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Injection mold question

saadracing

Plastic
Joined
Feb 22, 2016
I am making a mold to inject HDPE. The final product is just a flat rectangle sheet of plastic with a smaller rectangle on top of it by 0.250" thick. I am making a aluminum cylinder to melt the plastic and then use our press to inject to plastic into the cavity of the 3 piece mold (which the mold will also be heated to 350 Degrees). This will be a 3 piece mold with a flat plate on top and bottom of this cavity (all of the hole will be where it bolts together).My question is looking at the drawing of the cavity (the green colored area is where the plastic will fill), where is the best place to inject the plastic and how many/do I need small holes to allow air to escape?

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I would be filling it opposite the small rectangle to save the plastic front from having to fold back on itself to fill the corners and without knowing any better would probably vent the far corners as well. If your mold making skills are less than ideal chances are your mold will vent adequately without any additional venting. Also, don't forget a cold slug to prevent puckering around the sprue.


If you have a seat of premium solidworks you could simulate the injection, at least in the newer releases.
 
Cold slug? Not familiar with this. The mold should be fairly air tight as all of the mating surfaces will be lapped on a stone and then bolted together
 
Mold does not need to be heated to 350F. I had a new mold sampled today, using polypropylene (Similar resin to HDPE) We ran the mold at 75F. 1/4 inch thick is on the heavy wall size, but can be done. Have you considered having a molder to run the parts? Hot HDPE is dangerous!!! It sticks like glue to skin, and when it cools, skin peels off several layers deep....
 
Cold slug? Not familiar with this. The mold should be fairly air tight as all of the mating surfaces will be lapped on a stone and then bolted together

I was just ribbing you about the less than ideal mould making skills. The cold slug is to give the part a place to shrink without affecting the actual part. In an actual injection mold it will also hold the part in the mold until ejected by the pins.
 
Bolts will not hold injection pressure. I have seem a dozen 1/2 inch bolts snap when plastic is injected.

It seems like OP will be using a hydraulic press not an actual injection moulder, he might not reach the pressures we normally see.
 
Will just be using our hand hydraulic press. Says 20 ton but actual is probably around 10 to 15 ton
 
We are doing just a few parts. If it works as I thinks is should we will look into larger runs and have them done at an actual molder. HDPE is what we have available at the shop. Thanks for the information. I will be wearing protective clothing.
 
Am I missing something? Why don't you buy a 1/4" sheet of HDPE and cut it? Or even have it water jet cut.

Mold making isn't near as simple as you think it is. Buy the time you get done dinking around with you mold, a waterjet could have 100 of those cut. You can buy sheet in white, black, or gray for less than $5/sq ft.
JR
 
Am I missing something? Why don't you buy a 1/4" sheet of HDPE and cut it? Or even have it water jet cut.

Mold making isn't near as simple as you think it is. Buy the time you get done dinking around with you mold, a waterjet could have 100 of those cut. You can buy sheet in white, black, or gray for less than $5/sq ft.
JR

I understand that it is not simple nor did I mean to imply. Was trying to gain some new knowledge in the process as this is not a part of high precision.
 
I understand that it is not simple nor did I mean to imply. Was trying to gain some new knowledge in the process as this is not a part of high precision.

New knowledge? The knowledge you will gain by doing this on a shop press is simply: "It CAN be done."

Other than that, JR's suggestion of having them cut on a water jet, or even with a jigsaw, will yield a far superior part, will cost less, and be available sooner.
 
I understand that it is not simple nor did I mean to imply. Was trying to gain some new knowledge in the process as this is not a part of high precision.

You are underestimating the forces involved for injection molding. A 50 ton press runs tiny parts. That part has way too much surface area to run just bolted together. Just looking at the drawing, I'm thinking at least a 150-200 ton press.

You don't run holes to vent the mold, you machine or grind vents. Probably 10 or more for a mold that size. 1/2" wide and 0.001" deep near the cavity and 0.010" deep 1/2" away.

I think you need to take a tour of a mold shop and see how things are done.
JR
 
Agreed. Rule of thumb is injection molding requires between 2 to 7 tons clamp pressure per inch of cavity surface projected to the parting line. That part has about 22 sq. in. of projected surface, indicating it needs to be clamped with between 44 and 154 tons. While it's a simple part to fill, it's a thick part, which means material has to continue to be fed as it solidifies, or else it will have horrible ugly sink marks. That would indicate pressures toward the higher end of the range. Bolts are not likely to do the job. Anyway, what is the advantage of molding the part if you have to undo 28 bolts to get it out?

Dennis
 
assuming you have a reason to mold this vs cutting from flat stock. Consider compression molding,an approach I experimented with is rough cutting flat stock, then heating in an oven then placing in mold and closing mold. this very basic approach will give you a certain amount of contouring, embossing, etc. It takes alot of force to move material around!
In this case, you preheat material and chill it with the mold.
I experimented with this idea years ago trying to make molded name plates. IT worked reasonably well in ABS but I found a vendor that specialzed in making plastic name plates with cheap molds.
 
assuming you have a reason to mold this vs cutting from flat stock. Consider compression molding,an approach I experimented with is rough cutting flat stock, then heating in an oven then placing in mold and closing mold. this very basic approach will give you a certain amount of contouring, embossing, etc. It takes alot of force to move material around!
In this case, you preheat material and chill it with the mold.
I experimented with this idea years ago trying to make molded name plates. IT worked reasonably well in ABS but I found a vendor that specialzed in making plastic name plates with cheap molds.

Thanks for the info
 








 
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