Jackie

Jackie Lau, Senior Tooling Engineer and Expert of plastic injection molding, experienced in plastic injection molding and tooling for more than 15 years. The founder and Director of Eco Molding & Sositar Mould.

High impact polystyrene(HIPS)plastic injection molding resin material

by

HIPS plastic injection molding

HIPS(High impact polystyrene) is an impact-resistant polystyrene product created by adding butyl rubber particles to polystyrene. This polystyrene product is added with micron-sized rubber particles, and links the polystyrene and the rubber particles together by means of grafting.

When subject to an impact, the tip end stress of the crack propagation is released by the relatively soft rubber particles. Therefore, the crack propagation is hindered and the impact resistance is improved.

HIPS Engineering Plastic

Milky white opaque pellets, with a density of 1.05g/cm3, a melting temperature of 150-180°C, and a thermal decomposition temperature of 300°C. HIPS is one of the cheapest engineering plastics. When compared with ABS, PC/ABS and PC, the glossiness of the material is relatively poor, and so is the overall performance. HIPS is modified from PS plus butadiene. Since the impact strength of PS is very low, the product made from it is very brittle, but the toughness of butadiene is very outstanding. After joining with butadiene, the impact resistance of PS can be improved by 2-3 times. Although the impact strength of HIPS is much higher than that of PS, its comprehensive performance is not as good as that of ABS, and PC/ABS, etc. Among all engineering plastics, the impact resistance of HIPS is relatively low.

The flame-retardant high-impact HIPS material.

HIPS is a modified material of PS, which contains 5-15% of rubber component, so its toughness is about four times higher than that of PS, while the impact strength is greatly improved. It possesses the great advantages in moldability and tinting strength of PS. HIPS products are opaque. Thanks to its low water absorption rate, pre-drying is not required before production. Since the HIPS molecule contains 5-15% of rubber, the fluidity is affected to some certain extent, so the injection pressure and temperature are preferably higher. In view that it cooling rate is slower than that of PS, sufficient holding pressure, holding time and cooling time are required. Its molding cycle is slightly longer than that of PS, and the processing temperature is usually 190-240° C. There is a special “white edge” issue with the HIPS products, which can be improved by increasing mold temperature and clamping force, and reducing the holding pressure and time, etc., but welding lines is quite obvious on the product.

HIPS Properties:

HIPS are milky white and opaque pellets, with a high impact strength and toughness, and can be arbitrarily colored. In addition, it also features great moldability, chemical resistance, and electrical properties. Modified by rubber, although the impact strength and toughness of polystyrene are greatly improved, its tensile strength, bending, hardness, light resistance and thermal stability are lower when compared with the homopolymer.

Since it contains rubber content, the impact strength of high impact polystyrene is 5-10 times higher than that of GPPS, which has expanded the application range of polystyrene, allowing it to partially replace the expensive ABS materials.

HI-PS is a modified material of PS. It contains 5-15% of rubber component in the molecule. Its toughness is about four times higher than that of PS, with the impact strength greatly improved (high impact polystyrene). Now, there are flame retardant, stress crack resistant, high glossy, high impact, glass fiber reinforced and low VOC HIPS materials available. Other important properties of the standard HIPS: bending strength of 13.8-55.1MPa; tensile strength of 13.8-41.4MPa; elongation at break of 15-75%; density of 1.035-1.04g/ml; It possesses the great advantages in moldability and tinting strength of PS. HIPS products are opaque. Thanks to its low water absorption rate, pre-drying is not required before production.

Process Characteristics of HIPS:

Since the HIPS molecule contains 5-15% of rubber, the fluidity is affected to some certain extent, so the injection pressure and temperature are preferably higher. In view that its cooling rate is slower than that of PS, sufficient holding pressure, holding time and cooling time are required. Its molding cycle is slightly longer than that of PS, and the processing temperature is usually 190-240° C. The HIPS resin absorbs water very slowly, so drying is generally not required. Sometimes excessive moisture on the surface of the material will be absorbed, which will affect the surface finish of the final product. But, excessive water can be removed by drying at 160°F for 2-3 hours. There is also a special “white edge” issue with the HIPS products, which can be improved by increasing mold temperature and clamping force, and reducing the holding pressure and time, etc., but welding lines is quite obvious on the product.

Purposes:

High impact polystyrene (HI-PS) can be injection molded or extruded into various products, suitable for household appliances, electrical appliances, instrument parts & components, refrigerator lining, boards, television, tape recorder, telephone casing, stationery, toys, packaging containers, daily necessities, furniture, tableware, trays, structural foam products, disposables, instruments, household appliances and entertainment products, as well as the construction industry. The flame-retardant (UL V-0 and UL 5-V), impact-resistant polystyrene has been produced and widely applied in the production of TV cases, commercial machines and electrical products.

