This question is like asking how big of a car do I need? It depends mostly on two factors: platen size and inject volume. Both of those factors are indirectly related to how many and/or how large the parts are that you will ever want to run in the press.
Step 1: Determine Your Platen Size
The platens heat the mold and should be at least the size of the mold you want to run. The larger the platen the larger the mold you can run. The larger the mold, the more and/or larger parts you can produce. Because the surface area goes up by the platen size many more parts can
be run on a larger press at the same time and with the same operator. This results in much lower cost per part. A platen size of 16" x 16" for example, will result in 256 square inches of
surface area. A platen size of 32" x 32" will result in 1024 square inches, etc.
During the inject cycle, however, the rubber being injected into the larger platen size results in
more force driving the platens apart. Without increasing the clamping force of the platens
would leak rubber out resulting in excess flash and inconsistent fills. This is why larger
platens require larger clamping force.
To summarize; 1) Determine the size of the mold 2) This determines the platen size
3) This determines the clamp size.
Examples of our standard press clamping force size vs. platen size:
Clamping Force size 
Platen size 

5 ton
10 ton
15ton
20 ton
100 ton
200 ton
300 ton
500 ton
600 ton
800 ton
1400 ton 


8.0 x 8.0
13.5 x 13.5
15 x 10
17 x 12
16 x 16
18 x 22
24 x 25
30 x 30, 30 x 36
30 x 30, 30 x 36
32 x 40
42 x 44 

Step 2: Inject Size
Describes how much rubber can be injected in one shot. US Molding describes this in pounds (lb)
of rubber @ 1.25 specific gravity or when the displacement is in cubic inches (CI) at 22.5 CI/lb.
Therefore, if your part uses 5 ounces of rubber and you have 8 cavities in your mold your
parts require 40 ounces per shot or 2.5 lb.
Normally you might think our standard 2.7 lb. would be sufficient but don't forget injection
molds require a runner system, or a way to get the rubber from the inject nozzle to the
cavities. Also, this might not be the largest part you'll ever run this press. A safer choice
might be our standard 5,5 lb.
Examples of our standard press clamping force size vs. inject size:
Clamping Force size 
Platen size 

5 ton
10 ton
15ton
20 ton
100 ton
200 ton
300 ton
500 ton
600 ton
800 ton
1400 ton 


3 oz.
5 oz.
12 oz.
12 oz.
17 oz.
2.7 lb. / 5.5 lb.
5.5 lb. / 10 lb.
10 lb. / 15 lb. / 20 lb.
10 lb. / 15 lb. / 20 lb.
10 lb. / 15 lb. / 20 lb. / 30 lb. / 40 lb.
30 lb. / 40 lb. / 55 lb. 

Please note that we make many more sizes than are shown above. Also, we have some leeway
in being able to put different size platens with different inject sizes and with different
clamp sizes. These examples are just meant to allow you to calculate ballpark requirements.
