Blog 38: Introduction to Compression molding
The fabrication of thermosets, thermoplastics, elastomers, and natural rubbers all frequently use compression molding. It creates high-volume, dimensionally accurate, strong, temperature-resistant, and surface-quality parts. In comparison to other manufacturing processes, parts can be produced in a variety of lengths, thicknesses, and levels of complexity at lower per-part costs.
Compression molding is a technique primarily used for thermoset plastic molding in which the molding compound (typically preheated) is placed in the heated open mold cavity and the mold is closed under pressure to force the material to flow and completely fill the cavity. The pressure is maintained until the thermoset material has cured.
Process of compression molding
A measured amount of a plastic substance is shaped or produced using heat and pressure in this procedure. The molding powder is stored in an open mold chamber at a known weight or volume. The male and female sides of the two-part molds are generally warmed. Mold halves are fastened to a press’s upper or lower platens. The cavity and core of this mold are its two halves. Either direct heat application to the mold or indirect heat application via the platens is used.
The molding compound, which is typically in powder form, is added to the cavity of the mold before the mold is sealed. When pressure and heat are applied, the plastic substance softens, flows, and completely fills the hollow. Temperature ranges from 140°C to 250°C, and pressure ranges from 2000 to 10,000 psi, depending on the properties of the plastic material and the design of the mold. Until the plastic substance dries and hardens, the heated, closed mold stays in place. Generally speaking, a molding pressure of 2000 psi on the part’s whole anticipated area is advised. To get the total molding pressure needed, multiply this by around 800 psi for every inch of the molded part’s vertical height.
When the material is sufficiently dry, the mold is opened, and knockout pins discharge the piece. The size and thickness of the component affect the curing time. Anything from 20 seconds to 10 minutes may be used. The cure time must be determined by trial and error or experience as it cannot be reliably anticipated.
The step-by-step compression molding process:
- Preparation of machine: Installed in the press, the mold, or metal tooling, has a cavity and a core and may have inherent heating to regulate temperatures as the process progresses. The hydraulic press’s platens include temperature controls as well.
- Pelleting: Pelletizing makes it possible to gauge the charge to the mold accurately, which lowers contamination and makes preheating easier. With more automatic compression techniques, it may not necessarily work properly.
- Preheating: Preheating is advantageous since it shortens the pricey molding process and enables the quick heating of huge pellets or masses of powder. Prior to molding, it helps to eliminate moisture and other volatiles, and because it advances the cure, it has been asserted that molding shrinkage is decreased.
- The Molding Stage: The heated mold cavity is filled with the appropriate powder material, and once the mold is closed, a constant predetermined pressure is given to the molding. The moment the material touches the hot mold, it will begin to cure. Before the gel point is achieved, flow and shaping must be finished (i.e. the material shows the first signs of being cross-linked). The optimal treatment (cross-linking) for one attribute may not always be the most excellent treatment for another. In order to employ cure durations and temperatures that provide an excellent balance to the many objectives, including those of cost, it is vital to determine what attributes are essential in the finished molding.
The regulation of the flow and cross-linking processes, as well as their proper sequencing, are of utmost importance. Even while finer adjustments, such as breathing (venting), can frequently be made to an operation, they are useless if the fundamental procedure is flawed.
Finishing Operation: Depending on how the product behaves after curing, the portion finishing may be carried out. There are situations when extra material is seen flashing out of the molding and can be removed after curing. The finished product can undergo additional finishing operations to make it more appealing.
Advantages and disadvantages of compression molding
The benefit of compression molding is that it may be used with a wide range of compounds. Some polymers can be used to create devices at room temperature, which could help with drug-polymer interaction issues. With the mold geometry, flat devices can only be produced, and the matrix homogeneity varies depending on the drug, which can lead to inconsistent drug release patterns.
- Composite parts made by compression molding can be as sophisticated as those made of metal and come in a variety of sizes. They have comparable mechanical and strength properties to metals but outperform them in terms of weight-to-performance, anti-corrosiveness, and electrical insulation. Additionally, they need less post-fabrication machining to achieve geometric requirements.
- The ease with which ribs and other inserts can be integrated at the time of forming, reducing or completely eliminating the need for later procedures, is one of the compression molding technique’s major benefits. It can replace multiple assembly elements with a single procedure and create a complicated compression molded part.
- Compression molding is frequently the most economical manufacturing technique when producing large, straightforward, generally flat products. Designs can have some acceptable curves and pockets, but it can be difficult to obtain severe angles and deep draws by compression molding. Low pressures result in economical tooling costs and lengthy mold lifespans without warping or replacement requirements. Manufacturers can utilize a mold with numerous cavities to make a number of pieces in the same cycle, which helps to reduce the expense associated with compression molding’s lengthy cycle periods.
- Solid, flow- and knit-line-free pieces are produced via compression molding. Compression-molded components have very high structural stability. Compression molding is also utilized to create components from composite materials, making it simple to create robust, corrosion-resistant parts and goods utilizing this technique.
- Engineers and product designers can also benefit greatly from compression molding as a manufacturing technique. For instance, inexpensive compression molding can be used for prototyping. Simple compression molds can be created in computer-aided design (CAD) software, 3D printed, and then used with a straightforward desktop vise to form different kinds of materials.
- Compression molding has some drawbacks, including the potential for minor distortions or breakage due to process flow and pressure, which makes it unsuitable for parts with undercuts, side draws, or small holes. With the exception of cycle times, which are longer because of heating and cooling procedures that are crucial to component performance and quality when employing thermosets and thermoplastics in the compression molding process, compression molding is a process that rivals injection molding in most respects.
- In comparison to high-volume molding techniques, the cycle time, which can be several minutes, is slow. For instance, cycle times for injection molding are frequently just a few seconds.
- The slow cycle time that corresponds with increased working hours makes the labor cost for compression molding potentially rather high. Compression-molded parts need to have flash and burrs manually removed, which adds time and waste.
The Applications of Compression Molding
Compression molding can produce parts with weights ranging from one ounce to over 100 pounds that can match the strength and intricate geometries of metal while also having anti-corrosive and electrically insulative qualities. Products come in a variety of shapes and sizes, from thin-walled containers to substantial, solid objects like cookware, electrical housings, helmets, and parts for cars and airplanes.
Here are some of the many parts and products we interact with that have compression molded parts:
- Vehicle parts – Many large parts and panels of cars, tractors, and other vehicles are made using compression molding. Many plastic parts used in vehicle interiors and engine components can be compression molded, too.
- Computer and gaming devices – Components of video game controllers, keypads, and more can be compression molded.
- Kitchenware – Many kitchen tools, utensils, and appliances have compression molded parts. Dinnerware, including bowls, cups, plates, and more, especially those made out of melamine, is often manufactured using compression molding.
- Electrical components – Compression molding is often used to manufacture electrical sockets, switches, faceplates, and metering devices.
- Medical and dental device parts – Many plastic and silicone parts used in the medical industry are compression molded, including syringe stoppers and respirator masks.