Cost & Cavity Calculation fo injection moulding

Are you to spec out a machine for a particular job? The tonnage rating for an injection molding machine relates to the clamping pressure. The clamping pressure must be enough to keep the mold closed during injection. For example: If you have a mold cavity that has 10 square inches of surface area and you are going to inject at 1000psi there will be 10,000lbs of force or 5 tons of force trying to open the mold during injection. The clamping pressure must be enough to overcome that force plus some extra for safety. I do not know how much that safety margin should be but the machine manufacturer should be able to make recommendations. Then the injection capacity of the screw plunger is rated in maximum grams per shot, which of course must be more than the weight of the part combined with the runners and sprue . Here it is also good to have extra capacity or your machine will not be able to keep up with the expected production cycle time.

You will have to get the grams per cubic inch from your plastic material supplier and then calculate the weight of the part plus the runners and sprue based on their combined volumes. Then determine the square inches of the cavity and multiply times the recommended injection pressure to determine the necessary machine tonnage.

Thanks a millenium tones for giving an info.

Regards

Ajay tote.

Thanks Dear Mr.Garyceng,

Actually i have drawing of injection moulding parts. that weight is approx. 47 grams.

And its area is 4500 square mm.

If i gave this drawing to Mold maker.And revert back from the mold maker with, suppose the mould should make for 40 tonnage of injection moulding m/c. & mould will be 4 Cavity.

how i can rely on him where he is true or not?

for that only i need to know how we will calculate the above things for mould design for require tonnage of machine & cavity.

please send me a calculation of above specs.

i will be a very greatful to you.

Thanks & regards

Ajay Tote

Sorry for my weakness but I had to convert this to English. If my calculation is right we are talking abour roughly 7 square inches of cavity. For 4 cavities this would be 28 square inches times 1000 (1000psi is an average injection pressure for many common plastics) pounds per square inch. This means that there is 28,000psi of hydraulic force trying to open the mold. 2000lbs =1 ton. 28,000/2000 = 14 tons. A 40 ton press shold be able to handle it. The next question is the shot capacity usually rated in ounces. Your 4 parts add up to 6.63 ounces to which you must add the weight of the runners and the sprue to come up with the total weight of the shot. 1000psi is just a starting point. You will need to get a recomendation from the plastic manufacturer for the specific material you will use. The mold mker will also need to know the shrink factor of the material. You must get this right the first time or you will be paying to make the mold twice.

Please consider this very carefully and I am sure there are other mold makers and plastics experts on this forum who would agree. In order to design the most economical mold the mold maker should know how many parts you expect to make and how many parts per month you will need and of course what material. If this is going to be a big or complicated mold you should have prototypes made of your parts and test them to make sure of your design. It can be very expensive to change a mold after it has been made. It is a good idea if possible for the mold maker to also be the molder because it concentrates the accountability in one company. Try to get multiple quotes and require the mold maker to doccument what type of mold base he will use and what materials the cavity inserts will be made of and how many cavities. Have everyone quote the same thing so you can make an accurate comparison. If you would like I could help you review the quotes, or there are others on this forum who may be better qualified than me to offer their consultation.

Excellent points made here. I would highly recommend a full moldflow analysis prior to building any mold, if at all possible. VisuFlow (sp) is an excellent analysis software and can identify warpage and shrink (or no shrink) conditions.

Dear Mr.Garyceng,

Thanks for reply,

No i heve not checked out, but i have only the drawing that i need to devlop from moldmaker.Mold maker have been given the mold details which i have mention in my previous quote.

I want to be check it out whether he has given the m/c tonnage is write or wrong. For that only i need the data calculation for that.

please help me.

regards

Ajay Tote

The number of cavities that you need depends on the volume of parts you will be making over the lifetime of the program. The greater the number of cavities, the higher the cost of the mold, but the lower the cost of the parts. If you make only a small number of parts each year, and the life of the program is only a few years, you will spend much more money on a mold with many cavities than you will save with the lower cost parts. On the other hand, if you will be making hundreds of thousands or millions of parts per year, and the program will last a long time, you will probably want as many cavities in the mold as possible.