Solvent welding...

[Don Y]

I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

<https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1>

[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

I doubt that solvent welding will be sufficient to let the
two surfaces \"flow\" together. Or, if they did, it would
be geometrically uncontrolled.

Fiberglass seems like it would just build things up a lot
(and leave me with a tedious chore of trying to smooth that
irregular surface -- ribs, etc.).

Are there other plastic fillers that I can use -- even if I
have to resort to painting the finished product?

Grinding down the rounded edge may be a possibility -- but, I don\'t
know if there is enough material behind it to keep things structurally
intact.

 

[Jeff Layman]

On 20/08/2020 09:22, Don Y wrote:

I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1

[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

I doubt that solvent welding will be sufficient to let the
two surfaces \"flow\" together. Or, if they did, it would
be geometrically uncontrolled.

Fiberglass seems like it would just build things up a lot
(and leave me with a tedious chore of trying to smooth that
irregular surface -- ribs, etc.).

Are there other plastic fillers that I can use -- even if I
have to resort to painting the finished product?

Grinding down the rounded edge may be a possibility -- but, I don\'t
know if there is enough material behind it to keep things structurally
intact.

It depends on what the plastic materials are. If either is polyethylene
or polypropylene it won\'t be easy. There are adhesives which claim to be
able to stick to either, but how effective they are and what they cost I
am not sure.

One other possibility is car body filler - the sort of thing to fill
small dents. That can be smoothed to a nice finish, and, being epoxy
resin based, will stick to many things. It will also fill imperfections
and gaps in the join. The main problem is that it is usually a dark grey
colour, so you might have to paint it.

--

Jeff

 

[Martin Brown]

On 20/08/2020 09:22, Don Y wrote:

I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover).  Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs.  So, the outermost edge (that I\'ll have to preserve)
is already rounded.  I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1


[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

Your best bet is probably a two part epoxy glue and using the smallest
amount needed to wet out the joint with a suitable pigment filler in it
to match the colour of the plastic that is being glued.

You could be out of luck if the two plastics are seriously incompatible
with each other.

--
Regards,
Martin Brown

 

[Clifford Heath]

On 20/8/20 8:35 pm, Martin Brown wrote:

On 20/08/2020 09:22, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover).  Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs.  So, the outermost edge (that I\'ll have to preserve)
is already rounded.  I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1


[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

Your best bet is probably a two part epoxy glue and using the smallest
amount needed to wet out the joint with a suitable pigment filler in it
to match the colour of the plastic that is being glued.

I\'d be using a polyurethane glue. Gorilla make one that looks like
honey, foams a bit (make sure it\'s not too humid when you use it) but
sticks to lots of really difficult plastics pretty well, even PET.

If it\'s still available in your building store, PL Premium in a gun
canister is cheap and very good too - doesn\'t foam as much as Gorilla.

CH

 

[Martin Brown]

On 20/08/2020 13:11, Clifford Heath wrote:

On 20/8/20 8:35 pm, Martin Brown wrote:
On 20/08/2020 09:22, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover).  Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs.  So, the outermost edge (that I\'ll have to preserve)
is already rounded.  I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1


[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

Your best bet is probably a two part epoxy glue and using the smallest
amount needed to wet out the joint with a suitable pigment filler in
it to match the colour of the plastic that is being glued.

I\'d be using a polyurethane glue. Gorilla make one that looks like
honey, foams a bit (make sure it\'s not too humid when you use it) but
sticks to lots of really difficult plastics pretty well, even PET.

If it\'s still available in your building store, PL Premium in a gun
canister is cheap and very good too - doesn\'t foam as much as Gorilla.

If it was woodworking and going to be out of sight then it does have a
high strength but it discolours horribly after a few months in sun. I
used some as a space filling super strong glue once and it looked OK for
a while but now shows as a dirty brown line on the surface. The joint is
still strong but the cosmetic appearance is less than ideal.

Also with that stuff it is easy to become attached to the job if you
don\'t pay careful attention to what you are doing it is very high tack.

--
Regards,
Martin Brown

 

[Don Y]

On 8/20/2020 2:53 AM, Jeff Layman wrote:

On 20/08/2020 09:22, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Are there other plastic fillers that I can use -- even if I
have to resort to painting the finished product?

