A variety of processes are used to produce material suitable for moulding.
Harboro have an approach to quality control which ensures reliable parts through the application of high technologies for material analysis and inspection.
Harboro has concentrated on moulding in all its aspects and operates a wide variety of machines using all the common moulding processes.
Compression moulded and some injection moulded parts require finishing.
Harboro still carry out manual inspection where applicable but, where products or agreements dictate, we apply high technology automated inspection systems.
Finished part testing requires different equipment and testing and is not available in many rubber companies.
Finished part testing is therefore crucial for many components in today’s applications.
Raw rubbers have few uses in their natural state. To achieve the desired range of properties, the raw rubber must be combined with a range of additives. The selection of appropriate additives, and their skilful and consistent mixing, is known as compounding.
The additives in a rubber compound may vary from 2-3% (in the case of a rubber band) to over 60% by weight and will include some or all of the following:
Active chemicals which bring about the cross-linking of the long chain rubber polymer. Sulphur was the first to be discovered and is still commonly used.
Chemicals which vary the speed and timing of the curing reaction.
Materials which increase the strength of the material. Carbon black and silicas are the most commonly used.
Relatively inert chemicals, such as clays, which increase the bulk of the compound. (Excess use of inert fillers can cheapen materials but often has an adverse effect on performance.)
Added to produce specified colours. They can only be used with compounds which do not contain carbon black.
Added to aid processability or to produce specified properties.
Chemicals which are added to help the compound resist surface attack, especially by ozone.
Resins, soaps, low-weight polyethylene.
The constituents are weighed out and combined by a mixing process which must blend the ingredients thoroughly in a repeatable way. This is achieved either by an internal mixer, where the compound is mixed by two meshing rotors in an enclosed case; or by open mill mixing, adding the ingredients carefully into the “nip” between two steel rollers, typically of 30” diameter.
The result of either process is a batch of uncured rubber compound. This is allowed to settle for a time before undergoing Quality Assurance tests. Once passed, it can be formed into suitable shapes for moulding.
Each moulding process has its own requirements for uncured material. Compression moulding, for example, requires a “blank” of material in a size which will fill the cavity exactly. Direct injection moulding needs relatively large quantities of compound in a continuous strip. Due to the nature of the injection process, material properties must be precisely measured and controlled to achieve the planned flow and cure behaviour, as well as the desired final characteristics of the rubber.
A variety of processes are used to produce material suitable for moulding:
Uncured material is produced in sheets of the desired thickness. Sometimes “blanks” are cut from the sheet, like pastry cutting.
Extruders force warmed compound through a shaped die. Any reasonable length of shaped material can be produced. Once cooled this is fed into the direct injection presses.
Extrusions as above are cut to required lengths as they emerge from the die. This process can be accurately controlled to produce blanks of precise volume for compression moulding.
Rubber can be made into components by a number of processes, including extrusion, calendering, coating onto fabric and moulding. Harboro has concentrated on moulding in all its aspects and operates a wide variety of machines using all the common moulding processes.
The choice of moulding method will depend upon factors such as the finish desired, the number of components required, the money available for tooling etc., and should be discussed in detail with a rubber manufacturer.
The basic processes of moulding are:
A piece of uncured rubber of the correct geometry is placed between two halves of a heated mould. The mould is closed in a press under a pressure of around 14 MPa and the rubber is forced into the exact shape of the cavity. The rubber gains heat by conduction from the mould surfaces and “cures”. When the rubber has had sufficient time to cure, the mould can be opened and the part removed.
Compression moulding is a relatively simple process and is often used for components required in fairly low quantities. Mould costs are generally lower.
Parts moulded by this method will always have some flash because the mould surfaces are held apart by the necessary excess rubber in the “blank”.
The heated mould is closed in a press and the rubber injected by a hydraulic cylinder through a feed hole in the cavity. The cylinder can either be incorporated in the press or sometimes in the mould.
Provision must be made for air to escape from the cavity as the rubber enters, and the feeding method chosen to suit the operational requirements of the part.
