Thermoplastics can be foamed in a variety of ways to achieve various densities. In general, the blowing agent is combined with
Polymer melt mixing to obtain plastic articles of reduced density by displacing the polymer with a gas. However, density or weight reduction is only one of several advantages that foam has to offer. Other common benefits include sink removal, reduced warpage and increased production speed.
Blowing agents fall into two broad categories - physical and chemical. Various gases and volatile liquids are used as physical blowing agents. Chemical blowing agents (CFA) can be organic or inorganic compounds that release gases when thermally decomposed. CFA is commonly used to obtain medium to high density foams, and is often used in combination with physical blowing agents to obtain low density foams. This article presents basic processing information for CFAs in extrusion and injection molding.
CFA can be classified as either endothermic or exothermic, which refers to the 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 generate nitrogen gas upon decomposition. Exothermic blowing agents generally have higher total gas production and pressure than endothermic blowing agents.
Mixtures of these two types are sometimes used in certain applications. This is the case in profile extrusion, where high gas pressure and volume from the exothermic section helps fill the profile, while controlled gas production and cooling from endothermic decomposition reduce profile warpage.
Endothermic CFA typically decompose in the range of 130 to 230 degrees Celsius (266 to 446 degrees Fahrenheit), while some more common exothermic blowing agents decompose around 200 degrees Celsius (392 degrees Fahrenheit). However, the decomposition range of most exothermic CFA can be reduced by adding certain compounds.
CFA and polymer selection
Most CFA are designed with a certain polymer and application in mind. The previously mentioned range of decomposition, as well as the compatibility between the polymer and the decomposition products, should be considered when selecting a blowing agent. For example, endothermic CFA that release large amounts of moisture when decomposed may not be the best choice for polycondensates such as polycarbonate or PET.
Some polymers tend to foam more easily than others. For example, LDPE foams more easily than LLDPE, while copolymers of PP generally foam better than homopolymers. This is mainly due to the higher melt strength helping to support the foam structure. Resin suppliers typically offer "foamable" grades of resin and should be consulted for advice.
When dealing with foam, the basic principle is to keep the foaming gas in solution with the polymer melt until it leaves the mold or enters the mold cavity. With this in mind, any large pressure drop across the die lip or die should be avoided to ensure uniform expansion of the foam.
The ideal foaming extruder has a minimum of 24:1 L/D to allow complete decomposition of the CFA and dispersion of the gas in the melt. The screw design should create pressure on the screw profile, yet mixing is relatively gentle. This helps keep the gas in solution with the melt and prevents the polymer from overprocessing and reducing its melt strength. Strainer packs are not recommended as they can cause pressure drop and cause premature foaming. Strainer packs can also cause foam dispersion problems when they start to clog. Extruder vents or vents should be plugged as they allow foaming gas to escape.
There are exceptions to the real-world rule that calls for an ideal foam extruder. Excellent foams have been made on screws with blocking and mixing sections and on machines with screen assemblies. Foams can even be made on machines with degassing zones, such as foamed PVC extrusion on a conical twin screw. If a screen bank is required, a coarse screen is usually better than a fine screen for foam extrusion. In processes that do not allow clogged vents, the chance of success will be improved by increasing the screw speed, reducing the temperature before the vent, and choosing a blowing agent that will decompose after the vent.
A "bell-shaped" temperature profile is generally recommended for chemical foaming. Set the first temperature zone after the feed throat as cold as possible to reduce the chance of pre-foaming and gas escaping from the feed throat. The temperature should peak in the subsequent region to allow good melting of the polymer and complete decomposition of the selected blowing agent. Finally, lowering the temperature of the machine die or nozzle can increase melt strength, thereby preventing foam collapse.
One of the most critical factors, but probably one of the most often overlooked, is dosage. While weigh feeders are preferred, the higher cost makes these feeders harder to justify for some. More common volume feeders can be just as precise and accurate as long as a feedrate check is performed. It is recommended to generate a calibration curve for each volumetric feeder and each material used on that feeder.
