By: Brian Jung, Date: 02/12/2018
Summary
Injection molding is a popular manufacturing process for mass production of parts. Parts are created by injecting materials that are melted and injected into a mold to solidify into desired shapes. Injection molding has a wide range of applicability from aerospace parts to Legos.
Background
In 1872, John Wesley Hyatt, patented the first injection molding machine. The machine used a plunger to inject heated plastic into a mold.
Until the mid 20th century, injection molding machines used only the heating barrel to heat the plastic, which did not yield good results. The plastic pellets were either too cold on the inside or too hot which degraded the plastic. This issue was solved with the invention of the reciprocating screw.
Until the mid 20th century, injection molding machines used only the heating barrel to heat the plastic, which did not yield good results. The plastic pellets were either too cold on the inside or too hot which degraded the plastic. This issue was solved with the invention of the reciprocating screw.
Application
Advantages
Commercial(+/- .008) and fine(+/- .002)
Shrink rates depend on material
- High production rates
- Repeatable high tolerances
- Minimal scrap loss
- Low Cost
- High initial costs
- Long lead times
- Difficult to make changes
Process Details
- Desired part is designed
- Mold of desired part is made
- Material is fed into machine
- Reciprocating screw melts and moves material
- Screw injects material into mold
- Mold opens
Costs
- Injection mold machines and molds vary in cost
- High end machines cost millions of dollars
- Single cavity molds cost around $1,000-$2,000
- Unit cost of a Lego block: $0.10