By Design: Making more with less

By: 
August 11, 1998




In this bimonthly column, Glenn Beall of Glenn Beall Plastics Ltd., Libertyville, IL, shares his special perspective on issues important to design engineers and the molding industry.

In today's economy, the most efficient molder with the lowest part cost can expect to have that price hammered even lower before receiving a purchase order. The lowest bidder is subsequently expected to make annual cost reduction while providing additional customer services. Profit margins are being squeezed, but creative injection molders continue to find ways to reduce cost.

Cost Drivers

Once the design of a new plastic part is finalized and a mold is built, these two cost drivers are fixed. That leaves material and molding as major cost factors. Molding cost is dependent on how efficiently a molder operates his machines and how well he manages his company.

The material's cost per pound is established by the material manufacturer. Only the largest of the mega-molders is big enough to significantly affect the price set by even larger chemical companies. Smaller molders have little or no chance of influencing the dollar per pound cost of the material they are molding. A small molder can, however, reduce the amount of material used. Eliminating overpacking can save on material used. For example, the weight of a .080-inch-thick, 12-inch-diameter, polypropylene salad bowl can be increased by up to 20 percent by overpacking.

No spillage, optimum gate and runner design, elimination of rejects, runnerless molding, and the coring out of thick sections can all reduce the amount of plastic material being consumed. These are small savings, but you only have to give the customer a 5 percent reduction in order to keep the project another year.

Larger material savings can be realized by reducing a part's thickness. Thin walling is now in vogue, but it's not a new concept. Thin walling was pioneered in the 1950s as the packaging industry converted to plastic. Manufacturers of disposable medical devices and other single-use products quickly followed suit. The September/October 1981 issue of Plastics Design Forum featured an article entitled "Guideline for Thin-Wall Design." In 1995, the General Electric Co. published an excellent 35-page brochure entitled "Thin Wall: Technical Guide for Electronic Applications."

Why Less is Better

Thin walling has always been a method of reducing cost, but it has other benefits. Packaging can never be too low in cost, but thin walling also reduces shipping cost and disposal problems.

There is a need to reduce the cost of health care. In this field, thin walling allows a whole surgical device to pass through a small puncture site instead of a gaping incision.

Today, portable electronic devices are in the vanguard of thin walling, but this is an old concept. In the early 1970s, a state-of-the-art electromechanical calculator cost $1500 and occupied 9 by 16 in of space on a desk. By the late 1970s, Hewlett-Packard and Texas Instruments were producing $450 pocket calculators that did more, faster. Today, those same functions are performed on business-card-sized calculators so low in cost they are given away as advertising premiums. Solid-state electronics made these size reductions possible, but it couldn't have become a reality without thin-wall plastic parts.

Cameras, laptop computers, cell phones, and pagers are now pushing the limits of thin-wall molding. The Original Equipment Manufacturers (OEMs) appreciate the reduced material costs, but there are other, equally important, benefits. Portable products can never be too small in size or too light in weight. One OEM even toyed with the concept of marketing its cell phone as a form of jewelry to be worn as a necklace. OEMs are cramming more functions into increasingly smaller devices. Thin walling allows more functions to be included within a given space.

The renewed interest in thin walling is not limited to small electronic devices. Automobile bumpers and body panels get a few thousandths of an inch thinner with each model change. Business machines, appliances, and toys are also slimming down. The OEMs pursuing this approach are primarily looking for cost reductions. A thinner wall reduces both material and molding cost. Reducing a .125-inch wall to .090 inch could reduce the molding cycle from 40 to 30 seconds.

Traditionally, .125 inch was always considered to be a good compromise between ease of melt flow, cycle time, and stiffness. Product designers specified thicker or thinner walls, based on the size of the part, its load bearing requirements, and the type of material being molded. Some small parts have been molded with thicknesses of less than .010 inch. Some large parts have wall thicknesses of 4.5 inches.

Defining Thin Walling

The general shift toward thinner walls has now resulted in average thicknesses of .080 to .125 inch. Thin walling is considered to be in the range of .050 to .080 inch in thickness. Really thin-wall parts are in the range of .030 to .050 inch. Small parts, packaging, and small sections in thicker parts are even thinner.

Thin walling is a desirable technique that can reduce the cost, size, and weight of a molded part. Like all technologies, thin walling is used and misused by customers. There is more to thin walling than just specifying a thinner wall on the part drawing. This is a system that involves paying careful attention to all aspects of material selection, mold design, and molding. Too many OEMs go for the lower price without accepting the whole system.
Material selection. All other things being equal, the distance a material will flow through a cavity is directly related to the thickness of the flow path. The large flow-length-to-thickness ratios associated with thin walling may exclude the use of older, reliable materials a molder and his customer have become comfortable with. Material manufacturers are a good source for guidance on which materials can be relied upon to fill
a thin-wall cavity. Computer-aided mold filling analysis can also indicate whether or not the chosen material can successfully fill a cavity for a thin-wall part.

