Harboro Engineering in Excellence

• Introduction
• Value of Rubber
• Processing Rubber
• Designing with Rubber
• Specifying Rubber
• Material Data Chart
• Quality in Rubber
• Rubber Directory
• Glossary of Terms
• Measurement of Properties
• Health and Safety
• True Partnership

Designing with Rubber

Designing for Mimimum Costs

While every component is designed to fulfil a unique set of operational requirements, there are a number of common principles which will reduce the time and cost of obtaining an economic component. Many of these are self-evident but some require an understanding of the differences between moulding thermoset rubber and moulding plastics.

Parts will be obtained at minimum cost when there are no surprises in the design, supply or use. Good communication and early contact with suppliers, particularly during the development of new parts, will highlight potential areas of uncertainty and allow them to be overcome. Advice at an early stage can often add functionality to the design which saves cost in other areas e.g. easing assembly, simplifying design or reducing tolerances required in other components.

Producing a Specification

Designers are strongly advised to consider and record a specification covering the following points:

• What operating conditions are expected - normally and exceptionally?
• What substances will be encountered?
• What will be the likely material? (See Specifying Rubber)
• Will there be any movement or distortion?
• What are the price targets? Are there any tool cost constraints?
• What colour should the component be?
• What finish is required?
• What quality standards will it have to meet?
• How many are likely to be required?
• What amount and position of flash is allowable?

A specification is invaluable in selecting a suitable material for trials, as well as being a sound basis for producing parts which are safe and economic by design. Over-specification may lead to the use of an expensive polymer, an inappropriate tool or unnecessarily costly processing.

Prototyping

A single cavity prototype can be produced quickly and economically. This allows designs to be proved, materials tested and a small number of parts supplied for pre-production runs. Harboro offers a priority service for prototype moulds which can generally be obtained in less than four weeks.

Designing Components

The following should be borne in mind at the design stage:

• Be aware of where the split line will fall
• Think about removing the part from the mould when it is hot and soft
• Rubber parts do not generally need taper
• Re-entrant shapes are practical in rubber
• Core out thick sections which would extend cure times
• Geometric shapes make for economic tools
• Combine features - such as seals, springs, logos and tolerance take-up from other parts.

CAD

The days of hard copy drawings have been for many years apparently on the endangered list but thankfully for many they remain. Hard copy drawings still provide detailed information on specifications tolerances notations and something that everyone can use through the control documentation from goods inwards to despatch. Every customer has different methods for which part drawings and designs are transmitted. On a day to day basis Harboro are working with companies that do not have the resources to create CAD models and in some cases even hard copy drawings. This is not considered as being a problem as Harboro can work from just ideas, concept or sketches up to the most complex CAD model and turn it into reality. Harboro has the breadth of skills available to work with a customer to provide well designed solutions using in house CAD and FEA software. This is completed with the input of material technologists, process specialists and our in house tool design engineer. The combination of experience, knowledge and innovation is a rarely found package.

Tooling Design

There are significant differences between moulds for thermoset rubber and for plastics which the designer should take into consideration.

Thermosetting materials are cured by heating to around 150°C. Much of this heat is gained from the hot tool walls. As rubbers are good insulators, heat transfer can be slow where the part has thick sections.

Unlike thermoplastics, the flash, feed gates, runners and sprues of thermoset rubbers are cured irreversibly and are not reusable in any way. Flash is characteristic of normal rubber moulding, as rubbers do not “freeze” as plastics do when flowing into very thin sections and will run into gaps as small as 0.002mm.

Designers should not specify the number of cavities in a tool as economic production depends on a number of factors, including:

• the precision required
• the dimensions and orientation of the part
• press characteristics
• the quantities and rates required.

There is often a variety of possible ways to mould a component. For example, a cylinder can be moulded with the tool split line parallel to or at right angles to the axis. This decision affects the number of cavities possible and the appearance and price of the finished part. It will often depend upon the amount of visible flash allowable at the tool split line. The final choice will be governed by the function of the part, the appearance required, the economics of tool area and tolerances.

