# Plastic Injection Molding **URL:** \ **Category:** Factual Questions\ **Created:** [May 21, 2000, 12:05am UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079 "2000-05-21T00:05:04Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Imthecowgodmoo](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@Imthecowgodmoo](https://boards.straightdope.com/u/Imthecowgodmoo)\ **Post date:** [May 21, 2000, 12:05am UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/1 "2000-05-21T00:05:04Z")
So, anyone know how the process of Plastic Injection Molding works? I’m doing a project for my Intro to I.E.(industrial engineering) class and need some help. The project involves taking apart a desk lamp and finding the costs associated with the components and sub assemblies. I have come up with prices for all parts except the freakin plastic pieces. There doesn’t seem to be too many websites out there that are willing to tell me how plastic injection molding works and how much it costs. At least, I can’t come up with any websites. Before you all tell me to go to the UC’s library and look it up, I should remind you that this is a class wide project and by the time I got my lazy arse off my chair to go there, the books were all checked out. The web is my only hope now, either that or I drive 300 miles back home and put Mr. Public Library Card to good use which is something I’m not willing to do (the drive that is). I’m basically just looking for a little guidance, any help will be greatly appreciated. ps. I also did a search on the SDMB and came up with a heaping pile of Jack Squat. ---
**Author:** ![aseymayo](https://avatars.discourse-cdn.com/v4/letter/a/a88e57/32.png) [@aseymayo](https://boards.straightdope.com/u/aseymayo)\ **Post date:** [May 21, 2000, 1:17am UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/2 "2000-05-21T01:17:10Z")
Cowgod, you know I’d help you if I knew anything about this stuff. But I will tell you can drive a little less and get a library card at the Cincinnati Public Library (I hear the head librarian is a gorilla!). I know, not much help - but I tried. ---
**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\ **Post date:** [May 21, 2000, 1:32am UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/3 "2000-05-21T01:32:58Z")
Cowgod: Well, nobody’s giving it away cheap… [Plastic Injection Molding Bid Software](http://www.bews.com/home.htm) An introductory course for a mere [£500.00 + VAT](http://www.rapra.net/t05.htm) From the [Listo Corporation website](http://www.listo.com/plastic.htm): > [@](#): > > There are two costs involved in injection molding, the cost of the mold and the cost of each unit. The cost of the mold is different for every job. The majority of the cost however, is in the labor to build the mold. Molds can take as little as 50 hours or as much as 500 to 1,000 hours to build. A going rate at a tooling shop might be around $60/hour. The remainder of the mold cost is in the supplies; the mold base, inserts, and cores. > > The second cost involved in injection molding is cost of each unit. This is the cost that we can provide for you. Within the unit cost, there are three factors: material, machine, and set-up cost. > > The material cost is determined by the amount of the material needed to make the part. Materials usually cost between $1.00 and $3.00 per pound. If your part weighs 4 oz., for example, and the material cost 1.00/lb., the material cost per unit would be .25. > > The machine time cost is based upon cycle time and number of cavities in the mold. Our hourly rates range from $30/hour to $75/hour, for automatic operation, depending on the size of machine needed. If an operator is required, the rates increase $15/hour. The cycle time is the time it takes for the machine to make one shot, and the number of cavities refers to how many parts are made from each shot. If, for example, a machine can run automatic at 30/hr., at a 36 second cycle time (100 shots/hr.), and has a single cavity mold, the machine cost per unit would be .30. > > The set-up cost is determined by the number of shop hours needed to set-up and to begin production of a run. This time is usually between 2 and 4 hours, and at a shop rate of 75/hr. This cost is then distributed over the number of parts of the run. With the greater the quantity of parts planned for a run, the less expensive the set-up cost becomes. In our above example, if it takes 2 hours to set-up the mold, and we are planning to run 1,000 parts, the set-up cost per unit would be .15. > > If these three costs are added together, our example product cost per unit would be .70. We are always looking to reduce the unit cost of each product we manufacture, so we recommend to our customers that they maximize the number of cavities in their mold as well as the run quantity. These two figures are the biggest factors in determining the cost per unit. Again using our above example, if the number of cavities in the mold was increased to 4, meaning it makes 4 parts from each shot, and the planned run quantity was increased to 10,000 parts, the cost per unit drops significantly to .34. This is the power of injection molding; the ability to substantially reduce the unit cost to increase profitability Try a a search for “Plastic Injection Molding Cost” on [The Big Hub](http://www.thebighub.com). Good luck! ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [May 21, 2000, 1:56am UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/4 "2000-05-21T01:56:00Z")
