Processing conditions

Influence of important technological parameters on product properties

a) Drying

Too high moisture content leads to surface defects on injections. In the case of some materials (PC, PBT), hydrolytic decomposition also occurs, leading to a sharp decrease in injection properties.

It is necessary to observe the specified drying times and temperatures; it is not recommended, for example, to decrease the drying temperature and increase the drying time and in any case to increase the drying temperature, which may cause the material to burn in the drying equipment.

It is advisable to check the drying of the material (the simplest test is the so-called "slide method"). The drying time is also significantly influenced by the type of the dryer used; drying with dry air (molecular sieve dryers) is significantly shorter than drying with fresh or circulating air. Beware of circulating air without admixture of fresh air, where drying does not occur at all!

The dried material should be processed as quickly as possible (preferably within a maximum of 2 hours). Hoppers should be heated, especially for hydrolytically degradable plastics. Hot dried material becomes wet very quickly during cooling and can reach very high moisture contents, higher than before drying! Do not add fresh wet material to the drying equipment with drying material!

Dried material can only be stored in packaging that prevents air access. It is therefore advisable to dry a quantity of material that can be continuously processed (in addition, there is no energy loss).

Be sure to remember that plastic additives also need to be dried. This especially applies to color concentrates and mineral fillers!

If the material is packed in the original packaging preventing moisture from entering the packaging (AL-layer), drying is not necessary. However, in case of damage to the packaging it is absolutely necessary!

Insufficient drying of the material can be detected, e.g. at the PC, by easy breakage of the injection and a change in the color of the material (yellowing)

b) Tool temperature

The following table shows the effect of the tool temperature on the injection properties:

* Crystallinity affects the stiffness and toughness of the injection. A high degree of crystallinity gives higher ductility, E modulus and lower absorbency. A low degree gives higher toughness. Among crystalline and semi-crystalline plastics, PE, PP, POM, PA, PBT are the most commonly used.

The quality of the injection is significantly affected by the level of the internal stress, which is manifested by cracking of the injection after a certain period of time (stress corrosion).

It is therefore advisable to choose a higher tool temperature when the degree of orientation of the macromolecules and thus the stress corrosion is lower. Due to deformation and shrinkage, it is advisable to work with lower tool temperatures. It is therefore necessary to choose a compromise in specific cases. It is clear that lower tool temperatures reduce the cycle times and increase production economy.

c) Melt temperature

It also affects the shrinkage (to a minimum extent, unlike the tool temperature and injection pressures), deformation, internal stresses, but the effect is small compared to other parameters.

It is recommended to use the lowest possible melt temperature, also due to the possibility of using shorter cycles. It is very important not to exceed the melt temperature when processing POM, as there is a risk of polymer decomposition to form formaldehyde gas, which is a severe irritant to the mucous membrane. Similarly, plastics with combustion detergents can also be subject to decomposition, again with very unpleasant gaseous emissions.

It is important to observe the maximum temperature when processing PVC, again for the same reasons (release of hydrogen chloride gas).

d) Injection pressures and holding pressure

They strongly influence the orientation of the marker molecules and thus the size of the internal stress, so it is advisable to use the lowest possible internal pressure and holding pressure.

However, plastics with poor flowability require maximum injection pressure to fill the tool cavity. If the requirements are to minimize deformation and shrinkage, the injection pressure and holding pressure must be increased, but care must always be taken.

e) Injection speed

Generally, it should be high, reducing the injection speed at the beginning of the injection phase will prevent surface defects (dull spots and lamination).

However, the injection speed also affects the internal stress level (high injection speed increases the orientation of the macromolecules and thus the size of the internal stress)

The injection speed also affects air sealing (air bubbles and mass burning) and the strength of the cold joints.

f) Back pressure

Higher back pressure eliminates color inhomogeneity of the material, improving the homogeneity of the melt. However, it also prolongs the injection cycle and compromises the thermal stability of the material.

IDENTIFICATION OF PLASTICS

Note: PP can be distinguished from PE by the so-called nail test (PE has a noticeable indentation, PP does not)

TEST METHODS

1) ITT - melt flow index /ISO1133 , DIN 53735, ASTM D1238
is defined as the weight of material that will flow through a ø2 mm nozzle in 10 minutes (for a specific material type) at a given piston load and temperature

MVI (MVR) is defined as the volume of plastic that is extruded at a given load and temperature in 10 minutes expressed in cm³/10 min.

MFI (MFR) is defined as the weight of plastic that is extruded at a given load and temperature in 10 minutes expressed in g/10min.