Steel hardness test method and conversion table

by

Steel hardness describes the properties of steels that resist deformation, indentation, penetration, and scratching. steel hardness is important because the inherent resistance of the surface to withstand friction or erosion by oil, steam, and water from injection molding process,there are several harness test method as below:

steel hardness test

1.Brinell Hardness (HB)
Press a hardened steel ball of a certain size (usually 10mm in diameter) against the surface of the material with a certain load (usually 3,000kg) for a period of time. After the load is removed, the ratio of the load to the area of indentation is referred to as the Brinell hardness (HB), measured in kg/mm2 (N/mm2).

2．Vickers Hardness (HV)
Use a diamond square cone presser to penetrate into the surface of the material with a load of 120kg or less, and an apex angle of 136°. Then, divide the area of indentation in the material by the load, and we get the Vickers hardness (HV) value (kgf/mm2).

3.Rockwell Hardness (HR)
When HB>450 or the sample is too small, making it impossible to apply the Brinell hardness, the Rockwell hardness measurement will take the place. A diamond cone with an apex angle of 120° or a steel ball with a diameter of 1.59-3.18mm is pressed into the surface of the material to be tested under a certain load, and the hardness of the steels is determined based on the depth of the indentation. According to the hardness of the test material, it is measured by three different scales:

The Rockwell Hardness is comprised of three different standards, i.e., HRA, HRB and HRC, referred to A scale, B scale, and C scale.

HRA: The hardness obtained by using a 60kg load and a diamond cone indenter, applicable for materials with an extremely high hardness (such as cemented carbide).

HRB: The hardness obtained by using a load of 100kg and a hardened steel ball with the diameter of 1.58mm, applicable for materials with a lower hardness (such as annealed steel, and cast iron, etc.)

HRC: The hardness obtained by using a 150kg load and a diamond cone indenter, applicable for materials with a very high hardness (such as hardened steel).

Steel hardness conversion table

There are so many different types of steel hardness tests, a conversion table, such as the one below, allows us to take one measure and estimate the relative hardness on another measurement scale.

RmN/mm2 HV HB HRC RmN/mm2 HV HB HRC
250 80 76 – 1030 320 304 32.2
270 85 80.7 – 1060 330 314 33.3
285 90 85.2 – 1095 340 323 34.4
305 95 90.2 – 1125 350 333 35.5
320 100 95 – 1115 360 342 36.6
335 105 99.8 – 1190 370 352 37.7
350 110 105 – 1220 380 361 38.8
370 115 109 – 1255 390 371 39.8
380 120 114 – 1290 400 380 40.8
400 125 119 – 1320 410 390 41.8
415 130 124 – 1350 420 399 42.7
430 135 128 – 1385 430 409 43.6
450 140 133 – 1420 440 418 44.5
465 145 138 – 1455 450 428 45.3
480 150 143 – 1485 460 437 46.1
490 155 147 – 1520 470 447 46.9
510 160 152 – 1555 480 456 47.7
530 165 156 – 1595 490 466 48.4
545 170 162 – 1630 500 475 49.1
560 175 166 – 1665 510 485 49.8
575 180 171 – 1700 520 494 50.5
595 185 176 – 1740 530 504 51.1
610 190 181 – 1775 540 513 51.7
625 195 185 – 1810 550 523 52.3
640 200 190 – 1845 560 532 53
660 205 195 – 1880 570 542 53.6
675 210 199 – 1920 580 551 54.1
690 215 204 – 1955 590 561 54.7
705 220 209 – 1995 600 570 55.2
720 225 214 – 2030 610 580 55.7
740 230 219 – 2070 620 589 56.3
755 235 223 – 2105 630 599 56.8
770 240 228 20.3 2145 640 608 57.3
785 245 233 21.3 2180 650 618 57.8
800 250 238 22.2 660 58.3
820 255 242 23.1 670 58.8
835 260 247 24 680 59.2
850 265 252 24.8 690 59.7
865 270 257 25.6 700 60.1
880 275 261 26.4 720 61
900 280 266 27.1 740 61.8
915 285 271 27.8 760 62.5
930 290 276 28.5 780 63.3
950 295 280 29.2 800 64
965 300 285 29.8 820 64.7
995 310 295 31 840 65.3
860 65.9
880 66.4
900 67
920 67.5
940 68

TPE,TPR,TPU and TPV injection molding

by

Thermoplastic Elastomer (TPE)

TPE-TPV-TPR-TPU injection molding

Domestically in China, it is usually known as “TPE”, basically referring to the styrene-based thermoplastic elastomer.

The styrene-based TPE (known as TPS in foreign markets) is a copolymer of butadiene or isoprene and styrene block, so its performance is similar to that of SBR.

