In-Mold Assembly the Next Frontier

In this era of flexible automation, it may seem that investing in a niche technology such as a double-turning cube stack mold is stepping back in time to “hard” automation, but conceptually, it is a huge step forward for molders and part designers to know that operations such as in-mold assembly are a reality, not just a theory.

Referring to Milacron/Foboha’s double turning-cube stack mold system for in-mold assembly, Bob Strickley, marketing director for Cincinnati Milacron says, “This is a potent technology for adding value to a part while it’s still “between the tie bars”, and it’s being explored by a number of customers for applications on our larger machines.”

Arburg has done work in this area as well, calling it assembly injection molding. (See Feb. 05 Canadian Plastics)

Arburg’s premise is that two materials which do not bond can be molded one after the other to form one part. Because the materials are incompatible, they can be in contact but still move freely.

Milacron has added precision, cavity-registered assembly to the list of capabilities of an injection molding machine by using Fohoba’s double turning-cube stack mold system. With this set-up, there are effectively three “parting lines”, so a single injection machine can mold two precision parts, from two different materials, and then label and assemble the finished product while in the mold. Demonstrated at the K show, the system produced a two-component lid-within-a-lid. The double turning-cube concept is suitable for high-volume packaging, medical, personal care, cosmetic, electronic and optical components.

The K show demonstration system produced a lidded spittoon built into a lid for a tobacco can. The can lid is injected on one face of cube A at the stationary end of the molding machine, while a smaller lid is injected and in-mold labeled on the B-cube face that’s opposite the moving platen. As the cubes index with each cycle of the machine, the finished parts on each face are snap-fit together at the centre parting line. The finished assembly is removed by a robot.

A clever new development on the dual-injection system shown at K is a secondary injection unit mounted on the moving platen. This not only saves floor space compared with a side-mounted secondary injection unit, but also saves cycle time by allowing the injection nozzle to stay mated to the hot runner system during clamp movement.

Specialized technology appeals to custom molders

Bob Hare, the U.S. general manager of Milacron, explains that the double turning-cube stack mold is an evolution of Foboha and Milacron’s original turning cube stack mold introduced in 2001. “The original turning cube stack system has quickly become mainstream technology, and not surprisingly, most of those systems are in the U.S. because we have the population needed to consume the high output of these machines.”

“What might come as a surprise is that custom molders own more of these systems than proprietary-product companies.”

The part demonstrated at K is also being produced on two injection machines with off-line assembly and labelling so there’s a benchmark to compare the in-mold assembly concept. The result, according to Hare: “Output per square metre is higher, output per kWh is higher, and output per unit of capital cost is greater than any other technology for producing this part.”

More on the rotating mold theme: Gram Technology has developed a system that applies paint to the plastic part while it is still in the mold. The mold spins around a horizontal axis, and the part is painted at the top station. A self-contained paint chamber descends over the part while the paint is applied. Surface quality is improved compared with a paint line, and no surface treatment is required.

Jes Gram, of Gram Technology, explains that a special injection machine is not necessary; it must simply have enough daylight space for Gram’s Spin Stack mold.

Multi-component challenges

Tier One automotive supplier Visteon and Husky Injection Molding Systems Ltd. recently earned kudos for a two-color automotive instrument panel produced by what Husky calls simultaneous-shot injection molding. The process involves the simultaneous injection of two colors into the mold from two different injection units. For the instrument panel, knit lines were placed in areas covered by finish panels on the final product. The two-color process helps to reduce squeaks and rattles between instrument panel and finish panels, and improves scratch resistance.

The panel was molded on equipment supplied by Husky: a large-tonnage Quadloc Dual 1650 two-color injection molding machine with Polaris control and a Tracer TMB robot.

The fact that different materials have different cooling times can sometimes adversely affect the efficiency of multi-component applications. It is always the longest cooling time that determines the cycle time. A new development by Engel may break this rule –the eccentric rotary table.

Engel can install an off-centre, larger-than-standard rotary table, in a vertical orientation on the moving platen. Equipped with three or four stations, the rotary table technology for Engel’s Combimelt machines permits two stations to be used for injection while the other one or two stations remain accessible for other functions, such as cooling, part insertion or part removal, without adding to the cycle time.