Temperatures used - 220, 250, 260, 265, 280, 300, 320, 360 °C

Load - 1, 2, 2.16, 3.8, 5, 10, 21 kg

Usual MFR (MFI) conditions for individual materials:

2) Temperature resistance according to Vicat according to ISO306, DIN 53460, ASTM D1525

According to ISO 306, there are 2 methods:

Method A - 10 N load
Method B - 50N load

Depending on the nature of the increase of the thermostat temperature by 50 °C/hour or 120 °C/hour, the test is referred to as:

VICAT A50, A120 or
VICAT B50, B120

The test is carried out in a heated oil bath where the initial temperature of the bath is 23 °C and a load of 10 or 50N is applied after 5 minutes.

The shape stability/temperature resistance according to Vicat is a temperature, at which a ø 1mm² needle is pressed into the test body to a depth of 1mm+0.01mm

ISO versus ASTM comparative value

• the dimensions of the test body are different, therefore the ISO values are lower than the ASTM value

3) HDT temperature resistance - permanent thermal deformation according to ISO75, DIN53461, ASTM D648

Marking

HDT/A - heat resistance at a load of 1.8 MPa
HDT/B - heat resistance at a load of 0.45 MPa

Description of the test:
The test body supported as a beam with two supports 100 mm apart is placed in a heated siliconized oil bath and the tip applying on the test body is loaded according to ISO 0.45MPa (0.45 MPa/ASTM) or according to ISO 1.8MPa

(1.82 MPa/ASTM); the initial measurement temperature is +23 °C and the bath temperature is increased by 2 °C/min.

The heat resistance according to HDT ISO is a temperature at which the test body deflects by 0.32mm

The heat resistance according to HDT ASTM is a temperature at which the test body deflects by 0.25mm

4) Impact strength - ISO180, ASTM D256

The impact strength is tested either as notch toughness or notchless toughness.

The Charpy (horizontal) or IZOD (vertical) method is used depending on the nature of the test body location

IZOD ISO 180 distinguishes the types of bodies according to the shape of the notch:

IZOD ISO 180/1A - body 80x10x4mm, notch shape - radius 0,25mm

IZOD ISO 180/1B - the same type of body as 180/1A, radius 1 mm

IZOD ISO 180/1C - the same type of body as 180/1A, but the impact on the body is from the reverse side

IZOD ISO 180/1U - the same type of body as 180/1A, but without the notch

IZOD ASTM D256

• Dimensions of the body: 63.5×12.7×3.2 mm - the same notch radius as IZOD ISO180

Charpy according to ISO179, ASTM 256

The test is performed on bodies with or without a notch. Body dimensions of 80x10x4mm

There are 3 variants of a notch with radius:

ISO 179/1eA - A 0.25 mm
ISO 179/1eB - B 1.00 mm
ISO 179/1eC - C 0.1 mm
ISO 179/1eU - without a notch

The preferred type is type A

The unit of measurement of toughness is expressed in kJ/m² for the ISO method and in J/m for ASTM.

The conversion is 10, which means 100 J/m = 10 kJ/m²

The impact strength test can be carried out over a temperature range of -60 °C to +23 °C, typically

it is measured at +23 °C and -30 °C.

5) Hardness - shore according to ISO868, DIN53505, ASTM D2240

The measurement of the indentation resistance of elastomers or soft plastics is based on the penetration depth of the conical cone.

The shore hardness is measured by Durometre A for soft materials (10-90 shore A) and Durometer
D for hard materials (20-90 shoreD)

6) Bending strength and modulus strength according to ISO778, DIN53452, ASTM D790

The bending strength characterizes the "stiffness" of a material as opposed to the tensile strength.

In the bending strength test, all the force is in one direction .
Testing is done by holding the load on the body constant at 2mm/min.

7) Flammability

a) Flammability classification according to UL94

Test description:

The test body is positioned vertically for the V1, V2, V0 test and horizontally for the HB test

Body dimensions - 125+5mmx13+0.2mmx0.8mm or 1.6mm or 3.2mm

Conditioning:

• a set of 5 bodies/sticks is conditioned for 48 hours at 23 °C and 50 % of relative air humidity

• the second set of bodies is conditioned for 168 hours at 70 °C

The retest is performed when the burn time of a single body or of the whole set of bodies does not exceed the prescribed burn time value of 5s, i.e. for V0 =55s, V1,V2 = 255s

HB - the test body is fixed horizontally. The burning rate must be at a wall thickness of up to

3mm less than 76mm/min, for thickness above 3mm less than 38mm/min.

V2 - the test body is fixed vertically. Extinguishing of flame on the body within 30s after

pulling the flame away from the body; burning drops are allowed; smoldering must stop

within a maximum of 60 s

V1 - the test body is fixed vertically. Extinguishing of flame on the body within 30s after

pulling the flame away from the body; no burning drops may form; smoldering must

stop within a maximum of 60s

V0 - the test body is fixed vertically. Extinguishing of flame on the body within 10s after

pulling the flame away from the body; no burning drops may form; smoldering must

stop within a maximum of 30s

(b) GWFI (hot loop according to IEC 60695-2-1)

In this test, we investigate the behavior of plastics in burning by means of a hot loop that pushes on a test ring plate. The result is then considered as a characteristic value belonging to the properties of the tested plastic.

