Derived from both Latin and Greek (plasticus and plastikos), the modern usage of the term “plastic” is said to have been coined in 1907 by Leo Baekeland, the creator of bakelite. He really should have copyrighted the use of that word, so ubiquitous has it become in what we like to call the modern world. And what we call “plastic” in the modern world includes a vast array of artificial compounds largely derived from petroleum distillation, and falling into one of two broad categories: thermoforming and thermosetting. As we discussed before, thermoforming polymers are able to be molded by the application of heat to make them pliable, whereupon they form useful solids when cooled to room temperature. They can be recycled and remolded by melting, whereas thermosetting polymers undergo chemical reactions which alter the molecular structure in an irreversible manner.
Digging a bit deeper into this, we can also distinguish plastics according to chemical structure. One large subgroup is aliphatic, that is, there is a backbone of carbon atoms that link together in a single unbroken line. The length of such a chain of carbon-related atoms can be impressive, up to millions of atoms long in one molecule. Compare that to naturally occurring organic molecules, whose atomic weight is typically tens of atoms long. The addition of links in the molecular chain will alter the chemical and mechanical properties of the material, but all such aliphatic polymers are considered rather common, easily mass-produced and are widely used in all major molding processes.
The other main subgroup is heterochain, meaning that there are additional atoms of oxygen, nitrogen, sulfur etc. added to the mix. These are more complex, more expensive, and typically are found in engineering and specialty plastics. When they undergo processing there is often a chemical reaction that fuses this kind of material into a new compound, creating a molecular structure with properties unlike those of its constituent parts.
Industrial techniques for making useful objects out of plastic continue to be improved and refined, as does the material itself. The choice of which technique to use for a prototype or low-volume production part is not mutually exclusive: there are many possible, and overlapping, ways to make an acceptable finished part, so which method you choose is based on a combination of factors that include mechanical and aesthetic properties, tooling cost, part cost, length of the production run, turnaround time, etc.
The following therefore is meant to be a primer on the major plastic molding and forming processes that can be applied to your next project, along with some interesting historical information as well as the assorted pros and cons of each process to help make your decision an easier one.
1. Plastic Injection
By far the most common of all molding processes, accounting for perhaps 80% or more of the plastic objects that we find all around us in our daily lives. History tells us that Mesoamerican civilizations were using natural rubber for balls and other objects since the 1600s, but industrialized plastic use didn’t start to occur until the mid-19th century. Credit for the first artificial commercial plastic goes to Briton Alexander Parkes in 1861, who created Parkesine out of cellulose and nitric acid, to create a stable solid resembling ivory to be used in billiard balls. It was, unfortunately, somewhat flammable.
American John Wesley Hyatt improved upon Parkesine to create the more stable celluloid, and in 1872 patented a simple injection molding machine. The technology staggered along for many years until the fortunate outbreak of World War II created a major demand for cheap, durable products on a massive scale.
As with aluminum pressure die casting, the molding process requires a tool consisting of a cavity side and a core side. The machine will feed a measured amount of molten plastic into the plunger, which is then pressurized with a hydraulically-actuated screw. The core is pressed into the corresponding cavity, forcing the plastic to fill the void between. Once injected, the plastic is allowed to cool, the core is withdrawn, the part ejected and the machine prepared for another cycle.
injection molding diagram | Image Credit: noria.com
For prototype work, it’s possible to make a relatively simple mold for hand loading. Such a tool might only need to make a dozen or so parts, so allowance need not be made for automated slides and keyways. Plastic injection molding tools for mass production are the most expensive and complex. They must withstand heat and pressure and potentially millions of cycles, so the material from which they’re made must also be of a high grade. However, this high initial tooling cost can be amortized over a great many parts, making the the investment cost-effective in the long run.
The benefits of injection molding are manifold. First, most thermoforming plastics can be injection molded, so it’s possible to find the right material to suit most mechanical requirements. Commodity plastics are readily available on the open market which keeps material costs low.
Also, the surface finish of the part depends on the quality of the mold cavity but it can be very high indeed and can be fine-tuned to produce any desired result. Chemical photo etching processes are often used to produce faux leather, wood grain and other textured surfaces, not only for this but many other molding methods.
The part geometry can be complex, as long as sufficient attention is paid to draft angles and thermal stress (which is true for all molding processes). Once a tool is complete, millions of virtually identical parts can be made so the process is ideal for a stable, consistent and uninterrupted production line. The tolerance of the finished part is commonly in the range of +/- 0.1 mm with a maximum precision of 0.01mm but this is extremely dependent on the geometry and size of the component.
Some drawbacks might be the cost and complexity of the initial tool. For example, if a finished piece is meant to have internal threads, then a threaded core needs to be built for each hole. Automatic gear threading is possible for mass production, but physical space must be allowed for the gearing mechanism, so screw holes can’t be too close together. It’s possible to locate screw holes closer using manual threading, but then the process becomes that much more time-consuming and thus a limiting factor.
