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Test Methods for Plastic Raw Materials and Moldings
Chapter 11
Test Methods for Plastic Raw Materials and Moldings In this chapter we will describe the plastic raw material producer’s quality control data, the various material defects that a molder may find, as well as the test methods you can use when you want to analyze these kinds of defects.
11.1 Quality Control during Raw Material Production The plastic producers measure the quality of their plastic raw material at regular intervals (random sampling). Depending on the type of polymer and the included additives, they use different test methods during production. In general they are testing:
Figure 11.1 Here we can see the test results of 12 different batches of a 30% glass fiber reinforced grade. The aim is to be as close as possible to 30%, but as long as the result is within the green lines (30 ± 2%), the material is approved for delivery. Batches 7 and 11 are not acceptable and must be redone in order to fall within the de livery limits.
■■
Viscosity, which is dependent on the molecular chain length
■■
Fiber content, i.e. the ash content after complete combustion of the polymer
■■
Moisture content of each batch at the packing station Glass fiber content
32% 30% 28% Batch number 1
2
3
4
5
6
7
8
9
10
11
12
Table 11.1 In this table we can see that thermal and mechanical properties are tested at least once a year. These values are then used for the published values in the producer’s literature or in databases. It is only in exceptional cases that molders can get their material regularly tested with these types of testing.
Test Method (Unit)
Specified
A
Moisture content (%)
15512
≤ 0.20
A
Ash content (%)
3451
31–35
B
Melting point (°C)
3146
250–265
B
Density (g/cm3)
1183
1.34–1.41
B
Tensile strength (MPa)
527
≥ 157
B
Elongation (%)
527
≥ 1.8
B
Stiffness – E-modulus (MPa)
527
≥ 8,000
B
Impact strength Charpy (kJ/m2)
179
≥ 7.8
B
HDT at 1.8 MPa (°C)
D789
≥ 245
A: Every batch; B: Annually
102
ISO
11.2 Visual Quality Control of Plastic Granules The test values that the producers receive during the random sampling of the various production batches will as a rule be attached together with the material (or invoice) in the form of a delivery certificate. In this certificate you will find the same lot number (also called batch number) that you will find stamped on the bags or octabins. It is very important to keep these certificates in case of a complaint because the production plants always want information about the batch. Figure 11.2 Here you can see a delivery certificate from DSM for Akulon K224-G6 (natural PA6 with 30% glass fiber). Here they have measured: Moisture content of 0.050% and indicated the upper delivery limit to be 0.150% An ash content (glass fiber content) of 29.9% and indicated the limits of supply to be between 28.0% and 32.0% A relative viscosity in a solution of formic acid of 2.45, which is well within the tolerance limits.
11.2 Visual Quality Control of Plastic Granules It is very rare that an operator discovers that something is wrong with a material just by inspecting the granules. Only when the material is in production is it normal to discover defects.
Figure 11.3 The defects that you sometimes can see by inspecting the granules are different color, wrong size, clumping of granules, black specks in bright material, or metal particles on the surface. It may also be defects such as dust, dirt, or contamination within the bag of granules that affect quality and performance of production.
103
Chapter 11 — Test Methods for Plastic Raw Materials and Moldings
11.3 Visual Inspection of Plastic Parts If there are defects with the plastic raw materials, this is something that is discovered in most cases when the production of parts has begun. Most often you will discover defects by visual inspection, but there are also occasions when defects are discovered during measuring or mechanical testing.
Figure 11.4 Black specks Black specks occur due to thermal degradation in either the machine cylinder or within the production of the granules. If they already are present in the granules there is no doubt that this is a defect of the raw material, and a complaint must be made.
Figure 11.5 Metal particles Many molders use magnetic bars in the hopper to catch granules containing ferrous metal particles. If the metal particles are made of brass or stainless steel they may slip through, and they can damage both the screw and the cylinder or get stuck in the gate of the mold.
Figure 11.6 Discolorations If the color pigment in the raw material is poorly dispersed, you can get dark “shadows” of discoloration on the surface of the part. Increased back pressure and lower screw speed may in some cases help to solve this problem.
