Designing with Plastics

Firstly, designing with plastics can be addressed under four inter-related headings:

Function:	Will it do the intended job?
	For how long?
Form:	Does it have the shape to:
	Do the function economically?
	Will it please the user?
	Does it suit the manufacturing process?
Process:	Can it be made economically?
	How many units are required?
	Does the process suit the selected material?
Material:	Does the material suit the function, appearance, form, life, process, cost and also availability?

Design Process

Designing with plastics is similar to designing with other materials but more attention has to be paid to the time-dependant properties of plastics and how plastics behave with changes in temperature and environmental conditions. Likewise the properties of plastics materials in end products are a function of changes brought about by the manufacturing process.
The designing with plastics sequence of design development stages will normally be:

• Define function and form of the component
• Establish end-use requirements
• Preliminary design
• Material selection
• Process selection
• Modify design
• CAD/CAE analysis
• Prototype
• End-use testing

Designing with plastics will involve several iterative steps in which the design is continually being refined as constraints and also opportunities appear. As the design project evolves the ease of making design changes reduces while the cost of making changes increases.

Individual Tasks in the designing with plastics design process

1. Establish end-use requirements

1. Structural: applied loads, rate of loading, duration, impact, cyclic, possible misuse
2. Temperature extremes
3. Assembly and secondary operations (including storage and transport)
4. Cost limitations and also
5. Compliance with regulations and standards

2. Material selection: check list

1. Physical properties: density
2. Mechanical properties: elastic limits, strength, modulus v temperature, creep modulus, impact strength, fatigue limits, hardness, friction, wear
3. Thermal properties: coeff of thermal expansion, thermal conductivity, heat distortion temp, effect of temp on mech properties.
4. Electrical properties: resistance, permittivity, electrical strength and also antistatic
5. Optical properties: transparency, colour, gloss
6. Environmental properties: chemical resistance, water resistance, effect of uv, thermal stability, biodegradability and also fire performance.
7. Processability: mould shrinkage and finally
8. Cost: cost/unit weight, cost/unit volume, cost/property

3. Modify Design:

1. To suit the property balance of the material selected
2. To suit processing limitations (eg wall thickness)
3. For assembly (snap fits, push fits)
4. Cost optimisation (material cost, process cost and also assembly cost)

4. Computer Aided Design (CAD) and Computer Aided Engineering (CAE)

1. Stress analysis
2. Moulding simulation: flow analysis, cooling analysis, shrinkage and warpage

5. Designing for stiffness

1. Component stiffness is a function of part geometry and material modulus.
2. Establish mode of loading (tensile, compression, flex, torsion) and restraints.
3. Simplify geometry to beams and plates.
4. Apply strength of materials equations to give a first approximation of response; refine using more relevant data (eg creep modulus)
5. Apply an appropriate safety factor.
6. Increase part stiffness by increasing section thickness, ribs, corrugation etc
7. Optimise stiffness for: geometric considerations, stiffness to weight ratio, stiffness to cost ratio.

6. Designing for strength

1. Strength can be defined as ultimate strength (rupture), yield strength, strain to failure, toughness.
2. Strength of plastics is a function of: temperature, chemical environment (including moisture and process conditions).

7. Designing for precision

1. Mould shrinkage for amorphous thermoplastics is lower than mould shrinkage
2. for semi-crystalline thermoplastics.
3. Filled thermoplastics have lower mould shrinkages than unfilled thermoplastics
4. Mould shrinkage is a function of: melt temperature, mould temperature, packing pressure, cooling, orientation; part wall thickness and geometry; gate location, type and also size

8. Design for mouldability

1. Mould filling is controlled by melt viscosity, flow-path:thickness ratio
2. Cooling time (main contribution to cycle time) is proportional to [wall thickness]2
3. Warping is a function of part geometry, packing and also cooling
4. Design considerations: nominal wall thickness, parting line, undercuts, radii, ribs, bosses, coring, draft angles, texture, flow leaders, weldlines
5. Process considerations: cavity filling, gating, venting and also ejection

9. Design tips:

1. Avoid stress concentration factors
2. Avoid uncontrollable loads
3. Design for compressive stress rather than tensile stress
4. For tension, design for uniform cross-sectional area
5. For flex, design for moment of inertia
6. Be aware of processing issues (weld lines, moulded-in stress, anisotropy)
7. Use appropriate safety factors
8. Consider worst-case scenarios

10. Design for Automation

Labour costs in assembly of components can be a significant component of manufacturing costs. Robots and automated techniques for component handling and assembly can significantly reduce costs.

