Three Advancements That Have Driven Injection Molding Technology

Injection molding technology has made major advances in the last 25 years, from largely a world of shoot-and-ship to doing as much as possible at the press — using automation to remove parts and assemble them, two-shot molding, even assembling parts inside the mold.

Now instead of a big inspection department, constantly pulling parts off the line for testing, at many plastics factories the operator is responsible, backed up by in-mold sensors that can tell even before the mold opens if the part is good or bad.
The molding press has become the control center of the manufacturing cell, linking auxiliary equipment, material handling and other equipment together in the machine controller. That has been a constant evolution since 1989.

Three landmark changes have happened in injection molding in the last 25 years: all-electric technology, two-platen presses, and tie-barless machines. The tie-barless injection molding machine — just like Plastics News — is 25 years old this year.
None of the three were as revolutionary as the reciprocating screw, invented by William H. Willert in 1952, although the long-term impact of all-electric injection presses could come close. The screw was a sweeping, epic change that relegated the old plunger, if not to the scrap heap, then to jobs making marbling effects on cosmetic packaging and other narrow applications. (Decades later, the plunger found new life on some large-tonnage presses that used a screw to fill a shooting pot, then the plunger drives it home to the mold.)
Here’s a look at the evolution of the three technologies:

All-electric machines

All-electric injection presses sip energy, because the servo-electric motors only use power as needed to run machine movements. When the machine is not moving, no power is used. Electrics also offer precision and rock-solid repeatability.

Older hydraulic machines relied on constantly running pumps to create the oil pressure used to power machine movement — although manufacturers developed so-called “smart hydraulics” that use servo valves, variable-speed motors and displacement pumps.

All-electric technology was born in Japan, where environmental laws severely limited the disposal of used hydraulic fluid, and industry embraced energy efficiency to combat high energy costs. All-electrics have reached market penetration in Japan of more than 75 percent.

All-electrics account for about half of the U.S. market for injection presses, industry observers say. Now, most small-tonnage presses are all-electrics. There are some big all-electrics, too, but many machinery manufacturers offer hybrid systems, combining hydraulics for some motions and electric for others.

Milacron did the early missionary work for all-electrics in North America, introducing in 1990 its first all-electric, a press built in Japan by Fanuc Ltd. Most U.S. molders knew very little about the servo-driven process before then. Milacron launched its own U.S.-made all-electric at the NPE show in 1994.

The K show in 2001 marked a giant step forward in all-electrics, as molders worldwide were inundated with sales pitches for the technology. Yes, Milacron and the Japanese machinery makers already offered the presses. But at the 2001 trade show in Germany, more European companies came out with their own all-electrics. K 2001 made it clear that the all-electric injection molding machine was not a passing fad.

The K 2001 show kicked off the Great Hydraulic vs. All-Electric Debate that raged for years. You don’t hear it much anymore, probably because the two technologies have come closer together.
All-electrics cost more than hydraulic presses, although the price premium has declined as higher production of electric motors, ball screws and other components gives improved economies of scale. Even so, many molders in Germany and some other European countries still like the ever-improving hydraulics.

Looking ahead to the next 25 years, all-electrics could get a second look in Europe. Europeans are serious about energy efficiency — driven by relatively high German energy costs, that country’s aggressive moves to renewable energy, and the energy standards and labels pushed by Euromap, the machinery trade association.

Two Platens
Buyers of injection molding machines got inundated with two-platen press designs in the mid-1990s.
Most traditional injection presses used three of the heavy steel plates that support the mold assembly — one for each half of the mold, called the moving and fixed platens, and a third platen, usually called the end platen. The end platen’s job: transmit clamping force to the tie bars and moving platen.

Manufacturers brought forth a bunch of two-platen machines at the 1995 K show in Düsseldorf, Germany. Two years later, at NPE 1997 in Chicago, machinery executives proclaimed two-platen machines were here to stay.

By using two platens instead of three, they could greatly reduce the overall length of the machine. That becomes more important for bigger machines that generate larger clamping forces. Also, the machines were a bit less expensive to produce.

Ironically, back in the early days of plastics, many injection machines used just two platens. Later, the third platen was added for stability and to make the presses more robust.

Two decades ago, taking away that third platen required machinery engineers to come up with clamping systems, using locking nuts, special pressure plates and cylinders and pancake-shaped rams. The key challenge: Solve the problem of platen deflection — the tendency of the large steel plate to flex under clamping force.

The two-platen evolution also promoted big advances in platen design. Platens are no longer just a big plate of steel.

Tie-barless

Austria-based Engel Holding GmbH pioneered the tie-barless molding machine, introducing it at the 1989 K show. The tie bars are thick machined bars that link the platens together. They absorb clamping force and keep the platens parallel — a critical function for injection molding.
Making a press with no tie bars was a major breakthrough, because Engel had removed a major obstacle to getting access to the area around the mold. That enabled easier use of robots to remove parts. You also could use larger molds in a given machine — no longer limited by the tie bars.
It’s easier to change molds.

Engel engineers used know-how of physics and principles of mechanical engineering. In the past 25 years, Engel has sold thousands of tie-barless injection presses.

This article was written by By Bill Bregar
SENIOR STAFF REPORTER for Plastics News

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