Polymer molding

Thermoplastics are molded as viscous liquids. Injection molding and extrusion dominate, but all molding processes impose flow that can orientate the molecules; if the molding is cooled fast enough the alignment is frozen in (Figure 19.5).

If not, polymers mostly prefer to form an amorphous structure. In some polymers crystallinity may develop on slow cooling. All polymers shrink as the mold cools from the molding temperature to room temperature because of thermal contraction and the loss of free volume caused by crystallization. Allowance must be made for this when the mold is designed.

Materials

Engineering, Science,

Processing and Design

Michael Ashby, Hugh Shercliff and David Cebon

University of Cambridge,

UK

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD

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Butterworth-Heinemann is an imprint of Elsevier

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Design Process

Before any discussion of CAD, it is necessary to understand the design process in general (Fig. 3). What are the series of events that lead to the beginning of a design project? How does the engineer go about the process of designing something? How does one arrive at the conclusion that the design has been completed? We address these questions by defining the process in terms of six distinct stages: 1. Customer input and perception of need 2. Problem definition 3. Synthesis 4. Analysis and optimization 5. Evaluation 6. Final design and specification A need is usually perceived in one of two ways. Someone must recognize either a problem in an existing design or a customer-driven opportunity in the marketplace for a new product. In either case, a need exists which can be addressed by modifying an existing design or developing an entirely new design. Because the need for change may only be indicated by subtle circumstances—such as noise, marginal performance characteristics, or deviations from quality standards—the design engineer who identifies the need has taken a first step in correcting the problem. That step sets in motion processes that may allow others to see the need more readily and possibly enroll them in the solution process. Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition. Edited by Myer Kutz by John Wiley & Sons, Inc.

Variations and Extension to the Injection-Molding Process

Injection Blow Molding.

A preform (this looks like a test tube with bottle cap threads) is injection molded in one cavity, removed and then placed into another where it is pressurized with gas to stretch the hot preform into a thinnerwalled

seamless bottle or container such as a milk bottle or gas tank. This is depicted in Figure 7. This is an extension of injection molding more than a variation.

Injection Compression/Coining.

With this technique the mold is only partially closed during injection. At the appropriate time and with the right amount of plastic in the mold, the clamp is then completely closed, forcing (compressing) the plastic to the shape of the mold cavity. A variation on this is coining.

The clamp is closed but the mold has components that compress the plastic in the cavity as the plastic cools. Coining is where the cavity volume is changing during the solidification of the plastic. Plastic is injected into the cavity and then the movable platen closes completely, or a mold component moves to compress the plastic to compensate for shrinkage or densification.

Gas-Assist Injection Molding.

Here, plastic is injected into the cavity until it is 50–85% full, then high pressure gas, usually nitrogen, is  injected to finish filling the cavity by pushing the plastic flow front to the end of the cavity. This leaves a gas bubble or channel inside the part. This saves plastic, reduces cost, and often improves part strength especially in thick sections. Gas can be injected at the nozzle of the machine or directly into the mold as depicted in Figure 8. Gas-assist molding can be considered as a variation of co-injection molding where the outer layer or skin of the part is plastic and the core is a gas channel rather than another type of plastic.

“Injection Molding” in EPSE 2nd ed., Vol. 8, pp. 102–138, by I. I. Rubin,Robinson Plastic Corp.

JOHN W. BOZZELLI

Midland, Michigan

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Extrusion

General Features of Single Screw Extrusion

One of the most common methods of processing plastics is Extrusion using a screw inside a barrel as illustrated in Fig. 4.1. The plastic, usually in the form of granules or powder, is fed from a hopper on to the screw. It is then conveyed along the barrel where it is heated by conduction from the barrel heaters and shear due to its movement along the screw flights. The depth of the screw channel is reduced along the length of the screw so as to compact the material. At the end of the extruder the melt passes through a die to produce an extrudate of the desired shape. As will be seen later, the use of different dies means that the extruder screwharrel can be used as the basic unit of several processing techniques.

PLASTICS

ENGINEERING

Third Edition

R.J. Crawford, BSc, PhD, DSc, FEng, FIMechE, FIM

Department of Mechanical, Aeronautical

and Manufacturing Engineering

The Queen’s University of Belfast

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OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS

SAN DlEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO

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