It depends on what the plastic materials are. If either is polyethylene or
polypropylene it won\'t be easy. There are adhesives which claim to be able to
stick to either, but how effective they are and what they cost I am not sure.

I think neither. I\'ll try some solvents on it to see how readily it dissolves.
It almost feels like PVC (or a polycarbonate).

I can actually play with adhesives on the \"back\" portion of the case as it
is disposable. Perhaps I\'ll snap off a piece and try regluing it (?)

One other possibility is car body filler - the sort of thing to fill small
dents. That can be smoothed to a nice finish, and, being epoxy resin based,
will stick to many things. It will also fill imperfections and gaps in the
join. The main problem is that it is usually a dark grey colour, so you might
have to paint it.

I think I need something that cures \"less hard\". When doing bodywork,
you usually have easy access to the area being repaired and the material
in the adjoining area is \"robust\" -- you can put some elbow grease into
reforming/finishing it.

Here, I\'ll be trying to smooth the inner surface of a cylinder, essentially,
while avoiding damaging the surrounding soft plastic.

I was thinking along the lines of something that flows relatively easily...
and spending some time rotating the plastic piece (along the central
axis of the horn) to allow the material to flow into the gap all the way
around the inner surface. Then, once it was just barely thicker than
desired, *kiss* it with sandpaper, etc. to take off the \"flow marks\"
while trying to avoid scarring the adjoining virgin plastic.

Painting will also be a bit of a challenge, due to the shape. (Again,
autobody work is considerably more \"exposed\"). But, I might be able to use
an airbrush and patience to get into the various nooks and crannies.

But, Bondo makes a variety of different products so there may be something
that fits the bill. Thanks, I\'ll start some research along those lines.

 

[Don Y]

On 8/20/2020 3:35 AM, Martin Brown wrote:

On 20/08/2020 09:22, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1


[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

Your best bet is probably a two part epoxy glue and using the smallest amount
needed to wet out the joint with a suitable pigment filler in it to match the
colour of the plastic that is being glued.

I\'ve gone that route with some other \"modified plastic pieces\" (e.g., I have
cobbled together *recessed* 8P8C jack wallplates to locate behind appliances,
etc. without the mating plug interfering with the back of the appliance.

But, these are hidden; you\'d KNOW they existed but would never deliberately
go looking for/at them.

The horn is visible in much the same way that a smoke detector would be.
Granted, you won\'t continually notice it (you\'ll grow accustomed to
its presence) but WILL have a good look at it when you first \"discover\" it.

I thought of printing parts but the horn is folded and likely exponential
cross section. The unloaded driver has nowhere near the SPL of the
loaded form! A linear ASCII-art representation would be:

[courier]
/
/
/
/
/
/
| \\
| \\
|-- \\
|) >
|-- /
| /
| /
\\
\\
\\
\\
\\
\\

with the compression driver located at \")\" at the bottom of a deep throat
(with exponential contour).

I figured I could slice it through the center and digitize the cross section.
Then, revolve that to form a 3D model. Add the four ribs and done.

But, I\'d need to be sure the vendor \"washed\" the exterior suitably to
give it a nice smooth finish. (these are located low enough that you can
touch them if your eyes suggest they are \"rough\")

You could be out of luck if the two plastics are seriously incompatible with
each other.

(sigh) All of this cosmetic packaging is really an annoying detail!
I now have a better appreciation for that aspect of product development
(previously left to OTHERS!) :<

 

[Don Y]

On 8/20/2020 5:26 AM, Martin Brown wrote:

On 20/08/2020 13:11, Clifford Heath wrote:
On 20/8/20 8:35 pm, Martin Brown wrote:
On 20/08/2020 09:22, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1


[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

Your best bet is probably a two part epoxy glue and using the smallest
amount needed to wet out the joint with a suitable pigment filler in it to
match the colour of the plastic that is being glued.

I\'d be using a polyurethane glue. Gorilla make one that looks like honey,
foams a bit (make sure it\'s not too humid when you use it) but sticks to lots
of really difficult plastics pretty well, even PET.

If it\'s still available in your building store, PL Premium in a gun canister
is cheap and very good too - doesn\'t foam as much as Gorilla.

