Compression Moulding

Compression molding :
         Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly oriented fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Also, it is one of the lowest cost molding methods compared with other methods such as transfer molding and injection molding; moreover it wastes relatively little material, giving it an advantage when working with expensive compounds. However, compression molding often provides poor product consistency and difficulty in controlling flashing, and it is not suitable for some types of parts. Fewer knit lines are produced and a smaller amount of fiber-length degradation is noticeable when compared to injection molding. Compression-molding is also suitable for ultra-large basic shape production in sizes beyond the capacity of extrusion techniques. Materials that are typically manufactured through compression molding include: Polyester fiberglass resin systems (SMC/BMC), Torlon, Vespel, Poly(p-phenylene sulfide) (PPS), and many grades of PEEK.
          Compression molding was first developed to manufacture composite parts for metal replacement applications, compression molding is typically used to make larger flat or moderately curved parts. This method of molding is greatly used in manufacturing automotive parts such as hoods, fenders, scoops, spoilers, as well as smaller more intricate parts. The material to be molded is positioned in the mold cavity and the heated platens are closed by a hydraulic ram. Bulk molding compound (BMC) or sheet molding compound (SMC), are conformed to the mold form by the applied pressure and heated until the curing reaction occurs. SMC feed material usually is cut to conform to the surface area of the mold. The mold is then cooled and the part removed. Materials may be loaded into the mold either in the form of pellets or sheet, or the mold may be loaded from a plasticating extruder. Materials are heated above their melting points, formed and cooled. The more evenly the feed material is distributed over the mold surface, the less flow orientation occurs during the compression stage.
          In compression molding there are six important considerations that an engineer should bear in mind :

  • Determining the proper amount of material.
  • Determining the minimum amount of energy required to heat the material.
  • Determining the minimum time required to heat the material.
  • Determining the appropriate heating technique.
  • Predicting the required force, to ensure that shot attains the proper shape.
  • Designing the mold for rapid cooling after the material has been compressed into the mold.

 ADVANTAGES:

  • Lowest cost molds
  • Little “throw away” material provides advantage on expensive compounds
  • Often better for large parts
  • Lower labor costs
  • Minimum amount of wasted material & Improved material efficiency
  • Internal stress and warping are minimized.
  • Dimensional accuracy & stability is excellent.
  • Shrinkage is minimized and closely reprodcible.
  • Thick sections and large parts are practical.
  • Lower molding pressures allow molding of large parts on presses of lower tonnage.

DISADVANTAGES:

  • Offers least product consistency
  • Not suitable for fragile mold features, or small holds
  • Uneven parting lines present a mold design problem
  • High impact composites make flash control & removal difficult.
  • The depth of the molded holds is limited to 2 or 3 times their diameter
  • Shot weight must be tightly controlled
  • Dimension across the parting line may be difficult to hold but good accuracy may be obtained through tight process control.

Transfer moulding

Transfer Moulding :
          Transfer molding is similar to compression molding in that a carefully calculated, pre-measured amount of uncured molding compound is used for the molding process. The difference is, instead of loading the polymer into an open mold, the plastic material is pre-heated and loaded into a holding champer called the pot. The material is then forced/transferred into the pre-heated mold cavity by a hydraulic plunger through a channel called sprue. The mold remains closed until the material inside is cured.
          Transfer molded parts inherently have less flash (excess material that runs along the parting line of the mold) than their compression molded counterparts because the mold remains closed when the plastic enters the mold cavity. However, transfer molding still produces more waste material than compression molding because of the sprue, the air holes and the overflow grooves that are often needed to allow air to escape and material to overflow.
           One of the key advantages of transfer molding over compression molding is that different inserts, such as metal prongs, semiconductor chips, dry composite fibers, ceramics, etc., can be placed/positioned in the mold cavity before the polymer is injected/drawn into the cavity. This ability makes transform molding the leading manufacturing process for integrated circuit packaging and electronic components with molded terminals, pins, studs, connectors, and so on.
          In the composite industry, fiber-reinforced composites are often manufactured by a processed called Resin Transform Molding (RTM). Layers of textile preforms (long fibers woven or knitted in patterns) are pre-arranged in the mold. The resin is then injected to impregnate the performs. Vacuum is often used to avoid air bubbles and help draw the resin into the cavity. In addition, the resin used has to be relatively low in viscosity.
ADVANTAGES:
  • Loading a preform into the pot takesless time than loading preforms intoeach mold cavity.
  • Tool maintenance is generally low, although gates and runners aresusceptible to normal wear.
  • Longer core pins can be used and canbe supported on both ends, allowingsmaller diameters.
  • Because the mold is closed before theprocess begins, delicate inserts andsections can be molded.
  • Higher tensile and flexural strengths areeasier to obtain with transfer molding.
  • Automatic de-gating of the mold’s tunnelgates provides cosmetic advantages.
DISADVANTAGES:
  • Molded parts may contain knit lines in back of pins and inserts.
  • The cull and runner system of transfer molding leaves waste material, but this scrap can be greatly reduced by injection molding with live sprues and Runnerless Injection Compression (RIC).
  • Fiber degradation of orientation occurring in the gate and runner system reduces the molded part’s impact strength.
  • Compared to compression molding, high molding pressures are required for the transfer process, so fewer cavities can be put into a press of the same tonnage.