Ahah! I have a BS in chemical engineering. Injection molding is a fairly simple process. Obviously, you have a mold in the shape of the part. This is probably made out of steel or aluminum. Nothing exotic is needed, since any plastic that you can injection mold has a fairly low melting point, something like polypropylene, polystyrene, or ABS. Carving the mold is an exacting (and expensive) process that is computer controlled, and usually handled by a machine shop that specializes in mold cutting. The injection-molding machine itself is basically a screw that turns inside a tube. Little plastic pellets are fed in at the beginning of the machine, and friction is used to melt the plastic as it is moved along the tube by the screw. When it reaches the end, the melted plastic is squirted into the mold, and allowed to cool. This is the slow step in the process. When the part is cool enough, the mold is then opened, the part is removed, and the mold is closed for the next injection. As for the costs of the part, I can’t give you exact numbers, but most of the cost of the part is processing and labor. An engineering reference book can give you the cost per pound for the material the parts are made of. Try to find some guidelines in one of your textbooks about how much of a “mark-up” there is for injection molding. In my texts, this was given as a percentage of the raw material cost. If the references are all checked out, then call your classmates and ask to look at the copy they checked out. I don’t have my industrial processes book on-hand, or I’d look up the rule of thumb for you. I hope this helps. ---
**Author:** ![justwannano](https://avatars.discourse-cdn.com/v4/letter/j/7ab992/32.png) [@justwannano](https://boards.straightdope.com/u/justwannano)\ **Post date:** [May 21, 2000, 4:35am UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/5 "2000-05-21T04:35:51Z")
Hettinga uses resistance heater bands ,Three of them IIRC,to slowly bring the plastic beads to the correct melting point. This is critical since the plastic must be molten long enough to seep into every nook and cranney of the mold. But cool enough to take little time in hardening. The mold is cooled by running coolant,water, through a type of radiator,for lack of a more discriptive word.But not cold enough to halt the flow of molten plastic.The product can be removed from the mold while quite warm and cooled in a bath. ---
**Author:** ![Imthecowgodmoo](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@Imthecowgodmoo](https://boards.straightdope.com/u/Imthecowgodmoo)\ **Post date:** [May 22, 2000, 3:06pm UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/6 "2000-05-22T15:06:24Z")
You guys have just made my day! Thanks a million times over! ---
**Author:** ![horowitz](https://avatars.discourse-cdn.com/v4/letter/h/2acd7d/32.png) [@horowitz](https://boards.straightdope.com/u/horowitz)\ **Post date:** [May 22, 2000, 4:39pm UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/7 "2000-05-22T16:39:33Z")
Try this [http://www.polysort.com/discuss/injection.asp](http://www.polysort.com/discuss/injection.asp) ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [May 22, 2000, 5:28pm UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/8 "2000-05-22T17:28:13Z")
**justwannano:** I suspect that we’re thinking about different models of IM machines. The type we studied didn’t need thermal heaters, because those screws produced _alot_ of friction with all those little beads. Although it may have been made for plastics with low melting points, and extra heat would be needed for harder polymers. ---
**Author:** ![justwannano](https://avatars.discourse-cdn.com/v4/letter/j/7ab992/32.png) [@justwannano](https://boards.straightdope.com/u/justwannano)\ **Post date:** [May 22, 2000, 8:52pm UTC](https://boards.straightdope.com/t/plastic-injection-molding/20079/9 "2000-05-22T20:52:55Z")
Diceman Guess you must be right then. I spent about six months operating that machine and keeping the heat adjusted was my biggest headache. I sure would like to see one of those machines though. I can’t imagine how they keep the heat constant if they ever change the size of their shot. What is the machine used for. We were mostly making brush handles. Each of the zones was a little hotter.And when they mixed recycled material the heat settings would have to be changed.