Figure 11.7 Foreign granules Occasionally foreign granules end up in bags when packing. If these have a higher melting point and a different color it may look like this picture, where green polyamide granules have fallen into natural-colored acetal. The nylon melts about 80°C above the melt temperature of acetal.
If an error occurs when you are changing to a new bag or a new batch and there is an indication that the new material behaves differently it is a sign of defects in the material. In many cases you can solve this by making an adjustment of the process parameters. But if this is not successful you should test the material in another machine before filing a complaint.
104
11.4 Tests That Can Be Performed by the Molder
Figure 11.8 Silver streaks Polyamides are moisture sensitive in the melt. Vapor bubbles form and appear as silver streaks on the surface. The resin should be sufficiently dry in the bag, but if the bag has been punctured this is enough to cause normal pre-drying to be insufficient.
Figure 11.9 Unfilled parts If the plastic raw material has higher melt viscosity than normal, there can be problems with unfilled parts. Usually this can be corrected by using a higher pressure or increased melt or mold temperature.
Figure 11.10 Flash If the plastic raw material has a lower melt viscosity than normal, there can be problems with flash. Usually this can be corrected by using a lower pressure. In the polyesters PBT or PET, flash may occur due to insufficient drying.
Figure 11.11 Wrong size of the granules Sometimes the plastic producer offers a so-called “cube blend”. This is a masterbatch-colored resin. As you can see in this picture some of the granules are unusually long, and these granules may cause problems during the dispersion of color within the cylinder of the molding machine.
11.4 Tests That Can Be Performed by the Molder The eye is the best instrument for quality control! Besides visual inspection, most molders have other methods for testing their parts in order to determine if defects are present. Below there are some of the most common test instruments or tools used by molders: 1. Precision balance in order to determine if the weight deviates from normal 2. Measuring equipment in order to determine if the dimensions deviate 3. A saw in order to determine if the parts contain unmelted granules, foreign particles, or voids
105
Chapter 11 — Test Methods for Plastic Raw Materials and Moldings
Figure 11.12 Sometimes there are surprises when you saw apart a plastic part. This picture shows the presence of a large void within the acetyl part to the left and micro-pores present in the glass fiber reinforced polyamide part to the right.
Less common tests or equipment found among molders: 4. Drop-weight test for tests of toughness 5. Tensile testers for mechanical testing 6. Equipment for moisture analysis 7. Equipment for measuring melt flow index 8. Combustion testing (see Table 11.2 below) 9. Color testing
Figure 11.13 The equipment used for moisture analysis generally consists of a precision balance enclosed in an oven. You begin by weighing the granules before the oven is turned on. When all the moisture has been dried off from the granules you will weigh the granules once more. The difference in weight will show how much moisture the granules contained. This picture shows a HR83, a modern high tech instrument used to measure levels of moisture by the use of halogen technology from Mettler Toledo. [Source: se.mt.com]
106
11.5 Advanced Testing Methods
Figure 11.14 Most plastics have a very characteristic flame combined with smoke, smell, and fire, which can be used to determine the type of polymer. Table 11.2 This table describes the flame progression and odor of some common plastics.
Material
Fire progression
Odor
ABS
Yellow dripping flame with black smoke
Very typical for ABS
PA – Polyamide
Blue flame with yellow top. The flame melts and drips with clear viscous drops
Burned wood
PC – Polycarbonate Yellow flame with smoke The test bar melts and carbonizes PE – Polyethylene
Blue flame with yellow top. The flame melts and drips with clear burning drops
Candle
POM – Acetal
Blue flame without smoke
Ammonia
PP – Polypropylene
Blue flame with yellow top The test bar swells and drips
Sweetish
PS – Polystyrene
Yellow flame with black sooty smoke
Coal gas
PVC
Yellow flame with green edges The test bar is softening
Hydrochloric acid
SAN
Yellow flame with a black sooty smoke
11.5 Advanced Testing Methods Most major raw material producers use advanced test equipment for quality control and material development. Many offer analysis to their customers in order to determine the causes of defects on plastic parts. Some of these test methods are: 1. Accurate moisture analysis of granules or parts 2. Viscosity tests of granules or parts 3. Ash analysis after burning off the polymer to measure the level of reinforcement or filler 4. Material identification by the use of an infrared spectrophotometer (IR spectra) 5. Material identification using differential scanning calorimetry (DSC)
107
Chapter 11 — Test Methods for Plastic Raw Materials and Moldings 6. Error analysis by the use of scanning electron microscopy (SEM) 7. Error analysis using a microtome-cut sample in combination with light microscopy
Figure 11.15 The image shows an infrared spectrophotometer. With this equipment you can analyze organic material. The result is the type of spectrum shown below.