An additional consideration for the designer and mold engineer is whether to use hot or cold runner tooling.
Cold Runner
With the cold runner system, channels for injecting the plastic are an integral part of the mold tooling. Each time a part cools and is ejected from the mold cavity, the remains of the runners, called sprues, are also ejected. These sprues must then be cut off of the finished piece. Although that material can later be recycled it does add to processing time. In many cases, the amount of material can be greater in the sprue and runner system than in the part itself. In this case the economics of changing to a hot runner system needs to be studied. A cold runner is chosen because it is a less complex design and because certain engineering plastics require the relatively larger, open channels for the injection process to be successful.
Hot Runner
A hot runner has the runner channels as part of a manifold located between the machine screw and the cavity and is part of the tool itself. Individual nozzles are located in strategic areas to inject the plastic through a pin gate. This creates less waste and the finished part needs relatively little secondary work. However, the hot runner system is high precision and therefore expensive.
Processing time per piece is dependent mostly on wall thickness. Plastic is a good thermal insulator, so thick walls take a longer time to achieve equilibrium without deforming. Typical times will be around 30 seconds. Part size ranges from micro up to something the size of a car door.
2. Rotational (Roto) Molding
As the name implies, this technique requires that the molding tool be rotated during processing. And, unlike with other molding processes, the material is introduced into the tool in powder, rather than molten, form. The process was used at least as far back as the ancient Egyptians, who employed it for the making of uniform ceramics. It’s also claimed that Swiss chocolatiers used roto molding for making hollow chocolate eggs, but the present usage didn’t become common until around the 1940s, ostensibly for the making of doll’s heads.
The process can be broken down into four basic steps:
Roto holding process | Image Credit rotoworldmag.com
Preparing the mold / Charging
Assuming that a mold has already been made, the first step is to load plastic powder or resin into the hollow mold. Polyethelene is most common, but other forms of nylon, ABS or PVC can also be used. Color and other additives like UV stabilizers are formulated into the granulated powder before it’s put into the mold. The mold is then locked and loaded into a pre-heated oven.
Heating and Fusion
The temperature of the oven is dependent on the material used, but it’s typically between 500° ~ 700° F. Within the oven the mold is rotated on two axes, but it’s done slowly. Centrifugal force is not used to displace the material; rather, it’s allowed to coalesce onto the mold walls under gravitational pressure.
The temperature and timing must be controlled precisely during this stage. If the plastic is heated for too long it will degrade, and if it’s not heated enough it will form bubbles and will not achieve the proper wall thickness.
Cooling
The mold is then removed from the oven and cooled, either by natural or forced air, water, or a combination. As with heating, this process must be monitored closely. Done too quickly and the part may warp due to uneven thermal stress.
Unloading
Once the part has achieved equilibrium, which may take tens of minutes, the mold can be opened and the part removed and the process is repeated.
This process lends itself to large, hollow or concave shapes like tub enclosures, play sets, canoe shells and other such shapes. The advantage here is that it’s possible to make much larger forms than would be practical with other methods. Also, the tooling is much cheaper and less sophisticated. Polyethelene and polypropylene are the common choices with this method, which are cost effective and which stand up well to outdoor exposure. There is little material waste since the volume of raw material is pre-measured before going into the mold. And rotomolding makes parts that are stress free, under low pressure, and are one-piece (without seams).
The downside is that tools typically don’t withstand more than a few thousand uses before they must be replaced. The finished parts have only medium precision with marginal surface quality, but the main disadvantage for large-scale production is that it’s relatively slow, which is not such an issue with low-volume or prototype production.
3. Blow Molding
Once again the ancient Egyptians were ahead of the curve on this one, for heiroglyphics show them blowing glass at least as far back as 1600 BC. Cellulose was used in the 1880s for doll heads once more (what is it with doll’s heads?), but not until the invention of low-density polyethelene in the 1930s did the technique begin to achieve commercial application. Further improvements in the 1970s, and the invention of PET plastic, eventually brought about the now-ubiquitous plastic bottle that has made blow molding familiar to anyone who’s ever bought a soda pop.
This method can be divided into two main categories, extrusion and injection.
Extrusion blow molding | Image Credit designtekplastics.com
Extrusion Blow Molding
Molten plastic is introduced into a two-piece hollow die and formed into a droplet called a parison, which is the secret to the success of this method. The parison is open at one end, and is placed into a split mold. The parison is blown up like a balloon with forced air, and as it expands it contacts the inner surface of the mold, which is cold and therefore forces the plastic to solidify and become stable.
