Injection Molded Parts: Newsletter Challenge (04/10/07)

Doesn't "injection-molded plastic" infer that the pieces are made of plastic that have been injection molded ?

Size, shape, color, .... of injected plastic parts seems to me to be as varied as the size, shape and color of buterflies ......

I look forward to learning all there is on the topic of injection molding......

not sure about molded plastic .... but I have had something on my mind most of the day...... this being:

if lasers are reflected by mirrors, can a laser cut a mirror?

would a mirror be a good shield in a laser battle?

could Luke Skywalker have benefited by using a "mirror shield" when fighting his dad (Darth Vader)? .... or are light sabers different than lasers?

also, if light sabers didn't stop light sabers (when clashed together) how could you ever fight past the first couple strokes?

lasers can cut 'crappy' mirrors, but only as a function of the power of the laser and the crappiness of the mirror. Effectively, if a mirror is perfect, no light could effect it. However, if you were to use light at a different wavelength that is absorbed by the mirror, or a rock, then you might have more success.

Assuming that there is some force that results from changing the direction of the photons, I wonder if you might have some success with periodic application and removal of the beam on the mirror, creating some resonant effect that could vibrate the mirror into breaking?

As for the light sabers, i can't comment on them cause I have no idea how they would work.

Yes, there is a momentum change when you reflect light (2*energy_reflected*cos(theta)/c), which means that the force is (2*power*cos(theta)/c). (Theta the angle of reflection). So you could indeed affect a "perfect" reflector in this way. The amount of light you'd need might well fry everything else in the vicinity first, however.

Fyz

So ... it sounds like I'll need a "perfect mirror" mounted in either a Foamed Polyurethane RIM molded backing plate or possibly a Polypropylene (tough and flexible) backing plate to eliminate the potential for shield failure due to harmonics.

And that Acryonitrile Butadiene Styrene backing plate for the mirror should be avoided for this application because of their rigidity.

Thanks guys ..... I'll let you know how it goes.

Even a very high power laser, the laser beam needs to be focused to emmerge the beam energy before it can cut anything. Say the laser beam is 40mm diameter, you need to focus to 2mm or less in order to cut. And also depends on the laser mode (the profile of the laser beam), you may need a mode01 laser to cut as the intensity of the laser beam is high only at the beam centre.

Wavelength of the laser plays an important role as well. It determines whether the laser will be absorbed or reflected.

For example, CO2 laser uses copper (high thermal conductivity) mirrors to deliver the beam.

Where and why do people use 2-mm diameter laser beams for cutting? Wouldn't that generate rather a large shock wave? Perhaps the application needs a 4-metre depth of field? Or did you mean hole drilling?

No, it implies - only the reader can do any inferring.

The statement giving information implies

The interpretation using information infers

Pedantic English Users Incorporated

Were your bin?

Und 'ow bist 'ee me luvver?

bin busy bisy busy....still am but bored with it!

It's nice to be missed!

So are you located within the East of England , or to the East of England ? A statement has to be taken as read . I couldn't see an emoticon apt , but it was a rhetorical question anyway.

The closest option CR4 offered...and the ambiguity is endearing, I feel.

I live in what I believe is the largest county in England - it still runs from the Humber to the Wash (in days of yore it ran down to Biggleswade at least). All this is amusing since I was born in one of the smallest, although I find it hard to believe that the County of Bristol (pre-1974) was smaller than the county of Rutland

Hey , tread careful - grew up in Rutland !

I'm going to have to check my historical facts here. To make it worse , many of my relatives have the other rose (anybody looking check House of Lancaster&York /war of the roses - no NOT that film). SE is close enough a location , a bit like good old Etherville , (sorry Fyz , but this place is a gas , no A)

No roses here mate - and a different Minster! I believe those of a white persuasion lost the "largest county" status when North, South and the Ridings went their separate ways (but I could be wrong). Our county emblem does use both red and white though! (in a cruciform arrangement, if that helps)

My floral loyalties lie with red rose - having lived there a couple of times now...and I want to go back it's a wonderful county.

Rutland's nice too - I go through it regularly and have stayed in Oakham. very restful.

Roses are great , but they also have thorns - Oakham I know, but prefer Uppingham down the road , and was amazed when I went there recently for the first time in years. Happy memories- Can you believe , I went there Co's my brother wanted to see the Harley dealership (OK, it was a good one) ! A Harley's good , but....I've migrated South now - White cliffs of Dover and all that . If you've figured Aeros out keep it to yourself , I don't want to run out of excuses !

