The finished parts are stress-free and have no seams so they’re strong, and the tools are relatively simple and inexpensive to make. 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.
The process is easily controlled and repeatable and is commonly used for transparent plastic drinking bottles. It makes for excellent surface quality but it’s not ideal for thin walls.
PET (polyethylene terephthalate) or PEEK (polyether-ether-ketone) are the typical resin choices for drinking bottles, due to their clarity and durability, and because they’re rated as safe for consumables. They’re also easily recycled.
RIM is most often used in the automotive industry because it produces lightweight parts that have a rigid skin. This skin is easily painted to make body panels, dashboards, and other car parts. However, thermoforming plastics won’t work in this process. Instead, this process requires thermosetting plastic.
Thermosetting plastics undergo an irreversible chemical reaction inside the mold. This usually causes them to expand like foam, filling a mold cavity. When the chemical reaction is done the plastic sets into its final form.
Vacuum casting is a great choice for making a small number of high-quality rapid prototypes without a big investment in tools or material.
A master model of any rigid solid (often this is a 3D printed master pattern) is placed into a sealed box, which is then filled with a flexible urethane or silicone. When the master is removed, a cavity is formed inside the mold that can now be filled with plastic resin to form a copy of the original. Vacuum pressure is used to pull air out of the mold so that it fills completely with no air bubbles.
In this process, the surface finish quality and detail are excellent and pourable resins can imitate many engineering grades of plastic. Notice also that the pourable liquid resins aren’t quite the same as their counterparts used in plastic injection molding. In the former case, resins are solidified by a chemical reaction with a hardening agent, not by cooling down.
Polyurethane vacuum casting is fast because a silicone mold tool can be made in just a few days. It requires a much smaller initial investment, and the fidelity of the copies is excellent – even capturing fine grain texture.
The drawback is that the tools are fragile, and they react with the chemicals in the resin and must be replaced after about 20 copies or so.
This is a type of vacuum forming, where a thin or thick gauge plastic sheet is placed over a die, heated to a temperature that allows the material to become pliable, then 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, not thermosets, because the material needs to soften under heat and then become rigid again when it cools.
Thermoforming is very cost-effective and can be done with simple equipment. In fact, even home shops and DIY people use thermoforming to make models and prototypes. The process is also safe because there are no bad chemicals or high temperatures involved.
However, it’s only suitable for making simple shapes, and once the material cools it will shrink quite a bit so it’s not a high-precision application. Also, the relatively thin plastic stock is needed, although many sheets can be laminated together after forming to make a more rigid final product.
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. Making a compression molding die is much easier and less expensive than a plastic injection mold tool, and relatively simple equipment can be used for this plastic manufacturing process.
There are over 10,000 different types of plastic resins commercially available, each of them specially formulated to achieve very specific chemical and mechanical properties.
Although we can’t possibly list them all, there are ten major groups that account for the vast majority of industrial and consumer products. They are: Polyamide, Acrylic, Polycarbonate, POM, Polystyrene, ABS, Polypropylene, Polyethylene, Polyurethane, and Thermoplastic Rubber.
Plastic manufacturers can also add glass or carbon fibers to increase strength and stiffness, or mix two or more resins together to combine their properties. These plastic types provide a full range of benefits including strength, impact resistance, flexibility, chemical resistance, light weight, toughness, and optical clarity.
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