Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Kirk Wallace]

On 10/20/2016 06:15 PM, Dave Cole wrote:
... snip
> If it was wavy then they were probably having quality issues. The OD is
> the critical thing with PVC pipe that uses glued socket fittings.
... snip

I have noticed the same ID thing with PVC pipe here. The OD is very 
accurate, straight and smooth. The ID isn't quite as good and can be off 
center. Normally, it isn't obvious but is noticeable when machining.


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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Dave Cole]

All of the PVC Pipe I have seen has been extruded.PEX pipe is also 
extruded.  I'm sure that black poly pipe/tubing used for sprinkler 
systems is also extruded. Plastic extrusions are very common because it 
is a very cheap way to make a profile plastic product, once the machine 
is setup.  But its a continuous process

Google "PVC Pipe Extrusion" and you will see some machines and videos.
Here is a big one:
https://www.youtube.com/watch?v=zcBv_JvFDBI

If it was wavy then they were probably having quality issues. The OD is 
the critical thing with PVC pipe that uses glued socket fittings.
Schedule 80 is only a little heavier than schedule 40 in smaller diameters.

I've done work on extruders and plastics equipment controls.

Molding by spinning is frequently called rotomolding or rotary molding. 
   Many plastic Kayaks are rotomolded as are a lot of larger plastic tanks.

Dave

On 10/20/2016 8:00 AM, Todd Zuercher wrote:
> Is schedule 40 (or 80) extruded?  I don't think most of what I've used is.  
> While the outside is smooth the inner surface is usually a little wavy and 
> irregular, like it may have been spun in a mold???
>
> - Original Message -
> From: "Dave Cole" <linuxcncro...@gmail.com>
> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>
> Sent: Wednesday, October 19, 2016 11:49:08 PM
> Subject: Re: [Emc-users] Wat OT: Question about injection molding of PVC
>
> Ive never seen it poured.However think of all of the things that are
> extruded out of PVC in high volume.
> PVC pipe, gutter, fence posts, window frames, vinyl siding and trim.
> The trick will probably be finding a place that extrudes PVC in small
> quantities.   Most of the plastic pipe plants around here have silos
> outside of the plant full of PVC resin pellets ready to be fed into an
> extruder.Some of these silos are 30-50 feet tall.   The high volume
> plants usually have a pneumatic conveying system to deliver the PVC
> pellets to the extruders. Once they start up an extruder it typically
> runs 24x7 until they are done with that run.
>
> Dave
>
> On 10/19/2016 4:46 PM, Leonardo Marsaglia wrote:
>> 2016-10-17 9:50 GMT-03:00 craig <cr...@facework.com>:
>>
>>> I don;t know  what resources you have there.
>>>
>>> Minimum equipment approach
>>>
>>> 1 Heat the plastic and pour the rectangular shape oversize, Possibly
>>> flattening down  and spread it out as
>>> you do.   (How much it spreads naturally will be dependent on both
>>> material and surface temperature.
>>>If you have a big enough machine you want to use it to control the
>>> pour path.
>>> 2 cut off the edges and recycle the material  with mill or saw.
>>> 3. mill the top if the flattening does not leave the right height.
>>> 4 possibly heat polish (slightly melt the surface) the form if glossy
>>> surface it more important than sharp rectangular edges.
>>>
>>> You may want to reverse steps 2 and 3 and heat  polish the top for a
>>> glossy surface before cutting the edges.
>>>
>> Is it possible to pour PVC as you would do with molten metal? I mean
>> because of the viscosity of the plastic at that temperature I always
>> thought the only solution is to apply pressure wheter extruding it or
>> injecting it.
>>
>>
>>
>>
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Todd Zuercher]

Is schedule 40 (or 80) extruded?  I don't think most of what I've used is.  
While the outside is smooth the inner surface is usually a little wavy and 
irregular, like it may have been spun in a mold???

- Original Message -
From: "Dave Cole" <linuxcncro...@gmail.com>
To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>
Sent: Wednesday, October 19, 2016 11:49:08 PM
Subject: Re: [Emc-users] Wat OT: Question about injection molding of PVC

Ive never seen it poured.However think of all of the things that are 
extruded out of PVC in high volume.
PVC pipe, gutter, fence posts, window frames, vinyl siding and trim.
The trick will probably be finding a place that extrudes PVC in small 
quantities.   Most of the plastic pipe plants around here have silos 
outside of the plant full of PVC resin pellets ready to be fed into an 
extruder.Some of these silos are 30-50 feet tall.   The high volume 
plants usually have a pneumatic conveying system to deliver the PVC 
pellets to the extruders. Once they start up an extruder it typically 
runs 24x7 until they are done with that run.

