Many common rubber-based adhesives may be used to bond SAN parts to a variety of non-plastic materials such as metal, wood and glass, and to certain other plastics. Excellent results are obtained by sonic welding SAN parts to each other, and also to parts molded of ABS. Single-strip foaming with direct injection of physical blowing agent: (see Figure 1) With single-step/direct injection, the blowing agent, is injected into the extruder under high pressure.

Better mixing

The foaming process is influenced by the flow properties of the plastic melt, the gas permeability of the plastic, and the properties of the blowing agent. Important properties are the vapor pressure, the solubility and the decomposition temperature. It is important in foam extrusion to maintain the melt pressure in the partial pressure of the blowing agent.

Troubleshooting in foam extrusion

The mixing process that is critical in foam extrusion is the distributive mixing of the blowing agent into the plastic melt. It is well known that simple conveying screws do not have good mixing capability. Mixing can be improved significantly by adding mixing sections along the screw. Features that are important in obtaining efficient distributive mixing are - Frequent splitting and reorientation of the flow; The barrel should be completely wiped for efficient heat transfer; The mixer geometry should allow for streamlined flow; The mixer should have forward pumping capability.
There are numerous mixing elements used for single screw extruders. A comparison of some distributive elements is shown in Table 1
The Saxton mixer has forward conveying capability, a streamlined geometry, wipes the entire barrel surface, provides for frequent splitting and reorientation. Another mixer that provides for frequent splitting and reorientation is the cavity transfer mixer or CTM, developed Gale at RAPRA in England. The CTM has hemispherical cavities in both the screw (rotor) and barrel (stator). As the plastic melt moves through the CTM it passes from screw cavities into barrel cavities. Each time that happens, the fluid element is cut and reoriented by the screw, resulting in efficient distributive mixing. Because of the hemi-spherical cavities the flow is not very streamlined, there is no forward pumping, the mixer is difficult to clean and it is rather expensive. As a result, the overall characteristics of the CTM are not very attractive.
Another screw design issue that affects the heat transfer is the surface renewal of the melt. This is affected primarily by the helix angle of the screw flight, the number of flights, and the flights clearance. It has been found that multiple flights can improve heat transfer provided that appropriate values provided that appropriate values of the helix angle and flight clearance are used.
A small flight clearance is beneficial to good heat transfer between the melt and the barrel. It is obvious that for efficient heat transfer we should use not only barrel cooling but also screw cooling. The barrel consists of about 50% of the total available heat transfer surface; the screw surface makes up the other 50%
For efficient heat transfer the total available heat transfer surface should be used; thus active temperature control of both the screw and the barrel is necessary.

Better melting

The screw design issues affecting the melting process have been described in details; they are: the flight helix angle, the number of flights, and the flight clearance. A large flight helix angle is beneficial and multiple flights are beneficial as well. The benefit of multiple flights is due to multiple solid beds; as a result, this benefit does not apply to barrier screws that have only one solid bed. With multiple solid beds the average melt film thickness is reduced, resulting in more efficient melting. The initial melt film thickness is primarily determined by the flight clearance. Thus, a small flight clearance is important to achieve high melting efficiency. Another method of shortening the melting length in plasticating extruder is to preheat the plastic in the feed hopper. The most efficient melting is achieved in twin screw extruders and reciprocating single screw mixers. In these machines the melting process occurs by the dispersed soiled melting mechanism, which is more efficient than the contiguous solids melting mechanism that prevails in most regular single screw extruder.

Better melt temperature control

The most important requirements for screw design in foamed extrusion is good mixing, melting, and melt temperature control. Good mixing requires the use of efficient distributive mixing sections. Several distributive mixing sections have been described and compared. Efficient melting can be achieved with multiple screw flight combined with a relatively large helix angle and a small flight clearance. Melt temperature control requires low viscous heat generation and efficient heat transfer from the melt to the barrel screw. Viscous heat generation is primarily determined by the channel depth. Heat transfer is determined by the number of flights, the helix angle, the flight clearance, and the presence of mixing sections. Simply conveying screws have a tendency to develop a large temperature difference across the screw channel. This can be minimized by the mixing sections, multiple flights, and small clearance.

Courtesy: Mr. P.M. Jariwala, Kolsite Industries