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Computer assisted material selection
Computer assisted material selection
The plastics supply industry is commercially aggressive, highly competitive & provides the potential customer with a vast array of material choices & quite often contradicting advice. There are more plastics available now than at any previous stage in the history of the polymer industry with over 90 generic plastics, around 1,000 sub-generic modifications with 50,000 commercial grades from over 500 manufacturers. The diversity of plastics & the complexities of accurate material selection has become one of, if not the major reason for premature product failure.
The massive effort to cater for customer requirements has been the key factor in the continuing & impressive growth of plastics both in terms of consumption & product development & diversification. A product designer has the challenge of selecting the material that is “tailor-made” for their particular application.
In order to perform plastic material selection successfully a complete understanding of plastic material characteristics, specific material limitations & failure modes is required. Accurate material selection requires a judicious approach & careful consideration of application requirements in terms of mechanical, thermal, environmental, chemical, electrical & optical properties. Production factors such as feasible & efficient method of manufacture in relation to part size & geometry need to be assessed. In terms of economics the material cost, cycle times & part price need to be taken into consideration.
A practical approach to material selection is to select in a series of steps via material class, generic type & generic modification & to leave the selection of proprietary grade to the material supplier. This approach requires the minimum of effort yet relies upon the unbiased contribution from a supplier who is driven by sales targets & market share.
Computerised Material Selection
One of the selection tools available to the plastic scientists based within the consultancy centre at Smithers Rapra is an extensive plastics database. The data has been compiled over many years to provide comparative information in a format designed to facilitate the selection of plastics for given applications, taking into consideration processability & material performance. The database uses information based upon value judgements (rankings) which have been assigned to 62 of the principle qualities of 351 generic & modified plastic material types. These materials encompass thermoplastics, thermosets & thermoplastic elastomers, & are representative of 30,000 or more commercially available trade name & branded materials.
The data is an accurate, time saving resource allowing a Smithers Rapra plastic consultant to generate short-lists of potentially suitable candidate materials. These are then subject to further stringent evaluation in order to refine the list further before collaboration with suppliers to make the final material choice.
Smithers Rapra has provided independent plastics & rubber material selection services for over 30 years, successfully identifying the right materials for clients.
Data Search Methodology
The selection of the right plastics material for a particular application must take into account a number of factors, including:
- The operating environment
- The primary production method
- Assembly techniques
- Material & processing costs
- Aesthetic & decorative features
Generally each of these factors should be considered simultaneously to obtain a material that satisfies each of the requirements. In certain cases a material is used which has exceptional properties in just one or two areas. An example would be PTFE, the difficulties in processing, low strength & stiffness are accepted or designed around to utilise & benefit from its exceptional chemical resistance.
In a materials selection process there are likely to be a number of material requirements that are essential & a number that are considered desirable; it may be essential that a container is able to perform up to a temperature of 80°C for several years & be resistant to concentrated acids. In addition it might be desirable that it should be resistant to impact loads & exhibit fire retardant properties. Once the materials that meet the required acid & temperature resistance are selected, the list then requires optimising to obtain the best balance of toughness & heat resistance. The final list may contain a number of tough but flammable materials or it may contain heat resistant materials with slight impact weaknesses. The designer has to decide which of these two desirable factors is the more important & make a final decision.
A materials selection procedure must accommodate these two steps of selection; one to identify those materials that contain the essential qualities meeting the required specifications, the second to order or rank this short list against certain other desirable preferences to obtain a final working list of candidate materials for detailed scrutiny.
The first approach could be termed a series approach as demands are made to generate successive short lists & is the most efficient method of identifying suitable materials. The second approach which accounts for preferences could be termed a parallel approach as each of the materials in the original list is considered simultaneously against an input specification.
The Smithers Rapra database enables materials to be selected according to individual specifications. It does not provide a rigid routine like a 'decision tree' but gives the user complete freedom to define their requirements. The selection will only be as good as the interpretation & definition of the service conditions, the manufacturing methods & other product criteria.
Two search routines are implemented. The first is termed "“Search on Single Property" which enables those polymers that satisfy a particularly criterion to be identified. The second search routine termed 'Search on Combined Weightings' allows several requirements to be entered simultaneously to optimise or order the list of satisfactory materials.
This procedure results in a ranked list of materials which the consultant can then examine in detail. In almost every materials selection procedure the combination of the series search ‘Search on a Single Property' & the parallel search 'Search on Combined Weightings' is the most forthcoming in providing the consultant with information to make a recommendation.