Dave

On 10/19/2016 4:46 PM, Leonardo Marsaglia wrote:
> 2016-10-17 9:50 GMT-03:00 craig <cr...@facework.com>:
>
>> I don;t know  what resources you have there.
>>
>> Minimum equipment approach
>>
>> 1 Heat the plastic and pour the rectangular shape oversize, Possibly
>> flattening down  and spread it out as
>>you do.   (How much it spreads naturally will be dependent on both
>> material and surface temperature.
>>   If you have a big enough machine you want to use it to control the
>> pour path.
>> 2 cut off the edges and recycle the material  with mill or saw.
>> 3. mill the top if the flattening does not leave the right height.
>> 4 possibly heat polish (slightly melt the surface) the form if glossy
>> surface it more important than sharp rectangular edges.
>>
>> You may want to reverse steps 2 and 3 and heat  polish the top for a
>> glossy surface before cutting the edges.
>>
> Is it possible to pour PVC as you would do with molten metal? I mean
> because of the viscosity of the plastic at that temperature I always
> thought the only solution is to apply pressure wheter extruding it or
> injecting it.
>
>
>
>

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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Dave Cole]

Ive never seen it poured.However think of all of the things that are 
extruded out of PVC in high volume.
PVC pipe, gutter, fence posts, window frames, vinyl siding and trim.
The trick will probably be finding a place that extrudes PVC in small 
quantities.   Most of the plastic pipe plants around here have silos 
outside of the plant full of PVC resin pellets ready to be fed into an 
extruder.Some of these silos are 30-50 feet tall.   The high volume 
plants usually have a pneumatic conveying system to deliver the PVC 
pellets to the extruders. Once they start up an extruder it typically 
runs 24x7 until they are done with that run.

Dave

On 10/19/2016 4:46 PM, Leonardo Marsaglia wrote:
> 2016-10-17 9:50 GMT-03:00 craig :
>
>> I don;t know  what resources you have there.
>>
>> Minimum equipment approach
>>
>> 1 Heat the plastic and pour the rectangular shape oversize, Possibly
>> flattening down  and spread it out as
>>you do.   (How much it spreads naturally will be dependent on both
>> material and surface temperature.
>>   If you have a big enough machine you want to use it to control the
>> pour path.
>> 2 cut off the edges and recycle the material  with mill or saw.
>> 3. mill the top if the flattening does not leave the right height.
>> 4 possibly heat polish (slightly melt the surface) the form if glossy
>> surface it more important than sharp rectangular edges.
>>
>> You may want to reverse steps 2 and 3 and heat  polish the top for a
>> glossy surface before cutting the edges.
>>
> Is it possible to pour PVC as you would do with molten metal? I mean
> because of the viscosity of the plastic at that temperature I always
> thought the only solution is to apply pressure wheter extruding it or
> injecting it.
>
>
>
>

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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Leonardo Marsaglia]

2016-10-17 10:50 GMT-03:00 Dave Cole :

> Hi Leonardo,
>
> Unless you are going to be making thousands of these, I would buy or
> have PVC extruded into the shape profile and then weld the PVC together
> into that rectangular shape.
> You could injection mold that part but it will require a large press.
> You won't be making one of those as it is not economical.
> There are presses out there that can easily mold parts that large.
> Years ago I was at an industrial auction and they were selling injection
> molding presses that were used to mold childrens play cars.   The kind
> where the kid climbs into the car.The play "cars" are about 4 feet
> long.Anyway the company had several large presses and they were
> being auctioned.I wasn't interested in the presses but I was
> interested in how much they would sell for.The selling price started
> at $1000 or so and none of them sold.   The issue was the size and
> weight of the presses.   When the original presses were purchased they
> were brought to the facility via rail car.   The presses could not be
> easily broken down into a size that could fit onto a semi trailer.   The
> mold platens were about 6 feet square. Consequently there was no
> economical way to get the presses out of the building so they didn't
> even sell, which was way below their scrap value.   They didn't sell as
> scrap since it would cost more to cut them up then they were worth.
> Plus the machines were inside a building so the use of cutting torches
> would be a liability problem.
>
> If you want to make a lot of these parts, I advise you to find a large
> plastic injection molding company and show them your part and ask them
> how much it would cost to make _x_ number of parts.If they have a
> press that can handle the size, they will tell you.  Having molds made
> is routine for those companies, so they should be able to give you
> prices for everything.
>
> I'm in the Midwest USA and this area has many injection molding
> companies.Ohio, Indiana, and lower Michigan have many large
> injection molding companies and plants.
>

Well the idea is to make the parts on my own to reduce costs, since I have
several shapes similar to that It will require a lot of molds and that's
costly, at least the initial cost is to high.

