UNDERSTANDING SELECTION OF INJECTION MOULDING MACHINE

Continued from previous page.

ESTIMATION OF CYCLE TIME

Cycle time consists of

• Mould Close ∓ Clamp
• Injection Unit Forward
• Injection stroke (speed)
• Follow-up pressure
• Cooling time, dozing, injection unit retract
• Mould Open
• Ejection

Mould open/close time can be determined by dry cycle time. It could be slowed down if the mould has moving/sliding parts. It should be noted that number of dry cycles per hr could be with or without movement of injection unit.

Injection time can be estimated by dividing shot weight by injection rate. Injection rate could be slower for shear sensitive material, and higher for commodity polymer.

25% to 35% of cooling time could be considered as follow-up pressure time.

Cooling time can be estimated by the following formula.

tc = cc x square of (max. wall thickness mm)

cc = 3.5-5.5 for PS

1.9-4 for ABS

3.05-5 for POM

1.4-3.5 for PA

2.0-4 for LDPE

Ejection time could be 3 - 6% of cycle time if the ejection is automatic.

For thermally sensitive polymer

                         permissible residence time x % utilisation of injection capacity
cycle time <-------------------------------------------------------------------------------------------------------
                                                        K x 100
where K is a constant dependent on screw geometry

There are no simple generalised formula for determining the injection time, injection speed and cooling time. The complex equations for these parameters can be solved by powerful computer with MOULD FLOW and MOULD COOL program. The constant of proportionality will be different at different set up (mould design) for different polymer. Therefore to predict the power requirement and cycle time we should understand the followings.

• Flow ratio (maximum) of part to be moulded.
• Determination of Cavity pressure with the help of flow ratio..
• Relationship of cooling time with maximum wall thickness.
• Relationship of freezing time with minimum wall thickness.
• Relationship of injection rate ∓ time with minimum wall thickness.
• Relationship of power consumed with connected power.

Where L is flow path and T is wall thickness

FLOW RATIO

You will appreciate that the pressure is required to push the melt in to the cavity. It is directly proportional to longest flow path and inversely proportional to wall thickness. The ratio of L/T for a given part is called flow ratio. The maximum possible flow ratio for a given polymer melt at maximum possible injection pressure (of the machine) depends on the viscosity characteristics of the polymer melt. The maximum flow ratio can be about 250.

The prospective buyer will have to evaluate the requirement of his market. A list of items to be moulded should be prepared. The items should be listed in ascending order of shot weight and mould dimensions. Thereafter each parts should be evaluated in the following steps.

Steps for selection of Injection Moulding Machine

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1. a) Part

b) Drg.No.

2. Polymer-grade / make

a) Max.flow ratio

b) Degradation Temp.

c) Thermal Stability

3. Overall Size (in mm)

a) L x W x H

b) or m x mm

4. No. of Cavity

5. Weight

a) Part Weight

b) Shot Weight

(incl.of runner)

6. Estimated Shut

Mould Size (mm)

L x W x H

• Two plate mould
• Three plate mould
• Hot runner mould
• Insulated runner mould
• Core pullying
• Insert moulding

7. Projected Area

mm2 or cm2

8. Max.flow ratio of

Part

9. Most prevailing wall in thickness (mm)

10. Estimated cavity

pressure (bar)

11. Estimated Clamping

Force (tons)

12. Recommend IMM Model

a) Clamping Unit

b) Injection Unit

13. Screw size m mm

a) Hydromotor

b) Nozzle

c) Pump Power

14. %age utilisation of injection shot capacity should be such that Residence time < Thermal stability

CLASSIFICATION OF PARTS TO BE MOULDED

Wall thickness and flow ratio are very are important while evaluating the machine specification and cycle time. Therefore we can classify the parts -to be moulded - on the basis of wall thickness.

• Thin walled parts with maximum flow ratio. Wall thicknessequal to 1mm and less than 1 mm.
• Medium wall parts with wall thickness between 1 mm and 2 mm.
• Large wall parts with wall thickness larger than 2 mm.

CLASSIFICATION OF MOULDED PARTS - BASED ON WALL THICKNESS - TABLE

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Technical Papers by Prabodh C. Bolur

Technological Solutions for Quality in Injection Moulding of Plastics(1998)

Technological Tools for Part Design, Mould Design ∓ Mould Fabrication.(1999)

Understanding Selection of Injection Moulding Machine.This paper was part of authors lectures at CIPET since 1980. It has been regularly updated