Choosing a High-Speed Vertical Machine for Thin-Wall Parts

Thin-wall parts include connectors, waterproof plugs, eyewear nose pads, buttons, tags, overmolded shoe studs and small parts for clocks and cameras. How should you judge a high-speed vertical machine for this work?

Repeatability

Two measures are commonly used: repeatability of part dimensions and repeatability of part weight. Dimensions are hard to compare between parts of different size and thickness, so shot-weight repeatability is the better indicator of overall machine performance. A standard vertical machine typically repeats to around 1% by weight; a better machine reaches about 0.8%. Dimensional tolerances for such parts are often within 0.01–0.001 mm.

Injection pressure

• Standard machines: 147–177 MPa
• High-speed precision machines: 216–243 MPa
• Ultra-high-pressure machines: above 243 MPa, typically 243–392 MPa

Higher injection pressure brings three benefits:

• Better precision. Injection pressure has a marked effect on molding shrinkage; at around 392 MPa, shrinkage approaches zero and part accuracy depends mainly on the mold and environment. Tests show that raising injection pressure from 98 to 392 MPa can increase mechanical strength by 3–33%.
• Thinner walls and longer flow. With PC, a standard machine at 177 MPa can mold walls of about 0.8 mm, while 392 MPa allows walls down to about 0.45 mm and a higher flow-length ratio.
• Full use of injection rate. Rated injection rate is reached either by raising system pressure or by modifying the injection unit, including screw parameters.

Injection speed

High-speed vertical machines need high injection speed: above 200 mm/s for hydraulic machines and above 300 mm/s for all-electric machines.