I'm going to make lots of parts and I came to this solution because buying
the sheets is way too expensive for the final cost. At least here in my
country. I can always find better prices in China I guess but if I can make
the parts here it's going to be a lot easier. The problem is I really can't
decide wich solution could be simpler, if the extruder or the injection.

>From the top of my head the extruder looks simpler but I don't have any
experience with that kind of machines so I can't say I'm sure it's the
easiest way.


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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Leonardo Marsaglia]

2016-10-17 9:50 GMT-03:00 craig :

> I don;t know  what resources you have there.
>
> Minimum equipment approach
>
> 1 Heat the plastic and pour the rectangular shape oversize, Possibly
> flattening down  and spread it out as
>   you do.   (How much it spreads naturally will be dependent on both
> material and surface temperature.
>  If you have a big enough machine you want to use it to control the
> pour path.
> 2 cut off the edges and recycle the material  with mill or saw.
> 3. mill the top if the flattening does not leave the right height.
> 4 possibly heat polish (slightly melt the surface) the form if glossy
> surface it more important than sharp rectangular edges.
>
> You may want to reverse steps 2 and 3 and heat  polish the top for a
> glossy surface before cutting the edges.
>

Is it possible to pour PVC as you would do with molten metal? I mean
because of the viscosity of the plastic at that temperature I always
thought the only solution is to apply pressure wheter extruding it or
injecting it.




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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Leonardo Marsaglia]

2016-10-16 21:42 GMT-03:00 andy pugh :

> Sorry, no. I have absolutely no experience at either approach. I am
> interested in the technologies, but purely as an observer. The only
> thing I have ever looked at injection moulding was way outside normal
> parameters. We were considering the balance between £20k for tooling
> and £100 per part material cost for injection moulding polyimide
> versus machining glass ceramic. And 40 parts per year.  And they made
> me redundant before I found out what they decided.
>

I see, I think I'll try to collect the more info I can about extrusion of
PVC, seems the less complicated process but as you I don't have any
experience. Thank Andy as always anyway!


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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Gregg Eshelman]

Shear the pieces off a sheet then have a shop that does custom vinyl windows 
miter and weld the corners.
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Dave Cole]

Hi Leonardo,

Unless you are going to be making thousands of these, I would buy or 
have PVC extruded into the shape profile and then weld the PVC together 
into that rectangular shape.
You could injection mold that part but it will require a large press.   
You won't be making one of those as it is not economical.
There are presses out there that can easily mold parts that large.
Years ago I was at an industrial auction and they were selling injection 
molding presses that were used to mold childrens play cars.   The kind 
where the kid climbs into the car.The play "cars" are about 4 feet 
long.Anyway the company had several large presses and they were 
being auctioned.I wasn't interested in the presses but I was 
interested in how much they would sell for.The selling price started 
at $1000 or so and none of them sold.   The issue was the size and 
weight of the presses.   When the original presses were purchased they 
were brought to the facility via rail car.   The presses could not be 
easily broken down into a size that could fit onto a semi trailer.   The 
mold platens were about 6 feet square. Consequently there was no 
economical way to get the presses out of the building so they didn't 
even sell, which was way below their scrap value.   They didn't sell as 
scrap since it would cost more to cut them up then they were worth.
Plus the machines were inside a building so the use of cutting torches 
would be a liability problem.

If you want to make a lot of these parts, I advise you to find a large 
plastic injection molding company and show them your part and ask them 
how much it would cost to make _x_ number of parts.If they have a 
press that can handle the size, they will tell you.  Having molds made 
is routine for those companies, so they should be able to give you 
prices for everything.

I'm in the Midwest USA and this area has many injection molding 
companies.Ohio, Indiana, and lower Michigan have many large 
injection molding companies and plants.

Dave


On 10/15/2016 8:12 PM, Leonardo Marsaglia wrote:
> Hello to all.
>
> I'm sure here I'm going to find good answers so I decided to start from
> here.
>
> I'm planning to do some thermoforming production work but the sheet plastic
> I need to use is rectangular with a rectangular hole in the middle (I
> attached a picture so you can see). So to use regular PVC sheet would imply
> a lot of waste.
>
> I came up with the idea of injecting the PVC using a manual clamped mould
> and injecting it with a screw driven piston. The difficult part comes here,
> I need to inject almost 3 kg of PVC. The moulds are going to be pretty
> simple as you can see but I would like to know if there's any good source
> to determine the approximate power I would need to drive the screw that
> moves the piston.
>
> There's the possibility of using two pistons one on each side to make
> things easier. Off course I would need to reduce the motors with worm and
> gear and then connect the gear to the screw that drives the piston. But my
> main concern is if this approach is correct or if I should forget about it.
>
> I didn't even consider the hydraulic pump because the cost would be a lot.
> Also take into account that I'm not intending to make lots of these
> injections per hour, so injection times could be slower than the industry
> standard.
>
> I hope I've been clear about my doubts and I would be thankful if you can
> point me to any source of info about this, or help me to be more sure about
> the dimensions of what I need to do.
>
> Thanks as always!
>
>
>
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>
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[craig]

I don;t know  what resources you have there.

