This article discusses about how to measure clamping pressure. Clamping is needed to hold an object firmly against the force acting on it.
In injection moulding process, the mold needs to be clamped otherwise it will move during the injection process. As a result of which, the final product will have defects such as flash. Hence, applying clamping pressure is very beneficial.
What is injection moulding?
Injection moulding is a manufacturing process in which molten materialis injected in the mould at high pressure to make the final product.
Usually it is done at higher temperatures and thus can be called as hot moulding process. The final product is left to cool down before it is extracted from the mould. A mould is simply a hollow cavity which has the shape of part to be manufactured.
There are other moulding process too such as Spin casting, extrusion molding, casting, blow moulding etc.
Applications of injection moulding
Injection moulding has a wide range of applications.
The applications of injection moulding are as follows-
- Wire spools
- Bottle caps
- Automotive industry
The injection moulding process is a desirable manufacturing process for making a single product in high volumes that is same design in mass manufacturing. A mould design is kept fix that will be used again and again for making the same product in huge number. Examples include plastic bottles, toothpaste tubes etc.
Different types of injection moulding process
The different variations of injection moulding process are as follows-
- Die casting– Molten metal is forced into a cavity or mould with very high pressure.
- Metal injection moulding-Powdered metal is mixed with a binder and then shaped using traditional injection moulding process.
- Gas assisted injection moulding- An inert gas is used for inducing high pressure that pushes the plastic melts.
- Reaction injection moulding– This type of moulding is similar to traditional injection moulding except that in this thermosetting polymers are used that require curing reaction to take place inside the mould itself.
There are many other variations for injection moulding process such as cube mold technology, thin walled injection moulding, micro injection moulding etc. Which also work on the similar principle as injection moulding.
What is clamping pressure?
The term clamping pressure is most commonly used in injection moulding process. This term is important as it is used to choose a clamping machine of required capacity that will be used for clampng the part during the injection moulding process.
It is the amount of force required per unit area to hold the mould during injection process. In absence of clamping, the mould will open prematurely by the force applied by the injection pressure resulting in manufacturing defects like flash on the part.
How to calculate clamping pressure in injection moulding?
The clamping machines are often rated in terms of their capacity for example a 200 tons machine will exert a clamping force of 200 tons.
To find the clamping pressure, we will need the surface area of part on which clamping pressure is to be applied. The pressure can be calculated by the formula given below-
P is the clamping pressure
F is the Clamping force
A is the surface area of the part
How do you calculate clamping force from torque?
Calculating clamping force is necessary because it will be supporting the mould structure during the entire moulding process. Now, the magnitude of force depends on the tightening torque that is applied.
Clamping force can be calculated using a simple formula if torque is known. The formula is given below-
T is torque in N-m
K is constant to compensate friction losses
D is bolt diameter in m
P is clamping force in N
What is clamping tonnage?
Clamping tonnage is an important term used while selecting the appropriate clamp for the moulding process.
Clamping tonnage is the maximum clamping force that the clamping machine can offer to the injection tool while injection moulding process.
How do you calculate clamping tonnage for injection molding?
The clamping force is very important as it holds the mould tightly during the entire injection moulding process.
To calculate the clamping tonnage, a series of simple steps are followed. These steps are-
- Calculate projected area– The projected area of the part is calculated by the formula, A= lxb where, l is the length of the part and b is the width of the part.
- Multiply area by tonnage factor– Tonnage factor usually ranges from 2 to 8. tons per square inch. Tonnage factor is a material dependent quantity and changes from material to material. It is usually kept as 5.
- Add a safety factor– Around 10% of the total tonnage is added to the actual tonnage for a safe design. This adds extra capacity to the machine, extra capacity is needed in case of any mishap takes place.
Mathematically, Clamping tonnage, T=(Ax Cf)+S
A is the projected area (in square inches)
Cf is the tonnage factor or clamping factor
S is the safety factor which is 10% of the actual tonnage (in tons)
Example of clamping tonnage calculation
Let us consider a part whose length is 12 inch and width 3 inch. The tonnage factor to be considered is 5. Calculate the clamping tonnage.
The actual tonnage is given by multiplying area and tonnage factor.
that is 12x3x5= 180 tons per square inch.
Now, adding the safety factor which 10% of the actual tonnage.
that is 10% of 180=18 tons.
Implies 180+18= 198 tons per square inch.
Hence, a 200 clamping tonnage machine should be chosen for the work to be done flawlessly.