How to purchase injecting machine and decide the specification?

1. To calculate the relevant injection amount according to the weight of  the injection molding machine

(1)  when the product is made of polystyrene, the injection amount is Wps. Wps = (Product weight + Total weight of the casting head system)

When the requirement of product quality is high, the larger value of the coefficient should be applied.

(2)   if the product is made of other kind of plastics (call X plastics), according to the above method, first calculate the theoretical injection amount of the plastics Wx:

Wx = (1.3~1.1)×(Product weight + Total weight of the casting head system)

But Wx can not be simply used to decide the injection amount of the machine, it must be transformed into Wps value, here is the transforming equation:

Wps = W×1.05/vx

vx  —— the specific gravity of the plastics

Eg: if a article is made of polystyrene (PE), and the calculated weight is 175g, the casting channel system is about 20g, the with the above equation:

Wx = 1.2×(175)+20)=234g

You can look up the specific gravity of PE Vx =0.92 So, Wps = 234×1.05/0.92=267g

So the best choice is 10.5 ounce (200g) injection molding machine made by our company.

2.   Calculating Mold-locking force P according to the vertical projection area of the injecting product on the molding board (head board or second board):

Mold-locking force = mold-locking force constant  × projection area of the product

That is, P = KPS In the equation:

—— Mold-locking force (t)

S  —— Vertical projection area of the product on the molding board (cm2)

Kp —— Mold-locking force constant (t/cm2) Kp values given at the following table (Table 5)

Table 5 Mold-locking force constant Kp (for the injection of nice product)

Plastics Name













Glass Fiber

Other Engineering Plastics











the inner space between pull-rods is decided according to the outline size of the mold

the opening stroke of the injecting machine should be decided according to product height. The later should be larger than the former.


Opening stroke calculation

Other subsidiary factors that may need to be considered

(1)   The capacity of the injection molding machine should be consistent  with the height of your mold;

(2)   Ejector stroke, ejecting force;

(3)   Process factors;

(4)   The particles of plastics (including recycled materials) should be even and not be too big and long, otherwise it is very difficult to put them into the machine and the normal production may be influenced.

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