Toptenmoldingtips
ENGINEERING POLYMERS:
THE ‘TOP TEN’ MOULDING PROBLEMS
By R. Wilkinson, E. A. Poppe, Karl Leidig, Karl Schirmer
The ten problems listed below occur often when moulding semi-crystalline engineering polymers such as POM, PA, PBT and PET. In this series of articles the authors describe simple ways to identify and avoid them. Chapter 1. Moisture in the granules
1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners
9. Warpage
10. Mould deposit
Many plastics absorb moisture from the atmosphere; how much they absorb depends on the type of resin. Moisture in the granules, even if it is only surface condensation, can cause problems in parts moulded with engineering polymers. Many kinds of undesirable effects can occur, including processing problems, poor surface on moulded parts, or loss of mechanical properties. It is seldom possible to establish whether there is moisture present by means of visual examination alone. The authors prepared this article to give moulders who process a broad range of plastics some useful guidance on how to handle those polymers that are sensitive
to moisture.
Drying Plastic Materials
Most engineering polymers require the moisture in the granules to be below a certain maximum level for
processing. The need for drying depends mainly on how sensitive the raw material is to water. Naturally, the moisture content of the material as delivered, the type of packaging and the period of storage are also
important criteria. For example, polyamide is generally packed in bags with a barrier layer of aluminium, so that it can be used straight out of the bag. However, most processors of PA prefer to dry the resin in any case,even though drying is not necessary if the material is used within one hour.
PET and PBT, on the other hand, are far more critical where moisture is concerned and must always be dried to ensure that impact strength of the moulded parts is not affected. Another factor is that these resins pick up moisture very rapidly after drying, so that moulders should exercise special care when handling open
containers of PET and PBT, when they are in transport or conveyor systems, as well as regarding their dwell time in the hopper. Thus, in unfavourable climatic circumstances PET can absorb enough moisture in 10
minutes to exceed the maximum permitted moisture content for moulding of 0,02 per cent. Drying regrind and fully saturated granules (e.g. in the case of containers which were left standing around open) requires special care. In these cases the recommended drying times are usually not enough. Fully saturated polyamide may need more than 12 hours to dry. The yellowing associated with such treatment is practically unavoidable. The following guidelines should therefore be followed:? Always store sprues and regrind in closed containers.? Close containers or bags that have been partially used.? Keep a lid on the hopper.
How to Dry
It is important to follow correct drying procedures if you want good quality mouldings. Simple hot air driers of various types are not adequate for drying polyesters, for example, but dehumidified-air drier systems are
acceptable. Only these can provide the necessary constant and adequate drying, whatever the ambient climatic conditions may be. Apart from keeping the correct drying temperature, it is important to ensure that the dew point of the drying air remains lower than ≤–20°C. When operating multiple-container installations with
different filling heights and bulk density, it is also important to ensure that the air throughput in each container is sufficient.
Measuring Moisture Content
Moisture in the granules can be measured with commercially available measuring instruments, e.g. with the manometric or the Karl-Fischer method. To eliminate sources of error, the sample should be taken from well down in the hopper, and should be sealed in an appropriate container. Special heat-sealable sachets coated with PE and aluminium are suitable, as well as laboratory-type glass containers that can be hermetically sealed.
Sampling location
Source: DuPont Source: DuPont
1.5
Source: Brabender Messtechnik
Source: Metrohm
Recommended sampling location Container for sample of granules
Moisture measuring instruments Manometric method Karl-Fischer method
ENGINEERING POLYMERS:
THE ‘TOP TEN’ MOULDING PROBLEMS
By R. Wilkinson, E. A. Poppe, Karl Leidig, Karl Schirmer
Chapter 2. Feed system too small
1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners 9. Warpage
10. Mould deposit
a: structure with
b: structure with s t r o n g l y s h e a r e d r e g i o n
s o u n d s t r u c t u r e
Recommended diameters
Thin-walled part, large volume, high viscosity Thick-walled part, small volume, low viscosity
T + 2 T + 3 T + 4
T + 1
T + 1,5
T + 2
≥ C
> A
Recommended
cross-sections
10 B
A C
D1
2
D
Design of the feed system
In designing the feed system, the first point to be considered is the wall thickness (t) of the moulded part (see diagram). Nowhere should the diameter of the runner be less than the wall thickness of the injection moulding. Starting out from the gate, the runner diameter at each branch point can be widened so that an almost constant shear rate is maintained.