The source of the heat stress is represented by a special resistance wire loop, electrically heated to temperatures of 550 °C, 650 °C, 750 °C, 960 °C, at which the flames or smoldering spontaneously extinguish within 30 s after the removal of the hot loop. The tissue paper placed under the specimen must not be ignited by the burning particles, nor must the specimen catch fire.

Drip height - 200 mm
Pressing force - 1 N
Pad - tissue paper or wooden pine pad
The penetration depth is limited to 7mm.

The test specimens are usually round plastic plates with a diameter of 85mm and a thickness from 0.75mm, 1.5mm or 3mm or plastic parts (moldings)

The temperatures correspond to the appropriate classification of the tested part depending on the application, especially for electrode parts.

ADDITIVES TO PLASTICS

They are added to plastics for a variety of reasons, either at the producer's or when the plastics are modified at the processor.

LUBRICANTS AND SEPARATION AGENTS

They are used because of the better flow of plastics into the tools and easier removal of products from the tools. There are a number of these agents, such as paraffin waxes, metallic soaps, higher fatty esters and alcohols (zinc stearate, calcium stearate). The additive can affect the surface quality and the mechanical properties, so the lowest possible concentration is chosen. Silicone agents can also be used, both as a lubricant and a separation agent, but beware of applications in electrical engineering (they increase plastic conductivity). PP oil can also be used as a separator. The use of PTFE is suitable both as a lubricant and in spray form. The addition of PTFE is mainly ensured by plastics manufacturers.

STABILIZERS

There are thermal stabilizers (necessary to stabilize plastics against heat stresses during processing) and light stabilizers (to stabilize plastics against UV radiation). Heat stabilizers are added to the plastic by the plastic manufacturer, as are UV stabilizers, but it is also possible to add UV stabilizers by the processor to increase the product durability (but the UV stabilizer manufacturer must be consulted beforehand). UV stabilizers are always selected for a specific plastic.

The addition of a UV stabilizer (preferably in the form of a concentrate) is low, app. 0.1-1 %, therefore it does not affect the processing properties and physical and mechanical properties of the product.

Active carbon black is a very good stabilizer, while at the same time the conductivity of the plastic is increased (suitable for applications where a surface resistance of less than 10⁸ Ω is required). Carbon black cannot be added in powder form, as it heavily pollutes the working environment (manufacturers supply carbon black in concentrate form).

ANTISTATIC

It is preferable to use it as an additive directly into the plastic; it has a lasting effect. Antistats in the form of sprays that are applied to the surface of injections have only a temporary effect.

The essence of antistatic agents is to increase the conductivity of plastics (reduction of the surface resistance from 10¹⁵-10¹⁶ Ω to 10¹¹-10¹⁰ Ω). It is advantageous to buy plastics directly with an antistatic treatment.

Achieving a surface resistance < 10¹⁰ Ω for special applications of conductive plastics (with a value up to 10⁶-10² Ω) can be obtained by using active carbon black or metallic fibers. Pay attention to the orientation of these fillers; in the direction of the melt flow and perpendicular to the direction of the melt they show different surface resistance values!

RETARDANT ADDITIVES

They are used to reduce the flammability of plastics (according to UL 94, from HB values to the V1 to V0 values). They are manufactured either on the basis of chlorine, bromine or phosphorus containing compound, but there is a risk of significant deterioration of the working environment when the retardant system degrades (when processing temperatures are exceeded) or by using a system based on AL (OH)₃ or even other additives, where we record degradation of the retardant system, does not endanger the working environment.

In any case, however, the maximum melt temperature must necessarily be observed. Beware of the fact that, especially with older injection molding machines, the temperature set on the plastic cylinder and the melt temperature can vary considerably!

PIGMENT AND DYES

There are pigments based on inorganic compounds (these have very good heat resistance, are cheap, but are problematic for health safety) and based on organic compounds (lower heat resistance, satisfactory health safety, usually a higher price). Therefore, especially for organic-based pigments, it is necessary to observe the maximum processing temperatures and prevent the colored plastic from staying in the plasticizing cylinder for a long time (beware of the difference between the temperatures set on the plasticizing cylinder and the melt temperature).

It is advisable to work with lower screw speeds and lower injection speeds!

For hygroscopic color concentrates, the pigment must be dried together with the plastic!

