Injection Molding of Rubber

Injection molding is now a well-established fabrication process in the rubber industry. Its advantages in most situations over the older processes of compression and transfer molding have been amply demonstrated [1 to 7]. These advantages comprise reduced labor costs, shorter cure times, better dimensional control, and more consistent mechanical properties of the product.

This chapter will give an overview of the injection molding process and of the equipment used. More detail can be found in the references, which are the sources of much of the information in this chapter. The operation of an injection-molding machine requires: feeding, fluxing and injection of a measured volume of compound, at a temperature close to the vulcanization temperature, into a closed and heated mold; a curing period; demolding; and, if necessary, mold cleaning and/or metal insertion, before the cycle starts again. For maximum efficiency, as many of the above operations as possible should be automatic.

There are three main types of injection machine: the ram type, the in-line reciprocating screw type, and the out-of-line non-reciprocating screw type (Figs. 9.3±9.5) [2 to 8].

Simple ram machines cost less than screw machines and because the ram can be made to fit very tightly in the cylinder, they can develop very high injection pressures. However, as the mix receives heat only by thermal conduction from the barrel, high injection temperatures and thermal homogeneity are difficult to achieve, and they are not widely used.

The screw in the in-line reciprocating screw type acts both as an extruder and a ram. In this type of machine, the mix is heated and plasticized as it progresses along a retractable screw. When the necessary shot volume has accumulated in front of the screw it is injected by a forward ramming action of the screw. With this system, a more uniformly controlled feeding of the material can be achieved, together with more rapid heating of the stock from mechanical shearing, additional distributive mixing from the rotational screw action, a greater degree of thermal homogeneity, and a temperature 20 to 30 °C higher than the jacket temperature. However, during the injection stage, when the screw is acting as a ram, there is inevitably some leakage back past the flights and this limits the achievable injection pressure.

The out-of-line non-reciprocating screw machines have separate screw and injection chambers and combine the advantages of the above two types. The screw plasticizes the compound and delivers it through a non-return valve into a separate injection chamber. Machines of this type can generate injection pressures of up to 200 Mpa and can efficiently mold high viscosity mixes and effectively fill large volume molds.

In the standard injection process described above, the compound is injected into a closed mold; there are two variations on this:

• Injection-compression molding: The mold is partially opened and a vacuum applied to the cavity area which is sealed by a compressible silicone gasket. A measured amount of rubber is injected into the partially opened mold. The mold is then closed and the excess rubber is forced outward to flow off channels. This process is used for articles such as precision O-rings, where runner marks are unacceptable.
• Injection-transfer molding: The rubber is injected into a transfer chamber and then forced from the transfer chamber into the mold. This combination process uses the plasticization and heat generation advantages of the injection unit with the controlled flash pad and cavity layout advantages of the transfer press.

All of the above systems have been in use for many years. The equipment manufacturers are constantly improving the design, operation, and control of their machines, but in general, available equipment is based on the systems described above. The equipment manufacturers' concentration has been on data acquisition and process control systems to enable the processors to implement on-line statistical process control. This has been in response to the end-users demands, especially from the automobile industry, for defect-free products. Another major pressure on suppliers to the automotive industry is cost. This, in turn, is reflected onto equipment manufacturers to provide cheaper, more efficient machines to allow the processors to make parts more cheaply, but with no loss in quality.