LOL!! You've just reminded me of my school motto:

Sweete is the Rose but grows upon the Brière

(olde english spelling intentional!)

Harley?! Splutter! No use here coz they won't go round corners that require more than 5deg of lean! Oh, East England...land of the long Roman roads...hmm..ok (ish)

Alack, poor North Yorkshire, I knew it (moderately) well. (Has its government been devolved so it's no longer in England?)

Fyz

BTW, for the other extreme you might count the City of London; this is both smaller geographically and has a lower population than Rutland.

BTW, for the other extreme you might count the City of London; this is both smaller geographically and has a lower population than Rutland.

You've got to be kidding - come on , whats the catch ? Area /population ? I'm assuming you know , so drop the baited hook .

By the by , 'Alas ,poor Yorick , I knew him . '

The area is very small indeed. The population (those who have homes there) is also tiny. It's packed by over a million day-trippers* during the week - and practically deserted at weekends.

*Commuters?

Fyz

"By the by , 'Alas ,poor Yorick , I knew him . '

By the way, the complete quote, from Shakespeare's Hamlet, is 'Alas ,poor Yorick! I knew him, Horatio: '

For you and Kris - I suppose an alternative to the multiply altered quote would have been to leave Omelette unstirred and describe North Yorickshire in detail. And, as Kris said - enough is Enough!

Fyz

And, as Kris said - enough is Enough! ,Well , take 2...

But the City of London is not a county. Bristol has been both a city and county since 1373. I have the birth certificate to prove it (who'll be first to nod that one in?!)

As to the criterion, I believe it's area. Rutland is usually quoted as being England's smallest county. I ought to make the effort, now we have t'interweb, to check the relative areas of the Ccounty of Rutland and the City and County of Bristol as it stood prior to the 1974 carve up of administrative regions by the then Labour government in order to satisfy myself that Rutland is truely entitled to make this claim.

And since my clues have passed you all by, I'm in Lincolnshire, land of the Yellowbelly and Cabbage Eaters!

Strange as it may seem, some of us do know which county borders both the Humber and the Wash. However, North Yorkshire is still larger than Lincolnshire - but maybe my references to the alternatives of North York's demise or independence passed you by?

The City of London used to be a Ceremonial County; so far as I know, it still is. I may be wrong here, but I believe it to have been an administrative county up to about the time of the 1974 reorganisation

BTW, I realised I was mistaken about the daily numbers in the City - the area as currently defined (about 1-mile square) only reaches about 1/3 of that most days.

Fyz

ENOUGH - I admit this is a whole lot more interesting than Injection Moulding , but if you two don't desist I'm going to have to spend my whole weekend checking demographic stats for Rutland , and by the time I've done that Ruttles fans will appear . Have mercy.

Doesn't "Rutles" have only one "t", like "Beatles"? (not "Beetles")

Yes, I am one of those, a Monty Python fan!

For more info see the "Tragical History Tour" website!

This , is an ex- parrot ,it..

Submit - I want to sleep this w/e. How can I sleep when I start thinking Flatty Otters/Farty Towels..

Okay okay. Enough already. Right.

Get...........the Comfy Chair!!!

_{(the Comfy Chair? Comfy Chair? The COMFY CHAIR? The comfy chair?...)}

Just be sure to eat plenty of SPAM!

(lovely Spam, wonderful Spam.....)

Scene: A cafe. One table is occupied by a group of Vikings with horned helmets on. A man and his wife enter.

Man (Eric Idle): You sit here, dear.

Wife (Graham Chapman in drag): All right.

Man (to Waitress): Morning!

Waitress (Terry Jones, in drag as a bit of a rat-bag): Morning!

Man: Well, what've you got?

Waitress: Well, there's egg and bacon; egg sausage and bacon; egg and spam; egg bacon and spam; egg bacon sausage and spam; spam bacon sausage and spam; spam egg spam spam bacon and spam; spam sausage spam spam bacon spam tomato and spam;

Vikings (starting to chant): Spam spam spam spam...

Waitress: ...spam spam spam egg and spam; spam spam spam spam spam spam baked beans spam spam spam...

Vikings (singing): Spam! Lovely spam! Lovely spam!