Most production tooling for rubber parts is made of mild steel. A few compounds give off halogens as they cure which, in the long term, can corrode the mould surfaces but otherwise rubbers are not aggressive materials. When injected, they flow reluctantly and require large gates and feeds compared to plastics, so abrasive wear is minimal.

Tools are generally expected to last around 100,000 press lifts - a four cavity tool should produce 400,000 parts during a normal life. Precision parts and tight tolerances generally mean shorter tool life and higher maintenance costs. Frequent changes and short runs also reduce life expectancy. All tooling will require periodic refurbishment and should be reviewed annually.

Tool design is best discussed with the rubber manufacturer at an early stage.

Harboro’s in-house technical team includes an experienced toolmaker and tool designer.

Cost

The key determinants of cost are cycle time, the number of cavities in the mould, material cost and the need for manual operations before or after moulding.

Cycle times for rubber generally range from two to ten minutes, although the cure time for heavy parts may be much longer than this. Reducing the mass of a component not only reduces the material cost, but may also reduce the cycle time. This is especially true for parts with thick sections.

Where zero defects are required, due recognition of process capability is required in order to prevent unnecessary quality inspections after moulding. Checks that are not built into the process will inevitably add to the cost.

Colour

Rubber gains much of its strength and its resistance to heat and light from the addition of carbon black. Hence the vast majority of rubber is black.

Coloured rubbers can be produced using other reinforcing fillers and suitable colouring pigments.

However, the changes that take place during curing, and the nature of the moulding process, make it difficult to maintain perfectly even coloration, particularly with pale colours. Silicone rubbers are the most suitable for achieving reliable and clean coloured mouldings, even with pale colours and translucent.

Surface Finish

The finish of rubber can range from semi-gloss to extremely matt using the same mould, but it will always be less smooth and mirror-like than plastic. The matt, non-reflective nature of their surface means that coloured rubbers will have a different appearance to neighbouring “hard” materials.

Attractive appearances can be obtained by moulding a patterned surface into a component, such as a fine matt geometric pattern or “sparked” finish.

Materials

Harboro is happy to advise on the most economic materials for any application. The company will supply free samples of materials in 7 x 7cm (approx) sheets to allow different compounds and hardnesses to be trialed. Of course, any material proposed should be well tested before production commences.

Some materials are very expensive but on small complex parts the material cost may not be that significant. However, on tiny, thin parts, the weight of flash, sprues and runners may be large compared with the component itself.

Each mould is made to allow for the shrinkage of a specific compound. Rubber compounds of the same type have similar shrinkages - harder compounds shrink slightly less. It is possible to change compounds in a mould but the finished dimensions may vary if the shrinkage is different.

Tolerances

The finer the tolerances required, the more costly the part.

Tolerances for moulded products to ISO 3302 1995 (BS 3734)

Table classes M1 (Precision) and M2 (Commercial)

Values in mm, F = Fixed dimensions, C = Closure dimensions

Nominal Dimensions		Class M1		Class M2
Above	Up To	± F	± C	± F	± C
0	2.5	0.08	0.08	0.10	0.15
2.5	4.0	0.08	0.10	0.10	0.15
4.0	6.3	0.10	0.10	0.15	0.20
6.3	10.0	0.10	0.15	0.20	0.25
10.0	16.0	0.15	0.20	0.25	0.30
16.0	25.0	0.20	0.25	0.25	0.35
25.0	40.0	0.20	0.25	0.35	0.45
40.0	63.0	0.25	0.35	0.40	0.50
63.0	100.0	0.35	0.40	0.50	0.65
100.0	160.0	0.40	0.50	0.70	0.90

Top

Harboro can work from just ideas, concept or sketches up to the most complex CAD model and turn it into reality.

Tooling design and cavity configuration is vital in achieving quality parts at lowest cost.

Tool design is best discussed with the rubber manufacturer at an early stage. Harboro’s in-house technical team includes an experienced toolmaker and tool designer.

The matt, non-reflective nature of their surface means that coloured rubbers will have a different appearance to neighbouring “hard” materials.