Strengths: High elasticity of rubber and plastic injection moldability; environmentally friendly, non-toxic and safe; excellent coloring, smooth touch, weatherability, fatigue / temperature resistance, excellent processability, no need for vulcanization, and able to be recycled for cost reduction. It can be co-molded with such base materials as PP, PE, PC, PS and ABS, or molded separately.

Weaknesses: The biggest problem with SBS and SIS is their low heat resistance – the temperature should not exceed 80°C. At the same time, its strength, weatherability, oil / wear resistance, etc. are also incomparable to those of rubber.

Styrene-based TPR: Thermoplastic Rubber

TPR was originally named by the Americans. Back then, there was no uniformed nomenclature for thermoplastic elastomers. It was customary to use the English acronym TPR for thermoplastic rubbers, and TPE for thermoplastic elastomers (both are used in related articles), which was applied to the simple blends of PP and EPDM. Later, the PP/EPDM-based TPV (originally referred to as TPR, and later renamed as TPV) of AES also followed this nomenclature. After that, it became popular among the Taiwanese, as well as in mainland China. Regardless of the foreign names, the TPR and styrene-based TPE currently mean the same thing in China, indicating that it is a thermoplastic styrene-butadiene-styrene copolymer, which is commonly known as thermoplastic styrene-butadiene rubber, i.e., in China, TPE = styrene-based TPE.

However, due to historical and translation issues, the Chinese name for TPV (thermoplastic vulcanizate) is referred to as thermoplastic rubber, i.e., TPR. That is why we don’t tell our customers that our PP/EPDM-based TPV is referred to as TPR, because some customers take it for the styrene-based elastomer. If it is an SBS-based material, the price would be much lower than that of TPV.

Thermoplastic Polyurethane (AKA TPU)

The polyurethane-based TPE is a thermoplastic polyurethane rubber formed by block-bonding the hard segment of the isocyanate-reacted urethane and the soft segment of the polyester or polyether, abbreviated as TPU.

Strengths: TPU boasts an excellent mechanical strength, as well as resistance to wear, oil and flex. Especially, its wear resistance is the most outstanding; temperature resistance: -30-120°C.

Weaknesses: Poor resistance to heat, hot water and compression; prone to yellow discoloration, and mold sticking; requiring high precision processing, leading to a higher cost.

At present, wherever PVC is used, TPU can always be a substitute for it. However, in terms of the strengths of TPU, PVC is far behind. Its hardness ranges from 65A to 85D. The colors are natural, transparent, or highly translucent. TPU not only features excellent tension, tensile strength, toughness and aging resistance, but is also a mature environmentally-friendly material. At present, TPU is widely used in: shoe materials, ready-to-wear, inflatable toys, equipment for water / underwater sports, medical equipment, fitness equipment, car seat materials, umbrellas, luggage, and leather bags, etc.

TPV – Thermoplastic Vulcanizate

Dynamic vulcanization refers to the chemical cross-linking of the rubber phase under the action of the cross-linking agent when the thermoplastic resin is melt-blended with the rubber, and is sheared and broken into a large number of micro-sized and crosslinked rubber particles by a strong mechanical shearing action, which are then dispersed in a continuous phase of the thermoplastic resin.

Taking PP as the hard segment and EPDM as the soft segment for melt blending, a cross-linking agent for vulcanization is added during the melt blending process. The high shearing force of such machines as an internal mixer or a screw machine, is leveraged to disperse the fully vulcanized and crosslinked EPDM rubber particles into the PP matrix in micron size (less than 1 micron).

  •  TPV possesses the heat resistance and low-compression deformation properties of the vulcanized rubber;
  • Excellent processability: It can be processed by injection molding, extrusion and other thermoplastic processing methods. It is efficient, simple and easy to operate, and requiring no extra equipment, with a high fluidity and small shrinkage.
  • Low relative density (0.90-0.97), uniform appearance, high surface quality and smooth touch.
  • Usually, TPV can be used at the temperatures from -60°C to +135°C; when not subjected to load, it is able to withstand the high temperature of up to +150°C.
  •  A wide range of hardness, 25A-70D;
  • TPV boasts excellent acid and alkali resistance, and is strongly resistant to acid-based detergents. It is able to maintain its original characteristics after long-term application in acidic environment.

Warpage injection molding defects causes and solutions

by

Warpage as one of the most common defects in a plastic injection molded product, refers to the situation in which the shape of the injection molded part deviates from the shape of the mold cavity. The following is a brief analysis of the factors that affect the warpage of the injection molded products.

warpage injection molding defects

How the mold structure influences the warpage of an injection molded product

With regard to the mold, the main factors that affect the deformation of a plastic part are the gating system, the cooling system and the ejection system

Gating System

The gate location, type and quantity of a plastic injection mold affect the filling status of the plastic in the mold cavity, leading to changes to the plastic part. For some flat plastic parts, if only one central gate is employed, the molded plastic part will be distorted, as the shrinkage rate along the diameter is greater than that along the circumference; if multiple pin-point gates or the film gate is used, product warpage defects can be effectively prevented.