The constituency and wall thickness of the parison must also be carefully controlled, since as the material is forced to expand it will be drawn out and consequently thinner in cross-section. The designer must therefore allow for this progressive expansion so that the plastic can conform to the necessary geometrical details and achieve the desired uniform wall thickness when finished, though typically the complexity is not high and is limited to gently curved surfaces. Microprocessors and hydraulic valves were added in 1962 that allowed the parison to be “programmed” to account for the deforming effects of gravity and thermodynamics.
Injection Blow Molding
This method differs from the above in that the plastic is injected into the mold under gas pressure. This process is more controlled and repeatable, producing bottles and other hollow containers with uniform, thick and clear walls. The surface quality is excellent, although it is slower and is not ideal for making thinner wall sections.
Because this method is so often employed for disposable plastic drinking bottles the raw material is inexpensive and easy to recycle. PET (polyethelene terephthalate) or PEEK (polyether ether ketone) are the typical choices here, for their clarity, structural strength, and because such material is rated as safe for consumables. It’s also easily recycled.
4. Reaction Injection Molding (RIM)
This process is most often used in the automotive industry to produce body fenders, spoilers and other lightweight and easily painted pieces that have a relatively rigid skin with a softer and lighter foam core. In this case, thermosetting polymers are used.
RIM | Image Credit puremold.com
Two chemically reactive polymers are introduced separately into the die cavity, where they are heated and allowed to undergo a thermal expansion that fills the die cavity. Remember, these are thermosetting plastics which undergo a chemical change, so waste material cannot be recycled.
Various isocyanoacrylates, urethanes and elastomers can be used in this method. It also lends itself well to the introduction of fillers and reinforcing fibers like fiberglass, which are placed in the cavity first before the introduction of the chemicals. The entire molding process takes more time and the chemicals used tend to be more expensive, but the resulting parts are lightweight and strong by volume.
Tooling costs for prototypes are relatively low, while production tooling is moderately expensive. The main cost is in the material, bearing in mind that the resulting part must always be finished, usually with a urethane-based gel coat or by painting, so the process is more labor-intensive which increases the piece price.
5. Vacuum casting
Various casting techniques have been used for thousands of years; indeed, the earliest known casting is a bronze frog dating back to 3200 BC. Although modern polymer chemistry has changed the type of raw material we use, the process is still much the same, and lends itself well to low-volume and prototype production due to the simple and low-cost materials and tooling involved.
A master pattern must first be made. This can be of virtually any solid material such as metal, stone or wood, but for prototyping work it’s usually made of a photoreactive plastic using the SLA (stereolithography) method. The master is placed inside of a container and the container partially filled with liquid silicone or some other curable urethane elastomer. The level at which it’s filled will determine the eventual parting line of the finished mold.
The first half is allowed to cure, and once cured the top face of the silicone is coated with a releasing agent. Then the remainder of the master is covered with more liquid silicone and again allowed to cure. Once finished, the two halves are separated along the parting line and the master removed.
During this curing process, holes were located throughout the mold material, often using very simple methods such as ordinary household toothpicks. After the mold is cured, these stick can be removed, creating channels for the later application of vacuum pressure. Now with a finished mold cavity, liquid plastic can be poured into the hollow cavity to make an exact replica of the original master pattern. A low pressure vacuum will be applied to the holes, that insures the plastic completely fills the cavity without any empty pockets.
Vacuum casting is ideal for making solid prototypes, to test out form, fit and function of a design concept. A urethane mold is typically good enough to capture essential surface finish details and will withstand the heat of thermopolymers for dozens of parts without appreciable degradation and the investment costs are negligible. But it’s not suited for production volumes.
6. Thermoforming
This is a type of vacuum forming, where thin or thick gauge plastic sheet is placed over a die, heated to a temperature that allows the material to become pliable, then is stretched over the surface of the die while vacuum pressure pulls the sheet down and into its final shape.
Excess material is then trimmed off and can be easily recycled into pellets. Although industrial techniques can be highly automated and sophisticated, this process can also be done with simple dies and very basic equipment. It’s often employed with samples and prototypes of thin-walled, hollow-bodied parts. In industry, it’s used for plastic cups, lids, boxes and plastic clamshell packaging, as well as for auto body parts in thicker gauge material. Only thermoforming plastics are suitable for this process.
7. Compression Molding
Compression molding | Image Credit flickr.com
The raw material is often pre-heated and placed inside the open cavity of a die. A cap or plug is used to close the die and apply heat and pressure, causing the plastic to cure. Thermosetting polymers are commonly used here, along with the introduction of various fibers and tapes to increase strength. Thus this technique is common for high-volume production of large pieces in automotive applications as well as for consumer products like rubber boots.
It’s relatively inexpensive and wastes little material, although controlling the consistency of the finished piece can be difficult and much care needs to be taken in the preparation of the initial mold design.