Minimum equipment approach

1 Heat the plastic and pour the rectangular shape oversize, Possibly 
flattening down  and spread it out as
  you do.   (How much it spreads naturally will be dependent on both 
material and surface temperature.
 If you have a big enough machine you want to use it to control the 
pour path.
2 cut off the edges and recycle the material  with mill or saw.
3. mill the top if the flattening does not leave the right height.
4 possibly heat polish (slightly melt the surface) the form if glossy 
surface it more important than sharp rectangular edges.

You may want to reverse steps 2 and 3 and heat  polish the top for a 
glossy surface before cutting the edges.

Craig

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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[andy pugh]

On 17 October 2016 at 00:37, Leonardo Marsaglia
 wrote:

> That would be a good way of doing it too. Do you think that making the
> extruder could be less complicated than making the injection mold and
> piston?

Sorry, no. I have absolutely no experience at either approach. I am
interested in the technologies, but purely as an observer. The only
thing I have ever looked at injection moulding was way outside normal
parameters. We were considering the balance between £20k for tooling
and £100 per part material cost for injection moulding polyimide
versus machining glass ceramic. And 40 parts per year.  And they made
me redundant before I found out what they decided.

-- 
atp
"A motorcycle is a bicycle with a pandemonium attachment and is
designed for the especial use of mechanical geniuses, daredevils and
lunatics."
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Leonardo Marsaglia]

2016-10-16 19:12 GMT-03:00 andy pugh :

>
> Why not extrude the full width, cut out the centre, and put the
> cut-out part back in the extruder?
>
> It might even be possible to extrude with a hole on the middle if the
> extrusion nozzle had a gate in it. I don't even know if that is
> possible, though I know that plastic net is made by moving extrusion
> dies


Hello Andy.

That would be a good way of doing it too. Do you think that making the
extruder could be less complicated than making the injection mold and
piston? I'm a little lost because I don't have experience in this field and
it's hard to tell how big and costly things could be with one approach or
the other.





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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[andy pugh]

On 16 October 2016 at 16:01, Leonardo Marsaglia
 wrote:
> Another idea that comes to my mind but I don't know how doable it could be,
> is to have like an extruder than can make the turn and make the bend

Why not extrude the full width, cut out the centre, and put the
cut-out part back in the extruder?

It might even be possible to extrude with a hole on the middle if the
extrusion nozzle had a gate in it. I don't even know if that is
possible, though I know that plastic net is made by moving extrusion
dies


-- 
atp
"A motorcycle is a bicycle with a pandemonium attachment and is
designed for the especial use of mechanical geniuses, daredevils and
lunatics."
— George Fitch, Atlanta Constitution Newspaper, 1916

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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Leonardo Marsaglia]

Thanks to all for your answers!

I'll try to answer in one reply so I don't make a mess with the
conversation.

I've thought about pouring it as Marcus suggested but as you've said and
from what I was reading on forums and so, it's quite difficult in my case
because of the shape and lenght of my parts.

Welding the strips as Linden,Erik, and Craig pointed out together was my
first option, because I can polish the joints and that's it. But I found
the costs here in my country are way high doing it that way. Remember that
what I need to do with this rectangular shape is to thermoform it. Also, I
couldn't find sheets of more than 6 mm of thickness.

I came across with the idea of injection molding because it's a lot cheaper
than buying and welding the sheets. Another approach would be to extrude
the sheets and weld them but that would imply a lot of work in the
production stage. With injection I only have the head aches now, but if I
can make it possible it would be a lot simpler once it's ready.

Also please notice that the picture is an aproximation of the shape I need
to make the thermoforming, the final one is going to be wider on the sides.

Another idea that comes to my mind but I don't know how doable it could be,
is to have like an extruder than can make the turn and make the bend in the
sides to produce the rectangle with rounded corners (I don't mind about
that if I can cut them later), and then weld the last corner of the
rectangle. If possible I guess it could be a lot simpler that the injection
and I can save the costs.