Waitress: ...or Lobster Thermidor a Crevette with a mornay
sauce served in a Provencale manner with shallots and aubergines
garnished with truffle pate, brandy and with a fried egg on top and spam.

Wife: Have you got anything without spam?

Waitress: Well, there's spam egg sausage and spam, that's not got much spam in it.

Wife: I don't want ANY spam!

Man: Why can't she have egg bacon spam and sausage?

Wife: THAT'S got spam in it!

Man: Hasn't got as much spam in it as spam egg sausage and spam, has it?

Vikings: Spam spam spam spam (crescendo through next few lines)

Wife: Could you do the egg bacon spam and sausage without the spam then?

Waitress: Eewwww!

Wife: What do you mean 'Eewwww'? I don't like spam!

Vikings: Lovely spam! Wonderful spam!

Waitress: Shut up!

Vikings: Lovely spam! Wonderful spam!

Waitress: Shut up! (Vikings stop) Bloody Vikings! You can't
have egg bacon spam and sausage without the spam.

Wife (shrieks): I don't like spam!

Man: Sshh, dear, don't cause a fuss. I'll have your spam. I love it. I'm having spam spam spam spam spam spam spam baked beans spam spam spam and spam!

Vikings (singing): Spam spam spam spam. Lovely spam! Wonderful spam!

Waitress: Shut up!! Baked beans are off.

Man: Well could I have her spam instead of the baked beans then?

Waitress: You mean spam spam spam spam spam spam...

Vikings (singing elaborately): Spam spam spam spam. Lovely spam! Wonderful spam! Spam spa-a-a-a-a-am spam spa-a-a-a-a-am spam. Lovely spam! Lovely spam! Lovely spam! Lovely spam! Lovely spam! Spam spam spam spam!

Hey , I wanted the longest post here ! Admittedly , real spam is a whole lot more entertaining . Some restaurant opened in London a few years ago selling (you guessed it ) nothing but spam . Have you guys ever seen Fawlty Towers (John Cleese as a Hotel owner ) it's a scream

Anything starring John Cleese is a scream! Even his voiceover parts are a hoot, like his part as Cat R. Waul in An American Tail: Fieval Goes West. Yowl!

Have you seen his management training films? - pure genius!

Meetings, Bloody Meetings!

Why is it that no-one ever notices that the first two items listed by the "waitress" has no spam in it? I understand the joke, but this has annoyed my since I was small and heard this on the radio....

Foreplay ?

Yes please

There's an Oz joke , far too risque to post (and it's brief ! )

Time: 11pm, Place a Tesco Extra in north Lincolnshire - what's this? Can it be? Yes!! It's a DVD of All You Need is Cash by The Rutles!

That's this weekend's viewing sorted

I'll be right over. Does your cat bite?

Nope - she's a pussycat!

(can't guarantee her owner though!)

ER

Oh, that's solved easily enough. Do you prefer red, white, or champagne? I need to decide on which crystal to pack.

Silly, silly man...you've seen gremlins? Well don't feed ER any form of wine. Bring something distilled (perferably orange flavoured) and the 20oz glasses! Much safer

...... Just a quick check-in from the laser battle .....

....mirror shield appears to be holding ...... glad for the advise on foam polyurethane RIM molding backer-plates .....

.... well action is picking up a bit on the front, must run (literally and figuratively) ......

....cherrio for now........

Sounds to me like a "what color is a white rhino?", kinda question.

I'd say an injection-molded plastic part was made by injecting plastic into a mold.

But then again a white rhino is gray isn't it ..........

The white rhino is characterized by a wide mouth. The "white" is a misunderstanding from the german word for wide. White rhinos are indeed grey, but not so dark as the common rhino, lending some support to the white name.

However, I know nothing about injection-molded plastics.

Is this to be my next career?

There are at least two ways to make such lightweight parts, but SFIK only one of them is strictly injection moulding.

In pure injection moulding, a dense plastic would be injected initially, followed by a self-foaming material. Typically, the leading material will coat the surfaces, and the expanding foam will provide rigidity. This could produce a swirling pattern if the surfaces were suitably treated, but I wouldn't think this is what is intended. I think it may be possible to use several different density foams for optimum robustness.

Another method is to spray the inside of the mould with surfacing material, using an oscillatory sprayer, and follow that by injecting the foam. That would produce a swirling pattern, whose detail depends on the viscosity of he surfacing material at the point where the foam is injected.

