Jul 07 2007

The Continuous Quest For . . . .

Published by at 3:26 pm under Uncategorized

The never ending buzzwords of the Quest for . . . . .  have always been popular in the literature.  A quest is a journey with no end. But in industry each ‘quest’ has to have a goal and a means to get there.  Novel Idea yes? Here are some ideas.

It’s funny to hear someone chant the Mantra of the ‘Continuous Quest for Quality’.  All too often they look at the costing structure with a built-in scrap rate and pat themselves on the back when they beat it.  But they miss the point of ‘quality’.  Quality’s main goal is not to catch defective parts, but to make a better percentage of saleable parts per run.  Thus, quality’s real goal is to improve profits.

All ‘profit improvement’ programs must come under the critical eye of a return on investment calculation.  The cost of quality can become so high that it substantially erodes profits.  Obviously each molder must make the decision of when the expense exceeds the rewards.  Dr. Deming clearly put it in terms of when the results of being ‘In Control’ cease to give any more useful information, stop gathering data and making charts and do something else.

With ‘Quality’ at an optimized level; our Quest for Profit Improvement will shift to the molding process.  Mold setting and startups have come under the almost silly idea of the Single Minute Die Exchange that is easily done in the stamping industry but because of material changes and the requirements of bringing massive amounts of steel up to heat is impossible to do in a minute in molding.  However wasting several hours to start up a mold is truly eroding profits if (1) you could do it in a faster amount of time and (2) this time saved with the new changeover time can be filled with other production.  Faster mold changes also quickly burn themselves out because everyone gets used to thinking a half hour is too long.

Our next quest for profit is something that should be examined theoretically first before we run any experiments.  The best way to enhance profits is through the reduction of the cost to manufacture the part. The business of injection molding is the sale of material and the sale of machine time required to manufacture the part. Using the ‘best bang for the buck’ mentality we need to look at the gross contribution of both elements.

Many jobs are ‘machine’ heavy.  This means the percentage of cost is minimal for material and substantially weighted towards the cost of pcs/hour (machine cost).  With this information at hand the quest should be directed at not only process optimization but also whatever techniques can be used to further shorten the cycle time.  It is amazing what you can save by speeding up all the actions of a robot except the one that extracts the parts or runner directly from the mold.  Part ejection should be done by temperature, not time.  Do a study where you cut seconds off the cooling time and measure the ejection temperature of the part.  You’ll quickly find at some temperature the part will shrink or warp out of tolerance.  However you’ll also find a temperature where it doesn’t.  And, this temperature will usually mean a reduction in the cycle time.  Here’s an example:  You can successfully eject non-warped rigid PVC parts when the temperature is the boiling point of water.  Don’t believe me?  Try it.  NOTHING has to be ejected at room temperature.

With fast cycle times, multi-cavity molds, or large parts the equation tilts to be ‘material’ heavy:  where material is the main component of cost.  Speeding up a cycle has little impact on the cost, while improving yields can have a massive impact.  However let’s assume we’ve already Quality Ninja’d this particular part to death in terms of cycle time and our scrap rates are in decimal percentages.  What’s next?

Here is where we must reduce the amount of material used.  By reducing the part / shot weight we have two occurrences.  Obviously the part contains less plastic, but because there is less plastic in the shot, there’s also less plastic to cool.  Less plastic to solidify has the secondary benefit of a shorter cycle time.  Less Plastic is a concept under the umbrella of a technique called “Light Weighting”.  Here are four of several techniques used in Light Weighting.

1.  Install a hot runner / hot sprue bushing.  Since the sprue’s intersection with the runner is usually the thickest cross sectional area by eliminating the sprue or even the entire runner system you’ve lost shot weight and saved time.  The down side to this is residence time and shot inventory.  The less plastic injected into a mold for each shot; the more plastic in the barrel soaks under heat.  Because hot runner systems require wide daylight openings, in many cases the larger machines that accommodate these molds have too much shot capacity.  A heated sprue doesn’t particularly have this problem.  Heated sprues are relatively inexpensive.  Complete hot runner systems can sometimes double the cost of the mold.  Any expense of this type needs to be very carefully examined.

