About Plastic Injection Molding

The plastics industry is one of the fastest-growing major industries in the world. Every year there is an increase in the amount of plastics used in all types of products. A good example of this is the per cent of plastics used in today’s automobiles compared to 15 years ago.

The plastics injection molding field, at large, is volume-oriented. Vendor sources, particularly mold makers and custom molders, are geared to long tool life and high volume production. Plastics equipment manufacturers, likewise, have concentrated almost exclusively on fully automatic, sophisticated injection molding machines whose economics lie in single runs of 100,000 parts or more and in multi-shift operations.

The continued rate of growth in the industry hinges on the development of improved and new thermoplastics with greater physical properties. This has opened the door to applications never thought possible before. These are emerging both as product innovations and as existing products converted from materials such as metal, glass, wood or paper to plastics for competitive and economic advantage.

Theory of injection molding

The theory of injection moulding can be reduced to four simple individual steps: Plasticizing, Injection, Chilling, and Ejection. Each of those steps is distinct from the others and correct control of each is essential to the success of the total process.

Plasticizing – describes the conversion of the polymer material from its normal hard granular form at room temperatures, to the liquid consistency necessary for injection at its correct melt temperature.

Injection – is the stage during which this melt is introduced into a mold to completely fill a cavity or cavities.

Chilling – is the action of removing heat from the melt to convert it from a liquid consistency back to its original rigid state. As the material cools, it also shrinks.

Ejection – is the removal of the cooled, molded part from the mold cavity and from any cores or inserts. Repetition of these basic steps in sequence is the process of injection molding.

One of the best idea guides for “in-house” injection moulding applications is a book called “Cutting Costs in Short-Run Plastics Injection Molding” written by Morgan Industries, Inc..

This unique engineering manual covers most basic prototypes and short-run applications and mold making process in detail.  Presents actual cases with photos of parts, mold sketches, tool life, quantities and materials molded. Examples include aluminium and epoxy tooling, EDM cavity, encapsulation and inserts. Cost comparisons show savings achieved.

Table of Contents

1. Injection Molding Process
   • Theory in injection molding
   • The practice of injection molding
2. Tool Design and Building
   • Hand molds
   • Semi-automatic molds
   • Automatic molds
   • Tool quality designations
   • Making the tooling decisions
   • Mold design
   • Gating
   • Tool building
   • Tool making materials
   • Mold finishing
3. Part Design and Material Selection
   • Tolerances
   • Thermoplastic materials
   • Fillers and modified materials
4. Epoxy Tooling
   • Advantageous
   • Limitations
   • Types
   • Economics
   • Running of epoxy molds
5. Case Examples of Short-Run Molding Requirements
   • 2 plate tools emphasizing lathe work
   • 3 & 4 plate tools emphasizing lathe work
   • 2 plate tools emphasizing mill work
   • Multi-cavity & semi-automatic tools
   • Epoxy tooling
   • Tools made using EDM technique
6. Material Handling
   • Nylon
   • A.B.S.
   • Acetals
   • Troubleshooting
7. Injection Molding Equipment
   • Morgan-Press injection molding line
8. Appendixes
   • Draft angle table
   • Shrinkage factor table
   • Plastic properties chart