If it was woodworking and going to be out of sight then it does have a high
strength but it discolours horribly after a few months in sun. I used some as a
space filling super strong glue once and it looked OK for a while but now shows
as a dirty brown line on the surface. The joint is still strong but the
cosmetic appearance is less than ideal.

I\'ve been disappointed to find that to be the case with many adhesives.

I made my recessed 8P8C\'s by adhering the face of one of these \"single
keystone\" wallplates:

<https://www.homedepot.com/p/Commercial-Electric-White-1-Gang-1-Decorator-Rocker-1-Duplex-Wall-Plate-1-Pack-5001-WH/206428112>

to the rear of one of these low voltage feed-thru wallplates:

<https://www.homedepot.com/p/Commercial-Electric-White-1-Gang-1-Decorator-Rocker-1-Duplex-Wall-Plate-1-Pack-5018-WH/206456271>

(the curved/folded portion at the rear snaps off).

Despite being careful with the adhesive application (to keep it off
of the portion of the recessed face that would be \"exposed\"), there
is a noticeable discoloration at the join.

<frown> \"I\'ll fix it later.\" (modify the front piece so the adhesive needn\'t
be placed close to the opening to the adjoining back piece!)

Also with that stuff it is easy to become attached to the job if you don\'t pay
careful attention to what you are doing it is very high tack.

Yeah, and for something like this -- where I\'ll undoubtedly have to be
tweeking the relative positions of the two pieces -- you can bet that
adhesive will end up in places it shouldn\'t! (e.g., fingertips)

 

[whit3rd]

On Thursday, August 20, 2020 at 1:23:14 AM UTC-7, Don Y wrote:

I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

The usual way this sort of thing is handled in architecture, is molding
(a piece of trim that flexes, at the transition). Some of the plaster-corner
solutions, too, can work.

For this particular item, can you accurately cut the \'horn\' element,
as though in a lathe, to have a cirular rim? Then you could
make a transition piece of a tube: join the flat to the tube,
and fit the horn into the tube.

 

[Don Y]

On 8/20/2020 10:47 AM, whit3rd wrote:

On Thursday, August 20, 2020 at 1:23:14 AM UTC-7, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

The usual way this sort of thing is handled in architecture, is molding
(a piece of trim that flexes, at the transition). Some of the plaster-corner
solutions, too, can work.

For this particular item, can you accurately cut the \'horn\' element,
as though in a lathe, to have a cirular rim? Then you could
make a transition piece of a tube: join the flat to the tube,
and fit the horn into the tube.

Hmmm... make a sleeve into which the horn fits? That\'s an
idea I\'d not considered. (I\'d assumed adhering the flat plate
to the face of the horn).

I\'d still have a \"join\" to address -- the cylinder formed between the
sleeve and horn. But, this might be easier to \"patch\"/hide than
trying to hide a seam in the wall of a cylinder!

It also means the hole in the flat plate can be clean -- no need to
soften that edge (which would lead into/onto the top of the horn
in my original envisioning).

I was planning on cutting the horn \"cylinder\" out of the current
mount -- but had not planned on being very careful about doing that
as only the top/face surface would be important to my mating scheme.
In your solution, I\'d have to make it a reasonably CLEAN separation.

I\'ll look into that possibility. Maybe I can use a laser cutter
to carve that out...

Thanks!

 

[Tabby]

On Thursday, 20 August 2020 10:53:18 UTC+1, Jeff Layman wrote:

On 20/08/2020 09:22, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1

[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

I doubt that solvent welding will be sufficient to let the
two surfaces \"flow\" together. Or, if they did, it would
be geometrically uncontrolled.

Fiberglass seems like it would just build things up a lot
(and leave me with a tedious chore of trying to smooth that
irregular surface -- ribs, etc.).

Are there other plastic fillers that I can use -- even if I
have to resort to painting the finished product?

Grinding down the rounded edge may be a possibility -- but, I don\'t
know if there is enough material behind it to keep things structurally
intact.

It depends on what the plastic materials are. If either is polyethylene
or polypropylene it won\'t be easy. There are adhesives which claim to be
able to stick to either, but how effective they are and what they cost I
am not sure.

One other possibility is car body filler - the sort of thing to fill
small dents. That can be smoothed to a nice finish, and, being epoxy
resin based, will stick to many things. It will also fill imperfections
and gaps in the join. The main problem is that it is usually a dark grey
colour, so you might have to paint it.

it\'s not epoxy but should be a good option.


