Gaseous thinking -

Buzz - I had an opportunity to be thinking about injection molding, specifically using 'gas assist'. Anyway I was looking at some scrap and thinking about gassing and voiding in general. What had originally attracted me to the scrap was that there were some cut up sections where the center of the thicker rib areas were almost all void - as though they had been start up or R&D run in as 'gas assisted' parts - where a void is intentionally the outcome in the middle of thick sections. Does this have any additional affect on thinking during the design phase?

Skippy - the experts in the industry say that the overall effect of gas assist is the same as running a thinner wall part - a faster cycle with less shrink and less sink on the part; important to know when working with tooling people. Remember that recommendations concerning raw material shrinkage from the raw materials guys are just that - recommendations; as processors, we have to smarten ourselves up too. Anyway, when you get ready for gas assist, you should set up some trials on known parts from known tools dimensions and should intentionally shoot parts at the same cycle and barrel heats before gas assist and after to observe and measure the changes in SHRINKAGE which should occur reductions in overall shot/part weights - aka the shrinkage should be less. These tests could then be augmented with adjusting barrel temps (stock temp) and cycle time to pull hotter parts in an effort to 'shrink' them some more if too much larger.

Buzz - so if the mold isn't right on the first hit, we're sunk?

Skippy - When there is a question about shrinkage, the typical recommendation is to ask to 'section' molds where possible and put core pins in so that they can be replaced or at least moved around a little bit on an elliptical of some kind. The hotter the material, the thicker the wall, and the shorter the cycle - the more the material is going to shrink under normal circumstances - but you can only makes so much adjustment to these (plus perhaps annealing can be incorporated to 'shrink' an oversize part) processing parameters. The suggestion is that when ready to incorporate gas-assist, do some testing with various materials to see what changes in both 'the direction of flow' and 'cross flow direction' shrinkages that may occur.

finis

More 'Cutting Remarks'

Since posting thoughts earlier about ‘Cutting Plastic’ back in September, Skippy and Buzz have had a number of side discussions and have some additional inputs.

Skippy: Hey Buzz – Back on the idea of cutting plastics, someone suggested using a standard or carbide blade for cutting slightly abrasive PVC and Acrylic plastics but installing the blade ‘backwards’; any thoughts?

Buzz: The concept of 'blade in backwards' is a confirmation that a negative rake or tilt to the blade teeth imparts slicing versus gouging action in some plastic saw blade designs. This 'using of a blade improperly' is a bad idea all around – especially with carbide inserted blades – the carbide pieces are joined to the blade body, but are designed to push and cut into the product while cutting, not be pushed against backwards while trying to “exit” a product to be cut. Having the carbide pieces come loose and or be knocked off and slung at the high foot per minute speeds that saw blades run at is a potentially dangerous outcome. These blades just aren’t designed to be used that way. We are agreeing with negative rake in general for plastics along with the right tooth configurations, disagree with using wrong blades 'creatively'. Just buy the right kind of blade to start. Yes, PVC is slightly abrasive, generally due to the loading of talc or calcium carbonate or other fillers - sometimes they are in color and UV packages. While on the subject of abrasiveness, do your production folks a favor - remind them to remove jewelry while handling these plastic articles - particularly gold and silver rings while handling the slightly abrasive PVC - you know those grey marks they leave on the PVC? (come'on you want to test it right? Just rub a ring on the surface . . .) - yup, thats GOLD and SILVER being worn away - puts a new spin on the phrase 'value added' - eh?

Skippy: A few reminders again about safety. The sawn material removed will generally be larger than dust and can be airborne. Inexpensive paper dust masks to filter out the chips (which won't be much if you put together an inexpensive 'catch box' behind the chip path to catch 80% of the chips) and just as importantly, eye protection are always a good idea in a shop environment when using cutting and grinding tools. It may be that you also need a "Lock-Out, Tag-Out" procedure to disconnect all sources of danger when installing blades, doing set-up, and servicing the equipment.

Buzz: eventually someone recommends putting a lubricant on the blades as well. Recommendation - do not put petroleum products on PVC or other plastics in general - depending on the application, the by-products in the can (aerosol propellants etc) can create a variety of problems including UV and color fastness failures, and negative interaction with mating parts (not previously mentioned). If your final installation (what ever this does) needs to be warranted by you, not a good idea to create a future problem.

Skippy: what about ‘Silicone’ spray?

Buzz: Ah yes, good old "silly-cone" spray; next to ‘WD-40’ and Duct Tape, another of the great ‘cure alls’ - falls in line with the 'Bash to fit, Crush to assemble, Paint to hide' axiom - this is generally not a good material to be used in a shop environment as it will get into (and onto) everything eventually (worst is the floors and assembly counter surfaces) - again, if used on and impregnated into PVC can cause other problems - especially if you later want to use any adhesives with gaskets, tapes, labels etc. From a warranty standpoint, it is not a good idea to spray, coat or otherwise inject other stuff into your base compound; bare minimum it is likely to VOID the warranty from your profile supplier as well as their raw material manufacturer.