11. Design for Recyclability

To simplify end-of-life recycling, plastics should be preferably consist of only one polymer type, although individual components may have different additives, to achieve the required design properties.

Non-plastic parts should be avoided. Ease of disassembly can hold the key to reducing the cost of recycling.

Mechanical assemblies (snap fits, press fits) are easy to disassemble. Welded components are acceptable for similar or compatible thermoplastics but not for dissimilar plastics.

The same applies to adhesive bonding, provided the adhesive is compatible. Assemblies involving metal screws should be designed to make disassembly easy.

Beginner’s Guide to Designing with plastics for Moulding

Once the geometry of a component has been determined by function, form and appearance, a number of additional factors have to be considered to ensure that the design is in sympathy with the moulding process and to ensure that tooling and processing costs are minimised.

Parting Line

The position of the parting line is chosen for ease of moulding, tool design and part appearance.

Ejection

To assist ejection of the moulding, all surfaces normal to the parting line should be given a taper or draft angle of 0.5o to 1.5o . Ejector elements are selected on the basis of wall section and possible distortion. Components with small undercuts may be sprung from the mould if the material is flexible enough. Components moulded in rigid materials and components with larger undercuts may require form pins, side cores or split moulds — all adding to the tool costs.

Section Thickness

Wall section thicknesses may have been determined by form, function, stiffness or strength but should be kept as thin as possible to minimise cycle times (cooling stage), to avoid sink marks, voids and internal stresses and to minimise shrinkage and warpage. Thick sections should be cored out , leaving ribs to satisfy part rigidity. Rapid changes of section should be avoided to minimise sinking.

Limits on wall thickness depend on the material.

The ability to fill a mould cavity is related to the flow-path:thickness ratio. For many thermoplastics, ratios of over 200:1 can introduce problems for the moulder.

Corners

Sharp corners should be avoided on two counts. The extra material at corners results in more sinking. Sharp corners act as stress concentration points and can severely reduce impact strength. Recommended radii for corners are 0.5 t for inside radius and 1.5 t for external radius (where t is the wall thickness)

Stiffening

The stiffness of moulded components can be increased by increasing the wall thickness (stiffness ∞ t3) but this leads to longer cycle times (cooling time ∞ t2). Use of doming, corrugation and ribs results in reducing weight and reducing cycle times.

Thick heavy section ribs create sink marks, voids and internal stresses. As a guide rib thicknesses should be 0.4 – 0.6 t (where t is the main section thickness), height up to 3 t, spacing at least 2 t and taper at least 0.5o. There are similar recommendations for boss design.

Shrinkage

All injection moulded thermoplastics exhibit mould shrinkage (part dimensions relative to the mould dimensions) as a result of differential thermal contraction as the melt cools down in the mould. Shrinkage can be minimised by using high packing pressures and long pressure hold times.

Because of orientation effects the shrinkage is not isotropic and, for unfilled thermoplastics, the shrinkage will be greater in the flow direction. Semi-crystalline thermoplastics show greater mould shrinkage than amorphous thermoplastics and filled thermoplastics have reduced shrinkage.

Warping in a flat panel, as a result of differential shrinkage, can be minimised by controlling the cooling stage or by localising the shrinkage using multi-point gating.

In box shapes, warping leads to concave rims. To counter this, boxes can be designed with sides bowing slightly outwards.

Appearance

Gate (witness) marks, weld lines and parting lines should be hidden as much as possible in cosmetic parts and their location controlled in critical functional parts.

Flat surfaces should be slightly domed to give more acceptable reflections or deliberately textured. Texturing and design features can also disguise sink marks. Ejector pin marks can be disguised with trade marks or logos.

