Foreword
Preface
Contents
Summary

Selection of IM Machine

Technological Solutions

Technological Tools

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

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

ITEMS	POLYMER	APPLICATION	WALL THICKNESS	FLOW RATIO	POWER REQ
Disposable cup	PP, PS	Packaging	< 1 mm	highest	highest
Disposable syringe	PP	Health care	< 1mm	highest	highest
House wares	HDPE, PP, LDPE, PS	Domestic use	Bet 1 & 1.5 mm	depends on size	high or medium
Pipe fittings	RPVC, HDPE, PP	Water connection	> 2 mm	depends on size	medium or low
Pipe fittings	RPVC, HDPE, PP	Sewervage connection	> 2.5 mm	depends on size	low
Crates Tray	HDPE, PP PS	Storage, transportation	> 1.5 mm	high	high or medium
Industrial parts	ABS, PC, SAN, PA, PMMA, POM, PBT,etc.	Engineering	bet 1mm & 1.5 mm	varying	medium
- Tiny items	Delrin, PA	Clock / meter parts,	< 1 mm	low or medium	medium
- handles	CA, PP	Tools & industrials	> 5 mm	low	low
- Housing	ABS, PS, PP	Appliances, Instruments, TV, Eletronics	bet. 1 mm & 2 mm	high	medium
Cellular Phone parts	PC, PC/ABS, Mod.PPO	Telecommunication items	=< 1mm	high	high
Laptop & Notebook computer parts	PC, PC/ABS, Mod.PPO	Computer parts	=< 1mm	high	high

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Technical Papers by Prabodh C. Bolur

Understanding Energy Consumption in Injection Moulding Machine(1994)
Understanding Heat Exchange in Injection Moulds (1994)
Ideal Moulding Shop (2001)
Understanding Selection of Injection Moulding MachineThis paper was part of authors lectures at CIPET since 1980. It has been regularly updated.
Technological Solution to Injection Moulding of Plastics (1999)
Technological Tools for Part Design, Mould Design and Mould Fabrication (1999)
Extrusion of Thermoplastics(1998)
Archives.

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