Many concepts of foam extrusion also apply to injection molding applications. Additionally, the ideal molding press will have a shut-off nozzle to prevent drooling between shots. The gates and runners should be positioned for quick and even filling. Shorter flow lengths should be used when possible. Venting is essential to allow the foam to expand. Experience has shown that vent depths can range from 0.003 to 0.010 inches, but actual vent size may require trial and error. Filling the mold is a proven method for determining the depth and location of venting.
Chemical foam extrusion
Extruded thermoplastics are often foamed to reduce density. The same method should be used for extruded profiles or sheets. If possible, start by achieving a stable process without CFA. The blowing agent should be introduced in relatively low doses and increased slowly until the desired extrudate density is obtained. Each dose increase should be given time to reach a steady state. It is expected that the line speed of downstream equipment will be increased to compensate for the three-dimensional foam expansion.
During extrusion, the desired extrudate thickness may not be obtained for a number of reasons. Solutions can be as simple as increasing the level of blowing agent or increasing the extruder screw speed or even decreasing the downstream line speed. However, for more complex reasons, the problem may be related to downstream processes. For example, in sheet extrusion, how the sheet contacts the roll set as it leaves the die is critical. In non-foamed sheets, the nip is typically run in a "melt bank" fashion to create a bright and smooth surface on the sheet. This is not ideal for foamed sheets because the foam must be given a chance to expand. Therefore, the preferred method is to have the rollers "kiss" the sheet or make light contact as the sheet initially exits the die.
When extruding foamed profiles, attention must be paid to how the profiles are cooled so as not to freeze and stop foaming prematurely. The distance between the mold and the water bath or sizing equipment, as well as the temperature of these units, must allow the foam to expand.
Any large voids found in the cell structure may indicate cell collapse, which may be caused by excessive mold temperature or excess blowing agent. If an irregular honeycomb structure is obtained, it is likely due to insufficient mixing in the extruder or feed. Excessive feed throat temperature has also been shown to result in non-uniform cell structure.
Pre-foaming in the mold can also be related to poor cell structure. This is caused by excessive pressure drop within the mold and can be counteracted by tightening the mold gap or shortening the mold platform. Lowering the die temperature may also help build back pressure.
CFA injection molding
Weight reduction and the removal of sinks are the two main reasons why chemical blowing agents are used in injection molding. When using CFA to reduce weight, it is important to reduce the shot size for short shots and use foam to help fill the mold. For example, if the target weight is reduced by 10%, it is recommended to reduce the injection volume by approximately 10% by weight.
The amount of blowing agent added should be steadily increased until the part is filled. To the extent that increasing the blowing agent does not improve part filling, reducing the holding and holding pressure and time may allow the blowing agent to expand further. If the part is still short at this point, you may need to increase the shot size slightly.
To remove the sink, add a foaming agent to help fill the parts. If the addition of blowing agent alone does not eliminate sink marks, use the method above to reduce the holding and holding pressure and time.
The general rule of thumb for CFA injection speed is that the faster the better. Accumulators are often used to achieve this, such as in the case of structural foam molding. Faster injections allow for even expansion of the blowing agent. But it can backfire if the mold isn't adequately vented. In some cases, reducing clamping tonnage is a proven solution. It has been noted that high speed at the start of the injection and then reduced injection speed at the end of the injection works in other situations where venting and part geometry are limiting factors.
Post-expansion or post-foaming of a part after demolding can have a number of causes, including insufficient cooling and excessive blowing agent dosage. If increasing the cooling time or reducing the CFA dose does not work, the injection volume may be too large.
A rough surface or flared appearance is associated with parts foamed for weight savings on standard high and low pressure injection molding machines. Reducing the amount of blowing agent, increasing the injection speed and pressure, and even increasing the mold temperature are all attempts to improve the surface appearance of the part.
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