Figure 11.16 This diagram shows the IR spectrum of Delrin® 100 NC010 acetal. An IR spectrum can be compared to a fingerprint. From each IR spectrum you can directly see which polymer and organic additives are present in the sample. When looking at a possible contamination in the surface of a plastic part using this method, it is possible to see what it consists of, unless the contamination is organic. [Source: DuPont]
Figure 11.17 A different method where you can analyze organic material is DSC. Here you can see what happens when the material is heated. It gives you, for example, the melting point of the contamination of the green granulates present in the circle. [Source: DuPont]
Figure 11.18 A scanning electron microscope is a very expensive equipment used for analysis of inorganic inclusions in plastic. It also allows analysis of fractures on the plastic part surfaces or surface structure as shown in the illustration in Figure 11.19. [Photo: DuPont]
108
11.5 Advanced Testing Methods
Figure 11.19 This is a high magnification of the surface of a perfectly molded part in acetal. To the right you can see how the surface has changed after exposure to sulfuric acid. In addition to the surface becoming etched, the tensile strength will be significantly lower. [Photo: DuPont]
Figure 11.20 Microtome analysis is a good method for studying the structure of injection-molded parts. A very thin layer is sliced from the surface of the plastic part by using a microtome. The structure of the material is then illuminated from below by polarized light. In order to make thin slices of soft materials the material must be frozen. [Photo: DuPont]
Figure 11.21 The image at the top left shows the slicing of a frozen plastic layer. To the right there is a sample mounted between two glass plates. At the bottom left, the sample is illuminated with polarized light. At the bottom right you can see a black enclosure of degraded material in the bottom of the part (see red arrow). [Photo: DuPont]
109
Test Methods for Plastic Raw Materials and Moldings In this chapter we will describe the plastic raw material producer’s quality control data, the various material defects that a molder may find, as well as the test methods you can use when you want to analyze these kinds of defects.
11.1 Quality Control during Raw Material Production The plastic producers measure the quality of their plastic raw material at regular intervals (random sampling). Depending on the type of polymer and the included additives, they use different test methods during production. In general they are testing:
Figure 11.1 Here we can see the test results of 12 different batches of a 30% glass fiber reinforced grade. The aim is to be as close as possible to 30%, but as long as the result is within the green lines (30 ± 2%), the material is approved for delivery. Batches 7 and 11 are not acceptable and must be redone in order to fall within the de livery limits.
■■
Viscosity, which is dependent on the molecular chain length
■■
Fiber content, i.e. the ash content after complete combustion of the polymer
■■
Moisture content of each batch at the packing station Glass fiber content
32% 30% 28% Batch number 1
2
3
4
5
6
7
8
9
10
11
12
Table 11.1 In this table we can see that thermal and mechanical properties are tested at least once a year. These values are then used for the published values in the producer’s literature or in databases. It is only in exceptional cases that molders can get their material regularly tested with these types of testing.
Test Method (Unit)
Specified
A
Moisture content (%)
15512
≤ 0.20
A
Ash content (%)
3451
31–35
B
Melting point (°C)
3146
250–265
B
Density (g/cm3)
1183
1.34–1.41
B
Tensile strength (MPa)
527
≥ 157
B
Elongation (%)
527
≥ 1.8
B
Stiffness – E-modulus (MPa)
527
≥ 8,000
B
Impact strength Charpy (kJ/m2)
179
≥ 7.8
B
HDT at 1.8 MPa (°C)
D789
≥ 245
A: Every batch; B: Annually
102
ISO
11.2 Visual Quality Control of Plastic Granules The test values that the producers receive during the random sampling of the various production batches will as a rule be attached together with the material (or invoice) in the form of a delivery certificate. In this certificate you will find the same lot number (also called batch number) that you will find stamped on the bags or octabins. It is very important to keep these certificates in case of a complaint because the production plants always want information about the batch. Figure 11.2 Here you can see a delivery certificate from DSM for Akulon K224-G6 (natural PA6 with 30% glass fiber). Here they have measured: Moisture content of 0.050% and indicated the upper delivery limit to be 0.150% An ash content (glass fiber content) of 29.9% and indicated the limits of supply to be between 28.0% and 32.0% A relative viscosity in a solution of formic acid of 2.45, which is well within the tolerance limits.