2016-10-15 22:29 GMT-03:00 craig :

> On 10/15/2016 5:12 PM, Leonardo Marsaglia wrote:
> > Hello to all.
> >
> > I'm sure here I'm going to find good answers so I decided to start from
> > here.
> >
> > I'm planning to do some thermoforming production work but the sheet
> plastic
> > I need to use is rectangular with a rectangular hole in the middle (I
> > attached a picture so you can see). So to use regular PVC sheet would
> imply
> > a lot of waste.
> >
> > I came up with the idea of injecting the PVC using a manual clamped mould
> > and injecting it with a screw driven piston. The difficult part comes
> here,
> > I need to inject almost 3 kg of PVC. The moulds are going to be pretty
> > simple as you can see but I would like to know if there's any good source
> > to determine the approximate power I would need to drive the screw that
> > moves the piston.
> >
> > There's the possibility of using two pistons one on each side to make
> > things easier. Off course I would need to reduce the motors with worm and
> > gear and then connect the gear to the screw that drives the piston. But
> my
> > main concern is if this approach is correct or if I should forget about
> it.
> >
> > I didn't even consider the hydraulic pump because the cost would be a
> lot.
> > Also take into account that I'm not intending to make lots of these
> > injections per hour, so injection times could be slower than the industry
> > standard.
> >
> > I hope I've been clear about my doubts and I would be thankful if you can
> > point me to any source of info about this, or help me to be more sure
> about
> > the dimensions of what I need to do.
> >
> > Thanks as always!
> >
> I would expect that the temperature  of both the material and the mold
> will be important for the long thin shapes.  The viscosity will change
> as the material cools running down long thin paths.
>
>   It might be easier to extrude of cut the sides and heat bond them or
> mold just the corner to bond the pieces.
>
> craig
>
> >
> >
> > 
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[craig]

On 10/15/2016 5:12 PM, Leonardo Marsaglia wrote:
> Hello to all.
>
> I'm sure here I'm going to find good answers so I decided to start from
> here.
>
> I'm planning to do some thermoforming production work but the sheet plastic
> I need to use is rectangular with a rectangular hole in the middle (I
> attached a picture so you can see). So to use regular PVC sheet would imply
> a lot of waste.
>
> I came up with the idea of injecting the PVC using a manual clamped mould
> and injecting it with a screw driven piston. The difficult part comes here,
> I need to inject almost 3 kg of PVC. The moulds are going to be pretty
> simple as you can see but I would like to know if there's any good source
> to determine the approximate power I would need to drive the screw that
> moves the piston.
>
> There's the possibility of using two pistons one on each side to make
> things easier. Off course I would need to reduce the motors with worm and
> gear and then connect the gear to the screw that drives the piston. But my
> main concern is if this approach is correct or if I should forget about it.
>
> I didn't even consider the hydraulic pump because the cost would be a lot.
> Also take into account that I'm not intending to make lots of these
> injections per hour, so injection times could be slower than the industry
> standard.
>
> I hope I've been clear about my doubts and I would be thankful if you can
> point me to any source of info about this, or help me to be more sure about
> the dimensions of what I need to do.
>
> Thanks as always!
>
I would expect that the temperature  of both the material and the mold 
will be important for the long thin shapes.  The viscosity will change 
as the material cools running down long thin paths.

  It might be easier to extrude of cut the sides and heat bond them or 
mold just the corner to bond the pieces.

craig

>
>
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Peter Blodow]

PVC flooring is roilled out in long strips and then joint welded by 
little automatic machines, running along, following the joint by a 
sensor stick and equipped with a hot air fan and a roll of 4 mm PVC 
wire. Every floor working shop has been using them for a long time. PVC 
welding is easy. No CNC involved. Automation can be so simple.

Peter Blodow



Am 16.10.2016 03:44, schrieb Leonardo Marsaglia:
> 2016-10-15 22:33 GMT-03:00 John Kasunich :
>
>> Injection pressure will be trying to force the two sides of your mold
>> apart.
>> That's why injection molds are made of tool steel, and injection machines
>> have VERY sturdy construction to hold the mold closed.  I saw a machine
>> that might be big enough to make your parts - the mold closing cylinder was
>> about 2 feet (0.6m) in diameter, and the four steel tie-rods that held the
>> machine together against the clamp force were about 100mm diameter.
>>
>> Could you make it out of four strips with some kind of joining at the
>> corners?
>>
> Hello John.
>
> Yes the idea is to machine the four strips and then joing them strongly. To
> clamp the mold I was thinking about mechanical ways like eccentrics or may
> be screw clamps. That way I can hold the mold together when the injection
> takes place. I really don't worry about the time consumption on open the
> mold and close it again.
>
> The concerning part I have is if It's a good idea to move the pistons for
> the injection with a screw and a common 3 phase AC motor. My idea is to
> load the piston, wait for the plastic to reach the temperature and then
> inject no less than 30 seconds. I probably have to preheat the mold too.
>
>