Tolerances

Shrinkage is determined by a number of process variables (melt temperature, mould temperature, injection pressure, packing pressure etc) and there will be variations in shrinkage during a production run.

Tighter control of process variables can give tighter tolerances but the moulding cost will increase accordingly. Tight tolerances should be stipulated only on critical dimensions and not blanket tolerances. It is difficult to hold tight tolerances on dimensions across the split line of the tool.

Assembly

Assembly of moulded components can be either reversible or irreversible. Reversible assembly methods include moulded-in screw forms, self tapping screws, interference fits (push fits) and snap fits. Irreversible methods include chemical jointing (solvent or adhesive) and various methods of heat welding (hot-plate, friction, ultrasonic and laser welding). This distinction may be critical for disassembly and recycling.

Product Design Summary

For effective components produced by injection moulding, the designer needs to be aware of a range of factors:

optimise gate position; minimise runner length; assist ejection; avoid undercuts.

Material Selection	Suitability of material and grade for: mechanical properties; environmental properties (resistance to heat, light, chemicals, fire) colour; processability.
Orientation in mould	To: minimise projected area (and hence clamping force);
Parting line	for ease of ejection; minimise visible flash mark.
Gate position	minimise witness mark; uniform filling; flow marks; weldlines.
Taper	All surfaces at 90o to parting line require a taper (draft angle) to assist ejection. Typically 0.5 – 1.5o.
Section thickness	Minimum wall thickness to reduce cycle time and material usage; increasing wall thickness increases stiffness but not necessarily strength. Stiffness ∞ t3; cycle time ∞ t2. Uniform wall thickness avoids stress and sink marks. Core away unwanted material.
Sink marks	Localised indentations associated with differential shrinkage in thick sections, eg at rib intersections
Radii	Sharp corners act a stress concentration points; generous radii on all corners and at roots of ribs and bosses.
Ribs	Stiffening ribs allow thinner sections (less material and shorter cycle times); ribs 60% thickness of main wall.
Holes	Easiest if in line of draw; at 90o to draw requires side cores or split moulds (more expensive mould).
Surface Texture	Gloss finish or textured surface.
Secondary operations	Degating; painting; assembly (push-, snap-fit; adhesives, ultrasonic welding).

Common mistakes made by product designers are lack of radius on corners, over-specification of section thicknesses (inducing internal stresses and increasing cycle times) and inappropriate materials selection (usually resulting from under-specification of product service conditions).

Injection moulding is used to produce mouldings of varying complexity from as small as 1 mg (micromoulding) up to and over 6 kg for a range of market sectors.

Beginner’s Guide to Designing with plastics Mould Design

In its simplest form, a mould for injection moulding consists of two plates, a negative (cavity) plate and a positive (core) plate. The space between the two plates defines the shape and detailed geometry of the mould. In reality the mould is much more complex and will include some of the following features:

Mould alignment	pillars and bushes to ensure perfect alignment of the two halves in the closed position
Feed systems	A series of channels and sub-channels to convey the melt from the nozzle in the plasticisation unit to the mould cavities. Feed systems may be divided into sprues, runners and gates
Ejection systems	Devices to eject the mouldings from the cavities during the mould opening phase
Cooling systems	Channels in the mould plates, carrying heat exchange fluids (usually water), to extract heat from the melt in the cavities

To simplify the construction of the mould, the mould plates are usually assembled from a number of individually produced components. In complex moulds there may be many moving parts such as split moulds and side cores for moulding components with re-entry geometry (undercuts) and pins and plates in the ejector system.

Originally all mould part movements, ejector elements, angled cams for side splits and rotating cores for ejection of screw form mouldings, were linked to the opening and closing stroke of the mould platens but today many of the movements are independent of the opening stroke, by using servo motors and small hydraulic actuators.

Because of the pressures involved and the high wear from millions of operating cycles, moulds are usually constructed from hardened tool steel but it is possible to use metals that are more easily machined, eg softer steel and aluminium alloy, provided the moulds are well maintained.

The best moulds will have carefully designed feed systems and cooling systems, capable of producing consistent high quality mouldings with the shortest possible cycle time.