Thermoplastic

THERMOPLASTIC:

           A thermoplastic polymer is a type of plastic that changes properties when heated and cooled. Thermoplastics become soft when heat is applied and have a smooth, hard finish when cooled. There are a wide range of available thermoplastic formulas that have been created for many different applications.
          A thermoplastic polymer is made up of long, unlinked polymer molecules, generally with a high molecular weight. Because the molecular chains are unlinked, they rely on other interactions, such as dipole-dipole interactions, aromatic ring stacking, or Van der Waals forces. Thermoplastics generally form a crystalline structure when cooled below a certain temperature, resulting in a smooth surface finish and significant structural strength. Above this temperature, thermoplastics are elastic. As the temperature increases, thermoplastics gradually soften, eventually melting.
          The material properties of a thermoplastic polymer can be adjusted to meet the needs of a specific application by blending the thermoplastic resin with other components. For example, shape memory polymer can be mixed with thermoplastic polymer to create a material that has shape memory characteristics, but retains the basic properties of the thermoplastic. Plasticizers can be added to a thermoplastic polymer to keep the material flexible at lower temperatures. This mixture is often used in plastic automobile body parts to prevent them from cracking during periods of cold temperatures.
        Some of the most commonly found thermoplastic polymers include polyethylene, polypropylene, polyvinyl chloride (PVC), polystyrene, polytetrafluoroethylene (PTFE, commonly known as Teflon), Acrylonitrile butadiene styrene (ABS plastic), and polyamide (commonly known as nylon).
          Because thermoplastics can be melted and reused without any change in material properties, these polymers can be actively recycled. Beverage bottles and household containers with resin identification codes are generally thermoplastic polymers. These containers are ground into chips, melted, refined to remove impurities, and reused as reclaimed material.

Injection Moulding

INJECTION MOULDING:

          This is the most common method of producing parts made of plastic. The process includes the injection or forcing of heated molten plastic into a mold which is in the form of the part to be made. Upon cooling and solidification, the part is ejected and the process continues. The injection molding process is capable of producing an infinite variety ofm part designs containing an equally infinite variety of details such as threads, springs, and hinges, and all in a single molding operation.
          A plastic is defined as any natural or synthetic polymer that has a high molecular weight. There are two types of plastics, thermoplastics and thermosets. Thermosets will undergo a chemical reaction when heated and once formed cannot be resoftened. The thermoplastics, once cooled, can be ground up and reheated repeatedly. Thus, the thermoplastics are used primarily in injection molding.
There are four major elements that influence the process. They are:
• the molder
• the material
• the injection machine
• the mold