To prevent the inevitable cold slug reaching the moulding from the injection nozzle, the gate should always be extended so that the cold slug can be intercepted. This extension should have roughly the same diameter as the gate to ensure that the cold slug really is retained.
When moulding partially crystalline, unreinforced polymers, the minimum gate thickness should be 50 per cent of the wall thickness of the moulded part. This would also be adequate for reinforced compounds. To minimise the risk of damage to the fibres and also bearing in mind the higher viscosity of these compounds, the gate thickness should be up to 75 per cent of the wall thickness of the moulded part.
Gate length is especially crucial. This should be ≤1 mm to prevent premature solidification of the sprue. The mould will heat up near the gate, so that the holding pressure is at its most effective.
To summarise the basic rules:
? always provide a means of intercepting the cold slug;
? make the runner diameter bigger than the moulded part wall thickness
? gate thickness should be at least 50% of the moulded part wall thickness.
These principles take only the crystallising behaviour of engineering polymers into account. If one wants to estimate mould filling behaviour, data about flow lengths of the polymer can be used and, if needed, flow calculations must be carried out. There are probably certain applications where, for various reasons, gate design does not follow these recommendations. Here, one will generally have to compromise between quality and cost-effectiveness.
ENGINEERING POLYMERS:
THE ‘TOP TEN’ MOULDING PROBLEMS
By R. Wilkinson, Roger Wilkinson, E. A. Poppe, Karl Leidig, Karl Schirmer
Chapter 3: Wrong Gate Position
1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners
9. Warpage
10. Mould deposit
The position of the gate is decisive for the flow front profile and the effectiveness of the holding pressure and, as a result, for the strength and other properties of the moulded part.
Since the position of the gate is usually specified by designers and mould makers, this article has been written especially with these persons in mind. Nevertheless, injection moulders should be also involved from the planning stage, to prevent predictable problems.
Integral hinge
entrapment
fill tubular moulds longitudinally
Gate for cup-shaped moulds
near the bottom
ENGINEERING POLYMERS:
THE ‘TOP TEN’ MOULDING PROBLEMS
By R. Wilkinson, Roger Wilkinson, E. A. Poppe, Karl Leidig, Karl Schirmer
Chapter 4: Hold Time Too Short
1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners 9. Warpage
10. Mould deposit
In practice many injection moulders, working from their experience of amorphous polymers, tend to use shorter hold pressure times and longer cooling times. Unfortunately, this approach also tends to be used for semi-crystalline polymers such as POM (acetal), PA (nylon), PBT and PET (polyesters). This article discusses the most important points to help machine setters chose the most suitable hold pressure time.
Visual check for voids (POM)
Reflected light
Microtome section photograph
(Moulded part to be sawn through at thickest)
Source: DuPont
4.1
What exactly happens during the hold pressure phase?
Once the mould cavity has been filled, the polymer molecules start to crystallise, i.e. the molecule chains become aligned with respect to each other, resulting in higher packing density. This process starts in the outer zone and ends in the centre of the wall (see diagrams). The volume shrinkage caused by this can be as much as 14 %, as in the case of POM, and has to be made up again by further amounts of melt which are injected into the mould cavity during the hold pressure phase. If the hold pressure time is too short, it causes small voids to be formed (microporosity), which can have an adverse affect on moulded part properties in many ways.
How to find out whether the hold time is too short
Parts made in this way often show excessive shrinkage, warpage, sink marks, voids and, in some cases,enormous loss of mechanical properties. In addition, there may be considerable dimensional variations. In some cases a misguided attempt is made to compensate for these shortcomings by increasing the cooling time.This results in unnecessarily long cycle times.
One way of recognising the effects of inadequate hold pressure times, for unreinforced moulding compounds,consists of cutting through the moulded part at a point where there is maximum wall thickness. The polished cut surface can then be checked for voids and pinholes. A magnifying glass or reflected-light microscope are sufficient to form a first opinion. A more elaborate method consists of preparing microtome sections (see diagram). In these, even the finest defects can be made visible with a transmitted-light microscope.
With reinforced moulding compounds, defects can be easily detected on a fracture surface where there is
maximum wall thickness. If the hold time is too short, there will be a foam-like structure in the fracture region and an enlarged fracture photomicrograph will show exposed fibres which are not embedded in polymer.Another method consists of preparing a photomicrograph of a polished section, in which pinholes can be detected with a microscope.
The effective hold time can be determined on the injection moulding machine by weighing a number of
mouldings (see description). This is the best way of determining the hold pressure time for a given moulding under practical conditions.
