Compounding is the process to mix various raw materials together to create a plastic product. Typical compounding process of thermoplastic material is conducted by extruders (most often co-rotating intermeshing twin screw extruders). The key to most compounding processes is to achieve best distributive and dispersive mixing and create a homogeneous mixture of each raw material. However, there are some special cases where balancing is needed. For example, in compounding long glass fiber products, you don’t want to break the glass fiber to achieve the best dispersive mixing.
To mix with stabilizers
The most basic compounding operation is to compound the thermoplastic resin with stabilizers. Thermoplastic products in their simplest formulation are the neat resin compounded with stabilizers which include primary and/or secondary heat stabilizers, UV stabilizers if needed.
To make color
For plastic materials, they are easily made into various colors by compounding the transparent colorless resin or white/off-white resin with colorants and pigments. Special visual effects, such as high gloss, reflective colors etc., can be achieved by compounding the resin with special colorants and fillers.
To aid the next step in processing
Plastic materials are usually injection molded, extruded, thermoformed to make into shapes for various applications. These processes have specific requirements of material. For example, how easy the material flows at the processing temperature is an aspect for consideration. If the material is too viscous, the injection molding machine may not have the tonnage to push material into mold to form a high quality part. Compounding high molecular weight resin with low molecular weight resin at a ratio is a common way to adjust flow of the plastic materials. Also, flow promoter, mold release agent may be compounded with the resin to help the processing in the next step.
Concentrates, or master batches, are also produced by compounding. The master batch will later be compounded or mixed with other ingredients to make the final product. The master batches help the material handling, ensure good mixing and accurate dosing of small percentage additives.
To modify the plastic product
For many years, there has been no new plastic material (resin) introduced to the market. However, there are many ways to create new plastic products to meet customers’ needs by blending different types of polymer materials (polymer blends) and compounding the plastic resin with inorganic fillers (filled/reinforced plastics).
One general category polymer blends are to improve the impact performance of the resin by blending the resin with impact modifiers which are also types of polymer materials, such as ABS, MBS, EP rubber, Acrylic impact modifiers etc. For example, PC/ABS blend.
Plastic resins are also compounded with inorganic fillers to improve certain properties or reduce cost (some fillers are much cheaper than polymers.). Most common ones are filled with Talc or reinforced with glass fibers.
Another major category in plastic modification is to compound plastic resin with flame retardant additives to achieve flame retardancy required by customers and regulatory organizations such as UL.
There are many companies focusing on compounding and modification of plastics. The compounding businesses do not need huge amounts of investment to start compared to resin manufacturers. Small businesses can start with just one twin screw extruder and accessory equipment such as feeders, pelletizers etc..
In the previous posts, when the resin manufacturers were listed, the compounders are left out if they are not upstream integrated. During the pandemic, many companies faced supply chain issues and had difficulties getting the raw materials. When one searched the material on the internet, it would show many suppliers. However, if one company doesn’t have the resin production and only runs compounding business, most likely the company is facing the same situation with the short supply.