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The arrival of a new tool always brings a certain level of excitement. Often it is late and everyone is in a rush and pressured to produce “good” parts. So the history and culture of the molding industry has been to pull out every trick possible to work around design, resin, tooling, processing, and testing issues. Burns? No problem, just slow the injection rate. Flash, color streaks, blush, shrink, warp, etc.? No worries. We spend hours on profiling, fixturing, trimming, and other tricks of the trade to get around these problems.
The result of this approach is often a few good parts but no real process that meets the production level, quality, or profits expected. Even worse, one of these good parts produced “by luck” is often waved around by your boss, who wants to know why, since the mold can “obviously” produce good parts, can’t you get the process to run? This leads to even more experimentation, which wastes time, resin, and money.
Is there a better approach? Sure, but since each tool is unique and has its own peculiarities, it’s tough to have a standardized way of doing things. That said, there are certain requirements and tests that all tools should pass before any parts are sent out.
BEFORE THE MOLD IS BUILT
Let’s start with requirements before the mold shows up:
•Adequate ejection: One of my pet peeves is that if you make a short shot (on purpose or by mistake), you often have to dig the part out. Every shot a mold will see will not be a full and complete shot. So spend the money and put in sufficient ejection for shorts and problem areas like thin ribs.
The production hours lost as processors dig out parts would justify the extra costs. We are probably talking about hundreds of thousands of dollars each week across the country. And that’s not counting the tool damage caused by frustrated processors using screw drivers, pliers, hammers, ejector pins, etc. to yank parts out. If you add that in, maybe even the bean counters would begin to understand why better ejection makes sense. Scratch a mold surface while you’re digging and the problem gets worse. Some simple, safer solutions include adding a vacuum break, an extra ejector pin, a generous radius, polish in the direction of draw, or adequate draft.
(Why don’t we have a special brass tool that heats up like a soldering gun? Pull the trigger and a dull brass blade or probe heats up to melt the plastic. This would be a lot quicker and safer than all those torches heating screws, ejector pins etc.)
•Adequate venting: We have been using vents in molds for some 60 to 70 years. Most people acknowledge that there is air in the cavity that has to escape as the plastic fills the part, but we still see burns on parts, and many a cycle is extended to allow time for the air to escape. True, we are seeing more vacuum venting nowadays, but we have plenty of room for improvement. My favorite example is the mold builder who supplies you a tool with no venting but with a note asking you to indicate where the burn marks occur so he knows where to put the vents! If this happens at your shop, promise me you’ll take your tooling business to another moldmaker.
As general rule, 30% of the cavity perimeter should be vented. Vent depth is resin dependent; seek recommendations from your resin supplier or a good tooling shop. Vents should be ground with a 120 to 240 grit wheel in the direction of air flow. They should not be milled. Vent land should be no longer than 0.090 in. and lead to a wide dump channel 0.015-0.020 in. deep. It’s okay to mill the deeper dump channel.
•Clear labeling: Each water line should be labeled with its required gallons or liters per minute (GPM or LPM). Also, a diagram of the water-line circuits should be affixed to the mold.
•Time-saving hot-runner diagrams: With every hot-runner system, you should receive a diagram of the hot drops and manifold layout so you’ll know which heater is controlling which drop or flow channel. Unless, that is, you actually love puzzling out which temperature controller regulates which drop or runner.
AFTER THE MOLD ARRIVES
Once the new mold arrives, perform the following tests. These also apply to any current tool that consistently causes problems:
•Water-flow check: GPM or LPM of coolant for each channel should be documented for future runs. Ensure that each line has turbulent flow with a Reynolds number of at least 5000 in each channel. And make sure that all lines have open water flow. While not common, it is possible for water flow to be blocked in one or two channels in a new mold: Gun drills stray and plugs manage to get misplaced. It is worth the hassle to find any blockages before you try to develop a production process, since cooling is 95% of your cycle. It is important to get this right from the start and find a way to ensure that it is duplicated on each run. You might also want to investigate the benefits of positive-displacement water pumps over centrifugal pumps, which are more common in our industry.
•Flow-analysis check: If a mold-filling analysis was done, the calculated fill time should be communicated to the molder. You spent good money on this analysis, so you might as well shoot the tool at the targeted fill time. Compare predicted flow pattern and pressure loss to actual, and involve the analyst. This will provide insight into the process.
•Pressure loss study: This is a short-shot study to check the fill pattern for air traps, flow-front acceleration, uniform filling, and balance of fill. Plus you’ll get the pressure loss required for each of the flow-path components.
One such study on a new tool provided the data in the accompanying figure. In this case the machine data indicated the process was in control, yet the reject rate was high. Note the high pressure loss in the sprue and runner. Many molders simply open up the sprue, runner, and gates to solve flow problems. This time, a more targeted approach was required. There was no need to mess with the nozzle or gates—the sprue and runner were the problem. It was a good thing to find this out when the tool was new and under some type of warranty. If discovered later in production, costs would have been much higher. This 20-min test also can be used to compare the actual pressure loss to the prediction of a mold-filling analysis.