NT

 

[Carl]

On Thursday, August 20, 2020 at 1:23:14 AM UTC-7, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

The usual way this sort of thing is handled in architecture, is molding
(a piece of trim that flexes, at the transition). Some of the
plaster-corner
solutions, too, can work.

For this particular item, can you accurately cut the \'horn\' element,
as though in a lathe, to have a cirular rim? Then you could
make a transition piece of a tube: join the flat to the tube,
and fit the horn into the tube.

Rather than worrying about identifying the plastics involved and finding a
suitable glue, how about a simple round hole in the cover to fit the horn
and a strap around your device with the ends secured by a screw to the cover
plate on each side of the device? Pretty much certain to be mechanically
stronger and no worries about filler, runs, and color matching the glue to
the pieces.

--
Regards,
Carl Ijames

 

[Don Y]

On 8/20/2020 2:01 PM, Carl wrote:

On Thursday, August 20, 2020 at 1:23:14 AM UTC-7, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

The usual way this sort of thing is handled in architecture, is molding
(a piece of trim that flexes, at the transition). Some of the plaster-corner
solutions, too, can work.

For this particular item, can you accurately cut the \'horn\' element,
as though in a lathe, to have a cirular rim? Then you could
make a transition piece of a tube: join the flat to the tube,
and fit the horn into the tube.

Rather than worrying about identifying the plastics involved and finding a
suitable glue, how about a simple round hole in the cover to fit the horn and a
strap around your device with the ends secured by a screw to the cover plate on
each side of the device? Pretty much certain to be mechanically stronger and
no worries about filler, runs, and color matching the glue to the pieces.

I want the entire horn and mechanism to ride behind the plate
so that a user can run their hand over its surface and find it
smooth (with the exception of the hole!).

I want the resulting \"product\" to look like it would have been manufactured
that way -- no superfluous (visible) fasteners, etc.

The existing product is reasonably well designed -- for its ORIGINAL intended
use. The driver is attached to the rear of the case. The HINGED cover plate
folds down over it to perfectly align with the driver\'s design and \"complete\"
the horn.

A production version of my goal would likely have plastic \"catches\"
on the rear of the front plate to accept and hold the driver in place.
(I plan on just gluing the driver to the horn and worrying about
\"servicing\" at some later date -- if the driver fails, discarding the
cover plate won\'t be a big loss!)

 

[Jeff Layman]

On 20/08/2020 21:07, Tabby wrote:

On Thursday, 20 August 2020 10:53:18 UTC+1, Jeff Layman wrote:
On 20/08/2020 09:22, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1

[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

I doubt that solvent welding will be sufficient to let the
two surfaces \"flow\" together. Or, if they did, it would
be geometrically uncontrolled.

Fiberglass seems like it would just build things up a lot
(and leave me with a tedious chore of trying to smooth that
irregular surface -- ribs, etc.).

Are there other plastic fillers that I can use -- even if I
have to resort to painting the finished product?

Grinding down the rounded edge may be a possibility -- but, I don\'t
know if there is enough material behind it to keep things structurally
intact.

It depends on what the plastic materials are. If either is polyethylene
or polypropylene it won\'t be easy. There are adhesives which claim to be
able to stick to either, but how effective they are and what they cost I
am not sure.

One other possibility is car body filler - the sort of thing to fill
small dents. That can be smoothed to a nice finish, and, being epoxy
resin based, will stick to many things. It will also fill imperfections
and gaps in the join. The main problem is that it is usually a dark grey
colour, so you might have to paint it.

it\'s not epoxy but should be a good option.

<https://www.carrepairtools.co.uk/Body-Filler/Epoxy/Index.html>

But I take your point - there are other two-part fillers available.

--

Jeff

 

[Clifford Heath]

On 20/8/20 10:26 pm, Martin Brown wrote:

On 20/08/2020 13:11, Clifford Heath wrote:
On 20/8/20 8:35 pm, Martin Brown wrote:
On 20/08/2020 09:22, Don Y wrote:
I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover).  Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs.  So, the outermost edge (that I\'ll have to preserve)
is already rounded.  I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1


[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

Your best bet is probably a two part epoxy glue and using the
smallest amount needed to wet out the joint with a suitable pigment
filler in it to match the colour of the plastic that is being glued.