A materials selection choice is made on the basis of selecting a quality that is required in the material, such as resistance to dilute acid & specifying the level of resistance required. Materials listed in the database have been assigned a value judgement in the range of 0 to 9 based on the representation of the specific quality required in that material. For example, the fatigue resistance of polypropylene copolymer is excellent, hence a value judgement of 9 has been awarded; the resistance of polystyrene to impact is poor so it has a value of 1. If a quality is not represented in a material it is assigned a value judgement of 0. Examples would be, phenolic which has a rage of 0 for transparency & PTFE which has a rating of 0 for blow mouldability.
Where possible the value judgements have been assigned on a decile basis so that approximately 10 percent of the total number of materials has a value judgement of 9. It is therefore possible to identify the top 30 percent of materials matching a particular quality.
For certain attributes in addition to assigning a value judgement, a specific property value has been filed. This covers properties such as the "maximum continuous operating temperature" or "dielectric constant"
The searchable qualities included in the system are grouped into five selection sub-menus covering general, physical, electrical & thermal properties; mechanical properties; chemical properties; processing properties; & post-production properties.
General & electrical properties
| * | Maximum operating temperature | Flame spread | |
| * | Heat distortion temperature | * | Oxygen index |
| * | Expansion coefficient | Ease of flow | |
| * | Dielectric coefficient | * | Shrinkage |
| * | Dissipation factor (50 Hz) | Warpage | |
| Dissipation factor (1 MHz) | Surface finish | ||
| * | Dielectric constant | Transparency | |
| Arc resistance | Volume/unit cost | ||
| Tracking resistance | Flexural/unit cost |
Mechanical Properties
| * | Tensile strength | Fatigue index | |
| * | Toughness (20°C) | Surface hardness | |
| Toughness (-40°C) | Wear | ||
| Brittle temperature | Friction | ||
| * | Flexural modulus | Dimensional stability |
Chemical & radiation resistance
| Hydrolytic stability | Halogenated hydrocarbons | ||
| Detergent | Alcohols | ||
| Dilute acid | Phenol | ||
| Concentrated acid | Ketones | ||
| Dilute oxidising acid | Esters | ||
| Concentrated oxidising acid | UV radiation (weathering) | ||
| Aliphatic hydrocarbons | Gamma radiation | ||
| Aromatic hydrocarbons |
Production methods
| Injection moulding | Pultrusion | ||
| Compression moulding | RIM | ||
| Transfer moulding | Structural foam moulding | ||
| Blow moulding | Casting | ||
| Rotational moulding | Resin injection | ||
| Vacuum forming | Cold press moulding | ||
| Extrusion | Contact moulding |
Post-processing
| Bonding | Plating | ||
| Welding (med. freq.) | Machining | ||
| Welding (ultrasonic) | Panting |
*properties also have absolute values included.
The Smithers Rapra database contains data sheets & textural information of all the materials represented.
Material selection Case study - Selection of a Clock Gear Mechanism
The effectiveness of the collaborated materials data can be illustrated by considering the choice of polymer for manufacturing a gear wheel in a clock mechanism.
The critical requirements are injection mouldability, reasonably low shrinkage, & good dimensional stability. The material must also be cheap & have low tendency to warpage. Wear resistance is not particularly important as only low loads are envisaged. No lubrication is required so there are no chemical resistance considerations.
The search begins with the series search routine. Three successive elimination searches are made for injection mouldability, shrinkage & dimensional stability. The top 50 percent of materials with respect to mouldability & then shrinkage are selected & then the top 30 percent in terms of dimensional stability. This yields a short list of 26 materials. The search output is detailed below.
Output from the series searches routines
Single pass search on injection moulding
Conducted on new (thermoplastics only) materials list of 258 materials
Minimum value: 5
Maximum value: 9
103 materials identified
Single pass search on shrinkage
Conducted on current (thermoplastics only) short list of 103 materials
Minimum value: 5
Maximum value: 9
51 materials identified
Single pass search on dimensional stability
Conducted on current (thermoplastics only) short list of 51 materials
Minimum value: 7
Maximum value: 9
26 materials identified
1 ABS (medium impact)
2 ABS (high impact)
3 ABS (high impact; UV stabilised)
4 ABS (high heat)
5 ABS (high heat; UV stabilised)
6 ABS (fire retardant)
7 ABS (low gloss)
8 ABS (plating)
9 ABS (30% glass fibre reinforced)
21 Acrylic (general purpose)
22 Acrylic (high impact)
26 Cellulose acetate butyrate
37 PFA (20% glass fibre reinforced)
115 PPO
116 PPO (fire-retardant)
117 PPO (30% glass fibre reinforced)
118 PPO (structural foam)
149 Polystyrene
151 Polystyrene (30% glass fibre reinforced)
152 Polystyrene (2% silicone lubricated)
154 Polystyrene (medium impact)
155 HIPS
163 SAN (30% glass fibre reinforced)
165 SMA (copolymer)
185 PPS (glass fibre & bead reinforced)
187 PPE (20% PTFE lubricated)
At this stage materials have been identified that satisfy the essential criteria & these have been presented in the computer file record order with no discrimination between them.