2.  Parts with little depth and a lot of surface area offer an excellent opportunity for Light Weighting.  Let’s assume your part has a specified nominal wall stock of .085-095 inches. Most tooling engineers and mold builders construct molds that are as close to the specified mean as possible – which in this case is .090 inches.  BUT, how much material would you save (both in weight and cycle time) if you ground down the stand-off pads so that the part’s nominal wall stock in line of draw is now bordering on .085?  You’ll have to make sure the vents were still open, the parting line shut off was still OK, and all mechanisms still operate properly.  While this may not seem like a lot of material per part, remember we’re in the high-ultra high volume business.  On a 128 cavity bottle cap mold, saving .004” of an inch converts into a little less than a half ounce per shot.  Multiply that by millions of shots and we’re talking a few tons of material saved per year.

3.  Thick / heavy parts are ideal candidates for light weighting.  In this instance it is mandatory that you are running a robust process in a robust machine because you need highly accurate control.  The process of injection molding first starts with the filling of the mold as rapidly as possible.  If we stopped only at fill the part would be full of sink and voids.  For this reason we fill as rapidly as possible then pack the melt with enough material to offset shrinkage.  Pressure need only be applied to compress the plastic until the gate has frozen off.

By fully understanding the molding process we know that the instant hot plastic touches the cool mold surface it immediately forms a solid skin.  As more heat is removed the inner and outer surfaces cool making a continually thicker skin until it is a structural part although the interior may not be fully solid.

Most molders tend to over pack the parts.  This is done for two reasons:  (1) Most multi cavity tools are not balanced.  The additional pressure of packing over-packs the first cavity to fill but the last filled cavities have sufficient packing to make acceptable parts. (2) During the initial qualification runs the optimal packing times and pressures are usually not found experimentally.  Thus, the techs simply use more than necessary and get an acceptable part.

Light Weighting in this case requires that you fill the mold normally (as fast as possible) and pack it normally.  The difference here is to either use less packing pressure or use the normal pressure but cut the packing time so that the mold slightly depressurizes while the internal plastic mass is still molten.  With some experimentation you will usually find you can lighten the part by a few grams and not produce and sinks or voids.  With the cooperation of your customer, it is possible to make perfectly acceptable functional parts that are replete with sinks with very tangible material savings.  This is an ideal application for internal parts.

4. The last trick in Light Weighting is difficult but usually has substantial rewards: Change the material.  Going from a ‘natural’ material to one with fillers has less plastic to cool because the fillers don’t enter into the thermal equation.  Going to a material with better physical properties can facilitate much thinner wall stocks with no sacrifice in strength or durability.  While the cost per pound of non-commodity grade resin is more, you tend to use fewer pounds and ultimately end up with a net savings.

The most fun you can have with your Quest for Cost Reduction is when Machine and Material costs are about equal.  With neither component controlling the cost equation you have the luxury of playing with both variables and seeing fairly large rewards each time.

The rule of Thumb in the Quest for Savings is “with small parts look for time, with large parts look for weight.”  In the best of all worlds, the molder and designer have consulted with each other so that these savings have been already designed into the part.  When they haven’t the moldier is leaving money on the table by not trying to lower his costs.

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This article is virtual. But change is a fearful concept for most customers.  Making thinner parts before submitting your final samples for approval won’t scare anybody.  Optimally using your equipment is simply good business.  Changing materials is something you might want to foist off on a materials salesman so that you can feign ignorance if the product fails.  Or, you can leave ‘well enough’ alone and keep any of these suggestions hidden in your computer for when you might have to pull a miracle out of the sky to improve profits.  OR, you can forget everything and delete this article because you think you’ll be laughed at if you suggest any of this.

Your choice.

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