Thermoplastics can be foamed in a variety of ways to achieve different densities. Typically, blowing agents are mixed with polymer melts to produce reduced density plastic products by expelling polymer with gas. However, reduced density or weight is just one of several benefits that foams can offer. Other general benefits include clean chutes, reduced warpage, and increased production speed.
Blowing agents fall into two categories: physical agents and chemical agents. Various gases and volatile liquids are used as physical blowing agents. Chemical blowing agents (CFAs) can be organic or inorganic compounds that release gases upon thermal decomposition. CFAs are commonly used to produce medium to high density foams and are often used with physical blowing agents to produce low density foams. This article provides basic information on CFA processing in extrusion and injection molding.
CFAs can be classified as either endothermic or exothermic depending on type of decomposition they undergo. Endothermic types absorb energy and typically release carbon dioxide and moisture upon decomposition, while exothermic types release energy and typically release nitrogen upon decomposition. The total gas yield and pressure of gas released by exothermic blowing agents is generally higher than that of endothermic ones. A mixture of these two classes is sometimes used for certain applications. This is case in profile extrusion, where high air pressure and high exothermic volume help fill profile, and controlled gas production and cooling from endothermic decomposition reduce profile distortion. Enothermic CFAs are known to decompose in range of 130 to 230 C (266-446 F), while some of more common exothermic blowing agents decompose at around 200 C (392 F). However, degradation range of most exothermic CFAs can be reduced by addition of certain compounds. CFA and polymer selectionMost CFAs are designed with a specific polymer and application in mind. The previously mentioned degradation range is taken into account when choosing a blowing agent due to compatibility between polymer and degradation products. For example, endothermic CFAs that release a lot of moisture when decomposed may not be best choice for degrading polymers such as polycarbonate or PET. Some polymers foam more easily than others. For example, LDPE foams much more easily than LLDPE, and polypropylene copolymers generally foam better than homopolymers. This is mainly due to higher melt strength which helps to maintain foam structure. Resin suppliers usually offer "foaming" grades of resin and should be consulted. The basic principle of foam processing is to keep blowing gas in solution with polymer melt until it exits mold or enters mold cavity. With this in mind, large pressure drops in front of lip or head should be avoided to ensure uniform expansion of foam. The minimum L/D of an ideal foaming extruder is 24:1 to ensure complete CFA decomposition and gas dispersion in melt. The design of auger should create pressure on auger profile, but provide relatively gentle mixing. This helps to keep gas in solution with melt and also prevents polymer from being overworked and reducing its melt strength. Filter assemblies are not recommended as they will cause pressure drop and premature foaming. Foam dispersion problems can also be caused by clogged mesh bags. Extruder vents or vents must be closed as foam can escape through them.blowing gases. There are exceptions to rule that real world requires perfect foam extruder. Augers with barrier and mixing sections, as well as machines with sieve units, give excellent foam. Foaming can even be carried out on machines with a degassing zone, as in case of PVC foam extrusion on conical twin screws. If a screen is required, a coarse screen is usually better than a fine screen for foam extrusion. In a process that does not allow clogging of vents, chances of success will be increased by increasing speed of screw, lowering temperature before vent, and choosing a blowing agent that will decompose after vent. For chemical foaming, a "bell-shaped" temperature profile is generally recommended. Set first temperature zone after inlet as cool as possible to reduce chance of pre-foaming and gas escaping from inlet. The temperature should be maximum in next zone to ensure good melting of polymer and complete decomposition of selected blowing agent. Finally, lowering temperature of machine head or nozzle increases strength of melt, which prevents foam from breaking down. Dosage is one of most important elements, but probably one of most often overlooked. Although gravimetric feeders are preferred, higher cost makes them difficult to justify. The more common volumetric feeders can be just as accurate if a feedrate test is performed. It is recommended to create a calibration curve for each bulk feeder and each material used in that feeder. Many foam extrusion concepts also apply to injection molding. In addition, ideal forming press should have shut-off nozzles to prevent salivation between shots. The location of gates and runners should ensure quick and uniform filling. Where possible, short stream lengths should be used. Ventilation is critical for foam expansion. Experience has shown that depth of vents ranges from 0.003 to 0.010 inches, but actual size of vents can be determined by trial and error. Mold fitting is a reliable way to determine depth and location of vents. Chemical foam extrusionExtruded thermoplastics are often foamed to reduce density. The same method should be used for extruded profiles or sheets. If possible, start with a constant flow without CFA. The blowing agent should be added at a relatively low dosage and slowly increased until desired density of extrudate is obtained. Each dose increase should be given time to reach a steady state. It is desirable to increase line speed of downstream equipment to compensate for expansion of 3D foam.During extrusion process, desired thickness of extrudate may not be obtained for several reasons. The solution can be as simple as increasing amount of blowing agent or increasing speed of extruder screw, or even reducing speed of downstream line. However, problem may be related to downstream processes for other reasons. For example, in sheet extrusion it is very important how sheet contacts roll magazine as it exits die. In non-foamed sheets, gap is usually made in such a way as to create a "melt build-up" to give sheet a glossy and smooth surface. This is not ideal for foam sheets because foam must be allowed to expand. Thus, preferred method is to allow roll to "kiss" or lightly touch sheet as it initially exits die. When extruding foamed profiles, it is necessary to pay attention to how profile cools, so as not to freeze and stop premature foaming. The distance between mold and water bath or calibration equipment, as well as temperature of these devices, must allow foam to expand. Any large voids found in honeycomb structure may indicate cell failure, which may be caused by too high mold temperature or too much blowing agent. If an uneven cell structure is obtained, this may be due to insufficient mixing in extruder or in feed. It has also been shown that excessively high throat temperatures lead to inhomogeneous cell structures. Pre-foaming within matrix can also be due to poor cell design. This is caused by excessive pressure drop within mold and can be counteracted by narrowing mold gap or shortening mold surface. Lowering mold temperature can also help create back pressure. CFA Injection MoldingWeight savings and elimination of troughs are two main reasons for using chemical blowing agents in injection molding. When using CFA for weight reduction, it is important to reduce injection size for short injections and use foam to fill mold. For example, if target weight was lost by 10%, it is recommended to reduce injection volume by about 10% (by weight). The addition of blowing agent must be constantly increased before adding part. If it reaches a point where increasing blowing agent does not improve part filling, reducing seal pressure, holding pressure, and time may allow blowing agent to expand even further. If part is still short at this point, you may need to increase frame size a bit. To remove sink, a foaming agent is added to help fill part. If addition of a blowing agent does not by itself eliminate shrinkage, use methods above to reduce padding andpressure and time storage. The general rule of thumb for CFA injection speed is that faster better. Batteries are often used for this task, as in case of structural foam casting. Faster injection ensures uniform expansion of blowing agent. However, this can be counterproductive if mold is not sufficiently ventilated. In some cases, reducing clamping force is a proven solution. Maintaining a high speed at start of hardening and then slowing down hardening rate towards end of injection is considered useful in other situations where exhaust and part geometry are limiting factors. For a variety of reasons, including insufficient cooling and excessive use of foaming agent, parts may experience post-expansion or post-foaming after demolding. If increasing cooling time or decreasing CFA dose does not help, injection volume may be too large. On standard high and low pressure injection molding machines, foamed parts create a rough surface or flared appearance to reduce weight. Reducing amount of blowing agent used, increasing injection speed and pressure, and even raising mold temperature are all ways to improve surface appearance of part.