One application Engel has demonstrated with the rotary table is a strainer and stream shaper for sanitary tapware. The strainer is molded of PBT and liquid silicone rubber, and in-mold assembled on an Engel Victory Combi equipped with the eccentric rotary table.

Engel notes that the eccentric rotary table could also be used in conventional injection molding in the case where cooling is a limiting factor to cycle time reduction.

Aiming for the automotive glazing market, Battenfeld has designed a rotary platen, two-component injection molding machine which uses the injection-compression process. This technology permits the molding of glazing panels up to 1.8 m2 with items such as sealing lips, fastening elements or lamps directly integrated.

Improving on its IMPmore injection-compression process, Battenfeld has developed a new generation of machine, the HP 2P WE rotary platen machine. The machine has two movable injection units mounted facing each other on the machine’s central axis. Between these two units is an axially-fixed rotating platen, which can be turned 180 degrees on its vertical axis, bearing one mold half on each side.

The tie bars are completely retractable, allowing unrestricted access to the mold space, easy insertion of the mold and lots of possibility for automation.

Having the injection units placed on opposite sides of the rotating platen allows for simultaneous parallel functions, thus reducing cycle times. For example, while a frame is being molded by the second injection unit, simultaneous injection of the next glazing preform takes place at the opposite end. The rotary platen design also allows processes such as cooling to be more flexible; each mold could be maintained at a different temperature.

The rotary platen machine is currently available in three sizes: 800 tons, 1300 tons and 2700 tons.

Use cycle time wisely

Both multi-component molding and its more recent offshoot, in-mold assembly, have their roots in the quest for efficiency. Similarly, tandem molds aim to gain more productivity from a single machine.

A Husky Tandem molding system has also shown benefits for molding automotive door panels. The Tandem system is integrated with Husky’s Quadloc machine; it allows the simultaneous operation of two standard, single-face molds in one machine. It is ideal for high-volume production of two identical or similar parts. Adding even more flexibility, the latest generation Tandem system permits simultaneous operation of two molds with different shut heights and dissimilar opening strokes.

Sma
ll machine specialist Boy demonstrated at the K show a tandem mold which increased productivity by about 50% by putting the delay and pause times inherent in injection molding to good use. The tandem mold allows two molding cycles to be interwoven.The Simplex tandem mold shown at K is equipped with a special feed system — using two heated sprue bushings arranged one behind the other, the parts are filled on two parting lines.

While a molded part is cooling on one parting line, the other is being ejected from the mold and the next injection cycle is beginning. The melt for the furthest plate from the nozzle is injected through the part on the plate closest to the nozzle.

To make full use of this mold’s capacity, the Boy 35 was equipped with a servo-electric screw drive.

According to Boy, the system performs well for technical parts weighing up to 73.7 g.

Multi-component molding is not for everyone

While the part volumes may be high, machinery sales in this specialized market are not. Steve Elliott, manager machine sales for Engel Canada Inc., says industry-wide, multi-component machine sales only made up about 3% of the North American IMM market in 2004.

However, he adds, “We predict a 5% growth in the multi-component market over the next year.”

ENTRY-LEVEL MULTI-COMPONENT MOLDING

There can be tremendous variety in the details of multi-component molding, but typically, multi-component molding (also called two-shot molding) uses multi-sprue systems, where the molding machine has two separate injection units, and the melt is led separately into two distinct cavities. This method can be expanded into three, four, five or even six injection units.

Arburg’s Uwe Haupt separates molds for multi-component molding into several basic designs:

* rotation of the moving side;

* rotation of a section of the moving side;

* rotation of a centre plate of stack molds;

* no rotation, but moving one or several cores (sometimes called core-back technology).

Haupt says rotation of the moving side is, in many cases, the entry-level technology for mulit-component molding. This is typical of an overmold situation. The drawback is that the second shot can only make changes to the nozzle side of the part.

Haupt explains that machines for multi-component molding need independently regulated injection units, and a large enough space between tiebars to allow the rotation of the mold, or part of the mold. The stack height of two-component molds is significantly higher as there are frequently two independent hot runner systems to accommodate.

As well, Haupt notes that the machine control must be very flexible, because sequencing the core pulls and ejector movements can be a challenge. He cites one five-component application for toothbrushes in which each injection unit had independent coloring and feeding devices.