A guideline for the optimum hold time can also be obtained by using actual figures as comparison (see table).These apply only for a given wall thickness and cannot be applied to other factors such as temperature,nucleating additives and pigments, mould filling time, etc. For thinner walls, the figures will be lower, for thicker ones higher.
Correct setting procedure
To obtain optimum moulded part properties, the hold time should be determined by the weighing method, and the cooling time should be reduced to the required minimum (which is usually just above the plasticising time). This presupposes that the gate has been correctly positioned and designed (cf. parts 2 and 3 of this series, Plastverarbeiter 46 [1995] 6 and 7). It is also important to keep the pressure constant during the hold time. The correct pressure varies between 60 and 100 MPa, depending on the material used.
ENGINEERING POLYMERS:
THE ‘TOP TEN’ MOULDING PROBLEMS
By R. Wilkinson, Roger Wilkinson, E. A. Poppe, Karl Leidig, Karl Schirmer
Chapter 5: The Wrong Melt Temperature
1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners 9. Warpage
10. Mould deposit
Choosing the right melt temperature is vital for part quality when moulding semi-crystalline engineering
polymers. As a rule the margin of tolerance is less than when processing amorphous resins. The moulder at his machine directly influences the properties of the end-product. In the fifth chapter of this ten-part series, the authors consider the question of melt temperature when moulding POM (= acetal), PA (= nylon), PBT and PET
(polyesters).
What Happens when the Melt Temperature is Wrong?
Melt temperature can be too high or too low: both are wrong. In addition, even distribution of temperature in the melt is also a factor to be kept in mind.
Temperatures that are too high degrade the polymer, that is, destroy the molecular chains. Another consequence may be that additives in the melt, such as pigments, impact modifiers, etc., also decompose. The results are poorer mechanical properties (as a result of the shorter molecular chains), surface defects (caused by decomposition products) and unpleasant odours.
When the temperature is too low, the structure fails to achieve the required homogeneity. This drastically reduces impact resistance and leads in most cases to considerable variations in physical properties.
Apart from the melt temperature, the polymer’s dwell time in the injection unit also plays an important role. Experience has shown that dwell times of between two and nine minutes are normal. If the dwell time is longer, thermal decomposition may take place in certain circumstances, even if the melt temperature is correct. If the dwell time is very short, the melt usually does not have time to become fully homogeneous.
What are the Signs of Wrong Melt Temperature?
In the case of POM, excessive thermal stress generates decomposition products, causing bubbles to form in the melt. This can be observed clearly in the melt when it is purged. Other symptoms are increased mould deposit and an unpleasant odour. The physical properties of POM homopolymer are, however, hardly affected by too high melt temperatures.
PA discolours under extreme conditions, including if overheating occurs as a result of injection nozzles that are too hot. Thermal decomposition can be recognised in all PA types through reduced mechanical properties. In the laboratory, thermal decomposition can be established by measuring solution viscosity, but as a rule moulders are not in a position to apply this method.
PBT and PET react even more strongly to overheating, leading to reduced toughness. Faults are scarcely discernible during processing. If no suitable quality control measures are carried out, the damage usually becomes apparent only at the assembly stage, or when the part is in use. Discoloration indicates an unusually high degree of damage. In practice, there are tests on random samples with which certain toughness-related properties can be measured. Tests on the viscosity of moulded parts are time-consuming and expensive to carry out.
In the case of unreinforced PA or PBT, if unmelted particles are observed in the purge, it is a sign of too low melt temperature, or excessive shot size in extreme cases.
The data sheets for engineering polymers indicate the optimum melt temperature range for each. In general,the temperature setting of the barrel heating zones alone is not reliable because, apart from the temperature rise due to the heater bands, friction from the screw rotation also generates heat. How much heat is generated this way depends on screw geometry and rpm as well as on back pressure.
The following recommendations can help to achieve accurate temperature measurements:? Keep the diameter of the melt temperature probe less than 1,5 mm (response behaviour);? Pre-heat the probe;
? Collect the melt in a thermally insulated container;? Stir while taking measurements.
When taking initial temperature measurements or when there are no known values to rely on, a temperature profile should be selected which is 10 to 15°K above the melting point in the feed section and about 5 to 10°K under the required melt temperature in the metering zone. The temperature can be fine-tuned according to the measured melt temperature. In the case of long dwell times and short metering strokes, rising profiles are usually recommended. For short dwell times and long metering strokes flat profiles generally give the best results. A temperature zone should never be set at less than the melting point of the polymer.
