Thermoset Materials - Phenolic
Thermoset materials were the first commercial plastic materials developed in the early 1900’s to be processed into moulded form for many industrial and commercial applications.
The conversion of the raw Thermoset material into the moulded product requires the use of both heat and pressure, to start the chemical chain reaction of the material from its original form into a hard moulded product. To do so requires the mould tools to operate at 150°C to 180°C, and pressures of 23bar per square centimetre as a minimum. Usually, it is the tool size dictates the press size rather than the part size. The heat enables the resins to flow, and then cure solid. The pressure is to keep the mould closed and press out the volatiles/gas produced as the material flows to fill the tool cavity form.
Phenolic started life as a pure resin, being used for decorative objects of art, jewellery, and ornaments in the late 1800’s and early 1900’s. These objects have been much prised in the recent years for their value and appeal to collectors. Early examples were of an amber coloured resin, this was before other colours became fashionable for jewellery and objects of art. As a resin it was easy to create a design into a simple wooden mould and allow to cure to become a solid part.
In the early 1900’s in New York, a scientist turned the resin into what we now know as Phenol Formaldehyde, or its most common name Bakelite. It was not much later that the material became usable as a moulding material for many industrial and commercial products. At the same time the addition of other compounds into the original resin further improved/enhanced the properties of Phenolic. These ranged from wood powder, cotton fibres for improved impact resistance, minerals, such as silica, talc and mica, to increase the heat resistance, and glass fibres for impact and tensile strength, or a mixture of any of these as well.
Known for its Insulating properties it became widely used in many electrical applications, such as Bobbins for windings/coils, Fuse Holders, Pan Handles and Ash Trays for its heat resistance, Telephones, Radio Cases, Bottle Caps for its smooth solid appearance, along with many applications in both WW1 and WW2, and many more since.
Over the years the name Bakelite has remained as both a trade and company name, but the company name has become part of Sumitomo Bakelite. The original company and manufacturing facilities are now under the trade name of Hexion. There are other manufactures of Phenolic, mainly in USA – Plenco, Norplex Micarta, but also in Europe as Vyncolit and Hexion, and China.
Phenolic material is in a coarse granular form, which can be used as is, or for easier and speedier handling can be pre-formed into ‘pellets/slugs’. To do this requires the granular material to be pressed into a cold die in a pelleting machine/press. The ‘pellet/slug’ is ejected out and retains its shape. The shape in most cases is simply round but can be of the shape of the tool cavity form of the moulded part.
In the majority of cases the granules and ‘pellet/slug’ requires pre-heating, to soften the hard material prior to loading into the mould tool. This heating process is usually by plate heating using high frequency electrical energy, and passing a current through the material to create heat. In cases where just the granules are used, for small parts or difficult mould tool cavities, the granules are poured into the tool cavity and allowed to sit for 5 to 10 seconds to warm through, before the tool closes under slow speed. This allows for the material to soften before coming under the press pressure. In some instances, both granules and ‘pellets/slugs’ are used together. It is very much the choice of the best process and product design that dictates what is chosen by the setter and the quality.
The process of compression moulding Phenolic, requires the material charge to be placed into the tool cavity form, the tool closes slowly to compact the material and starts the chemical chain reaction, tool opens partially, for a few seconds, to allow gas/volatiles along with ‘flash’ – additional material to ensure the whole cavity is full – the tool re-closes and holds firm for a set time to allow for the material to fully cure, and then opens. At this point the part is ejected by the tool and press movement and removed by the operator. The tool is then blown clean of debris, before the cycle starts again.
Another process option is transfer moulding of Phenolic. This involves a chamber for the Phenolic to be placed in, the tool closes, and the material is forced into the impression detail by a hydraulic ram as part of the press function. This is often supported with a vacuum arrangement to suck the air and gas out as the material is forced into the tool. This method reduces the flash, presents a cleaner moulding, but also enables the part design to be machined into both halves of the mould tool. The part split line can then be in a suitable position for the part gate feed and offers improved quality as the process eliminates porosity through gas trapping. Some of the compression press’s at Merriott Plastics have this facility, 80t, 150t, 300t and 400t sizes.
Merriott Plastics also injection mould Phenolic, in a process known as DST -Direct Screw Transfer. The material is feed into a rotating screw along a heated barrel, at a temperature that softens the material, the same as the pre-heating mentioned above. The material is then forced into a closed tool, operating in the same manner as a conventional injection machine. The tool is heated to the temperature of 150/170°C. The benefit of this process is increased yield for high quantity demands, improved cycle times, and in most cases run fully automatically, thus reducing the labour cost.
Merriott Plastics produce many components using Phenolic material, despite many other thermoplastic materials being available, but they do not have the feel, strength, rigidity, insulation, heat resistance and long lasting as Phenolic. Parts that we produce today are Fuse Carriers and Holders, used in electrical cabinets for industrial use, also Insulators for many different electrical applications, where both insulation and high heat are specified, other mouldings for hazardous environmental areas, due to the chemical and hardness of Phenolic, and many more.
Picture of both granules and pellet/slug’s pressed from the granules.