Process overview

Dip moulding is exactly what the name suggests; A former is preheated and then dipped into the PVC plastisol and allowed to dwell. The former is withdrawn with a gelled coating of the plastisol around it and placed into an oven where it is cured. After cooling the moulding is stripped from the former by hand, tool and/or compressed air. It is probably the most simple of all the moulding processes but certainly it is a very flexible and cost effective one that has applications in many if not all industries.

The above is a typical although simplified description of a dip moulding cycle. Not mentioned for the sake of simplicity are: Pre heat , dwell, cure and cooling times which would need to be generic in any case as setup and production running differs from job to job. In addition, tooling and certain materials also have their effect as well as processing equipment, release agents and post moulding operations that may have effects upon the dipping cycle.

The pros and cons of the Process

The most obvious limitation of dip moulding is the use of a male only type former which gives a hollow moulding. As can be appreciated, the action of immersion, dwell and withdrawal will lead to a tapered wall thickness which will be more prominent over longer lengths. However, this can be reduced to a minimum and, in some applications, is desirable. Internally however, faithful reproduction of the former's dimensions and detail can be reproduced as long as the dip and/or tooling has been carried out to meet the process requirements.

Having reviewed the disadvantages it is time to take a look at the positive side of dip moulding. The time and cost required to prototype, tool and go into production is very much where the process shines. The simplicity of tooling and its ease of manufacture will show a significant reduction in both these areas. Where a component starts life as a low volume requirement and so did not warrant a high investment in tooling it is a simple matter of adding formers to the tool mass as the requirement grows. The larger tool mass will enable the part to be run on larger capacity automatic plant giving a twofold improvement as output will be increased by the extra tooling and faster cycle times available from the automatic plant. This of course is based upon CPL's approach and automated plant design.

Dip moulding produces very attractive high gloss finished items that display no seams or flash. Internally they will reproduce tool detail to the extent where mouldings can be turned inside out to display a detailed surface. This can be taken a step further as this detail can be used as reinforcement, stiffening or a physical stop.

Material

Although other materials are used with the dip process, PVC is the most prolific. CPL's expertise lay with mouldings produced from PVC only and so other materials are not covered.

Dip moulding grade PVC plastisol is a dispersion of PVC polymer in plasticiser (D.O.P.) along with stabilisers and pigments. Plasticiser content determines the Shore hardness, measured on the A scale and tested on a cured moulded test piece, in a general purpose range of 50 to 80 Shore. However, specialist grades are available that give either softer or harder finished mouldings.

Manufacturers are obliged to attach a warning label to the Plastisol containers because of the plasticiser content and this is as follows:
EEC 117-81-7
R62 Possible risk of impaired fertility to unborn child.
It should be noted however, that this applies only to the plastisol and not to mouldings produced from it.

Properties

The insulation properties of PVC are extremely good and the material is used extensively throughout the electrical and related industries. A 1mm thickness of PVC will offer resistance to 10Kv while coloured, (non carbon black pigments) materials offer close to twice this figure. Such figures presume that the material is completely cured and free of pin holes.

Fire retardant properties are also good and most general purpose grades would achieve a UL94/V1 rating with V0 being achieved with a wall thickness of 2.5mm and above. Specialist fire retardant formulations are available that give a V0 figure regardless of wall thickness.

The splash resistance to many acids, alkalis and detergents is also very high and again special purpose grades are available for certain applications such as:-
Oil resistant, Food grade, Toy grade, Medical grade, UV stable and High temperature to name but a few. The latter will give a working temperature of around 90 degrees Celsius with intermittent bursts into the low 100's. General purpose grade working temperatures fall into the -40 degrees Celsius to 60 degrees Celsius with intermittent usage in the low 80's.
As with most plastics solvent contact should be avoided.

A wide range of standard colours are available and accurate colour matching is a simple process. Alternative finishes to that of the standard high gloss are also available with matt and texture finishes becoming quite common.