This method of moulding can produce high-precision parts in moderate quantities without high tooling costs. In the simplest case, the mould can be the same as a compression mould with the addition of a feed hole. Maximum weights and number of cavities are governed by the capacity of the transfer cylinder and the clamp pressure.
A screw injection system delivers a metered quantity of rubber into the closed mould. The injection unit is fed from a continuous strip or a reservoir of uncured rubber and pre conditioned to provide the very best material characteristics for introduction into the cavity.
This process is generally used for multi-cavity moulds and can produce hundreds of components per press cycle. Because of the amount of rubber in the system, it is inadvisable to change materials frequently.
Large moulds require complex feed systems to balance the pressures in each cavity. Generally these are in the heated top half of the mould and cure at the same time as the components. Unlike thermoplastics, cured thermoset rubber cannot be reground or reused and the additional waste has to be included in the material usage per piece. Where very large volumes of mouldings are required, cold runner systems should be considered. These are justified by material savings over £10,000 pa.
Direct Injection lends itself to relatively large quantities, a large number of cavities and infrequent changes of materials or moulds. Parts are repeatable and can be made to a high level of precision.
Compression moulded and some injection moulded parts require deflashing. This is done in various ways depending upon the shape and size of the component and the type of rubber used.
The most modern and efficient method of finishing uses cryogenics. Parts are frozen to temperatures as low as -120°C and then tumbled and/or bead-blasted while cold to remove the brittle flash. The machines are individually programmed with the optimum temperature and running times for each particular type and number of components, which are tested and proven during the development stages.
The tool is designed to produce a very thin section of flash around the part which is torn off at the press during de-moulding.
If required, components can also be hand finished by the following methods:
Parts can be smoothed using a variety of abrasive belts, wheels and mops.
Parts are die cut by machine using a range of tools from precision steel knives to wood-formes, or in extreme cases are trimmed with scissors.
Tight manufacturing controls with well designed processes coupled with operator knowledge and vigilance make it possible to produce parts with very low defect levels. While this is common at Harboro Rubber, the variability of materials in rubber processing prevents many from achieving zero defects. Unlike plastics and metals, variability in rubber processing is very high since the material cross links with time and temperature and radically changes its flow properties (doubling the speed of change for every 10°C rise). Variations in mixing, storage and preparation and changes of temperature with flow (particularly through restrictions such as injection gates where temperatures can rise 20-30°C) prevent most factories from achieving zero defects. This leads to the need for expensive visual inspection.
Human visual inspection methods are limited even with multiple inspections.
50-100PPM is probably the best achievable on non critical parts where faults are readily recognisable. This is a satisfactory level in many commercial situations. However, with the trend for increasing liabilities there is growing demand for zero defects. Harboro Rubber employs computerised camera inspection where near zero defects is truly required.
The liability in manufacturing grows as each year passes. A manufacturer must be certain of their products and able to demonstrate this both by testing and through the traceability of the materials and processes used. Harboro have one of the best equipped laboratories in the industry and an approach to quality control which ensures reliable parts through the application of high technologies for material analysis and inspection.
It is difficult even for highly qualified material technologists or chemists (of which Harboro employs four) to determine the characteristics of a material without the use of testing equipment. Traditionally tests are carried out on carefully prepared test sheets under supervised conditions but these are not always truly indicative of the parts being manufactured. Finished part testing is therefore crucial for many components in today’s applications. Finished part testing requires different equipment and testing and is not available in many rubber companies.
Harboro has been involved in global procurement for some parts since 1996 and has developed expertise and acquired specialist equipment suited for such testing. A dedicated laboratory team control the supply of both in house manufactured and globally procured mouldings. In- house manufacturing materials are not an issue as all compounds being supplied to Harboro are carefully tested but globally supplied products needed a different approach. An investment program over a number of years was undertaken in order to elevate the laboratories quality control and analytical capability, allowing detailed analysis of finished parts by the determination of not only the physical attributes but the composition of finished parts.
