Advantages of Injection Molding and Monomer Free Thermoplastics

There is a worldwide demand for non metal removables and monomer free dentures. Complaints of sensitivity to acrylic monomers is a daily occurrence. Patient awareness of superior esthetics and comfort with Flexite has created this demand. Thermoplastics differ from regular thermosets such as acrylic powder and liquid because they are already polymerized (cured) and do not require a cure. Thermoplastics are manufactured in many forms; from sheet, to powder, to pellets. By introducing heat we soften the plastic to the desired state and inject the plastic into a mold. Since we only change its physical shape or form, there is no joining of chemicals or chemical changes.

Thermoplastics differ in characteristics just as metals do. Depending on its molecular composition, some will require higher temperatures to reach a moldable state. Some require greater injection pressures since the viscosity and melt flow varies from plastic to plastic. In appearance some are clear, opaque or white. Impact and tensile strength vary as do the flexural and elastic module characteristics. Water absorption and dimensional stability will also differ according to the plastic.

Comparable to metals, (example, palladium, silver and gold), plastics have different melt indexes and characteristics. There are two basic systems used in injection molding. There are hopper fed systems and cartridge filled systems. The hopper contains a large funnel type opening where the pelletized plastic is fed. Hoppers vary in size from small one ounce shot cavities to large commercial capacities where multiple molds are used. For commercial use the hopper type is popular because the same plastic is continually fed to the hopper into multiple molds for an “injection run.” As much as 10,000 units can be done in a single day.

In the dental laboratory we have different requirements. Every patient represents a different mold. We constantly change our molds and require intervals when the machine is not running. The hopper type previously used in the dental lab was discarded due to the necessary purging and cleaning requirements to keep the plastic flowing. This required extra time and excessive costs for materials used in purging. The cartridge type prevails in dental technology because it omits these inconveniences and lends itself to variety. We can switch from one type of plastic to another without fuss; no cleanup or purging is necessary. In commercial molding, where metal molds are used, hydraulic force is popularly used to drive the molten thermoplastic to the mold. In the laboratory, where molds are made from stone and where sometimes critical porcelain teeth are part of the finished product, some injection systems use nitrogen as its source of power. Nitrogen, being inert, poses no environmental threat. Our new Flexite injection machines use a 150 lbs. air compressor (10 Bars) but compressed air can introduce moisture and other impurities into the injection machine cylinder if you do not use an air filter. The Flexite Company sells a low cost air filter to keep impurities from clogging up the air cylinder. Small inexpensive dehydrators are also available for compressors if your laboratory operates in a high humidity environment.