How to Determine the Optimum Clamping Force? - 3 ways

The optimum clamping force can be set at the stage of mold design and development and also when testing mold. Keep reading and you will know why and how.

There are three ways:

1st Way: Determine the optimal clamping force at the stage of mold design and development.

When we calculate the mold expansion force of mold products, we always take the maximum value, so the calculated mold expansion force is the minimum critical clamping force for products without ridges, and it is also the best clamping force.

Calculation formula:

F (critical clamping force) = P cavity average pressure bar × S projected area of product and flow channel cm2

First understand the cavity pressure: the pressure of the screw acting on the plastic at the front end of the check ring passes through the nozzle, runner, gate, cavity, and finally acts on the pressure in the cavity.

This pressure is very complicated, related to the viscosity of the polymer material, the size, position and size of the runner and gate, the size and thickness of the product, the injection speed, mold temperature, barrel temperature, mold exhaust, etc.

For example, a product: ABS material, the main nozzle length is 50mm, the square flat into the water is 1.5mm, the wall thickness is 2.0mm, the shape is as shown in the figure below.

cavity pressure example product

Two proven knowledge points for the plastic

Grade

Thermoplastic

Flow Coefficient(K)

1

GPPS、HIPS、LDPE、LLDPE、MDPE、HDPE、PP、PP-EPDM

×1.0

2

PA6、PA66、PA11/12、PBT、PETP

×1.30~1.353

3

CA、CAB、CAP、CP、EVA、PUR/TPU、PPVC

×1.35~1.45

4

ABS、ASA、SAN、MBS、POM、BDS、PPS、PPO-M

×1.45~1.55

5

PMMA、PC/ABS、PC/PBT

×1.55~1.70

6

PC、PEI、UPVC、PEEK、PSU

×1.70~1.90

(2) The relationship between cavity pressure, wall thickness and flow length ratio.

The relationship between cavity pressure wall thickness and flow length ratio

Step 1: Calculate the flow length ratio.

The longest flow of material flow is approximately equal to: 200+30/2+50=265mm, and the thinnest wall thickness is 1.5mm at the water inlet.

Flow length ratio = longest flow of material flow / thinnest wall thickness

=265/1.5

=177:1

Step 2: Calculate the average pressure P of the cavity through the relationship diagram.

The thin wall is 1.5mm, the flow length ratio is 177, and the cross corresponding curve point is P1 = 250bar.

P cavity average pressure=P1 * K flow coefficient = 250 * 1.55 = 387.5bar

Step 3: Calculate the projected area.

This projected area can be calculated in the mold design software when the mold comes out in 3D, and it needs to be clearly marked on the mold manual and the mold nameplate.

S = product projected area + runner projected area

S=20*15*2+3*1

S = 603cm2

Step 4: Calculate the critical clamping force (best clamping pressure)

F (best clamping force) = P cavity average pressure bar × S projected area of product and flow channel cm2

F (best clamping force)=387.5bar*603cm2

F (best clamping force) = 233662.5kg

F (best clamping force) = 234Ton

The critical clamping force required for this ABS material product has been calculated here. And the maximum value of the coefficient has been taken in the above calculation process, so there is no need to multiply the safety factor.

This is the theoretical optimal clamping force. It needs to be clearly marked on the mold manual and on the mold nameplate, so that the adjustment personnel of injection molding production have a reference standard.

Clamping Force setting

Under our current industry conditions, this algorithm corresponds to the actual situation of the mold and the habitual action of our technicians, which is difficult for some small and medium-sized enterprises to implement.

And the correct use of this clamping force is very critical to the mold injection industry!

2nd Way: When testing molds at the injection molding production site, use this test to determine the best clamping force.

Using this method, it can be quickly tested on any machine and any mold. And only one micrometer scale is needed, and correspondingly set different clamping forces on the machine.

Step 1: Set the clamping force to 90% of the maximum pressure, inject with medium pressure (about 60%~70%) and medium speed (30%~60%), set the holding position and holding pressure, and confirm that the appearance of the product is free from defects .

Produce 3 molds, record the weight and appearance according to the following table.

Step 2: Reduce the clamping force by 10Ton in turn, and record the weight. Besides, confirm whether the appearance is OK, until the weight of the product increases by about 5% and the burr appears.

Clamping force /Ton

First mold weight /g

Second mold weight /g

Third mold weight /g

Appearance

110

20

20

20.01

OK

100

19.99

20.01

20

OK

90

20

20

20.02

OK

80

20.01

20.02

20.03

OK

70

21.1

21.11

21.2

Small burr

60

21.3

21.3

21.5

Burr

50

23.3

23.9

23.4

Burr

It can be seen from the above table that the optimum clamping force parameter of this product on this machine is 80Ton~90Ton.

3rd Way: By experience.

In the injection molding production process, if there is no special requirement for mold products, our PMC personnel generally arrange production according to the size of the mold and the number of machines. And the adjustment technician sets the clamping force according to 70% of the maximum clamping force of the machine may solve lots of issue.

It is fast and effective.

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