I\'d be using a polyurethane glue. Gorilla make one that looks like
honey, foams a bit (make sure it\'s not too humid when you use it) but
sticks to lots of really difficult plastics pretty well, even PET.

If it\'s still available in your building store, PL Premium in a gun
canister is cheap and very good too - doesn\'t foam as much as Gorilla.

If it was woodworking and going to be out of sight then it does have a
high strength but it discolours horribly after a few months in sun. I
used some as a space filling super strong glue once and it looked OK for
a while but now shows as a dirty brown line on the surface. The joint is
still strong but the cosmetic appearance is less than ideal.

PL Premium doesn\'t have those problems. We have used it to splice PET
softdrink bottles together into long cylinders (up to six bottles joined
to over a metre long) and the result can withstand 120PSI for a
significant period (in case you\'re wondering, for water rockets).

Even though PET has almost no functional groups and is extraordinarily
difficult to glue...

CH

 

[Don Y]

On 8/20/2020 11:07 AM, Don Y wrote:

On 8/20/2020 10:47 AM, whit3rd wrote:

For this particular item, can you accurately cut the \'horn\' element,
as though in a lathe, to have a cirular rim? Then you could
make a transition piece of a tube: join the flat to the tube,
and fit the horn into the tube.

Hmmm... make a sleeve into which the horn fits? That\'s an
idea I\'d not considered. (I\'d assumed adhering the flat plate
to the face of the horn).

I\'ll look into that possibility. Maybe I can use a laser cutter
to carve that out...

This looks like the winning approach! Unfortunately, the OD of the
portion of the folded horn is not a standard ID for PVC, ABS, etc.
But, I think I can get creative by using a portion of a fitting,
instead -- note that I don\'t have to \"sleeve\" the entire horn...
just a big enough portion of it to secure it to the face plate!

And, my measurements may be at the narrow end of the draft angle
for the molded part so there may be less discrepancy than it
initially appears.

Thanks!

 

[Tabby]

On Friday, 21 August 2020 08:20:28 UTC+1, Jeff Layman wrote:

On 20/08/2020 21:07, Tabby wrote:
On Thursday, 20 August 2020 10:53:18 UTC+1, Jeff Layman wrote:
On 20/08/2020 09:22, Don Y wrote:

I\'ve got a plastic \"horn\" that I\'d like to site behind a flat
plastic plate (4\" round Jbox cover). Superficially, this means
cutting a hole in the plate and adhering the horn to the back.

Of course, the horn was designed to be a part of it\'s *own*
\"front piece\" -- which is mechanically incompatible with my
needs. So, the outermost edge (that I\'ll have to preserve)
is already rounded. I.e., mounting it to a flat surface will
result in a visible discontinuity between the rounded and
flat surfaces.

https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.s5U3jecaAtNtBV2pXWro9AHaGT%26pid%3DApi&f=1

[not sure how persistent that link is!]

I want to ONLY expose the \"hole\" in that image -- resting the
flat plate across the topmost extent of it.

This is essentially at eye level so cosmetics are important.

I doubt that solvent welding will be sufficient to let the
two surfaces \"flow\" together. Or, if they did, it would
be geometrically uncontrolled.

Fiberglass seems like it would just build things up a lot
(and leave me with a tedious chore of trying to smooth that
irregular surface -- ribs, etc.).

Are there other plastic fillers that I can use -- even if I
have to resort to painting the finished product?

Grinding down the rounded edge may be a possibility -- but, I don\'t
know if there is enough material behind it to keep things structurally
intact.

It depends on what the plastic materials are. If either is polyethylene
or polypropylene it won\'t be easy. There are adhesives which claim to be
able to stick to either, but how effective they are and what they cost I
am not sure.

One other possibility is car body filler - the sort of thing to fill
small dents. That can be smoothed to a nice finish, and, being epoxy
resin based, will stick to many things. It will also fill imperfections
and gaps in the join. The main problem is that it is usually a dark grey
colour, so you might have to paint it.

it\'s not epoxy but should be a good option.

https://www.carrepairtools.co.uk/Body-Filler/Epoxy/Index.html

But I take your point - there are other two-part fillers available.

