding cannot be used.

A disadvantage of compression molding is that some material can seep through the parting line of the mold (called flash). This flash can affix to the molded part and must be trimmed from the part after molding. Another disadvantage is that cycle times are longer than for injection molding.

 

In many cases, blocks or sheets of UHMWPE are compression molded or consolidated, and the desired part is machined out of the block or sheet of molded material. This process prevents excessive tooling costs because replacement joint parts are made in many different sizes. So instead of having 1000 different molds to make various joint sizes, a machining center can be programmed with 1000 different programs to machine the different joint sizes.

 

In other instances, orthopaedic implants are compression molded to their net shape. Called direct compression molding, this process can result in a finished or almost-finished part with a highly polished part surface directly out of the mold. In either case, the properties of the final part can be influenced by the technology available in today’s compression molding presses.

 

A number of factors must be considered when selecting the correct type of press construction for an application. With a heated press, the first consideration is thermal growth of the moving and fixed platen relative to cylinder platen. Since the cylinder platen stays at room temperature as the fixed platen rises in temperature, the press tie bars are no longer parallel.

Another consideration is the problem of clearance required between the guiding and the tie bars, which allows the moving platen to shift freely out of parallelism. The next issue to consider is the possibility of an offset load (the mold or material is not centered in the platen) putting a side load into and deflecting the tie bars, allowing the moving platen to go out of parallelism. The last, but not least, problem is the rigidity of the frame itself. When the offset load is transmitted into the tie bars, the cylinder platen wants to shift due to the side loading, making the connection between the tie rods and platens crucial.

 

There are two methods for constructing presses used for molding joint replacement parts. The first method uses round tie rods. Bronze bushings are inserted into the moving platen and ride up and down on the tie rods as the press is opened and closed. The main problem with this type of guiding is that it is not adjustable. Clearances for bushings must consider thermal expansion conducted from the heat platen into the press platen, as well as manufacturing variance of all of the piece parts and running clearance. To replace a round bushing, the press frame usually needs to be taken apart. Initially, because the bushing diameter is larger than the tie rod, the bearing area is very small, which results in much higher wear until the tie bar wears into the bushing, creating an undesirable result of more clearance.

 

The second method uses rectangular tie rods or slab sides machined with 45° gib guides. This type of guiding is more costly but has several advantages. Contact area is higher and predictable. When used properly, the 45° angle can help offset most of the thermal growth of the fixed and moving platen. The press can be designed so that when the moving platen grows from front to back it increases the guiding clearance relative to the cylinder platen, which is offset by the left-to-right platen growth to maintain good guiding. This type of guiding is simple to replace and is adjustable, so that clearances can be run tight with easy adjustment for wear. Regardless of the type of construction used, it is important that the press closes in a parallel condition to mold high-quality parts. Parallelism across the platen within 0.002 in. is desired when molding joint replacement parts.

 

The construction of the press must also be robust enough to keep deflection caused by the clamp to a minimum, which is usually no more than 0.0015 in./ft of platen area. In addition to the thickness of the press platen, another way to reduce deflection is to use multiple clamp cylinders to spread the load out over the press platen area as shown in Images 1-3.

 

The basic theory of compression molding thermoplastics is that the materials are heated above their melting points, formed, and then cooled. When using medical-grade UHMWPE to make joint replacement parts, the material comes in powder form and must be consolidated. The proper consolidation of the material is a direct result of the heat, pressure, and time combination used by the processor.³ The equipment is commonly located in a clean and controlled environment to prevent any extraneous debris from getting molded into the part.

 

In many cases, because of the long cycle times, double-daylight presses are used. A double-daylight press allows for two molds in the press and doubles the output of the press. In other cases, vacuum presses, which can also have two daylights, are required.

 

During operation, virgin UHMWPE powder is loaded into the mold. This can be done outside the press with the mold placed onto heating and cooling platens in the press after material loading. The press is closed. Pressure and heat are applied at a controlled rate. After several hours, the press is opened and the molded part or parts are removed from the mold. The form and structure of the molded UHMWPE joint replacement part is greatly influenced by the rate and accuracy of the pressure and temperature applied to the mold during the molding cycle. This is where the technology in the modern compression press can be the key factor in producing high quality parts.