Good
Bad
PA 66
PA 66
Melt flows
homogeneously
Melt is not homo-geneous
unmelted particles
Bubble formation due to decomposition
Good
Cross-section of purge
Evaluation of melt quality
POM
Source: DuPont
5.5
ENGINEERING POLYMERS:
THE ‘TOP TEN’ MOULDING PROBLEMS
By R. Wilkinson, Roger Wilkinson, E. A. Poppe, Karl Leidig, Karl Schirmer
Chapter 6: Wrong Tool Temperature
1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners
9. Warpage
10. Mould deposit
When moulding semi-crystalline engineering plastics such as POM (acetal), PA(nylon), PBT and PET (polyesters), it is important to make sure that the surface temperature of the tool is correct. The basic requirements for optimum processing are in the design of the tool. Only if the tool design is right can the moulder produce good quality parts with the help of temperature control equipment. This calls for close co-operation in the tool design and planning phase, in order to avoid production problems at a later stage.
Tool at 40 °C Tool at 90 °C
Outer layer insufficiently crystallised Optimum structure
The influence of tool temperature on the structure of POM
Source: DuPont
6.1
Possible Negative Consequences of the Wrong Tool Temperature
The symptom that is easiest to recognise is poor surface finish of moulded parts. The cause is often too low surface temperature in the tool.
The mould shrinkage and post-moulding shrinkage of semi-crystalline polymers are strongly dependent on tool temperature and the wall thickness of the part. Uneven heat dissipation in the tool can thus lead to differential shrinkage. This in turn can lead to inability to maintain part tolerances. In the most unfavourable circumstances shrinkage can be beyond correction, whether working with unreinforced or reinforced resins. When dimensions of parts in high-temperature applications become smaller with use, this is generally due to mould surface temperatures that are too low. This is because with too low mould surface temperatures mould shrinkage may be lower, but post-moulding shrinkage is substantially higher.
If a long start-up phase is needed before the dimensions settle down, it is a sign of poor temperature control in the tool, since the tool temperature is probably rising for a long time until equilibrium is reached.
Poor heat dissipation in some regions of the tool can cause substantial lengthening of the cycle time, leading to increased cost of the moulding.
Incorrect tool temperatures can sometimes also be established from the moulded parts by means of analytical methods such as structural analysis (e.g in the case of POM) and differential scanning calorimetry (DSC) examination (e.g. with PET).
Recommendations for Setting the Correct Tool Temperature
Tools are becoming more and more complex, and as a result it is getting ever more difficult to create the proper conditions for effective mould temperature control. Except in the case of simple parts, mould temperature control systems are always a matter for compromise. For this reason, the following list of recommendations should be seen as rough guidelines only.
?Temperature control of the shape to be moulded must be taken into consideration at the tool design stage.?When designing moulds that have a low shot weight and large mould dimensions, it is important to allow for good thermal transfer in the construction.
?Be generous when in dimensioning flow cross-section in the tool and in the feed pipes. Do not use fittings that cause a major restriction to the flow of the mould temperature control fluid.
?Use pressurised water as the temperature control medium, if possible. Provide flexible pipes and manifolds that are capable of withstanding high pressures and temperatures (up to 8 bar and 130°C).
?Specify the performance of the temperature control equipment to match the tool. The tool-maker’s data sheets should supply the necessary figures for flow rates.
?Use thermal insulation plates between both halves of the tool and the machine platens.
?Use separate temperature control systems for the moving half and the fixed half of the mould.
?Use separate temperature control systems for any side actions and the core, so that you can work with different start-up temperatures to get the mould running.
?Always connect different temperature control circuits in series, never in parallel. If circuits are in parallel, small differences in the flow resistance cause different volumetric flow rates of the temperature control medium, so that bigger temperature variations can occur than with circuits connected in series. (This series connection will work properly only if there is less than 5°C difference between mould inlet and mould outlet temperatures.)
?It is an advantage to have a display showing the supply temperature and return temperature on the mould temperature control equipment.
?For purposes of process control it is recommended to have a temperature sensor built into the tool, so as to be able to check its temperature during actual production.