It is now useful to optimise the list of candidate materials against certain desirable features. These are good ease of flow, low tendency to warp, reasonable wear resistance & low cost. Weighting factors are applied according to the relative importance of these qualities. The output is shown below.
Search Output from the Optimisation Routine
Combined weightings search on current shortlist of 26 (Thermoplastics only) Qualities & weightings ( ).
- Volume/unit cost, (9)
- Ease of flow (7)
- Warpage (7)
- Wear (7)
26 materials selected for current short list.
| Materials | Rating | Quality | |||||
|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | ||||
| 152 | Polystyrene (2% silicone lubricated) | 226 | 8 | 8 | 7 | 7 | |
| 149 | Polystyrene | 207 | 9 | 8 | 9 | 1 | |
| 154 | Polystyrene (medium impact) | 200 | 9 | 8 | 8 | 1 | |
| 1 | ABS (medium impact) | 198 | 8 | 8 | 8 | 2 | |
| 22 | Acrylic (high impact) | 196 | 7 | 5 | 9 | 5 | |
| 21 | Acrylic (general purpose) | 189 | 7 | 5 | 8 | 5 | |
| 4 | ABS (high heat) | 189 | 7 | 8 | 8 | 2 | |
| 5 | ABS (high heat, UV stabilised) | 189 | 7 | 8 | 8 | 2 | |
| 3 | ABS (high impact; UV stabilised) | 189 | 7 | 8 | 8 | 2 | |
| 2 | ABS (high impact) | 189 | 7 | 8 | 8 | 2 | |
This shortlist shows the relative strengths & weaknesses of the materials against the input optimisation specification. The rating factor permits broad comparison between the various materials & is the sum of the products of the weighting factor & value judgements. Examination of the value judgements shows the balance of qualities exhibited by each material.
Examination of the texts & datasheets given below for the top two generic materials confirms that the selection is reasonable. In view of the price difference initially it is decided to evaluate polystyrene.
Generic Texts for Polystyrene & ABS - Material: 152 – Polystyrene (2% silicone lubricated)
Generic group: PS (Polystyrene)
Advantages
Cheap, rigid, transparent, easy to mould with good dimensional stability. Good electrical properties, low dielectric loss. Excellent resistance to gamma radiation.
Disadvantages
Brittle, poor chemical resistance especially to organics. Susceptible to UV degradation. Flammable.
Applications
Toys, light diffusers, beakers, cutlery, general household appliances. Electronic housings, refrigerator liners. Structural foam PS mouldings used for business machine housings, tools, cases & boxes. Expanded PS beads used for packaging & cushioning. Foams for food trays, dishes, egg boxes.
Material: 1 – ABS (medium impact)
Generic group: ABS (Acrylonitrile Butadiene Styrene)
Advantages
Hard for a thermoplastic. Reasonably tough (maintains impact resistance to low temperatures). Easily processed (may be electro-plated), easily bonded. Good glass, surface scuff resistant. Low shrinkage & warpage.
Disadvantages
Poor solvent & fatigue resistance. Poor UV resistance, unless protected. Maximum continuous use temperature 70°C. Poor bearing properties (high friction & wear). High smoke evolution.
Applications
Cabinets & cases, particularly for domestic & industrial instruments e.g. TV cabinets, food mixers, telephones, vacuum cleaners. Vacuum forming for baths, shower trays, etc. Extruded into pipe. Used in preference to PVC for high (50 – 70°C) or low (< -20°C) temperatures. Mouldings may be electro-plated for bathroom or automotive applications.
The Smithers Rapra consultancy centre is available to assist product designers & developers with accurate material selection. Further information on Smithers Rapra's material selection services can be obtained by emailing info@rapra.net or by calling +44 (0) 1939 250383