One Way Out
By Jim Anderton,technical editor
Certainly, the screw and barrel are critical components in any injection molding machine. But the moneymaking end the machine -- where the polymer's journey finally and truly ends -- is the mold. Howe...
Certainly, the screw and barrel are critical components in any injection molding machine. But the moneymaking end the machine — where the polymer’s journey finally and truly ends — is the mold. However, before we examine the mold, we have to consider how getting the resin from the screw into the mold’s entry point can affect the quality of a part. That’s why we’re going to spend time looking at the sprue bushing.
Remember how the screw on most general-purpose machines both rotates and plunges to fill the mold? The rotation moves, homogenizes and heats the melt. Pushing that viscous mass through runners and gates then into mold cavities takes considerable pressure. To achieve this the press stops screw rotation then pushes a volume of melt, like a syringe, into the mold. Astute observers might wonder how we prevent the melt from simply flowing back through the screw’s flights during the packing process. The answer is mechanical: use a check valve.
The non-return valve is deceptively simple, usually a sliding ring with a sealing surface — a little like a water faucet or a captive ball that covers and uncovers a fill port. Both operate similarly. During screw rotation, the relatively low- pressure melt unseats the ring or ball, allowing easy flow ahead of the screw. As the screw moves forward, the pressure buildup ahead of the valve and pushes the ring or ball backwards, thereby sealing the opening. It’s simple, but there are a couple of things worth considering.
One is contamination. Junk in the melt stream could prevent the valve from sealing, with predictable results. Very low melt temperatures can also cause the valve to stick open or cause operations to be erratic. This might not be immediately apparent if the heating systems downstream of the valve — nozzle and hot runner system — adds enough heat to mask the issue. Shot-to-shot repeatability can be frustratingly inconsistent as a result.
Another thing to think about is machine timing. The screw needs to move forward to seat the valve, a motion that’s ‘wasted’ from a mold filling perspective. It may be just millimeters, but it’s something to consider if you’re shaving fractions of a second from the cycle time.
As always, it’s about ‘pounds on the ground,’ so when specifying a tip — often on the recommendation of a material supplier — consider the implications.
Some tips allow easy melt flow, but promote ‘floating’ of the check ring resulting in poor shot volume consistency. If necessary, it’s possible to program a quick initial surge as injection begins to slam the door shut, or decompress to urge it back onto its seat. But in a perfect world, we’d set up injection profiles for clean, quick mold filling and packing without having to worry about tip flow.
If you change the tip or valve configuration you may end up revising your machine’s running parameters. It’s great if changing the tip or valve configuration drops a second or more in an injection molding operation.
If you’re adding time to an injection molding operation, then make sure to check the economics of the change. Remember that a $60 an hour machine running steady state costs 1.7 cents a second, so low margin, fast cycling parts can really take hit profit margins.
Also keep in mind that there’s a lot more to this simple technology than meets the eye. Having a good rapport with your resin and component suppliers is essential. X-ray vision would help but in the absence of Superman, we’ll have to fall back on brains and experience.