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Thin-wall molding.

Nowadays, you may be faced with molding thinner walls than you're used to. In order to you meet that challenge, a maker of engineering resins shares what it has learned about thin-wall processing through years of experimentation at its Polymer Process Development Center.

"Thin is in" for reducing part costs - not just with makers of portable electronics, but for a variety of consumer products as well. Manufacturers are designing ever-thinner plastic parts and demanding that injection molders keep pace. And it's a trend that shows no sign of letting up.

The conventional view of thin-wall molding is that it isn't easy. These parts have long flow lengths in thin wall sections - typically at length/thickness (L/T) ratios ranging from 100:1 to more than 125:1. (These are rough guidelines for parts made of engineering materials.) To fill them at all requires injection pressures and injection velocities far above those needed in conventional molding (see table).

Applications on the cutting edge of thin-wall molding, where pressures exceed 30,000 psi and fill times drop into the tenths of a second, require dedicated molding machines capable of high pressures and high speeds. But there is hope for those wanting to mold thinner without having to invest in specialized equipment. Thanks to the improved capabilities of modern off-the-shelf molding machines, thinner walls with length/thickness ratios of 100:1 are very often within the reach of a large number of molders.

Whether or not your applications test the limits of current technology, successful thin-wall processing comes down to a systems approach that carefully takes into account the interactions between the part design, the material, and the process. These three interrelated topics are each worth an article on its own. Here, we'll focus on processing - the area that molders have most control over. In this review of mold and machine requirements, note that some are unique to thin-wall molding and represent recent technology developments, while others are already common practice for many molders.

A Capable Machine

The good news is that you don't necessarily have to have a specialized machine in order to join in the thin-wall action unless you aim to ride its leading edge. Many machines in the field have the pressure and speed capabilities to handle parts with walls that are often 30% thinner than current applications. But as the flow-length-to-wall-thickness ratio continues to rise, fewer and fewer nonspecialized machines will be able to do the job.

When evaluating the suitability of your press for a particular thin-wall job, it's important to look at each aspect of the machine and compare it against the capabilities of a specialized thin-wall molding press.

Downsize injection units

If you want to use a standard machine to mold thin-wall parts, pick an appropriately small barrel size. Look at this example from our own shop floor at GE's Polymer Processing Development Center (PPDC). Our Husky 550-ton thin-wall machine has a 10-oz barrel, far less than the 60 oz you might otherwise find on a machine that size. Likewise, our 1700-ton Sandretto thin-wall machine has a barrel size of only 90 oz. Besides not giving your machine the pressure boost it needs, too large a barrel will result in more residence time than many thin-wall materials can tolerate.

The optimum barrel size will change from machine to machine and part to part. To pick the proper size, however, focus on the relationship between the shot size and barrel size. The shot size should be at least 40% of the barrel size - and that's a bare minimum for engineering resins.

While you're downsizing your injection unit, don't forget to downsize the dryer as well. You don't want resin sitting in the drying hopper for extended periods of time or it could degrade. So either use a smaller hopper or install a level switch that matches the projected material throughput.

Specialized thin-wall machines typically offer injection pressures above 30,000 psi, and we've run some jobs up to 35,000 psi on our Husky and Sumitomo thin-wall machines. Good off-the-shelf standard machines now offer injection pressures over 20,000 psi, which is adequate for a range of thin-wall parts - including some notebook-computer and cellular-phone housings.

If you have a conventional machine, downsizing the barrel can boost the available injection pressure by changing the hydraulic intensification ratio. However, our experience has been that most standard machines, even with a downsized barrel, are ultimately more limited by injection speed than by pressure. Speed is very important in filling thin-wall parts because they need to fill fast to avoid freeze-off before the cavity is filled and packed. Thin-wall parts will need injection velocities of more than 3 in.3/sec, and many jobs will require more than 6 in.3/sec. If your machine can deliver enough speed for thin-wall molding, it's unlikely to be pressure limited.

We have found that thin-wall applications require screws similar to those used for any of our engineering materials. The same goes for check rings. However, more development is needed to get the right screw design for faster screw recovery with a minimum increase in shear.

Thick platens for thin walls

Dedicated thin-wall machines often will adequately handle applications requiring up to 5-7 tons/sq in. of clamp force - more than the 2-4 tons/sq in. applied by many ordinary machines. The more clamp tonnage you have available on a standard machine, the less likely you'll be limited by clamp force. Specialized thin-wall machines also tend to have accumulator-assisted clamps because thin-wall molding's fast cycle times require fast clamp movements.

Extra-thick platens help the machine withstand thin-wall injection pressures. The Husky thin-wall machine at the PPDC, for instance, has platens about a third thicker than a standard machine of the same [TABULAR DATA OMITTED] tonnage. On the best thin-wall machines, the ratio of tiebar distance to platen thickness is 2:1 or less.

