Circles in Plastic Castings


Paul Hillman
 

Just a question here. Nothing we can do about it except fill them in but, why are there small circular holes or impressions in various plastic castings? They can't be for material-flow in the casting process ???


As example, I have several Proto 2000 War Emerg. hoppers (and other kits) with these holes in the interior slope sheets, etc. Will have to file/fill them in. Just curious. What are they really for? Would be nice if they weren't there.


Thanks, Paul Hillman


Richard Brennan
 

At 09:43 AM 8/7/2016, chris_hillman@msn.com [STMFC] wrote:
Just a question here. Nothing we can do about it except fill them in but, why are there small circular holes or impressions in various plastic castings? They can't be for material-flow in the casting process ???
Ejector pins...
without them... it would be extremely difficult to get the warm injected part out of a complex mold.

Description at: https://www.protolabs.com/resources/injection-molding-design-tips/united-states/2010-06/


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Richard Brennan - San Leandro CA
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Schuyler Larrabee
 

They are caused by ejector pins. Moldings don’t automatically eject from the dies and have to be pushed out. I agree they can be a pain, especially in older dies where you can find flash standing normal to the surface, which has to be shaved off.



Schuyler



From: STMFC@yahoogroups.com [mailto:STMFC@yahoogroups.com]
Sent: Sunday, August 07, 2016 12:44 PM
To: STMFC@yahoogroups.com
Subject: [STMFC] Circles in Plastic Castings





Just a question here. Nothing we can do about it except fill them in but, why are there small circular holes or impressions in various plastic castings? They can't be for material-flow in the casting process ???



As example, I have several Proto 2000 War Emerg. hoppers (and other kits) with these holes in the interior slope sheets, etc. Will have to file/fill them in. Just curious. What are they really for? Would be nice if they weren't there.



Thanks, Paul Hillman





[Non-text portions of this message have been removed]


Paul Hillman
 


Thanks Schuyler, ---- So THAT'S what they're from. Strange thing is many other hoppers & other cars show no signs of these markings, at least not in clearly visible areas. Either the other cars were ejected differently, or the Proto 2000 (and others) were ejected too soon when the plastic was still too soft ???
 
Paul Hillman
 

----- Original Message -----
Sent: Sunday, August 07, 2016 12:11 PM
Subject: RE: [STMFC] Circles in Plastic Castings

 

They are caused by ejector pins. Moldings don’t automatically eject from the dies and have to be pushed out. I agree they can be a pain, especially in older dies where you can find flash standing normal to the surface, which has to be shaved off.

Schuyler

From: STMFC@... [mailto:STMFC@...]
Sent: Sunday, August 07, 2016 12:44 PM
To: STMFC@...
Subject: [STMFC] Circles in Plastic Castings

Just a question here. Nothing we can do about it except fill them in but, why are there small circular holes or impressions in various plastic castings? They can't be for material-flow in the casting process ???

As example, I have several Proto 2000 War Emerg. hoppers (and other kits) with these holes in the interior slope sheets, etc. Will have to file/fill them in. Just curious. What are they really for? Would be nice if they weren't there.

Thanks, Paul Hillman

[Non-text portions of this message have been removed]


Dennis Storzek
 




---In STMFC@..., <chris_hillman@...> wrote :


Thanks Schuyler, ---- So THAT'S what they're from. Strange thing is many other hoppers & other cars show no signs of these markings, at least not in clearly visible areas. Either the other cars were ejected differently, or the Proto 2000 (and others) were ejected too soon when the plastic was still too soft ???
 
Paul Hillman
================

Where the ejector pin prints end up is dictated by how the mold designer orients the part in the mold. There have been many molds built over the years, and each designer has to weigh all his options, then make what he feels are the best choices.

Aside from where the ejector pins will go, the other consideration is where the plastic will be injected into the part, the "sprue" and "gate". Unless this is a really complicated mold, the "E pins" will always be on the side opposite the gate.

If you want the pin prints to be on the underside of the slope sheets, that means the gate needs to be in the center of the interior of the part, which either puts it on the cross ridge, or the centersill over the center hopper on a triple. In addition, it puts the gate deep into the part, which means a long sprue. Since sprues need to be tapered, the longer it is, the bigger the scar it leaves on the part. Heated sprue bushings can allow a "pin point" gate, but accommodating the heated bushing in the narrow confines of the core that makes the car interior becomes problematic.

The other way to design things is to orient the part so the gate is on the underside, between the hoppers. This puts the e-pins on the slope sheets, but since the slope sheets are the traditional location for car weights, they  can be covered by separate slope sheet parts.

Indented pin prints are not from ejecting the part too hot, but is rather intentional to make little seats so the part stays on the pin ends and comes out of the mold straight, without dragging a detailed surface on part of the mold. It also helps insure that the pin prints don't stand above the surrounding surface and form shallow bumps that get in the way of other parts. Ejector pins in the typical mold for an HO scale body are somewhere about 6" or 7" long, and change length relative to the rest of the mold as the mold temperature changes, so a perfect match is seldom possible.

Dennis Storzek