Skippy: So in general, a user of saw blades to cut plastics is looking to key in on

a) Correct blade design; differs per material; and correct equipment set up and use
b) Correct Cutting speeds (often measured in foot per minute surface speeds)
c) Adequate product support during cutting operations
d) Personal protection correctly applied and used at all times

Buzz: that's right Skip, hopefully you have already used the information to acquire the correct blades and are producing satisfactory results.

Working With ‘Air’; Routing and Trimming

Skippy: We seem to get a lot of inquiries about trimming and routing in small job shops. This last month on the road, it felt a lot like that scene from 'The Graduate'; although instead of hearing the word 'plastics' everywhere, it seemed like a lot of folks are thinking that they need to look at 'Robotics'.

Buzz: Well, let's back up a little bit - for plastics and composite materials, air trimming tools are often easy to assemble machines that can be used either by hand or in semi-automatic mode with a little shop engineering to produce good quality parts at a fraction of the cost of robotic trimming.

Skippy: Doesn't that fall into the category of just 'throwing people at a problem'?

Buzz: Easy now, like you, I believe in increasing the productivity of our shop associates using high technology, but not all shops have volumes to justify high dollar investments into Robotics early on. On the other hand, one important item to consider ergonomically about cutting and trimming items with hand operated air tools is that user fatigue can increase when trying to extend cutting tool life and this must be carefully considered during tool selection and implementation (along with labor per unit costs.) Trying to work too fast with 'single fluted' tools can take less time than with 'multi-fluted' tools but quality will suffer. On the flip side, higher quality finishs can be obtained by making multiple passes: one roughing pass and a second finishing pass, but here again you need to consider the additional work for the operator or the semi-automatic machine.

Skippy: What are the critical shop issues?

Buzz: Air Pressure is probably first of the the most two critical items to consider and like a few other essential shop items (electricity?) when the supply drops a ‘little’ the resultant performance and quality drops more precipitously. For instance, these devices often need 90 psi or more at 30 cfm or greater of dry, clean, lubricated air to operate. If in this example with a drop in supply, the tool is receiving less than 70 psi or 20 cfm, then its usable horsepower is cut in half, as well as exhibiting a drop in its rpms. This compounds the quality and time related problems because router bits cutting edge surface speeds are designed for specific feet or inches per minute or second based on their rpms and do not perform as well at the wrong speed; just like the saw blade discussion earlier – time, pressure and surface speed all work together with the tool cutting surface design to yield a ‘sweet spot’. Some times air losses happen during the most innocent of 'plant improvement opportunities'. In the ‘Leaning’ process, shops often add more quick disconnects at multiple work stations to reduce travel time. These changes should be considered carefully as too small a supply line or too many users on a line can cause these pressure problems.

Skippy: You mentioned two critical items -

Buzz: - ah yes, Supply of air as above was the first and the second most important item is a proper maintenance schedule. Air tools are most often hampered or destroyed by particulate damage coming from their operating environments in shops where regular maintenance is not performed often enough. So, with the correct tooling, a well thought out implementation plan and a well executed maintenance program, air routers can be a very cost effective solution in small job shop settings.

There's (often) 'not much new under the sun'

Buzz: This question came up from a application that was using a "Blow Molding" process to produce parts. Instead of de-necking the resultant product, they wanted to end up with a hollow part that was sealed - their thought process included 'gas injection' of course to inflate the hollow section(s):

"I'm trying to find a way to close the hole made by the gas injection needle in a plastic part. This part must come out of the mold without any hole! Can someboby help me with this issue?! Thank you!"

Skippy: Ok, here's an idea, but can I have a piece of the patent?

Buzz: Sure if it already hasn't been invented before -

Skippy: Design the needle injection/retraction point through essentially a hot runner area - sequence the activity so that the needle is retracted through hot plastic after the gas injection then freeze the aperature -

Buzz: Good answer; now for the bad news, based on your small amount of feedback, a quick search of the internet based hot runner folks yielded a quick note back from the questioner - "Thank you for your cooperation!"

It seems that the quick and easy answer was to use a Gunther System (these folks already have a huge knowledge base assembled on hot runner components and applications) - information can be found at:

http://www.guenther-hotrunner.com/php/start.php?lang=us

http://www.guenther-hotrunner.com/Download/Katalog/englisch/KAP-01-02-()-EN-NEEDLE-VALVE-TECHNOLOGY.pdf

Skippy: glad to be of help - sometimes just a nudge is as good as a push -