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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[TJoseph Powderly]

if the picture is the final product...
why not air weld 4 strips together?

chamfer the mating edges and fill with 'rod' of same material

google 'weld pvc' for more

this is used on plastic window sets as example

parts of Linuxcnc Hal and tricks from machinekit/reprap extruders can be 
used to automate this

( plus a jig ;-)

HTH
tjtr33/tomp

On 10/16/16 07:12, Leonardo Marsaglia wrote:
> e pictureHello to all.
>
> I'm sure here I'm going to find good answers so I decided to start from
> here.
>
> I'm planning to do some thermoforming production work but the sheet plastic
> I need to use is rectangular with a rectangular hole in the middle (I
> attached a picture so you can see). So to use regular PVC sheet would imply
> a lot of waste.
>
> I came up with the idea of injecting the PVC using a manual clamped mould
> and injecting it with a screw driven piston. The difficult part comes here,
> I need to inject almost 3 kg of PVC. The moulds are going to be pretty
> simple as you can see but I would like to know if there's any good source
> to determine the approximate power I would need to drive the screw that
> moves the piston.
>
> There's the possibility of using two pistons one on each side to make
> things easier. Off course I would need to reduce the motors with worm and
> gear and then connect the gear to the screw that drives the piston. But my
> main concern is if this approach is correct or if I should forget about it.
>
> I didn't even consider the hydraulic pump because the cost would be a lot.
> Also take into account that I'm not intending to make lots of these
> injections per hour, so injection times could be slower than the industry
> standard.
>
> I hope I've been clear about my doubts and I would be thankful if you can
> point me to any source of info about this, or help me to be more sure about
> the dimensions of what I need to do.
>
> Thanks as always!
>
>


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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Marcus Bowman]

I machine moulds for a local company which pours PVC into the mould (think a 
deep groove, open at the top). The finish on the top surface (open to the air) 
is very good, but maybe not quite as consistent over such a long length as you 
need, compared to injection moulding. The company do injection moulding too, 
but I have not made any of those.
The challenge would be in machining such a large groove. I machine out of 
aluminium tooling plate, 450 x 450mm, or thicker blocks. The main cost is the 
mould. Once that is made, the cost of pouring the PVC is quite low.
If I had to make any quantity of frames like yours, I would have them poured 
commercially.
Tooling for injection moulding is much more expensive, and, as has already been 
pointed out, needs to be much stronger. Low quantity runs can get by with 
aluminium moulds, but higher quantities require steel moulds.
Three other factors to bear in mind with poured PVC:
the hardness of the PVC can be varied to suit the end product, but only up to 
around 70 Shore A;
it is hand poured (at least in the factory doing the moulds I have made);
pouring requires the mould to be heated, and the arrangement for doing that may 
impose a size limit on the moulds, in at least one dimension. Mine are all 
450mm wide because they sit on fixed rails;
undercuts are no problem in most moulds. Once the PVC has cooled and set, it is 
blown out of the mould using compressed air (usually injected from the top, in 
a poured mould).
With the manual pouring and extraction, no CNC is involved.

Larger objects like car front scuttles, buckets etc, are moulded by using a 
male mould then heating a sheet of PVC and draping/vacuum sucking over the 
mould. Easy; but large moulds are expensive to make (3D CNC using large blocks 
of cast aluminium). Draping is not appropriate for your shape.

Oh; poured PVC shrinks as it cools in a mould..

Marcus
 
On 16 Oct 2016, at 07:10, linden wrote:

