Big mold size ? Not sure if clamping force is enough ? Try out the general clamping force calculator formula !
Angela W.
Rapid Prototyping /Mold Making /Spare Part Manufacturing/ Injection Molding Supplier
Published Apr 6, 2022
The clamping force of an injection molding machine, also known simply as the clamping force, is crucial for molding injection products. Insufficient clamping force can lead to product defects such as flash or material shortages. Conversely, excessive clamping force results in resource wastage and accelerates the wear and premature aging of hydraulic components and mechanical structures.
There are numerous formulas for calculating the tonnage of injection molding machines. Below, we will explain how to calculate the clamping force using four different formulas, one by one.
Empirical Formula 1
Clamping force (T) = clamping force constant Kp product projected area S (cm cm)；
Kp experience value:
PS/PE/PP - 0.32;
ABS - 0.30~0.48;
PA - 0.64~0.72;
POM - 0.64~0.72;
Add glass fiber - 0.64~0.72;
Other engineering plastics - 0.64~0.8;
For example, a product has a projected area of 410 cm² and material is PE, calculate clamping force.
Calculated by above formula: P = Kp S = 0.32 410 = 131.2 (T), 150T machine tool should be selected.
Empirical Formula 2
350bar*S(cm²)/1000.
As above, 350*410/1000=143.5T, choose 150T machine.
Empirical Formula 3
Two important factors in calculating clamping force:
1. Projected area (S) is the largest area viewed along mold opening and closing.
2. Determination of cavity pressure (P)
Cavity pressure is affected by following factors
(1) Number and location of gates
(2) Gate size
(3) Wall thickness of product
(4) Viscosity characteristics of plastics used
(5) Injection speed
3.1 Grouping of thermoplastic flow characteristics
Group 1: GPPS HIPS TPS PE-LD PE-LLD PE-MD PE-HD PP-H PP-CO PP-EPDM
Group 2: PA6 PA66 PA11/12 PBT PETP
Group 3: CA CAB CAP CP EVA PEEL PUR/TPU PPVC
Group 4: ABS AAS/ASA SAN MBS PPS PPO-M BDS POM
Group 5: PMMA PC/ABS PC/PBT
Group 6: PC PES PSU PEI PEEK UPVC
3.2 Viscosity grade
Each of above groups of plastics has a viscosity (flow capacity) rating. Relative viscosity grades of each group of plastics are as follows:
Group multiplication constant (K)
Group 1 * 1.0
Group 2 * 1.3～1.35
Group 3 * 1.35～1.45
Group 4 * 1.45～1.55
Group 5 * 1.55～1.70
Group 6 * 1.70～1.90
3.3 Cavity pressure is determined by ratio of wall thickness, flow to wall thickness
Look up table P0•P=P0•K (multiplication constant)
3.4 Determination of clamping force (F)
F=P•S= P0•K•S
For example, part: calculation of clamping force of polycarbonate (PC) lamp holder
As shown in figure is a round PC plastic lamp holder with outer diameter of 220mm, wall thickness range of 1.9-2.1mm, and pin-type center gate design. The longest process for parts is 200mm.
Place where flow resistance of melt is the largest occurs at position where wall thickness is thinnest (1.9 mm), so value of 1.9 mm should be used when calculating required injection pressure.
Process / wall thickness ratio calculation Process / wall thickness = longest melt process / thinnest part wall thickness = 200mm / 1.9mm = 105:1
Application of cavity pressure/wall thickness curve Relationship between cavity pressure and wall thickness, process/wall thickness ratio is provided. It can be seen from figure that cavity thickness of 1.9mm, cavity pressure of flow/wall thickness ratio of 105:1 is 160Bar. Data is applied to the first group of plastics. For other groups of plastics, we should multiply corresponding multiplication constant K.
Determination of cavity pressure value of PC Flow properties of PC belong to viscosity class of sixth group. Compared with the first group, viscosity of PC is 1.7-1.9 times of that, different viscosities are reflected in cavity pressure, so cavity pressure of PC lamp holder should be 160 bar*K (the viscosity grade of PC). P = 160*1.9 bar = 304 bar for safety reasons, we take 1.9 times.
Projection area value of PC lamp holder S = π*lamp holder outer diameter 2 / 4 = 3.14* 22*22 / 4 (cm2) = 380cm²
Clamping force of PC lamp holder F=P•S=304bar•380cm2=304kg/cm2•380 cm2=115520Kg or 115.5Ton, so 120T can be used.
The primary criterion for classifying injection molding machines is their clamping force. Traditionally, a larger machine is associated with a greater clamping force, while a smaller machine has a lower clamping force. This has long been the conventional view, with clamping force being the main indicator of a machine's size. However, as the market becomes increasingly specialized, clamping force alone is no longer sufficient to fully evaluate the size and capability of a machine. Consequently, more specialized machines are emerging to meet diverse market demands.
Therefore, while clamping force remains a key parameter for measuring an injection molding machine, it is essential to consider a comprehensive evaluation of other parameters when assessing a machine's overall capabilities.
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