         Of these four, the injection machine and the mold are the most varied and mechanically diverse. Most injection machines have three platens. Newer models use just two platens and may be electrically operated as opposed to the traditional hydraulic models. They can range in size from table top models to some the size of a small house. Most function horizontally, but there are vertical models in use. All injection machines are built around an injection system and a clamping system.
The injection system mechanism may be of the reciprocating screw type or, less frequently, the two-stage screw type. Also included is a hopper, a heated injection barrel encasing the screw, a hydraulic motor, and an injection cylinder. The system’s function is to heat the thermoplastic to the proper viscosity and inject it into the mold. As the resin enters the injection barrel, it is moved forward by the rotation of the screw. As this movement occurs, the resin is melted by frictional heat and supplementary heating of the barrel encasing the screw. The screw has three distinct zones which further processes the resin prior to actual injection.
          Injection is accomplished through an arrangement of valves and a nozzle, all acted upon by the screw and the hydraulic pump that pushes the resin into the mold. This so-called “packing action” occurs at pressures from 20,000 to 30,000 psi and higher. The temperature of the resin at this time is between 320o and 600o F. The clamping system’s function is to keep the plastic from leaking out or “flashing” at the mold’s parting line. The clamping system consists of a main hydraulic pressure acting on the mold platens and a secondary toggle action to maximize the total clamping pressure.
The platens are heavy steel blocks that actually hold the mold tightly closed during the injection phase. Most injection machines have three platens. The “stationary” platen has a center hole that receives the injection nozzle and holds the cavity half of the mold. This platen also anchors the machine’s four horizontal tie bars. The “movable” platen holds the core half of the mold. This platen moves back and forth on the tie bars and as the mold opens, the mold’s ejection system of pins and posts expel the finished part. The “rear stationary” platen holds the opposite ends of the tie bars and anchors the whole clamping system.
          All injection machines have some sort of safety interlock system that prevent access to the molds during the clamping and injection phases when the machine is operating semi-automatically. The operator removes the finished part, closes the door or gate, which sets in motion the next molding cycle. In full automatic operation, finished parts fall into a container, conveyor, or are removed by robot mechanisms.
ADVANTAGES:
1. Injection molding allows for high production output rates.
2. When producing your product you may use inserts within the mold. You may also use fillers for added strength.
3. Close tolerances on small intricate parts is possible with Injection Molding.
4. More than one material may be used at the same time when utilizing co-Injection Molding.
5. There is typically very little post production work required because the parts usually have a very finished look upon ejection.
6. All scrap may be reground to be reused, therefor there is very little waste.
7. Full automation is possible with Injection Molding.
DISADVANTAGES:
1. High set up costs – Moulds etc.
2. Complicated process.
3. can only be used for large quantities due to costs.

Thermosetting Plastics

THERMOSETTING PLASTICS:
          Thermoset, or thermosetting, plastics are synthetic materials that strengthen during being heated, but cannot be successfully remolded or reheated after their initial heat-forming. This is in contrast to thermoplastics, which soften when heated and harden and strengthen after cooling. Thermoplastics can be heated, shaped and cooled as often as necessary without causing a chemical change, while thermosetting plastics will burn when heated after the initial molding. Additionally, thermoplastics tend to be easier to mold than thermosetting plastics, which also take a longer time to produce (due to the time it takes to cure the heated material).
          Thermosetting plastics, however, have a number of advantages. Unlike thermoplastics, they retain their strength and shape even when heated. This makes thermosetting plastics well-suited to the production of permanent components and large, solid shapes. Additionally, these components have excellent strength attributes (although they are brittle), and will not become weaker when the temperature increases. 
          Thermoset plastic products are typically produced by heating liquid or powder within a mold, allowing the material to cure into its hardened form. These products can be removed from the mold even without allowing it to cool. The reaction used to produce thermosetting plastic products is not always the result of heating, and is sometimes performed by chemical interaction between specialized materials. Typical types of thermosetting plastics are epoxies, polyesters, silicones and phenolics. Vulcanized rubber is also an excellent example of a thermosetting plastic; anyone who has ever driven an automobile can attest to the properties of a superheated tire—it burns but does not mold into a new shape. 
          Each type of thermosetting plastic has a unique set of properties. Epoxies, for example, exhibit elasticity and exceptional chemical resistance, and are relatively easy to cure. Phenolics, while fairly simple to mold, are brittle, strong and hard. Because of their wide range of characteristics, thermosetting plastics find use in an extensive variety of applications, from electrical insulators to car bodies.