Feed System

The function of the feed system in a mould is to transport the polymer melt from the nozzle of the injection moulding machine to the individual impressions (cavities) in the mould with:

minimal cooling;
minimal loss of pressure;
and a minimal disruption to the laminar flow.

In cold feed mould systems, the melt is carried through the sprue channel to the parting line and through runner channels in the parting face to individual cavities.

The final part of the feed system is the entry to the cavity, the gate. The sprue is tapered for ease of removal and may incorporate sprue puller designs to ensure the feed system stays in the moving half of the mould.

The length of the runner channel is determined by the lay-out of the cavities but should be as short as possible to minimise pressure drop.

The cross-section (full-round or trapezoidal) is traditionally generous in dimension (3 -6 mm) but small cross-sections can take advantage of shear heating of the melt to give better cavity filling conditions. Runner lay-out should be symmetrical to ensure each cavity fills identically and at the same instant.

The gate is the smallest cross-section in the feed system and the first part to “freeze” during the cooling cycle. Once the gate is frozen it is impossible to transfer any more melt into the cavity or apply packing pressure to compensate for mould shrinkage. There are many different designs for the gate, the choice being dictated by a number of factors.

• Injection time (large gates fill more readily)
• Packing time (large gates give more effective packing)
• Witness marks (smallest gates)
• Degating (large gates difficult to degate; some give automatic degating)

Gate position determines the filling pattern, flow marks, jetting, burn marks and weld lines.

In hot-runner systems the melt temperature is maintained at all times in the feed distribution system, thereby making cavity filling easier, giving better quality mouldings and saving the cost of recycling the sprues and runners.

In hot runner moulds the feed system is maintained at an elevated temperature so that there is minimum cooling of the melt before it enters the actual mould cavity.

Although this makes the mould more complex and more expensive, there are significant savings in material waste, reduction in cycle times and improvements in the quality of the moulding.

Ejection

After the cooling stage and mould opening, the mouldings can be difficult to remove because of mould shrinkage onto the core and the creation of a vacuum between the moulding and the mould. Ejection is accomplished mechanically using a number of ejector elements to ensure minimum distortion of the warm moulding. The ejector elements (pins, sleeves, blades, valves, stripper plates) may be actuated by the mould opening stroke or actuated independently using hydraulics, pneumatics or servo-motors. Robotic devices can be used to remove the moulding in a controlled manner.

Cooling

To shorten cycle times and to control cooling, the design of heat transfer systems in a mould is critical. The heat transfer fluid (water or oil) is circulated in channels close to the cavity surfaces in a number of separate circuits. Cooling systems are at their most effective when the inlet and outlet temperatures differ by only 2 -3oC.

Complex Moulds

Intricately shaped mouldings require complex moulds with more than just two plates. To allow ejection of components with undercuts, parts of the mould have to open at 90o to the main mould opening direction.

The additional “splits” can be actuated through the main mould opening action using finger cams, dog-leg cams (for delayed opening) or springs. Hydraulic actuation gives more control. Local undercuts can be accommodated more simply using side cores and form pins.

Designing with plastics – Plastics Material Selection

Material selection for plastics products can be split into two levels:

1. selection of the appropriate family of material (GENERIC)
2. selection of the appropriate grade within a family (GRADE SPECIFIC)

In the past, designers have selected materials on the basis of previous experience for products of similar size and similar function. Any attempt at a formal selection procedure was heavily subjective and constrained by the knowledge and experience of the designer.

Objective materials selection requires, as a starting point, a clear profile of the design requirements in the form of a check list of important properties and their relative importance in the design.

Searching through brochures from materials suppliers and data tables to rank materials in each individual property would give sufficient data on which to base an objective selection. However, the process is time consuming and suffers from limited available data.

The emergence of computerised materials property data, preferably in the same units and generated by the same test method, has made the task of selection easier.

Some databases even have a selection algorithm built in, usually in the form of a ranking procedure, a system of eliminating materials property by property or by creating a league table by weighting several properties at once.

CAMPUS is a system in which individual suppliers provide data for their grades in standard format (units and test methods) either in CD format or down loaded from the Internet.