>  When I worked  for a semi conductor tool manufacture we used to 
> plastic weld the drip trays out of PVC and Chemistry tanks out of other 
> plastics for HF, H2SO4 and other nastiness.
> 
>   The process is very similar to Tig welding metal. We used a hot air 
> gun and a filler rod to fuse and fill the seems. As with Tig welding 
> joint prep and weld temp is key. With a little practice it is not hard 
> to get sound joints and it is much easer than metal as you can see what 
> you are doing and work with out gloves. The only real issue are the 
> fumes released from the melting plastic.
> 
> You probably already have a miter saw to cut with and a good table to 
> clamp your PVC to the cost of a welder filler rod and some scrap to 
> practice on would most certainly be less than building an injection 
> machine and certainly more versatile.   Not as fun mind you;-)
> 
> 
>  n 2016-10-15 08:33 PM, Erik Christiansen wrote:
>> On 15.10.16 22:44, Leonardo Marsaglia wrote:
>>> Yes the idea is to machine the four strips and then joing them strongly. To
>>> clamp the mold I was thinking about mechanical ways like eccentrics or may
>>> be screw clamps. That way I can hold the mold together when the injection
>>> takes place. I really don't worry about the time consumption on open the
>>> mold and close it again.
>> <2c>
>> If joining by solvent is too weak or unsightly, have you considered "pvc
>> welding"?. I haven't used the process, but google offers myriad hits.
>> I've seen ¼" thick PVC with a rippled joint looking much like a weld in
>> steel. It may have been done on both sides, though, for complete
>> penetration. (I don't recall.)
>> 
>> OK, if molten plastic injection into the joint were a viable option,
>> then the mould restraining force would be _much_ less than if moulding a
>> large flat object from scratch, but just clamping four sheets for
>> welding is orders of magnitude simpler & cheaper. I can't imagine that
>> plastic welding gear is in the same ballpark as injection moulding,
>> either.
>> 
>> I suspect that molten filler material would need to be run straight from
>> nozzle to every point of joining, i.e. run a nozzle along the joint. If
>> injected at a central sprue, it would be too cold to melt the PVC strips
>> for adequate fusing. (But that is speculation.)
>> 
>> 
>> Erik
>> 
>> --
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>> engaging tech sites, SlashDot.org! http://sdm.link/slashdot
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[linden]

  When I worked  for a semi conductor tool manufacture we used to 
plastic weld the drip trays out of PVC and Chemistry tanks out of other 
plastics for HF, H2SO4 and other nastiness.

   The process is very similar to Tig welding metal. We used a hot air 
gun and a filler rod to fuse and fill the seems. As with Tig welding 
joint prep and weld temp is key. With a little practice it is not hard 
to get sound joints and it is much easer than metal as you can see what 
you are doing and work with out gloves. The only real issue are the 
fumes released from the melting plastic.

You probably already have a miter saw to cut with and a good table to 
clamp your PVC to the cost of a welder filler rod and some scrap to 
practice on would most certainly be less than building an injection 
machine and certainly more versatile.   Not as fun mind you;-)


  n 2016-10-15 08:33 PM, Erik Christiansen wrote:
> On 15.10.16 22:44, Leonardo Marsaglia wrote:
>> Yes the idea is to machine the four strips and then joing them strongly. To
>> clamp the mold I was thinking about mechanical ways like eccentrics or may
>> be screw clamps. That way I can hold the mold together when the injection
>> takes place. I really don't worry about the time consumption on open the
>> mold and close it again.
> <2c>
> If joining by solvent is too weak or unsightly, have you considered "pvc
> welding"?. I haven't used the process, but google offers myriad hits.
> I've seen ¼" thick PVC with a rippled joint looking much like a weld in
> steel. It may have been done on both sides, though, for complete
> penetration. (I don't recall.)
>
> OK, if molten plastic injection into the joint were a viable option,
> then the mould restraining force would be _much_ less than if moulding a
> large flat object from scratch, but just clamping four sheets for
> welding is orders of magnitude simpler & cheaper. I can't imagine that
> plastic welding gear is in the same ballpark as injection moulding,
> either.
>
> I suspect that molten filler material would need to be run straight from
> nozzle to every point of joining, i.e. run a nozzle along the joint. If
> injected at a central sprue, it would be too cold to melt the PVC strips
> for adequate fusing. (But that is speculation.)
> 
>
> Erik
>
> --
> Check out the vibrant tech community on one of the world's most
> engaging tech sites, SlashDot.org! http://sdm.link/slashdot
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Gregg Eshelman]

Look up how PVC house window frames are made. They use extrusions, cut the 
corners at 45 degrees then melt them together.
Here's a video on a portable corner welding machine. 
https://www.youtube.com/watch?v=cni6XdgOTj0



 
  From: John Kasunich <jmkasun...@fastmail.fm>
 To: emc-users@lists.sourceforge.net 
 Sent: Saturday, October 15, 2016 7:33 PM
 Subject: Re: [Emc-users] Wat OT: Question about injection molding of PVC
   
Injection pressure will be trying to force the two sides of your mold apart.
That's why injection molds are made of tool steel, and injection machines have 
VERY sturdy construction to hold the mold closed.  I saw a machine that might 
be big enough to make your parts - the mold closing cylinder was about 2 feet 
(0.6m) in diameter, and the four steel tie-rods that held the machine together 
against the clamp force were about 100mm diameter.

