Processing, thermoplastics clamping pressure

The molds in a RIM-process plant are located in a separate assembly, which has devices for opening and clamping a mold and, in some cases, for changing it. The pressure in the RIM-process is considerably lower than in traditional injection molding of thermoplastics thus, the requirements for the mold material are less rigid. Molds may be made from aluminum or reinforced plastics, since high-strength steel is not necessary. 

With the pressures in the mixing head at between 1,500 to 3,000 psi (10.3 to 20.6 MPa), the in-mold pressures are significantly lower than in many of the other molding processes. When comparing a typical RIM in-mold pressure of 50 to 150 psi (0.4 to 1.1 MPa) with the 5000 to 30,000 psi (34.5 to 206.7 MPa) required for thermoplastic injection molding (Chapter 4), it becomes apparent why RIM is particularly suitable for larger parts. Automotive bumpers are routinely produced on RIM presses with 100 to 150 tons of clamping force, while comparable injection molded parts require presses of 3500 tons or more. 

Blow molding is now a highly developed process with many variant forms (Fig. 10.31). At its most basic, the process involves melt processing a thermoplastic into a tube that is generally referred to as a parison. While still in a heated ductile plastie state, the parison is clamped between the halves of a eooled mold, so that the open top and bottom ends of the parison are trapped, compressed, and sealed by the mold faces. A blowing tube is also trapped in one of the parison ends, creating a chaimel through which air pressure is introduced within the sealed parison. Air pressure eauses the parison to expand like a balloon, so that it takes the form of the mold cavities. Contact with the eooled mold chills the thermoplastic to its solid state, so the form is retained after the mold is opened and the part removed. 

Injection molding of thermoplastics is probably the most important and popular plastics forming process. In order to manufacture high quality parts, it is important to assure balanced flows during the filling of the mold. In this condition, the boundaries of the mould are filled at the same time with more uniform pressure leading to a decrease of the maximum required injection pressure and clamp force. 

---