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

l E I N E M A N N

OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS

SAN DlEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO

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Drawing Number

The drawing number is used for part identification and to ease storage and retrieval of the drawing and the produced parts. While there is no set way to assign part numbers, common systems are nonsignificant, significant, or some combination of the two previous systems.

Nonsignificant numbering systems are most preferred because no prior knowledge of significance is required.

Significant numbering systems could be used for commonly purchased items like fasteners. For example, the part number for a washer could include the inside diameter, outside diameters, thickness, material, and plating.

A combination of nonsignificant and significant numbering systems may use sections of the numbers in a hierarchical manner. For example, the last three digits could be the number assigned to the part (001, 002, 003, etc.). This would be nonsignificant. The remaining numbers could be significant: two numbers could be the model variation, the next two numbers could be the model number, and the next two could be the series number while the last two could be the project number. Many other possibilities exist.

Dimensioning and Tolerancing

Handbook

Paul J. Drake, Jr.

McGraw-Hill

New York San Francisco Washington , D.C. Auckland Bogata

Caracas Lisbon London Madrid Mexico City Milan

Montreal New Delhi San Juan Singapore

Sydney Tokyo Toronto

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DEFINING PLASTIC PART REQUIREMENTS -2

Weather Resistance Temperature, moisture, and UV sun exposure affect plastic parts’ properties and appearance. The end-use of a product determines the type of weather resistance required. For instance, external automotive parts such as mirror housings must withstand continuous outdoor exposure and perform in the full range of weather conditions. Additionally, heat gain from sun on dark surfaces may raise the upper temperature requirement considerably higher than maximum expected temperatures. Conversely, your requirements may be less severe if your part is exposed to weather elements only occasionally. For example, outdoor Christmas decorations and other seasonal products may only have to satisfy the requirements for their specific, limited exposure. Radiation A variety of artificial sources — such as fluorescent lights, high-intensity discharge lamps, and gamma sterilization units — emit radiation that can yellow and/or degrade many plastics. If your part will be exposed to a radiation source, consider painting it, or specifying a UV-stabilized resin. Appearance Aesthetic requirements can entail many material and part-design issues. For example, a need for transparency greatly reduces the number of potential plastics, especially if the part needs high clarity. Color may also play an important role. Plastics must often match the color of other materials used in parts of an assembly. Some applications require the plastic part to weather at the same rate as other materials in an assembly. Engineering Polymers Part and Mold Design THERMOPLASTICS A Design Guide Bayer Corporation • 100 Bayer Road • Pittsburgh http://www.bayer.com/polymers-usa