Now all we need is someone who really knows the answer and the detail to tell us what actually happens.

Let see if I can make a guess at it:

The part is made by first heating the plastic pallets inside the barrel of the injection mold machine to the point of a paste. Then, the plastic is injected (or screwed out) of the barrel into the mold under high pressure. After a set amount of time passes, the mold is open and the parts fall down to be collected. The swirling pattern came from the mold release spray that was sprayed on the mold to help the plastic parts from sticking onto the mold.

MidniteFighter

Are you saying that the light weight is irrelevant to the answer?

I never say light weight is irrelevant to the answer, but I have seem some injection plastic parts that are over 4 feet long that aren't very heavy. So, in order to fully answer the question, I suppose we need to know what is "light weight??" 5 lbs?? 20lbs?? Also, the question stated that some area is 3/4" thick, but how much of the parts is 3/4" thick?? 2%?? 5%?? 50%?? I think those data are also important, don't you??

MidniteFighter

It says you were surprised by the light weight. Obviously, you know something about it if you're going to make a sensible shot at answering the question. That means the part was indeed at the lightest end of what you should expect from its dimensions. I think that should mean you don't need chapter and verse of the detailed dimensions.

---------------------
Logic is a fine thing - when it is misused, a fine should be incurred

It says you were surprised by the light weight. Obviously, you know something about it if you're going to make a sensible shot at answering the question. That means the part was indeed at the lightest end of what you should expect from its dimensions. I think that should mean you don't need chapter and verse of the detailed dimensions.

Actually, I never say I was surprised by the light weight. The challenge question stated that. I am simply taking a guess at how the injection part was made. I could very well be wrong in this matter. I answer the question base on what I know and what I have seen on the injection mold process. What the actual name of the process I have no clue, but that does not mean I can not make an attempt to answer the question.

MidniteFighter

Try reading what I wrote - i.e. "It says" not "you say". That is an intrinsic part of the question as posed, and that you are supposedly tying to answer. Of course, whenever there is a satisfactory answer to the question as posed, it's great to go off on any interesting tangents you see fit (not that ignoring the exceptionally light weight has so far added anything in this case)

Good basic understanding of the plastic injection molding process!

I believe structural foam molding is typically used to create thick-walled, lightweight structures. This process is know for its swirled surface finish due to the foaming of the plastic used in the molding process.

I believe you're correct about Structural Foam Molding.

The foaming is developed by blowing a gas with the plastic mixture into a mold at low pressure. This results in porosity which contributes light weight; the plastic quickly soldifies at the mold surface with a smooth skin that is notoriously swirled--requiring painting for a professional cosmetic appearance (when appearance is a consideration). As the approach is effective for making large parts, the illusion of solidity given by the smooth outer surface will lead the uninitiated to misperceive the part as being equally solid, and heavy (roughly analogous to a light & fluffy Milky Way candy bar versus a heavy & dense Snicker's bar--both of which appear roughly the same from the outside).

A quick internet search using the following search terms will yeild more than most of you ever wanted to know about the subject: "Structural Foam Molding" +swirl

Now that the snack bar downstairs has opened I'm off for some candy bars (they're not just for breakfast anymore!).....

"Injection moulding is a manufacturing technique for making parts from thermoplastic material in production. Molten plastic is injected at high pressure into a mould, which is the inverse of the product's shape. After a product is designed by an Industrial Designer or an Engineer, moulds are made by a mouldmaker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars. Injection moulding is the most common method of production, with some commonly made items including bottle caps and outdoor furniture. Injection molding typically is capable of an IT Grade of about 9-14.

The most commonly used thermoplastic materials are polystyrene (low cost, lacking the strength and longevity of other materials), ABS or acrylonitrile butadiene styrene (a co-polymer or mixture of compounds used for everything from Lego parts to electronics housings), nylon (chemically resistant, heat resistant, tough and flexible - used for combs), polypropylene (tough and flexible - used for containers), polyethylene, and polyvinyl chloride or PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser)."

http://en.wikipedia.org/wiki/Injection_molding

How were these parts made?

If the answer is just going to be a description of injection moulding technique , does it qualify as a 'Challenge Question' ? Are we to suppose something anomalous in the parts , and seek a mechanism to cause it ?