The composition is critical to the functionality of the product and a key test is using Thermo Gravimetric Analysis (TGA). This test is described below. Although TGA has become an indispensable tool for the analysis and characterization of materials, its scope is limited and at Harboro Rubber Company is used for high level quality control. As a standalone application the data can be used as a comparator against the approved material but no information is obtained about the qualitative aspects of the evolved gases during the thermal decomposition. For processes involving mass loss, a powerful technique to provide this missing information is Fourier Transform Infrared spectroscopy (FT-IR) in combination with TGA. This supplies a comprehensive understanding of thermal events in a reliable and meaningful way as data are obtained from a single sample under the same conditions. This provides extremely accurate and detailed information about the composition of the material thus allowing comparison against stored material data base and analysis for development and reverse engineering.
These tools allow us to ‘see into’ any finished part material. We can pick up any changes or deviations that exist in the material. This resource ensures that suppliers abroad cannot change materials without notification and our prior agreement. Many companies have come to serious grief with suppliers who have changed materials without notification, often for cost or convenience reasons, which is a common practice in Far Eastern manufacture. We can ensure that such practices do not take place amongst our suppliers.
Thermo Gravimetric Analysis (TGA)
The Q500 is the world’s number 1 research-grade thermo gravimetric analyzer. Its field-proven performance arises from a responsive low-mass furnace, ultra-sensitive thermo balance, and efficient horizontal purge gas system (with mass flow control). Its convenience, expandability and powerful, results-oriented software make the Q500 ideal for the laboratory where a wide variety of TGA applications are conducted, and where future expansion of analytical work is anticipated.
Fourier Transform Infrared Spectroscopy (FT-IR)
FT-IR – Nicolet 380. Harboro combine the quantitive analytical attributes of the Q500 (TGA) with the high level qualitative capability of the Nicolet 380 (FT-IR) to provide a detailed analysis of any material which decomposes below a 1000 degree ceiling at molecular level. The reflective chamber can also be used for non-destructive testing to provide a rapid surface composition analysis. This equipment is invaluable for quality control, fault analysis and reverse engineering.
One very obvious and much used application for the TGA and FTIR is that of reverse engineering. Both types of analysis when combined provide very detailed information about the back bone of the material used to manufacture products and is only limited to the thermal ceiling of 1000°c.
In conjunction with the FTIR chamber Harboro installed a Reflective Infrared Surface analyser and this provides a very quick and useful test to determine further information on surface contamination and material composition.
Hardness Analysis on Small Components & O Rings
Further addition to Harboro’s well equipped laboratory to improve control and analysis is the inclusion of Bariess Digital hardness testing. This equipment removes the manual element from hardness testing and therefore result variability is reduced. Its multi head design offers great versatility and is used for goods inwards control of compounds, development and for control of globally procured products.
PPM targets are ever decreasing and constantly have to be revisited. Manufacturing has to respond in the most appropriate manner. The PPM figure in most cases was originally deemed as a target but as most manufacturers know, targets quickly turn into measures and cost.
Understanding a customer’s real needs from the onset of any project is paramount in planning to achieve the lowest possible PPM figures. Continuous improvement and total quality management are deemed as processes to achieve the illusive actual zero defects. Many companies have spent a lot of resources working to reduce the level of defects from a process only to find that the number of variables which they have to contend with within the commercial boundaries mean that zero defects cannot be achieved solely through process improvement. Manual inspection is costly and provides unpredictable and low confidence results (100ppm is the best achievable on non-critical parts where faults are readily recognisable). The cost and unreliability has driven a different approach at Harboro.
Harboro still carry out manual inspection where applicable but, where products or agreements dictate, we apply high technology automated inspection systems. Harboro has invested in three automated systems which can process a broad range of products and are planning for further additions in the near future.
This does not mean that zero defects can be guaranteed but extremely low PPM figures can and will be achieved where required, thus reducing costly reject management exercises at our customers. The cost of initial set up can prevent it initially seeming the lowest cost option but over time it will provide the lowest total acquired cost and a concern free product life.
The Harboro Rubber Company Automatic Inspection Department
3 camera inspection system. This equipment is used for the inspection of high specification thermostat seals at thousands of parts per hour.
O Ring & Circular Product Checking System
This equipment carries out instant non-contact dimensional checking of circular products and provides full statistical analysis.
Semi automated Modular System
This non-contact measuring device can be programmed to accommodate irregular shaped products for high speed single face inspection.