SHRINKAGE

SHRINKAGE

Image hosted from www.autodesk.com

Definition:-

When a hot plastic cools inside a mould, it contracts by an amount depending on the material being processed and the final product is smaller than the mould size. This contraction is called shrinkage. In other words, it is the dimensional difference between the moulded part and the corresponding impression. Thus, when a tool is being designed from a dimensional component drawing, a shrinkage allowance must be added to give the corresponding mould dimension.

Often, a range of shrinkage is quoted, the exact value depending upon the material grade, mould temperature, particular material based on wall thickness, profile, mould temperature and moulding machine conditions including cooling or heating or both.

SHRINKAGE of plastic depends on material and its processing.

Various factors that affect shrinkage

1. Basic plastic material
2. Filler used and percentage
3. Part wall thickness
4. Melt temperature
5. Mould temperature
6. Injection pressure
7. Injection time
8. Hold on pressure
9. Hold on time
10. Gate area

Shrinkage increases with

•         Increase of  material temperature

•         Increase of mould temperature

•         Increase in wall thickness

•         Low injection pressure

Shrinkage decreases with

•         Low melt and mould temperature

•         High injection pressure

•         Long injection time

•         Presence of filler material  and its content

To assess the exact amount of shrinkage, it is very important that the production is near to the optimum and that the cycle has settled down to study conditions. The moulding should be checked only when it is fully set. Measurements should be made in all dimensions because shrinkage varies with the direction of flow. Differential shrinkage rates in the longitudinal and transverse planes may have rise to distortion of flat mouldings. Because of all these factors it is difficult to give the exact amount of shrinkage for moulding. Generally the grade of moulding is IT 11.

Shrinkage value

The chart gives the percentage of shrinkage for some of the commonly used materials

Material	% of shrinkage
Thermoplastic Materials
ABS high impact	0.5 – 0.7
ABS medium impact	0.4 – 0.6
Acetal	2.0 - 3.5
acrylic General purpose	0.2 -0.9
Acrylic high impact	0.4 – 0.8
Cellulose acetate (CA)	0.3 – 0.8
Cellulose acetate butyrate (CAB)	0.2 – 0.6
Ionomer (I)	0.3 – 2.0
Nylon (PA)	0.7 – 1.5
Nylon glass filled	0.5 – 1.0
Polycarbonate (PC)	0.5 – 0.7
Polyethylene low density (LDPE)	1.5 – 3.0
Polyethylene medium density	1.5 – 3.5
Polyethylene high density (HDPE)	1.5 – 3.5
Polymethyl methacrylate (PMMA)	0.2 – 0.8
Polypropylene (PP)	1.0 – 3.0
Polystyrene general purpose (PS)	0.2 – 0.8
Polystyrene high impact (HIPS)	0.3 – 0.6
Polytetrafluroethylene (PTFE)	5.0 – 10.0
Polyurethane (PU)	0.8 – 1.0
Polyvinyl flexible (PVC)	1.5 – 3.0
Polyvinyl chloride rigid	0.2 – 0.4
Styrene acrylonitrile (SAN)	0.2 – 0.6
Thermoset materials
Phenol formaldehyde general purpose (PF)	0.5 – 1.1
Phenol formaldehyde impact grade	0.2 – 1.1
Phenol formaldehyde heat resistant	0.2 – 0.9
alkyds	0.2 – 0.8
Allycis (DAP)	0.3 – 1.0
Glass filled Phenolics	0.1 – 0.7
Injection grade Phenolics	0.5 – 1.1
Glass filled Epoxy	0.1 – 0.7
melamine	0.4 – 0.7
Urea formaldehyde	0.6 – 1.3

Post Moulding Shrinkage:

When mould shrinkage occurs in cavity and part reaches room temperature, further shrinkage may occur as time passes. This post-moulding shrinkage is irreversible and it is determined by cooling rate, that is, by mould temperature and part thickness. Post –shrinkage is minimum in Delrin material.