For a higher quality of coloring of the molding, it is advisable to use concentrates with a lower pigment content and work with a higher back pressure of the injection molding machine. Powder pigments also give a better quality coloration, but the accuracy of pigment dosing into the hopper of the injection molding machine is worse; it is more difficult to clean the machine or change to another pigment and the working environment deteriorates.

In addition to the colored concentrates, colored plastics are also used, but the coloring is not optimal,

cleaning the machine is also more demanding. For every plastic, it is necessary to use a concentrate with the carrier of the plastic being processed; in exceptional cases it is also possible to use a concentrate on a different carrier if the two materials are compatible. Concentrates based on special waxes are versatile, but the coloration does not reach the same quality as with a concentrate with the same carrier as the plastic being processed.

A number of pigments and concentrates, especially those based on organic compounds, are affected by the crystallinity of the plastic, which results in deformations and higher product shrinkage!

BLOWING AGENTS

They are based on either a physical or chemical principle. As a plastic additive, the chemical principle comes into play. This consists in the fact that the added blowing agent is decomposed into gases by the action of temperature and the gases cause the plastic to swell. The decomposition temperature of the blowing agent must correspond to the processing temperature of the plastic.

There are a variety of blowing agents, from the simplest ones (e.g. sodium carbonate) to complex compounds. These are more costly, but give a homogeneous structure, which is crucial for the final quality of the mounding.

In the foaming technology, it is absolutely necessary to follow the principle that the decomposition of the blowing agent must take place in the last zone (or nozzle) of the heating cylinder of the injection molding machine. The injection speed is chosen as high as possible and the cooling long enough to prevent the molding from deburring. The best results are achieved with wall thicknesses above 3 mm.

Blown injections can be produced on conventional injection molding machines, but the best results are achieved by using special injection molding machines that allow the fastest possible injection (achieving the lowest injection density).

FILLERS

They include actual fillers and reinforcing additives.

Wood flour is widely used as a filler (but mainly for thermosets); it can also be used for plastics, but it is necessary to dry these fillers before processing and to monitor the heat resistance of the wood flour.

Inorganic fillers, e.g. crushed chalk, limestone, marble, silica flour, are more widely used. All these additives increase heat resistance of plastics, increase the E-modulus, notch toughness, stiffness. At the same time, in many cases, they positively influence the price of plastic. Silica flour gives the highest corrosion resistance, but is very abrasive (the most of all fillers and reinforcing additives).

The most commonly used is talc (either alone or in a mixture of short-fibre minerals), which increases the E-modulus, impact strength and bending strength, and affects the quality of the molding surface. Mica is used to obtain the so-called pearlescent shade.

The fillers used must be applied as finely ground particles with a diameter < 0.7 mm to achieve the desired effect.

Many fillers (e.g. limestone) are hygroscopic, so pre-drying before processing is necessary. Reinforcing additives are used the most, namely in the form of short and long fibers and balls (the uniform shrinkage of the molding in all directions is used).

The most commonly used are fibreglass, aramid fibers, carbon and profile fibers, steel fibers.

All of them, especially the long-fiber fillers, give plastics particularly high E-modulus, stiffness, heat resistance, chemical resistance, impact properties and rank among structural plastics with the highest requirements for their properties.

They do not cause any difficulties in processing (slightly worse fluidity due to the lower shrinkage and more difficult removal from the molds), but it is necessary to choose the addition of the regenerate carefully, because crushing of unsuitable moldings causes breaking of fibers and thus a change in the properties.

The abrasion of the machinery is not much higher than that of unreinforced plastics (glass fillers contain a corrosion-reducing finish).

CLEANING OF INJECTION UNITS

For problematic transitions from opaque to transparent plastic, especially PC, PMMA and for transitions to color changes, cleaning materials in the form of granules are used.

The cleaning granulate enables the removal of stuck dirt on the surface of the screws and chambers of the injection and extrusion machines. It is also suitable for cleaning hot sprues, allowing to reduce the amount of rejects and shorten the transition time when changing color. One very effective cleaner is the Ramclean granulate.

Ramclean 101/204

• Operating temperature range: 150 °C - 250 °C
  
• Suitable for PP, PE, PA, PBT, PC/PBT, PC/ABS, ASA/PBT, PC/ASA, EVOH, colored PC

Ramclean 206 transparent

• Operating temperature range: 230 °C - 320 °C
  
• Suitable for transparent PC, Acryl

Ramcelan 500

• Operating temperature range: 160 °C - 230 °C
  
• Suitable for PVC & ABS

Ramclean 600

• Operating temperature range: 120 °C - 280 °C
  
• Suitable for single- and multi-layer films

Ramclean 800

• Operating temperature range: 230 °C - 360 °C
  
• Suitable for PA, PC, PS, EVOH, ULTEM, PEX, PPO, PPS