View Full Version : Plastic injection molding

08-20-2002, 10:58 PM
Why is the pressure used for plastic injection molding so high? Is the high pressure for picking up the details or does the actual plastic need to be under high pressure for strength?

Is there any fairly easy and affordable way of making small run plastic injection molding?


08-21-2002, 06:55 AM
I'm trying to understand that too.

So far, my guess is that the higher pressure probably adds economy to the process by speeding up time for injection.

I may also help drive air out of the mold and plastic.

08-21-2002, 06:56 AM
<font face="Verdana, Arial" size="2">Originally posted by debequem:
I'm trying to understand that too.

So far, my guess is that the higher pressure probably adds economy to the process by speeding up time for injection.

It may also help drive air out of the mold and plastic.</font>

08-21-2002, 08:39 AM
Basically, the higher pressure gets the hot plastic into the molds before the stuff can cool below the flow temperature. A higher plastic temperature will allow lower pressures but there is a limit as to how hot you can get the plastic before it degrades. Thats really noticable when blues become greens and reds become oranges because of yellowing from heat.

Anyway, HSM ran a four-part article on building a hand-operated plastic injection molding machine back in May/June 1991 thru Nov/Dec 1991. The article was reprinted in our Projects 5 book since the back issues are no longer available. The author, Rodney Hanson, had described a good amount of detail involving the plastic injection process. The article didn't say how large of "shot" can be made but the objects represented were golf tees, bottle caps,etc.

Good luck with your project!


08-21-2002, 08:41 AM
The pressure is to get flow to all the details. Usually the injection site is small in diameter vs the part cross-section, and it is necessary to use some extra pressure because of that also.

The plastic is cooling as it enters, so pressure also get the mold packed full before it is too cold to flow.

And, if there is a hole molded into the plastic, it has to "knit" on the other side of the pin that forms the hole, requiring it to be hot and forced together.

08-21-2002, 06:20 PM

I used to work with High and Low density Polyethylene Injection molding machines (yogurt containers, flower pots, etc). You need high pressure for three reasons. Fine details, an injection mold with chilled water running through it all the time, and the melted plastic has the consistancy of play doh - only a hell of a lot hotter. The pressure is not the most important thing however. It is the injection time and the dwell time after filling the mold. Milliseconds change in these has dire effect the fit of lids on containers (they can be too tight or too loose). The QC department would constantly monitor the output and have the operators adjust accordingly.

I can attest to the high pressure, we had contamination on one barrel and the plastic was streaking - so I retract the barrel, put an empty resin bag in front of the tip to catch the waste and eject the fouled plastic. Sometimes they plug up if they sit too long - this one did. When the plug popped out I was sprayed on my left hand with molten PE - I ran to the chiller and dumped my hand in (the water is about 40*F). Worst burn I ever had.

Rob Frink
08-22-2002, 07:48 AM

I have a small benchtop machine that I picked up from a High school auction a few years ago. It looks like an arbor press with a vice. There is a heated pot where the plastic shot is loaded, then the ram/plunger inserts into the top of the pot and forces the molten platic thru a sprue to the vice. You clamp you mold tooling in the vice.

This machine is very simple, it plugs into a std wall outlet for the heater, and the ram pressure is from your arm.

The more heat that is used to melt the plastic, then the easier it is to flow into the mold...less pressure, however, more heat means more shrinkage during cooling. Basically, you can heat the plastic hot enough so it'll flow into the mold from gravity alone....but the shrinkage at this heat is detrimental.

The idea it to warm it up just enough to get it to flow, then use high pressure to coax it along...this minimizes the shrinkage and stresses from cooling. The parts have better dimensional tolerances.


08-22-2002, 12:41 PM
Morgan industries Inc. 3311 E. 59th St.-Long Beach, CA 90805. Prints a book entitled CUTTING COSTS IN SHORT-RUN PLASTICS INJECTION MOLDING. This book, which is used to promote their small injection presses (expensive machines) is an excellent treatise on the entire craft of mold making and on making very inexpensive molds. I can't praise it enough. I used several of their suggested mold designs to make simple molds for a customer.

08-22-2002, 04:51 PM
Thanks for all the leads. I'll follow-up on them.