The most reliable method of evaluating a thin-wall part is a good quality prototype mold. A prototype mold will prove moldability while providing test parts. Properly done, the prototype mold can also sustain market introduction while a production mold is being built.

Mold Design and Construction.

Conventional but thin parts can be successfully molded with a good quality standard mold. Attempting to use a conventional mold to produce thin-wall-and especially the really thin-wall-parts is courting disaster. When a molder sees a part drawing or database showing a thin-wall part, it is given that high injection pressures and rapid cavity filling will be required to mold that part.

High injection speed and packing pressures can combine to cause conventional molds to flex and flash. The most common approach to overcoming this problem is to reduce packing pressure. With conventional molding, this particular approach produces filled but underpacked parts. With thin-wall molding, the same approach results in short shots. The molds for thin-wall parts have to be designed with increased stiffness. Hardened tool steel cores and cavities, thicker plates, and additional pillars are all recommended.

A minor misalignment of the core and cavity or shifting during molding becomes a major variation in wall thickness on these thin-wall parts. High cavity filling pressure and speed requires the use of core, cavity, and parting line interlocks.

Every part of the melt flow path must be optimized in order to fill these cavities before the material freezes off. Hot runner molding systems are favored for their improved flow characteristics. Cold runner systems must be smoothly contoured and wide open from the nozzle orifice all the way through the gate.

The thickness of the gate must be equal to or greater than the part's wall thickness. Gates that are too thin will freeze off before the cavity is packed out. A dimple opposite the gate will improve melt flow. The thicker wall at the dimple also reduces cracking of the wall during gate removal. Multiple gates can be used to shorten the melt flow path, but this increases the number of weld lines and gate scars.

With cavity filling as fast as .5 second, there is little time available for venting. The depth of vents has to be the minimum in order to avoid flash. Since vents cannot be deep, more
of them are required. Some really thin-wall parts are molded with a vacuum assist.

If a molder is successful in getting a thin-wall part filled, vented, and packed, he then has to eject the part. These thin parts are more susceptible to distortion and ejector pin perforation than conventional parts. More and larger ejector pins are required. This is not easy, considering all the detail packed into small, portable electronic devices.

The high packing pressure associated with thin-wall molding increases the force required to eject these parts. These forces can be minimized by providing a 1°/side draft angle on all surfaces perpendicular to the parting line. A good polish, including drawing polishing, is also helpful. Plating the cores and cavities with one of the easy-release coatings is desirable.

In fact, in the case of thin-wall molding, these special tooling considerations are more than just desirable, they are mandatory. Thin-wall molding requires a good mold and certainly not just the lowest cost, quickest delivery tool that can be found. Some practitioners have estimated these additional features can add 30 to 40 percent to the overall cost of a mold. OEMs usually resist spending any more than necessary for a new mold, but sometimes a bigger investment will increase profits.

Molding. There is nothing mysterious about molding a thin part. Any good molder can learn how to mold these thin-wall parts by simply following the established guidelines. A molder should not, however, get involved in thin-wall molding unless he has state-of-the-art molding machines or is prepared to purchase new equipment. The primary molding machine requirements are high injection pressure and speed, increased clamp tonnage, and accurate, repeatable cycle controls.

Conventional but thin parts can be processed on conventional machines with injection pressures of 9000 to 14,000 psi and cavity filling time of 2 to 5 seconds. Mold clamping forces will be in the range of 21Ú2 to 4 tons/sq inch of projected part area. Thin-wall parts require injection pressures of 16,000 to 20,000 psi, fill times of .5 to 2 seconds, and clamp tonnage of 4 to 6 tons/sq inch. Really thin-wall parts can require injection pressures of 20,000 to 35,000 psi. Fill time has to be reduced to less than .5 second in order to fill and properly pack the extremities of the cavity. Accumulators may be required to achieve the rapid fill rates. Clamp tonnage increases to 5 to 7 tons/sq inch.

One common mistake made by molders is to overheat the plastic material in an attempt to fill and pack the cavity with an inadequate molding machine. This encourages thermal degradation with a reduction in physical properties. There is little or no safety factor in thin-wall parts. These parts require all the strength the plastic material has to give.

Another mistake is to put a thin-wall part in a large molding machine in order to get the higher clamp tonnage. These machines have larger injection cylinders. Thin-wall parts use less material, and that can result in an increase in hopper dryer and injection cylinder residence time. To avoid thermal degradation, shot size should be equal to at least 40 percent of the injection cylinder's capacity.

Mutual Benefits

Not all injection molders are capable of processing thin-wall parts. Those molders who have the knowledge and equipment can offer their customers thin-wall molding as an added service. The OEM will get the cost benefits, and the molder will have set himself above his competitors who have not learned how to mold really thin-wall parts.

Any molder who is looking for a competitive advantage should be on the lookout for conventional parts that could be redesigned to be thin-wall parts. Calling this opportunity to an OEM's attention and helping develop a successful thin-wall application can be mutually beneficial to both parties.

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