The data refer to specific, identifiable grades from participating raw materials manufacturers. It is now possible to search CAMPUS data from several suppliers at the same time using MCBase software. Unfortunately many grades have incomplete data.

www.campusplastics.com

CES Polymer Selector, from Granta Designs, combines the generic approach and the grade-specific features of CAMPUS in one system, with graphical representation of properties in ranges rather than points and no missing data

www.grantadesign.com

MATWEB is a web based system covering metals, ceramics and other engineering materials as well as plastics. Searches can be carried out on-line.

www.matweb.com

IDES Prospector is another web-based system

Searchable properties include physical, mechanical, thermal, optical and electrical properties as well as chemical resistance, process rheology, processing methods and even post-processing techniques.

Designing with plastics – Material Selection Check list

Material selection is best carried out in parallel with the design development, in contrast to the common procedure of completing the design and then looking for an appropriate material.

Developing material selection in parallel allows the design to adapt to the material capabilities and constraints.

Data on the design criteria and service conditions of the component can be entered into a check list, indicating properties which are not relevant, and then matched to possible materials using data sheets and material selection software.

The check list ensures that no property, manufacture detail or service criteria is overlooked.

1. Service Conditions

Firstly, establish:
– maximum and minimum temperatures to which the component will be exposed;
– loads (high or low mechanical loads);
– time scales.

2. Mechanical Properties

3. Thermal Properties

4. Environmental Properties

5. Aesthetic Factors

6. Electrical Properties

7. Processing

8. Post Processing operations

Material Selection: Check List

1. Service Conditions
	Max temp
	Min temp
	Loads
	Time scale
2. Mechnical
	static loading:
		load
	permissible deflection
	stiffness
	strength
	elongation at break
	creep:
		load
	time
	deflection
	dynamic loading
	impact :
		load
	temp
	speed
	friction:
	wear:
3. Thermal
	heat distortion temp: (stress)
	brittle temp
	continuous use temp
	coeff thermal expansion
	conductivity
4. Environmental
	chemical resistance
		water
	detergent
	hydrocarbons
	chlorinated
	ethers
	esters
	acids/alkalis
	inks
	adhesives
	oxidation
	uv
	fire performance
5. Aesthetics
	transparency
	colour
	surface finish
	tactility
	density
6. Electrical
	volume resistivity
	surface resistivity
	conductivity
	static dissipation
7. Processing
	drying
	process conditions
		melt temp
	mould temp
	ejection temp
	cycle time
	mould parameters
		nominal thickness
	max thickness
	flow path length
	cold/hot runner
	gate type
	ejection
	rheology
		MFR (temp, load)
	Viscosity at 1000 s-1 (temp)
	shrinkage
8. Post-Processing
	assembly
		insert
	snap
	push
	ultrasonic
	adhesive
	printing
	labelling

Generic Selection

Having established the important properties for the component, the first step is to identify the appropriate plastics family. A basic designing with plastics approach is to compare properties for a range of plastics families, using comparison tables similar to the ones shown below.

In these tables, the property values are typical or average values for unreinforced grades. In practice, the values for different grades of the same family will be spread around the average values and this spread will have to be checked at a later stage.

Codes for 1-5

LDPE	low density polyethylene
HDPE	high density polyethylene
PP	polypropylene
GPPS	general purpose polystyrene
TPS	toughened polystyrene
ABS	acrylonitrile-butadiene-styrene
PVC-u	unplasticised PVC
PVC-p	plasticised PVC
PMMA	polymethylmethacrylate (acrylic)
CA	cellulose acetate
PC	polycarbonate
PA 6.6	polyamide 6.6 (nylon 6.6)
PET	polyethylene terephthalate
PBT	polybutylene terephthalate
POM	polyoxymethylene (acetal_
PPO	podified polyphenylene oxide

Table 1. Modulus (GPa) (stiffness)

Table 2. Tensile Strength (MPa)

Table 3. Elongation at Break (%)

Table 4. Impact Strength (J/cm) [Izod notched, 23oC]

Table 5. Heat Deflection Temperature under Load