You also discover the surface has a swirling pattern in it.

Can a surface have a pattern in it ? It is not clear whether you are looking at a variation of colour or contour.

low pressure roto-molded, like a kayak. Swirls induced by the rotation.

PVC sewer pipe has a foam like interior so it is light in weight.

Pipe is usually extruded rather than molded. In extrusion the cross-section is constant and continuous, whereas injection molding produces produces one or more discrete parts for every cycle of the machine, over and over, plastic in, parts out, plastic in, parts out, etc. Molded parts also have one or more "gates" where the plastic enters the mold, resulting in small extra bits of the "sprue" or "runner" which must be trimmed off, unless the gate is recessed and a "hot runner" is used that results in elimination of the "sprue". Molded parts also have a "parting line" where the mold halves come together, which may result in "flash", thin scrap material that must be trimmed off also, if the halves do not fit together precisely or separate slightly under the pressure of the injected polymer. Extrusions have no gate and usually no parting lines, unless the extrusion tooling or "die" is segmented for maintenance or other reasons.

Pipes may be injection molded, but usually only very short ones with special features, such as angles, sharp curves, or pipe connections would be molded.

Sounds like a RIM process see http://www.rimmolding.com/rim/index.html

Yes, it does sound like foamed polyurethane RIM. Not my field at all, but I imagine that distortion of bubbles flowing near the surface could make the swirls particularly visible. According to Bayer ( thhttp://www.rimmolding.com/composites/tech_foam.html ) the process can be viable with a foam density as low as 15-lb/cuft (S.G. ~ 0.18). RIM product also tends to have walls that are thinner than they look at first sight, because the edges of sections tend to be thicker than the remainder, and the transistions are quite smooth.

Fyz

I would say that short of finding some new light weight injection molded plastic material that there are 2 possibilities for this. My first choice would be that the parts had a foaming agent applied at the time of injecting the material. This reduces the weight and increases the strength, prevents sink marks and shortens the cycle time. My second choice would be that the part was made using gas-assisted molding. This uses a high pressure gas, usually nitrogen, that seeks out the thick spots in the mold and fills them with gas pockets. This also makes a lighter part, strengthens the part, prevents sink marks and reduces cycle times. However, this process does not typically cause a swirling pattern in the surface of part. Therefore I would tend to think that this part was produced using a foaming agent.

It is posible if the part is molded in multiple step.With inner material having higher moulding temperature(Reprocessed material).

First inner parts gets molded.It may have two pieces or more depending on the part nature.Later those parts will be joined together forming a base for final product which will be hollow.

This base product will be kept in the mold and final part will be molded just like insert moulding. This way it is possible to mold part that will have low weight compared to direct molded part

I am not touching this one.

Back to the bb.... Kris, your estimate and the one I finally come up were the closest I saw there, what steps did you take to achieve your result?

The result posted did not show the estimated point of impact it just stated the bb will impact.The graph showed it about 2/3s of the way down .This would show a close average to the distance we both figured.

I love flattery , even if not fully deserved ! I've posted a reply back at #197 in the bb thread . Like yourself , I'm outta this one - It only involves speculation. Kris

Air is injected into the plastic as it enters the mold this will reduce the weight. The swirling pattern can be caused by the material cooling while the plastic is still injected into the mold if the mold does not have hot runner system. The parts could also be extruded this could also leave a swirling pattern.

Air injected into the mold is the answer, called air entrainment. As far as the swirling pattern, likely just the tool marks left after making the mold, ie., not likely a consumer product so the mold was not polished after it was made.

Light sabors generate a form of plasma that has properties of solids. That's why they can cut through any "cold" material but are blocked by each other. Only the blade of a light sabor can withstand the heat of another light sabor, which is so hot it ionizes the air around it enough to glow and generate a buzzing sound that increases with pressure such as when a sabor is swung through the air.

Is that why Luke's arm don't bleed when his hand was cut off??

MidniteFighter

The question clearly states injection moulded parts, which eliminates the process of rotomouldning which does not inject molten plastic but rather melts powder in a heated rotomoulded form. I will also exclude the possibility of blow moulding which is a related but different process to injection moulding.

The idea of foaming agents is valid, and justifies the swirling, but the effective weight reduction is not as significant as with the use of alternative injection moulding technologies.

