Bent wagon sides.

Discussion in 'Talk' started by Scale7JB, 6 February 2018.

  1. Scale7JB

    Scale7JB Western Thunderer

    Hi all,

    Recently got one of the Lionheart wagons and they really are very nice indeed.

    Just one problem though, the sides bend inwards, rather than straight or pushing out.

    image.jpeg

    Has anyone a tip for softening the plastic so I can bend them out? I was thinking 10 minutes in a very low oven to soften the plastic?

    JB.
     
  2. Yorkshire Dave

    Yorkshire Dave Western Thunderer

    I use a hair dryer for this purpose as you can localise the heat.
     
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  3. Oz7mm

    Oz7mm Western Thunderer

    JB

    Cut a piece of plastikard a bit wider than sides and force it between the top of the sides and then apply the heat as suggested. If the sides end up bowing out that wouldn't be so unprototypical. I do that with some of my Australian wagons.

    John
     
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  4. Scale7JB

    Scale7JB Western Thunderer

    Thanks Dave/John,

    Will give it a go..

    JB.
     
  5. unklian

    unklian Western Thunderer

    This is interesting as it has happened to me many times. Can anyone give a sensible and scientific reason for why the sides bow inwards and not outwards like the prototype does. It seems to happen most with one piece moulded bodies .
     
  6. Osgood

    Osgood Western Thunderer

    The 'what' is likely to be the corners trying to 'close up' i.e. go under 90 deg. - the ends being short remain straight so more stess is transmitted to the sides from each end.

    For the 'why', here is an explanation of the stress at tight corners from a guide to injection moulding:

    "The main enemy of any injection moulded plastic part is stress. When a plastic resin (which contains long strains of molecules) is melted in preparation for moulding, the molecular bonds are temporarily broken due to the heat and shear force of the extruder, allowing the molecules to flow into the mould. Using pressure, the resin is forced into the mould filling in every feature, crack and crevice of the mould. As the molecules are pushed through each feature, they are forced to bend, turn and distort to form the shape of the part. Turning hard or sharp corners exerts more stress on the molecule than taking gentle turns with generous radii. Abrupt transitions from one feature to another are also difficult for the molecules to fill and form to.

    As the material cools and the molecular bonds re-link the resin into its rigid form, these stresses are in effect locked into the part. Part stresses can cause warpage, sink marks, cracking, premature failure and other problems."​
     
  7. Chuffer

    Chuffer Active Member

    I think a simpler reason is that with planked wagons in particular, the exterior planks are correctly grooved between each one whilst the interior 'grooves' aren't, having slightly raised mould lines instead so the exterior is thus structurally weaker and is drawn inwards. If the inside were grooved instead, the pressure would be equalised and the sides/ends should retain an even appearance. With plastikard buildings I have to ensure I scribe the interior as well as the exterior to help stop the walls bowing - bracing spacers obviously help too.

    Like John, I've also had to resort to forcing in place slightly wider plastikard spacing pieces or gently convex curved on all sides false floors pressed in place midway up the sides but didn't try the hairdryer routine - should have thought of that. :rolleyes:
    Glued in place they make the sides and ends bow outwards just a tad, which is far more prototypical than bowing inwards. I subsequently fitted cosmetic coal loads on top of the spacers so they have to run loaded, which is a shame. May try the hairdryer routine next time.

    Incidentally, I seem to recall Slaters printed private owner wagons were also grooved only on the outside due, I understood, to the printing processes involved. Their non printed wagons, however, were correctly grooved both inside and out so the bowing problem didn't occur with those.

    Martyn
     
  8. Brian McKenzie

    Brian McKenzie Western Thunderer

    This would be a good question to put to Steven Leathers with his new injection moulding service addition to LLC, he having access to very expensive "Moldflow" software - which can analyse such situations and amend the mould shape to counteract.

    I'm more inclined to go with Osgood's explanation, as this phenomenon can be seen in many similar shaped mouldings, one example being Kodak slide boxes. Chuffer's reasoning is very valid for plasticard though.
     
  9. michl080

    michl080 Member

    that's what I learned many years ago. In simple words, the material tries to take the shape with the lowest tension which is typically a ball.

    I think an important aspect is that the actual effect can be influenced through process parameters. The higher the troughput, the injection temperatures, the pressure etc., the worse the result. Some of the manufacturers supply amazing quality with very thin walls. Brawa Germany is a good example.

    Michael
     
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  10. Osgood

    Osgood Western Thunderer

    I must stress (no pun intended) I have no experience of the process - I had a quick look around for an explanation. It seemed quite plausible.

    Martyn's explanation also seems quite plausible - although a secondary factor might be the process of scribing one side which introduces stresses that were not present in the sheet in its manufactured form. If the sheet were moulded with thin grooves representing planking on one side, would it still be inclined to bend, or would the moulding stresses be far less than those caused by scribing?

    Have any of the Lionheart mineral wagons suffered bowing? They are essentially plain inside and out - although:
    a) the corners are slightly thinner than the planked wagonso less able to exert a bending load on the sides
    b) however the outer sides have upright ribs around the door which might act in same way as the corners to exert a bending force trying to bow sides inwards.

    Fascinating that there is now software out there dedicated to resolving such issues at the design stage.
    Who'd be a manufacturer, hey? :)
     
  11. Chuffer

    Chuffer Active Member

     
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  12. Osgood

    Osgood Western Thunderer

    The obvious approach is to apply a hairdryer to the sides and push out - but if the corner stress concept is valid, perhaps it would be better to apply heat to the corner whilst pushing the sides out at the middle?