11.2 Visual Quality Control of Plastic Granules It is very rare that an operator discovers that something is wrong with a material just by inspecting the granules. Only when the material is in production is it normal to discover defects.
Figure 11.3 The defects that you sometimes can see by inspecting the granules are different color, wrong size, clumping of granules, black specks in bright material, or metal particles on the surface. It may also be defects such as dust, dirt, or contamination within the bag of granules that affect quality and performance of production.
103
Chapter 11 — Test Methods for Plastic Raw Materials and Moldings
11.3 Visual Inspection of Plastic Parts If there are defects with the plastic raw materials, this is something that is discovered in most cases when the production of parts has begun. Most often you will discover defects by visual inspection, but there are also occasions when defects are discovered during measuring or mechanical testing.
Figure 11.4 Black specks Black specks occur due to thermal degradation in either the machine cylinder or within the production of the granules. If they already are present in the granules there is no doubt that this is a defect of the raw material, and a complaint must be made.
Figure 11.5 Metal particles Many molders use magnetic bars in the hopper to catch granules containing ferrous metal particles. If the metal particles are made of brass or stainless steel they may slip through, and they can damage both the screw and the cylinder or get stuck in the gate of the mold.
Figure 11.6 Discolorations If the color pigment in the raw material is poorly dispersed, you can get dark “shadows” of discoloration on the surface of the part. Increased back pressure and lower screw speed may in some cases help to solve this problem.
Figure 11.7 Foreign granules Occasionally foreign granules end up in bags when packing. If these have a higher melting point and a different color it may look like this picture, where green polyamide granules have fallen into natural-colored acetal. The nylon melts about 80°C above the melt temperature of acetal.
If an error occurs when you are changing to a new bag or a new batch and there is an indication that the new material behaves differently it is a sign of defects in the material. In many cases you can solve this by making an adjustment of the process parameters. But if this is not successful you should test the material in another machine before filing a complaint.
104
11.4 Tests That Can Be Performed by the Molder
Figure 11.8 Silver streaks Polyamides are moisture sensitive in the melt. Vapor bubbles form and appear as silver streaks on the surface. The resin should be sufficiently dry in the bag, but if the bag has been punctured this is enough to cause normal pre-drying to be insufficient.
Figure 11.9 Unfilled parts If the plastic raw material has higher melt viscosity than normal, there can be problems with unfilled parts. Usually this can be corrected by using a higher pressure or increased melt or mold temperature.
Figure 11.10 Flash If the plastic raw material has a lower melt viscosity than normal, there can be problems with flash. Usually this can be corrected by using a lower pressure. In the polyesters PBT or PET, flash may occur due to insufficient drying.
Figure 11.11 Wrong size of the granules Sometimes the plastic producer offers a so-called “cube blend”. This is a masterbatch-colored resin. As you can see in this picture some of the granules are unusually long, and these granules may cause problems during the dispersion of color within the cylinder of the molding machine.
11.4 Tests That Can Be Performed by the Molder The eye is the best instrument for quality control! Besides visual inspection, most molders have other methods for testing their parts in order to determine if defects are present. Below there are some of the most common test instruments or tools used by molders: 1. Precision balance in order to determine if the weight deviates from normal 2. Measuring equipment in order to determine if the dimensions deviate 3. A saw in order to determine if the parts contain unmelted granules, foreign particles, or voids
105
Chapter 11 — Test Methods for Plastic Raw Materials and Moldings
Figure 11.12 Sometimes there are surprises when you saw apart a plastic part. This picture shows the presence of a large void within the acetyl part to the left and micro-pores present in the glass fiber reinforced polyamide part to the right.