Could you make it out of four strips with some kind of joining at the corners?

John Kasunich

   
 
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Erik Christiansen]

On 15.10.16 22:44, Leonardo Marsaglia wrote:
> Yes the idea is to machine the four strips and then joing them strongly. To
> clamp the mold I was thinking about mechanical ways like eccentrics or may
> be screw clamps. That way I can hold the mold together when the injection
> takes place. I really don't worry about the time consumption on open the
> mold and close it again.

<2c>
If joining by solvent is too weak or unsightly, have you considered "pvc
welding"?. I haven't used the process, but google offers myriad hits.
I've seen ¼" thick PVC with a rippled joint looking much like a weld in
steel. It may have been done on both sides, though, for complete
penetration. (I don't recall.)

OK, if molten plastic injection into the joint were a viable option,
then the mould restraining force would be _much_ less than if moulding a
large flat object from scratch, but just clamping four sheets for
welding is orders of magnitude simpler & cheaper. I can't imagine that
plastic welding gear is in the same ballpark as injection moulding,
either.

I suspect that molten filler material would need to be run straight from
nozzle to every point of joining, i.e. run a nozzle along the joint. If
injected at a central sprue, it would be too cold to melt the PVC strips
for adequate fusing. (But that is speculation.)


Erik

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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[Leonardo Marsaglia]

2016-10-15 22:33 GMT-03:00 John Kasunich :

> Injection pressure will be trying to force the two sides of your mold
> apart.
> That's why injection molds are made of tool steel, and injection machines
> have VERY sturdy construction to hold the mold closed.  I saw a machine
> that might be big enough to make your parts - the mold closing cylinder was
> about 2 feet (0.6m) in diameter, and the four steel tie-rods that held the
> machine together against the clamp force were about 100mm diameter.
>
> Could you make it out of four strips with some kind of joining at the
> corners?
>

Hello John.

Yes the idea is to machine the four strips and then joing them strongly. To
clamp the mold I was thinking about mechanical ways like eccentrics or may
be screw clamps. That way I can hold the mold together when the injection
takes place. I really don't worry about the time consumption on open the
mold and close it again.

The concerning part I have is if It's a good idea to move the pistons for
the injection with a screw and a common 3 phase AC motor. My idea is to
load the piston, wait for the plastic to reach the temperature and then
inject no less than 30 seconds. I probably have to preheat the mold too.


-- 
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Re: [Emc-users] Wat OT: Question about injection molding of PVC

[John Kasunich]

Injection pressure will be trying to force the two sides of your mold apart.
That's why injection molds are made of tool steel, and injection machines have 
VERY sturdy construction to hold the mold closed.  I saw a machine that might 
be big enough to make your parts - the mold closing cylinder was about 2 feet 
(0.6m) in diameter, and the four steel tie-rods that held the machine together 
against the clamp force were about 100mm diameter.

Could you make it out of four strips with some kind of joining at the corners?

John Kasunich


On Sat, Oct 15, 2016, at 08:12 PM, Leonardo Marsaglia wrote:
> Hello to all.
> 
> I'm sure here I'm going to find good answers so I decided to start from
> here.
> 
> I'm planning to do some thermoforming production work but the sheet plastic
> I need to use is rectangular with a rectangular hole in the middle (I
> attached a picture so you can see). So to use regular PVC sheet would imply
> a lot of waste.
> 
> I came up with the idea of injecting the PVC using a manual clamped mould
> and injecting it with a screw driven piston. The difficult part comes here,
> I need to inject almost 3 kg of PVC. The moulds are going to be pretty
> simple as you can see but I would like to know if there's any good source
> to determine the approximate power I would need to drive the screw that
> moves the piston.
> 
> There's the possibility of using two pistons one on each side to make
> things easier. Off course I would need to reduce the motors with worm and
> gear and then connect the gear to the screw that drives the piston. But my
> main concern is if this approach is correct or if I should forget about it.
> 
> I didn't even consider the hydraulic pump because the cost would be a lot.
> Also take into account that I'm not intending to make lots of these
> injections per hour, so injection times could be slower than the industry
> standard.
> 
> I hope I've been clear about my doubts and I would be thankful if you can
> point me to any source of info about this, or help me to be more sure about
> the dimensions of what I need to do.
> 
> Thanks as always!
> 
> -- 
> *Leonardo Marsaglia*.
> --
> Check out the vibrant tech community on one of the world's most 
> engaging tech sites, SlashDot.org! http://sdm.link/slashdot
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> https://lists.sourceforge.net/lists/listinfo/emc-users
> Email had 1 attachment:
> + shape.png
>   360k (image/png)


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