I suspect the lightest large injection moulded parts can be made with GIT (Gas Injection technology) or equivalent gas assisted injection moulding technologies, (eg CGI Cinpres). The parts are moulded as per typical injection moulded process with the addition of injected gas (typ. Nitrogen) that pushes out unsolidified polymer in the core of the section, followed by the normal cooling stage. This technology, however does not induce surface defects in the form of swirling, .... unless of course foaming agent is added to the polymer and it is subsequently gas injection moulded ;-)

Carefully, me thinks!

Gas Injected

Ah!! The answer is in the question folks........injection-molding of course!!

Also. what position would I have where I was working on a factory dock illegally going through pallets?

Is this dock in Liverpool??

The parts are light weight because they were moulded with a gas assist injection process leaving the parts "hollow" in the thicker, 3/4" in this case, areas of the part.

Scott Reiner

Sounds like structural foam injection molding.

These parts were roto molded

Come on guys! They gave us the fact that they were injection molded, the true question is what variation of injection molding was used. The use of foam is probably the most logical given the fact we were given the clue of the part being light.

the walls are actually hollow, to to a process that injects gas into the melt flow during the injection phase of the process.

Roto-molding is a rotational molding process that may leave swirls. However it generaly creates a uniform wall thickness.

Be always at war with your vices, at peace with your neighbors, and let each new year find you a better man."

Franklin, Benjamin

The parts are made by magic.

Although different processes could be used to injection mold the parts, it is most likely that they were molded using an endothermic blowing agent that is metered into the feed screw along with the resin and colorant.

The blowing agent is activated by heat (thermal decomposition) which causes a chemical reaction creating pockets or cells of gas (usually CO2) within the resin (more cells=less resin=less weight).This reaction also creates the swirls on the surface of the part as the flow front is pushed through the mold cavity under pressure. Getting rid of the swirls is always a challenge for molders, and injection velocity is critical in keeping a resin rich surface in order to minimize the swirls.

Blowing agents are typically used when the plastic has to flow long distances (read 4 ft long) through cold cavities, as well as for reducing part weight and cycle times.

I think this guy #40 nailed it. We shall see.

" Ideally, this microcellular co-injection molding process

enables components with class "A" surfaces and

lightweight microcellular cores to be molded while

eliminating the swirling patterns typical of microcellular

injection molded parts on the surfaces...."

Hmm .

Wow, some great answers and also some very lame ones. Let's recap:

You are working on a factory dock...For those of you who do not know, a factory dock is not usually used for boats, but for trailers so that the floor of the trailer is at the same height as the floor of the factory and material handling equipment can easily roll in and out of the trailer.

...when a pallet loaded with large injection-molded plastic parts is delivered. Clearly, injection-molded parts are specified, so anyone who proposed anything else (extruded, rotomolding, etc.) is JUST PLAIN WRONG!

You are amazed at the wall thickness of the parts, almost 3/4" thick at some points. As each part measures almost 4 ft long, you expect it to weigh quite a bit but are surprised that it is relatively light. OK, this is the part where you have to make some assumptions. Logically, "you" are some kind of engineer, manager, or factory worker who is used to handling and hefting plastic parts, and you know your injection molded from other types. By inference, you also know that typical injection molded parts do not have large wall thickness, because of the waste of material, heat retention, and warping in thick-walled parts, so "space savers" are typically used to create a "swiss cheese" effect, or the thick part is made using multiple thinner walls joined by structural ribs, webbing, or other connecting sections. These "space savers" must be open on one side for the tooling to withdraw or the part to eject from the mold, so they are readily apparent when handling the parts. "You" apparently see nothing of the kind so of course are surprised by the lightness.

You also discover the surface has a swirling pattern in it. How were these parts made? Any answer that does not include an explanation of the "swirling pattern" can only be a partial answer and cannot truly explain "How were these parts made."

From my experience, it does sound like a "structural foam" part that could be made with a chemical reaction of a foaming agent releasing gas or a direct injection of a non-oxidizing gas. Air would NOT be used because oxygen in air would lead to oxidation of the resin at the high temperatures, which would merely weaken the plastic at best or even cause the resin to catch fire and burn in the worst case.

The swirling occurs as the gas absorbs heat as it expands and allows some resin to cool and re-solidify or stiffen while other bits remain molten. Turbulence of the gas release combined with the flow of new incoming material and gas, causes the cooler bits (with gas trapped inside giving it a lighter color) to stretch and swirl as more molten and less gas filled (and therefore darker) material streams around it, eventually cooling and hardening as well.