Less common tests or equipment found among molders: 4. Drop-weight test for tests of toughness 5. Tensile testers for mechanical testing 6. Equipment for moisture analysis 7. Equipment for measuring melt flow index 8. Combustion testing (see Table 11.2 below) 9. Color testing
Figure 11.13 The equipment used for moisture analysis generally consists of a precision balance enclosed in an oven. You begin by weighing the granules before the oven is turned on. When all the moisture has been dried off from the granules you will weigh the granules once more. The difference in weight will show how much moisture the granules contained. This picture shows a HR83, a modern high tech instrument used to measure levels of moisture by the use of halogen technology from Mettler Toledo. [Source: se.mt.com]
106
11.5 Advanced Testing Methods
Figure 11.14 Most plastics have a very characteristic flame combined with smoke, smell, and fire, which can be used to determine the type of polymer. Table 11.2 This table describes the flame progression and odor of some common plastics.
Material
Fire progression
Odor
ABS
Yellow dripping flame with black smoke
Very typical for ABS
PA – Polyamide
Blue flame with yellow top. The flame melts and drips with clear viscous drops
Burned wood
PC – Polycarbonate Yellow flame with smoke The test bar melts and carbonizes PE – Polyethylene
Blue flame with yellow top. The flame melts and drips with clear burning drops
Candle
POM – Acetal
Blue flame without smoke
Ammonia
PP – Polypropylene
Blue flame with yellow top The test bar swells and drips
Sweetish
PS – Polystyrene
Yellow flame with black sooty smoke
Coal gas
PVC
Yellow flame with green edges The test bar is softening
Hydrochloric acid
SAN
Yellow flame with a black sooty smoke
11.5 Advanced Testing Methods Most major raw material producers use advanced test equipment for quality control and material development. Many offer analysis to their customers in order to determine the causes of defects on plastic parts. Some of these test methods are: 1. Accurate moisture analysis of granules or parts 2. Viscosity tests of granules or parts 3. Ash analysis after burning off the polymer to measure the level of reinforcement or filler 4. Material identification by the use of an infrared spectrophotometer (IR spectra) 5. Material identification using differential scanning calorimetry (DSC)
107
Chapter 11 — Test Methods for Plastic Raw Materials and Moldings 6. Error analysis by the use of scanning electron microscopy (SEM) 7. Error analysis using a microtome-cut sample in combination with light microscopy
Figure 11.15 The image shows an infrared spectrophotometer. With this equipment you can analyze organic material. The result is the type of spectrum shown below.
Figure 11.16 This diagram shows the IR spectrum of Delrin® 100 NC010 acetal. An IR spectrum can be compared to a fingerprint. From each IR spectrum you can directly see which polymer and organic additives are present in the sample. When looking at a possible contamination in the surface of a plastic part using this method, it is possible to see what it consists of, unless the contamination is organic. [Source: DuPont]
Figure 11.17 A different method where you can analyze organic material is DSC. Here you can see what happens when the material is heated. It gives you, for example, the melting point of the contamination of the green granulates present in the circle. [Source: DuPont]
Figure 11.18 A scanning electron microscope is a very expensive equipment used for analysis of inorganic inclusions in plastic. It also allows analysis of fractures on the plastic part surfaces or surface structure as shown in the illustration in Figure 11.19. [Photo: DuPont]
108
11.5 Advanced Testing Methods
Figure 11.19 This is a high magnification of the surface of a perfectly molded part in acetal. To the right you can see how the surface has changed after exposure to sulfuric acid. In addition to the surface becoming etched, the tensile strength will be significantly lower. [Photo: DuPont]
Figure 11.20 Microtome analysis is a good method for studying the structure of injection-molded parts. A very thin layer is sliced from the surface of the plastic part by using a microtome. The structure of the material is then illuminated from below by polarized light. In order to make thin slices of soft materials the material must be frozen. [Photo: DuPont]
Figure 11.21 The image at the top left shows the slicing of a frozen plastic layer. To the right there is a sample mounted between two glass plates. At the bottom left, the sample is illuminated with polarized light. At the bottom right you can see a black enclosure of degraded material in the bottom of the part (see red arrow). [Photo: DuPont]
109