View Full Version : rubber or nylon or polycarbonate injection moulding- edumacate me

07-17-2008, 01:05 PM

It's a dog toy...that's why it's at doggoneclean.com GO figure.

What kind of process would be involved in manufacturing something like that in the home shop environment??

How about basic processes of injection molding, the dogs don't tend to give a rats patooty if there are mold lines or rough surfaces, so I don't really need a hugely expensive moulding machine, but if I could injection mould say a 10 oz shot of plastic with basic home equipment, I could make somepretty sweet toys.

Dog toys get expensive. For one person, it's not a big deal to spend $8 for a dog toy. For a humane society or an animal shelter that has 40 kennel spaces and dogs that range from cute little puppies to big ole boys, it can add up. Especially when you want enough of a variety that they aren't getting bored and making asses of themselves. When the dogs make asses of themselves, they reduce their adoption potential to nil, basically signing their own death certificate.

Peter N
07-17-2008, 01:18 PM
To get any kind of homogenous mix, not counting witness lines or rough surfaces, but to make it as once consistent piece and not like lots of lumps of modelling clay stuck together, you need both high heat and high pressure.

We inject molten plastics at pressures around 20,000 psi at high speed, to fill a mould cavity in around 1 second, then continue to pump it in at high pressure until the melt 'freezes' or sets. Whatever the projected area (cavity area directly perpendicular to the the injection axis) in square inches gives you a multiplication of the opposing clamp force needed to stop it blowing open, so 10 sq. inches would need a clamp force in excess of 200,000psi.
Hence it is difficult to get good mouldings at a larger size on home shop type equipment.

Happy to chat more later, but right now I'm just off to see 11yr old daughter doing Richard III as the school play.


07-17-2008, 01:21 PM
I had done some research on this topic few years back, and decided not to undertake such a project owing to expense and complexity. If that's solid plastic, you'll need fair size injection setup.

The only kind of plastic molding that I've seen done successfully at a reasonable price is vacuum molding, but that's really only good for thin profile section.

You may consider using castable polymer instead of thermoplastic.

07-17-2008, 02:27 PM
What about a two part RTV silicone? I just bought some kind of rubber bone for my dog. Don't know what the material is but it seems pretty indestructable. Most RTV is pretty soft but you might find something that is higher strength.

You could just take a real bone, mold it, and then pull silicone bones out of that mold. I always wanted to have a human femur molded and give doggy one to carry around the neighborhood when we walk. Is that funny? It's funny to me, I forget the movie it comes from.

07-17-2008, 03:03 PM
That product may have been Blow Molded. I have a small DiAcro vacuum forming machine that is also capable of blow molding. There is one of these machines on ebay right now. Auction number is 310067100309 . I use this machine several times a week to vacuum form parts for my product line. Have never pressure formed any parts although the manual says it can be used for that purpose.

07-17-2008, 03:14 PM
vac forming and pressure forming is relatively easy in the home shop, I've done it before, fully aware of how to do it. Only thing I've never been able to find are the little "blanks" for doing pressure moulding, I imagine they are custom ordered.

So 10 ton/in^2, is that cumulative? It would seem that the face furthest from the injection port would take the least amount of pressure to melt, whereas the face closest to the port would take the most...assuming your port was say 1/10th the diameter of the part and fixed in the center.

I've actually got experience around moulding, but never really paid any attention....and it was always for HUGE thin parts, like windows in cars.

07-17-2008, 03:15 PM
ok wait, that product may have been blow moulded.....

that product is rubber, I'm having trouble understanding the processing of rubber at all.

07-17-2008, 03:47 PM
Almost everything that feels and looks like rubber is actually plastic now. Think of electric cord and CV boot. They are all synthetic polymer.

Rookie machinist
07-17-2008, 04:07 PM
The cost of setting up an injection machine in your home would probably be very costly. Even if you found a small machine at 5 to 10k molds start out at 20k and go up from there. I don't see any parting lines on that part so it may have been blow molded. Most small items like that are made overseas where costs are much cheaper. There are quite alot of hidden costs in injection moulding and even simple parts can be a pain.

07-17-2008, 06:11 PM
Even blow molds have parting lines.
I would guess that it is blow molded knowing that most dog toys are hollow. I build injection molds for a living, have for 27 years. A product like that would not be something for a do it yourselfer to take on in his garage.
Used molding presses however are pretty cheap for the most part. The company I work for just sold a very nice fully working 300 ton Cincinatti Milacron for something like $6000.00 dollars 2 weeks ago. We didn't use it much anymore and it was taking up valuable space that was needed for a 750 ton press. But to go along with the machine you would need an oven to dry material and quite a bit of power.