Gas pockets, ranging in size from pebble and pea-sized voids, down to microscopic "porosity", allow the parts to be significantly lighter. At the same time the parts appear on the outside to be solid, as injection molding has a "skin effect" where the more liquid molten plastic tends to coat the mold cavity walls first and cool in place, while the core remains more molten and flowing. However, occasionally gas bubbles may appear at the surface, or the skin is so thin that it is translucent, allowing the bubble to be easily seen as a defect.

The control of temperature, pressure, mold design, material and gas selection is quite an art and cannot always be scientifically controlled or predicted due to complex interactions.

A long time ago in a high school far, far away I actually did some injection moulding in a basic way. We made handles for such things as BBQ tongs and forks also for different types of screw drivers. etc. The injection machine was basically a large hopper at the top in which you placed the plastic balls (we had orange and blue plastic balls) you plug it into the power point. You waited until it heated up then you pulled a handle (like a one armed bandit) melted plastic came out the nozzle at the bottom. We had basic 2 part moulds. We formed the working end of the tool that we were making, we then either twisted the end with an oxy or filed groves on the shaft where we wanted the handle. We then placed this into the mould. Screwed it together, placed in a special vice and jammed it under the nozzle so there was a tight fit so plasic under hand cranked pressure when melted would go into the mould and not over the ground. Pumped lever until excess plasic oozed out of top of mould. Removed mould and let cool for 20 min. Ta daaarrr. One Tool.

Plastic is NOT heavy. Period

The Swirly look came from the low heat of the melted plastic. Ever seen

pahoehoe. And the fact that you had to hand crank it.

No gas. No fancy dancy foam.

We used to work on the KISS theory.

Keep It Simple Stupid.

Always look for the simpliest answer before trying to find the most complex. No one is impressed with a complicated answer when you miss the right one.

Geomech, let me explain why your answer is very, very lame:

"A long time ago in a high school far, far away....Pumped lever until excess plasic oozed out of top of mould. Removed mould and let cool for 20 min. Ta daaarrr. One Tool." Hey, that's great. You made ONE tool!! Now back to the reality of REAL production quantity manufacturing: Your cycle time, to close the mold, pump in plastic, and hold until cool enough to release the part from the mold, had to be almost a half hour or more. Production cycle time for a part like those described in the problem is measured in seconds and is more likely to be around a half minute, not a half hour.

"Plastic is NOT heavy. Period" Plastic is not heavy? Period? Oh, good. Can I drop a barrel or a Gaylord (very large box) of plastic resin pellets on your head, since plastic is not heavy? It's all relative. The parts described were much lighter than what "you", an experienced factory employee who has handled many other plastic parts, would have expected given the size, shape, and design of the parts.

The Swirly look came from the low heat of the melted plastic. Ever seen pahoehoe. And the fact that you had to hand crank it." You are comparing apples to oranges. Comparing your hand-cranked, low-temp process versus a modern, high-pressure injection molding process is not a good way to draw a conclusion. Yes, "swirly patterns" can appear in parts with standard injection molding due to low melt temperatures, but there are usually other problems that go with that, such as incomplete filling of the mold or oxidation (burning) because hot plastic stayed in contact with air in the hot mold (from incomplete filling) for too long. Your mold was probably not heated, nor retained any significant amount of heat by the time you were ready to mold another part. On the other hand, production molds are either heated, or stay hot due to short cycle times and short open times (when the might otherwise air cool) to eliminate the effects of a cool mold (premature cooling, incomplete filling, etc.) In fact, determining the optimum cooling time (not to short, not too long) is critical to producing good parts that do not warp or bend when they fall out of the mold into a bin or onto a conveyor.

"No gas. No fancy dancy foam." Hmmm... "Dancy"? Is that even a word? If it is I can't see how it applies here! Besides, foamed plastic produced by chemical reaction or direct injection is the most likely answer, and much more likely than yours. And if you READ, you will see I am not the only one saying this, but you are the only one giving your "answer".

