Sep 06 2007


Published by at 5:18 pm under Uncategorized

Some people think that building replacement tooling is a simple and placing a purchase order then showing up at the tryout to approve the parts. This is the time to realize new tools make different parts than older tools. If you want the introduction of the new tooling to be reverse compatible and seamless there are some things you need to consider:

You are one of the lucky companies that have a product(s) that has been in demand and will continue to be in demand essentially forever. Initially it was tooled to what the marketeers said would be the market volumes and as time went on and volumes increased you only had to run the mold more often. Despite your heroic efforts at maintenance some of the molds that produce the components are worn out. As an engineer your assignment is to retool some of the parts.

When retooling from an old design there’s always the temptation to improve things by adding draft, a radius here and there and perhaps thinning a wall stock just a tad. The parts from the old tooling work well and the new parts want to slip seamlessly into production replacing the worn-out tooling parts. They also want to be ‘identical’ in their form, fit, and function in any combination with other mating parts some of which are also being retooled. Your last goal is to not have to open tolerances when the new parts come into production. The only argument you want to have with the quality folks is that although they work interchangeably, they do in deed look different. Here’¢s how you do it:

Like every other project the first thing you need to do is your homework. Keep in mind the overall premise is that your current production of products (although being made on old, tired, worn out tools) do work. Get a full dimensional inspection of the old parts as well as the other components in the assembly that your new parts will be mating with in the final product. What you normally find is that many dimensions in the existing assembly are not to the print tolerance. Put through an engineering change modifying the old part designs to reflect these functioning parts. I’ve actually had clients who built replacement molds that came out “to print’ and didn’t work, because the engineer ten years earlier modified the mold but didn’t put through an engineering change so that the old parts would work.

The trick in the retooling game is that you don’t have to make all the dimensions identical. Only the functional surfaces need to be kept in the same tolerance as the old parts. This means everywhere else, so long as you don’t interfere with something in the assembly, you can make whatever changes you want. Here are the easiest changes to do and the trouble you might get into:

Draft: A basic Newbie Mistake is forgetting what draft does:  something usually gets bigger at a rate of .017 inches per inch (.4318 mm/mm) per degree. Draft (and the part’s change in dimensions) must be calculated from the mating surface. With deep draw parts you must pay attention to the bottom of the part because it will get noticeably smaller with only a few degrees draft. This may interfere with something.

Radius: Round corners are always better than square ones Again we have fitment and interference concerns. Usually this is inconsequential until you get into really big radii.

Wall Stock:To a point, thickness is a cube function of strength – twice as thick is eight times as strong. This equation also reverses when you begin to thin wall stocks down. In many cases thinning the wall stock to the print minimum won’t make any difference in the product’s dimensions or function. If the product is large, you’ll save a lot of material. Since injection molding is a ‘heat in – heat out’ manufacturing process, the less material you process per shot, the less heat is in the equation and therefore the cycle times are shorter.

Material Change: Many people find more expensive materials with thinner wall stocks perform as well as the old parts and in the end analysis are less expensive. Not changing the wall stock but changing to a less expensive material with about the same melt point, melt index, and shrink rate is almost a transparent change. There should be few surprises. HOWEVER, changing to a completely different material with different properties is as though you are building a new tool with a new material. Build steel safe and make as few adjustments as possible.

Gate location: This is where things get tricky. Let’s assume you are making something that looks like a small box lid. The old tooling is an edge gated tool.

Scenario #1: PROBLEM – Your new tool wants to center gate with either a three-plate or hot runner mold. Since the plastic will flow different lengths, will exhibit different stresses and see different cooling patterns; simply changing the gate locations will change the dimensions. SOLUTION – Build the new mold’s core and cavities steel safe. Using a height gauge, scratch a set of grid lines on each cavity. Start up the new mold and let the process settle in. Take about a dozen parts and inspect them. Note where your gridlines are compared to (for instance) exactly where a locating peg or the center of a hole in a boss should be. With this data in hand, now go back to your mold and put in the details for the mating parts.

SCENARIO #2: PROBLEM – You duplicate the gating scheme and cavity layout but you also know that gate sizes that are even slightly different will exhibit different fill patterns and the cooling on your new mold will be more efficient than the old one and therefore the shrinkage will also be different. SOLUTION – see the solution for SCENARIO #1

Redesign: Retooling is seductively tempting to introduce improvements that go well beyond the implementation of Best Practice Standards. If you have a few million units already in the field making a new generation of parts that combine two parts into one will be a nightmare for those wanting spare parts. If you do this you’ll be doing spare part lifetime replacement runs forever on your worn out money losing tools.

Retooling to make interchangeable parts that are reverse compatible with any unit in the field has a number of advantages:

1. Because this is done as an engineering change the part numbers of the products coming off the old tool and new tools don’t change.
2. Building everything steel safe and sneaking into the critical assembly dimensions keeps the folks in quality happy.
3. You don’t have to maintain a stock of the old parts as Service Replacement parts for the older units in the field.
4. If you are in a regulated industry (medical, defense etc.) it is easier to get an engineering change approved than a new part number.’ In the medical field trying to say it’s different but actually it’s identical will guarantee a long and convoluted argument with the folks who do regulatory compliance.

Redesign and retooling has its best success when the product is not designed to be repaired. If in the field when the product ceases to function it must be replaced, go ahead and redesign.The only problem will be managing the change within your manufacturing facility.

It’s always fun to listen to the marketeers talk about products that will go on forever because if you’re in the proper market sector they are absolutely correct. The fun comes from their complete misperception that tooling and machines last forever without continual and sometimes expensive maintenance. Things that move either of their own accord or are subjected to some kind of force will eventually wear out and need to be replaced. The key to replacement tooling is transparency : you bring a new tool on line and the product seamlessly continues to do what it always did.

The biggest problems with retooling are (1) convincing the bean counters, managers, and other people in High Level Closed Door meetings that the product needs to be retooled in the first place. (2) The folks with the spreadsheets and PowerPoint presentations are also highly resistant to funding new tooling because the “don’t want to buy the same real estate twice.” I’ve always been baffled by both of these problems. Continuing sales are continuing profit streams. Because nobody complains about the marketeers advertising and promoting the same product over and over again, it’s hard for me to see why justifying new tooling is such a political hot potato.

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This article is virtual. Retooling is an investment in profits. You can print it out and use it to scare the marketeers and they’ll do the work for you by only whispering “missed shipments” mean lower bonuses . Or you can wait for your Just In Time supply chain to evolve into We Just Got Lucky supply chain and then make your tools during this disaster.Or, you can just sit back and giggle.

Your Choice.

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