 

The long molding times are necessary to maintain slow, uniform heating and cooling rates and pressure ramping throughout the cycle.  The most effective results are achieved when the ramping accuracy of the pressure throughout the entire tonnage range is within 450 lbs. on a 75-ton press (accuracy within 0.3%). Until now, such tight pressure accuracy was unheard of in a hydraulic press. The hydraulic system must have a number of features to achieve this accuracy. Among these features are the following:

The pressure accuracy of the hydraulic clamp is calibrated with portable load cells at ambient temperature. The pressure works in conjunction with the temperature to achieve the optimum molded part. Because UHMWPE is a thermoplastic material, it must be heated and then cooled. With orthopaedic parts, the mold is not normally equipped with direct heating and cooling. The heating and cooling is done with heating and cooling platens in the press. Each daylight has top and bottom heating/cooling platens that conduct the temperature into the mold. The press is equipped with a separate, dedicated controller that specializes in heating and cooling. This dedicated controller communicates with the programmable logic controller and human interface of the press,so the heating and cooling values can be input on the color touch screen and are a part of the recipe set up for a particular mold. Heating in the platens is done with electric Calrod heaters that are fitted tightly into holes drilled in the platen. The dedicated temperature controller senses the platen temperature with thermocouples in the platen and sends the proper amount of current to the heaters to achieve the desired heat. The heat platen is typically heated to 450ºF. The wattage and arrangement of the Calrod heaters in the platen must be calculated to achieve heating that is within ±10ºF across the surface of the platen. The platens are heated from ambient temperature to 450ºF at a controlled rate. The platens are also gun-drilled with passages for cooling air and water. To start the cooling cycle, the electric heaters are turned off. Cooling begins with the dedicated heating/cooling controller, sending a signal to the compressed air and water solenoid valves to send the proper amount of air into the cooling passages. Then based on the desired rate of cooling, it is followed by an air/water mix, and then water. With the accuracy of the controller, the rate of cool down can be controlled to the optimum rate that is most beneficial to the molded UHMWPE part.

 

During the past several years, vitamin E has been blended with UHMWPE powder for molding orthopaedic insert parts. Vitamin E is an excellent biocompatible stabilizer for UHMWPE as it goes through the molding process.⁴  However, exposure to oxygen during the molding process can break down vitamin E. To eliminate this problem, the material is molded in vacuum presses. A vacuum press can have all of the features of a conventional press except that is constructed with solid-steel side plates (slab sides). These take the place of the tie rods. The moving platen rides up and down the slab sides on 45º gib guides. Front and back sealed doors are attached to the slab sides to create an air-tight chamber (see Figure 1). A vacuum pump draws down the press/platen chamber to a programmed Torr value that is best for the molded part. The vacuum can be maintained for the entire length of the cycle.

 

In addition to controlling press movement, pressure, and temperature, there are other valuable tools on modern press control systems, such as real-time graphing, data collection, and Ethernet capability. Real-time graphing (RTG) is a Windows application that displays a graph of process data in real time and collects and archives process data for every machine cycle. When the machine cycle begins, a real-time graph collects each data point (i.e., position, pressure, and temperature) 20 times per second and saves the information when the cycle ends. A new data file is created for each machine cycle. RTG allows the operator to see exactly how the press is performing at any time during the cycle. The user can go back and observe the data collected if there is ever a perceived problem with a molded part.

 

Compression molding joint replacement parts is a time-consuming and technically demanding process. To mold high-quality orthopaedic parts, it is essential that the selected compression-molding press has all of the necessary features to meet process challenges. Those features include very fine pressure control, accurate temperature control, tight parallelism, and low deflection. Other helpful features that will enable the user to keep track of the manufacturing process include real-time graphing, Ethernet capability, and data collection. There may be other features required of your press. It is a good idea to work closely with the press manufacturer to discuss the properties you will require in the finished molded parts.  Choosing a press manufacturer with experience building presses for joint replacement parts can help. Their knowledge can assist in designing and building a press that will produce high quality joint replacement parts for many years.

 

1. B Davis et al., Compression Molding (Cincinnati: Hanser Gardner Publications 2003),  1 – 18, 

2. M Allen, “Perplas Medical UHMWPE Processing Techniques and Problems,” (paper presented at UHMWPE meeting, University of Torino, Italy,  September 19, 2003).

3. MB Turell et al., “New Processes to Improve the Mechanical Performance of UHMWPE,” (Harvard Harvard Medical School, March 18, 2005, presented  at UHMWPE meeting, University of Torino).

4. P Bracco, “Stabilisation of UHMWPE with Vitamin E Chemical Mechanisms,” (paper presented at Fourth UHMWPE International Meeting, 2009).