I don\'t know whether that list is correct but most car body fillers aren\'t epoxy.

I suspect Don\'s best bet is to 3d print something.


NT

 

[Don Y]

On 8/21/2020 1:25 AM, Don Y wrote:

On 8/20/2020 11:07 AM, Don Y wrote:
On 8/20/2020 10:47 AM, whit3rd wrote:

For this particular item, can you accurately cut the \'horn\' element,
as though in a lathe, to have a cirular rim? Then you could
make a transition piece of a tube: join the flat to the tube,
and fit the horn into the tube.

Hmmm... make a sleeve into which the horn fits? That\'s an
idea I\'d not considered. (I\'d assumed adhering the flat plate
to the face of the horn).

I\'ll look into that possibility. Maybe I can use a laser cutter
to carve that out...

This looks like the winning approach! Unfortunately, the OD of the
portion of the folded horn is not a standard ID for PVC, ABS, etc.
But, I think I can get creative by using a portion of a fitting,
instead -- note that I don\'t have to \"sleeve\" the entire horn...
just a big enough portion of it to secure it to the face plate!

And, my measurements may be at the narrow end of the draft angle
for the molded part so there may be less discrepancy than it
initially appears.

Well, the math isn\'t great but I think this WILL work. My plan
of attack:

- adhere a nominal 2\" PVC \"cap\" to the rear of the face plate.
Take care to position it where the horn is intended to reside.
Note that one can be a little sloppy with adhesive as this
is on the backside (so slop beyond the extents of the cap
will never be seen AND slop *inside* the cap\'s extents will
be handled, below). Note, also, that the open end of the
cap is \"manufactured true\" so should sit flat on the front
plate. (this wouldn\'t be the case if you\'d used a CUT end!)
- after cured, remove (cut/grind) the closed end of the cap off.
Note that this can also be crude as the internal end of the
\"sleeve\" need not be pretty nor perfectly \"square\".
- using the adhered sleeve\'s interior surface as a fence,
remove the material from the face plate (i.e., cut the hole
in the face plate knowing that the sleeve will constrain the
extents of that hole to coincide with the sleeve\'s interior)
- excise the horn \"assembly\" from the original cover. Note that
the horn assembly has a cylindrical envelope:

[courier]
----------
/
/
/
/
/
/
| \\
| \\
|-- \\
|) >
|-- /
| /
| /
\\
\\
\\
\\
\\
\\
----------

Relying on the envelope\'s structure, it should be
relatively easy to trim the assembly\'s outer dimensions to
those of the envelope with crude tools (e.g., medium file)
Note that none of the cutting -- other than the face plate
hole -- is cosmetic!
- wrap the front end of the horn assembly with teflon tape.
This is necessary because the OD of the horn envelope is
2-3/16 while the ID of the PVC cap is 2-3/8. The tape should
build up the difference in diameters WITHOUT adhering to
either surface
- place the front plate on a flat surface with the sleeve up.
This ensures that the surface acts as a \"stop\" for anything
inserted into the sleeve.
- slide the horn assembly into the sleeve, adding or removing
teflon tape to ensure a snug (but not tight!) fit. The
goal is to keep the horn oriented along the axis of the
face plate hole
- pour flowable adhesive into the gap between the horn assy
and the sleeve, from behind (i.e., \"above\"). The teflon
tape should keep any viscous adhesive from flowing out
beyond the front plate
- once adhered, flip over. Using a dental explorer, fish
the teflon tape out of the gap from the front.
- with front plate on top, carefully fill gap with adhesive.
Adhesive previously used blocks the adhesive from flowing
THROUGH to the back of the sleeve.
- once adhered, use *formable* filler to clean up the
exposed gap between the horn and sleeve.
- paint
- relieve sleeve proximate to mounting holes in front plate
(as distance between the corresponding mounting holes in
the Jbox will not accommodate the ~3\" OD of the PVC cap)

The beauty of the approach is it eliminates the need for any
\"fancy\" tools (e.g., I can cut the cap COARSELY with a
hacksaw instead of needing to cut it \"square\" with a band
saw; I can file-out the front plate hole instead of
needing to laser cut a 2-3/16 hole; etc.) and reduces
the critical operations to:
- centering the sleeve
- \"sanding\" the finished front plate