There are several ways to form a finished plastic item, so we’d like to tell you about the major techniques and their pros and cons so that you can make an informed decision about which is right for your next project.
1. Plastic Injection Molding
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.
At it’s most basic, injection requires making a tool, or injection mold, that consists of a male and female half – the core and cavity. This mold is placed inside the machine. Plastic pellets are fed into a hopper where they are heated until molten. A power-activated screw then drives this molten plastic into the empty space of the cavity, wherein the core is pressed to form the final shape. Once cooled, the now-solid part is ejected and the process repeated.
The advantage of this process is that thousands or millions of identical parts can be made quickly and economically, with excellent surface finish and complex geometries. Potential drawbacks are that molding tools can be a high initial expense that is recouped only with high production volumes. Also, careful engineering of the mold is required to ensure thermal stresses are balanced out to avoid warping and other defects during injection.
2. Rotational (Roto) Molding
The most common plastics used for rotational molding are nylon, ABS, polyethelene and PVC. As with injection molding a core and cavity mold tool is used, but the process is quite different. There are four basic steps:
Preparing the mold / Charging
Plastic in powder form, along with any other additives like colorants or UV stabilizers, is placed inside the mold which goes into a pre-heated oven.
Heating and Fusion
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.
The mold is then removed from the oven and cooled, either naturally 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.
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.
Rotational molding is ideal for making large, hollow or concave shapes, often for outdoor use like canoes and tubs. The finished parts are stress free and have no seams so they’re strong, and the tools are relatively simple and inexpensive. The downside is that tools don’t last more than a few thousand cycles before they need to be replaced, and the part finish quality is average at best so it’s not suited for precision forming.
3. Blow Molding
Molten plastic in the form of a large droplet, called a parison, is placed into a two-piece clamshell mold. After the mold closes, the parison is inflated like a balloon until it fills the empty cavity. The walls of the mold are water cooled, so the plastic quickly solidifies and the bottle is ejected.
This method is fast and inexpensive, and it’s good for making thin walled bottles. But the surface geometry is not complex nor is it especially precise.
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.
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.
The entire molding process takes more time than injection molding 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
This method is best for prototyping small runs of about 20 pcs or less, since the mold is not especially durable.
A master pattern needs to be made of some durable material. It’s then placed into a casting box, which is half-filled with liquid silicone rubber. Once that half is cured, a second half is poured over it, with the two halves separated by a releasing agent.
After the two halves are dried the mold can be opened up and the master removed. Now the empty cavity is ready to be filled with the liquid plastic of your choice. Once filled, the mold is put into a vacuum chamber which sucks the air out of the mold, ensuring that the cavity is completely filled without bubbles.
Vacuum casting is ideal for making solid prototypes and to test out form, fit and function of a design concept. A silicone mold is typically good enough to capture essential surface finish details and will withstand the heat of thermopolymers for dozens of parts without degrading while the investment costs are negligible. But it’s not suited for production volumes.
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.
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
The raw material is 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.
This covers the basic techniques but of course there are a lot of special details involved in getting just the part you’re looking for. To find out more information about what we can do for you or to get a quotation, contact us today and talk to our manufacturing experts.