    Goes off to inspect some loose Lionheart planked and minerals in deep storage
     
  13. Bill Bedford

    Bill Bedford Active Member

    Yes, but the main source of stress will be the join between the sides and the floor, mainly because the floor will be thicker than the sides.
     
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  14. richard carr

    richard carr Western Thunderer

    My Lion heart (now Dapol) 16 ton minreal wagons do not show any sign of bowing in, but my JLTRT MDV wagons the larger 21 ton mineral wagon all do bow in and now the kit comes with a pre cut plastic card insert to prevent this, the first 6 bought didn't and I have had to fit my own.

    Richard
     
  15. Osgood

    Osgood Western Thunderer

    Of course, thanks - why is it so easy to miss the obvious?

    I've just checked the deep storage Lionhearts - bit of a mixed bag but seems to be more noticeable in the recent production ones, which might perhaps tie in with michl080's "other parameters", like quicker throughput, less cooling time in mould?

    Mineral - early production
    Mineral early prod.jpg

    Mineral - recent production (not latest batch)
    Mineral recent prod.jpg

    Coke (without rails which are all bowed) - assume main production
    Coke less rails.jpg

    I've pulled out a bad mineral to attack later today with hair dryer / hot air gun....
     
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  16. Osgood

    Osgood Western Thunderer

    An interesting experiment.

    Taking on board Bill Bedford's words, I wondered if de-stressing the floor might allow the sides to return to straight.
    So I took a de-chassis'd body and applied heat aimed directly onto the floor.
    Dryer was on max. heat, min. flow settings.
    Given the size of air nozzle one could argue the whole body was being heated up, but max. heat was being directed down into the floor area and into the lower part of the sides as the hot air flowed up and out.

    Heat1.jpg

    Within 2 or 3 seconds the sides started to straighten of their own accord and then bow out - seemingly the heat was de-stressing the moulding. When the heat source was removed the sides immediately started moving inwards again - applying heat once more and they again moved out.

    Repeating this a few times resulted in one side eventually staying slightly bowed out, with the other repeatedly returning to slightly bowed in, albeit better. All this time no external force had been applied to the body.

    So a further go with sides held out by constant finger pressure as soon as heat was removed did the trick. Cold air flow from the dryer helped to speed the body back to normal temp. The side which had stayed out before is now very slightly bowed out (looks good) but no doubt perfection could be attained if desired.

    The ends remain ever-so-slighty bowed in as before - not really noticeable on the steel minerals but I've seen some wooden bodied wagons that are quite badly affected by end bow.

    Just noticed my yellow boxes are a classic example themselves!
     
    Last edited: 8 February 2018
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  17. PMP

    PMP Western Thunderer

    My solution with 4mm wagons is similar, the only additional thing I do is to run a very sharp blade along the inside angle, floor and side/ends. I then place a widener inside the wagon body and apply hair dryer heat until the body is hot to touch. I leave the widener in place while the body cools, and don’t remove it for a day or so.
     
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  18. Osgood

    Osgood Western Thunderer

    Lionheart Minerals - a Postscript and recommendation:

    I had removed the body before letting loose with the hairdryer, and left it in a stable condition with one side back to straight, one very slightly bowed out.

    The next evening I checked the sides had not moved, and then reassembled the body to the chassis.
    When admiring my handiwork, I discovered the slightly bowed out side was still slightly bowed out, but the straight side was now slightly bowed inwards!
    So I removed the body once more, only to find the bowed out side was still the same, but the bowed in side had reverted to straight again.

    It looks very much like the body may have been moulded nice and straight, but the chassis plate - when chassis rail sides are clipped onto the outer faces - is very slightly too wide for the recess moulded into the body underside. So when the thing is assembled the chassis exerts an outward force on the side below floor level, thereby causing the upper sides to bow inwards!

    I tested this theory by inserting the die-cast chassis plate into the underfloor recess without the plastic chassis rails in place, and twisting a screwdriver in the resultant gap between plate and body moulding - this caused the straight side to bow in. Further twisting caused the bowed out side to go straight.

    On the other hand, the more likely scenario is that by heating and bending the body sides without the chassis in place, the gap for the chassis plate could have closed up too tight :rolleyes:

    So a file was taken to the die-cast plate to remove paint and a small amount of metal, and the lower part of the four door side stanchions were eased a little where they protrude below floor rail and meet the chassis side rails.

    Reassembly proved a success. :)

    The recommendation here must be to LEAVE THE :rant:BODY attached to the chassis and simply apply heat and muscle to straighten sides. :))

    I'll have a look at the wooden bodied coke wagons next when time allows. Not sure the above cause will apply to them.
     
    Last edited: 11 February 2018
  19. Scale7JB

    Scale7JB Western Thunderer

    Good to know and thanks for all your testing.

    Little man isn't too keen on the noise of the hairdryer, so I'll have a go when he's out with mum..

    JB.
     
  20. bradshaw

    bradshaw New Member

    Did something similar myself . Cut some 5mm ply about 20mm wide, long enough to fit the wagon and force the ends out a bit. Used a clamp behind the doors again to force the doors out a bit.

    Filled the wagon (still on it's chassis) with boiling water. Left it a few minutes then replaced the hot with cold water.

    The sides move back in a bit after a while, so a bit of experimentation is required on how far to force the sides. Oh, and if you get it wrong repeat the process without the wood, and the sides should come back in.

    Cheers

    IMG_0477resize 800x600.jpg IMG_0478resize 800x600.JPG
     
    Last edited: 13 February 2018
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