"We used to work on the KISS theory. Keep It Simple Stupid." Of course, that is the Engineer's Credo. But there is also the NIH (Not Invented Here) syndrome which you seem to subscribed to. NIH says that if something is not within the realm of your own experience it can't be any good, and it can't be right. This has been the downfall of many engineering projects, and many corporations as well. Sometimes, complex problems require complex solutions, however, RIM or any of the other processes described above involving gas or foam is not that complex anyway. It is done every day by thousands of molders around the world. So KISS my Yank bum, you Limey!

"Always look for the simpliest answer before trying to find the most complex. No one is impressed with a complicated answer when you miss the right one." Well, no one is impressed with your oversimplification that totally ignores several key points in the challenge question. I am sorry that your experience is so limited, yet you feel you must draw a conclusion that is in line with that limited experience.

Now then Yankie, just a quick point....us Limeys are from the UK (so named by your forebears as our forebears ate Limes to avoid scurvy on long sea voyages). The gentleman to whom you addressed your reply is from Australia - he's an Auzzie, and will be as offended at being called a Limey (Aus = Pom) as you would being called a Confederate - or any Southern (US) Gentleman would be at being called a Yank.

If anyone could remind me where P.O.M. comes from (no, not Production operation Management!) I'd be grateful - I know there's a good reason why the thieving Auzzies call us Pommie B******* !

Pomegranate (because of colour and wrinkled skin of new arrivals after a few days of sun), Prisoner of England (=new arrival), Port of Melbourne (ditto - and possibly going away again)??? Take your pick, as no-one really knows.

or , POME - Prisoner Of Mother England , alledgedly stamped on the shirts of early convict settlers.

Have you never seen the very funny "Donovan's Reef" with John Wayne and Lee Marvin? In one memorable bar fight scene, Lee Marvin's character, (Thomas Aloysius) 'Boats' Gilhooley starts the fight by referring to three Australian naval officers as "Limeys". Whether or not they took offense to the term "Limey" or to being compared to Brits is unclear, but the effect was the same. And I believe that was his intent. My intent was the same also!

Besides, didn't the Aussies (or Auzzies, or Ozzies) forbears all come from the UK via a long sea voyage, and probably ate limes to avoid scurvy as well?

By the way, a "Southern (US) Gentleman" of today would probably not want to be called a Confederate either, since it would be pretty obsolete. "Rebel", maybe, "Southerner", definitely, but never "Confederate". And my state, Missouri, was a Border State, which had been a slave state prior to the War, but had an almost equal share of Union and Confederate sympathisers, and despite the wishes of the pro-Southern Governor, never did officially secede from the Union. I have been called both a Yankee (note spelling please) by co-workers when I lived in Arkansas, and a Southerner by Chicagoans, one of whom told me, "Youse guys talks funny down he-are."

My brother moved to Texas, where someone told him, "If we'all like y'all, yore a Nawrtherner. If we'all don't care one way or t'other, yore a Yankee. If we'all dislike y'all, yore a Damn Yankee!"

The point is, don't take yourself so seriously!

I think in future ER should just let you aggressive colonials fight it out between yourselves.

Now that's what I'd call yanking the chain (No spoonerisms/puns intended) !

"Colonials"? Them's fightin' words, boy!

If you take the Cambridge dictionary's definition "a country or area controlled politically by a more powerful and often distant country", you might conclude that Britain looked more like a colony at the moment.

Fyz

Chambers :colony-" a name vaguely applied to a state's dependencies overseas or abroad "

Looks like the internet is going to be the end for us all . My local supermarket would crash within hours of the plug being pulled - 2 days later we'd have anarchy . I wonder which country could effectively do this .Oooh ! And for my next trick - what if everybody used their air-miles at once (supposedly the biggest currency in the world ) ?

I may just disappear to grow turnips on a remote Scottish croft .

(Sorry folks - I'm joking . Not gone yet )

Take myself seriously?! <ER picks self up after ROLFHAO>

That's be a first...spelling of Yankee duely noted. And you did refer to yourself as a Yank...

Just to add to the confusion and hilarity : according to Chambers dictionary Dutch settlers called English settlers Jan Kees (John Cheese ). Ha . Now I'm in the running to be a Limey/Pom/and Yankee ! My forebears must have come to England at some point , so I'm going to claim Colonial as well .

My solicitor has been instructed to act if my new found allegiances and sensitivities are hurt .

Ha! This is good!

I was just telling a friend the other day that Missouri needs to change it's state motto to something else. My friend is from St. Louis.

What do you think of "Missouri loves company?" hehehe

muiporue-