Thermal equilibrium is established in the tool after a number of shots on cycle, normally a minimum of 10 shots. The actual temperature at equilibrium will depend on many factors. This actual temperature of the surfaces of the tool in contact with the plastic can be measured either by thermocouples within the tool (reading 2 mm from the surface) or more commonly by a hand-held pyrometer. The surface probe of the pyrometer needs to be fast-acting, and the tool temperature needs to be measured in a number of places, not just once on each side. Corrections may then be made to the set temperatures of the control units to adjust the mould temperature to what it should be. The data sheets for the various raw materials always give the recommended tool temperature. These recommendations always represent the best possible compromise between a good surface finish, mechanical properties, shrinkage behaviour and cycle times.
Moulders of precision parts and of parts that have to meet exacting optical or safety-oriented specifications generally tend to use higher tool temperatures (giving lower post-moulding shrinkage, shinier surface, more uniform properties). Technically less critical parts which have to be produced at the lowest possible cost can probably be moulded at somewhat lower tool temperatures. However, moulders should be aware of the drawbacks of this option and they should test the parts thoroughly, so as to be sure that they still meet the customer’s specifications.
ENGINEERING POLYMERS: THE ‘TOP TEN’INJECTION MOULDING PROBLEMS
R. Wilkinson, E.A. Poppe, K. Leidig and K. Schirmer
Part 7: Poor mould surface finish
1. Moisture in the granules
2. Feed system too small
3. Wrong gate position
4. Hold time too short
5. Wrong melt temperature
6. Wrong tool temperature
7. Poor surface finish
8. Problems with hot runners
9. Warpage
10. Mould deposit
Partially crystalline engineering thermoplastics such as POM (acetal), PA(nylon), PBT and PET(polyesters) are used primarily because of their outstanding mechanical, thermal and electrical properties. Further advantages over amorphous materials include their excellent chemical resistance and low tendency towards stress cracking. In many kinds of applications a high quality surface finish is an additional requirement. This article is intended to help eliminate possible surface defects.
Localisation and definition of surface defects
To solve the problem of surface defects one must first examine the precise location of the defect, and when it actually became evident. Here it is advisable to observe the surface during the actual injection moulding process. The points that need clarification are listed below.
? Does the defect occur with every shot or irregularly?
? Does the defect always occur in the same cavity?
? Does the defect always occur at the same place in the moulding?
? Can the defect be predicted already during a mould filling study?
? Is the defect already evident on the sprue?
? How does the defect react when a new batch of moulding compound is used?
? Does the defect occur with only one machine or with others as well?
Analysis of possible causes of surface defects
Surface defects may be caused by many different factors such as:
? Compounding: drying, compound quality, presence of contaminants (foreign bodies)
? Injection moulding conditions: melt temperature, injection speed and change-over point
? Condition of the injection unit, e.g. wear and dead spots
? Design of the hot runner system (runners, material stoppage etc.)
? Mould design, position of gate and gate cross-section, cold slug interceptor, venting etc.
? Additives such as pigments
? The polymer contained in the moulding compound.
Conclusions to be drawn from surface defects
1. Regular local defects
If surface defects regularly occur in the same place this indicates that there is a problem in the injection nozzle or the hot runner nozzle. The shape and design of the runner, gate or the moulded part itself may be responsible, e.g. sharp edges, sudden changes in wall thickness etc. Another cause may be moulding conditions such as the injection profile or the change-over point.
2. Irregular local defects
Where surface defects occur irregularly in different places, one should look at compounding (compound quality, presence of dust). Factors such as low melt temperature, back pressure, screw speed and screw retraction can also play an important part.
3. Surface defects covering large areas
This kind of defect usually extends over the entire moulding and is often visible already on the sprue. Here one should check whether melt decomposition has occurred. This is done by forcing a shot of melt into the open and observing whether, for example, it contains bubbles. In the case of hot runner systems, this method can be used with only limited success. Melt decomposition may be due to polymer degradation or decomposition of additives, caused by overheating or excessively long residence times. In the case of hygroscopic polymers, an important part is also played by hydrolytic degradation if the moulding compound has not been dried sufficiently.
General recommendations
Parts made from partially crystalline engineering polymers should preferably not be made by hot runner injection moulding if a perfect surface finish is essential. It is advisable to make use of a subsidiary runner, which thermally isolates the nozzle from the moulded part, thereby reducing the risk of surface defects.
The cold slug coming from the injection or hot runner nozzle should be intercepted by a special device opposite the sprue so that it cannot get into the moulded part.
The following table lists various surface defects and ways and means of eliminating them. In practice however, different surface defects appear simultaneously, which makes any investigation as to their origin – and their elimination – much more difficult.