Are your controls up to it?

With fill times of less than i sec, thin-wall injection machines are usually microprocessor controlled. Typical control resolution is 0.04 in./sec on injection speed, 1 bar on pressure, 0.004 in. on position, 0.01 sec on time, 1 rpm on rotation, and 0.10 tons on clamp force.

Less capable controllers on standard machines will also support thin-wall molding if they offer closed-loop control of injection speed, hold pressure, screw rpm, backpressure, and all machine temperatures.

A Capable Mold

Tooling for thin-wall molding must be built to withstand high injection pressures and velocities. A good thin-wall mold must be more robust than a conventional tool or it will deflect during injection, causing flash and premature mold wear. Glass-filled materials, which are not uncommon in thin-wall applications, only magnify these problems.

Build it hard and thick

The entire tool should be constructed from steels harder than P20. Hard steels such as H-13 and D-2 have been used successfully for gate inserts.

Mold plates should be thicker than those used for a conventional tool. For example, leading-edge thin-wall applications - those with pressures over 30,000 psi - would call for mold-support plates at least 50% thicker than for standard molds.

To ward off deflection in the cavity region, use as many of the heaviest support pillars as you can fit in the tool. Preloading between 0.004 in. and 0.005 in. is common. Mold interlocks will likewise help minimize tool shifting.

Use ample venting and cooling

Short fill times make efficient venting essential. Use vent sizes recommended for the material but increase the amount of vents around the part. The most demanding thin-wall applications require venting the core pins and ejector pins, in addition to venting along the parting line. Some molders have tried sealing off the parting line with an o-ring and drawing a vacuum in the tool. But use caution: Most machines start injection before full clamp force is reached. So unless you start drawing the vacuum before injection, this process will not work.

Another tooling challenge is to provide for adequate mold cooling when cycle times are so short. Helpful cooling aids include non-looped cooling lines in the core and cavity blocks; larger cooling lines to increase flow through the tool (rather than relying on lower coolant temperatures); and, where necessary, inserts made of special high-conductivity metals for faster heat transfer.

Be generous with ejectors

Thin-wall part ejection presents some hurdles for a couple of reasons. First, thin walls and ribs are more easily damaged than thicker parts. Second, decreased shrinkage through the minimal wall thickness can interfere with part release.

To get around these problems, a thin-wall tool should have bigger ejector pins and more of them than a conventional tool. As a rule of thumb, a thin-wall tool will have twice as many pins that are twice as large as those found on a conventional tool. Many thin-wall parts will need blade- or sleeve-style ejectors on ribs and bosses to avoid damage to these features.

Because ejection forces are necessarily higher, ejector pins should be backed up with support buttons to avoid hobbing the ejector plate.

Part release can also be aided by draw polishing all internal features, by employing mold surface treatments (such as nickel-PTFE), and by using sufficient draft angles (minimum 1 [degrees]/side + 1 [degrees]/mil depth of texture).

And don't forget that robots can be very useful for thin-wall parts removal because many of the high-speed robots on the market can remove the part faster than gravity alone can achieve.

Gates and runners

Many standard gates, sprues, and runners can withstand the pressures of thin-wall molding. The trick is sizing these components to keep pressure drops and residence times to a minimum. Large sprues, runners, and gates will cut pressure drop, but they can boost residence time. Keep the gates small, and pressure drop may increase to the point where the tool won't fill. To find the optimum balance between residence time and pressure drop, a computer-aided mold-filling analysis can help. But be careful to use the right injection profile for the filling analysis. In general, a thin-wall mold's gates will be larger - and often more numerous-than in conventional molds.

Hot-runner manifolds are typically sized so that passages offer minimum pressure drop. Flow passages must have no sharp corners where resin can hang up.

Reverse gating sometimes presents another obstacle in that it may require reverse ejection, which can interfere with the manifold. The resulting mold can have the ejector pins protruding through the manifold, changing the manifold's heat distribution. Instead, consider moving the manifold to the stationary platen side of the ejector plate, though you will have to accommodate the longer drops and residence times that this move entails. We estimate that such "A"-side ejection could increase tooling costs by 50-60%.

Thin-wall Costs, but it pays

Thin-wall tools, which demand high quality and robust construction, can cost 15-25% more than conventional molds. Thin-wall machines are specialty models that may cost more as well, though it is more difficult to say how much, given the range of machines on the market. But for any increase in tooling and machine expense, thin-wall technology can still provide a lower system cost through faster cycles and reduced material usage.
COPYRIGHT 1998 Gardner Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1998, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:plastics molding
Author:Tremblay, Greg
Publication:Plastics Technology
Date:Apr 1, 1998
Words:1862
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