Cooling System

During the plastic injection molding process, the ununiform cooling rate of the plastic part will also cause ununiform shrinkage in the part, which leads to the generation of the bending moment, and therefore warpage of the plastic injection molded part.

During the plastic injection molding of a flat plastic part (such as the mobile phone battery case), if the temperatures in the mold cavity and the core differ greatly, the molten plastic close to the surface of the cold mold body is quickly cooled down, and that close to the surface of the hot mold cavity continues to shrink. Such an ununiform shrinkage will cause part warpage. In addition to considering the temperature balance inside and outside the plastic part, it also needs to be ensured that the temperature on each side of the plastic part is consistent, i.e., during mold cooling, the temperatures of mold core and the mold cavity should be kept as consistent and uniform as possible, so that an uniform cooling rate throughout the plastic part is ensured to effectively prevent the occurrence of warpage.

Ejection System

The design of the ejection system also directly affects the warpage of a plastic injection molded part. If the ejection system is not designed with a balanced layout, it will cause an imbalance in the ejection force, which leads to plastic product warpage. The cross-sectional area of the ejector pin should not be too small, so as to prevent the plastic part from being deformed by excessive force per unit area (especially when the temperature is too high during ejection). The ejector pins should be placed as close as possible to the area with a large ejection resistance.

How plasticization process influences product warpage

Plasticization refers to the process in which the plastic is converted from vitreous granules into a viscous melt. During this process, the temperature differences of the polymer along the axial and the radial directions (relative to the screw) will cause stress in the plastic material; in addition, the injection pressure, speed and other parameters of the injection molding machine have a great impact on the degree of molecular orientation during plastic filling, which causes warpage, or deformation.

How plastic filling and cooling influence product warpage

cooling factors for warpage

The effect of temperature on product warpage is reflected in the following aspects:

  • The temperature difference between the upper and lower surfaces of the plastic part may cause thermal stress and therefore thermal deformation;
  • The temperature difference between different areas of the plastic part will cause uneven shrinkage between different areas;
  • Different temperature conditions influence the shrinkage of a plastic part.

How product ejection influences product warpage

The imbalanced ejection force, unsteadiness in the movement of the ejection mechanism, or the inappropriately designed ejection area easily causes product deformation . At the same time, the stress that is “frozen” in the plastic part during the filling and cooling processes will be released in the form of “warpage” due to the loss of external constraints, resulting in product deformation.

How product shrinkage influences product warpage

The immediate cause of warpage defects of an injection molded product is the uneven shrinkage of the product. If the influence of shrinkage during the filling process is not taken into consideration during the mold design stage, the geometry of the product will vary greatly from the design requirements. For warpage analysis, shrinkage itself is not important, but what is important lies in the shrinkage difference.

How residual thermal stress influences product warpage

During the plastic injection molding process, residual thermal stress is an important factor that causes product warpage, and thus exerts a great influence on the quality of an injection molded product. Since the influence of residual thermal stress on product warpage is very complicated, it will not be detailed here.

How the metal insert influences product warpage

With regard to plastic injection molded products with inserts, it is easy to cause distortion (even cracking), since the shrinkage of plastic is much higher than that of metal; to improve this, the metal part needs be preheated (generally at a temperature of no lower than 100°C), before being put into production.

Conclusion

There are many factors that affect the warpage defects of an injection molded product. The structure of the mold, the thermophysical properties of the plastic material, as well as the conditions and parameters of the molding process all exert different influences on the product warpage defects. Therefore, when handling the warpage of a molded product, we must take all the above factors into consideration.

Methods for Coloring Plastics resin

by

A variety of plastics coloring systems can be used to get color for the plastic materials, each possessing its unique features, benefits, and weaknesses.now we share some plastics coloring process as below:

Select a Color Scheme

According to the plastic properties, the molding process, the toner characteristics, the color matching principles, the product requirements and other comprehensive considerations, match the various toners, and then achieve the required color requirements. Each color has a certain hue, a certain brightness, and a certain degree of saturation, so you can call up tens of thousands of different colors by changing these three factors. Therefore, during toning, it is the most basic technique to adjust the three factors of depth, hue and difference. The toning principle requires us to adjust the depth first and then the hue, because when the depth changes, the hue will definitely change.

The most porpular color matching systems include Panotone and RAL,please click below links for detail:

PANTONE-color chart

Pantone color chart

RAL Color Chart

coloring plastics

We always use this two systems to coloring plastics resin.

Adjust the Depth

According to the target sample, observe the degree of penetration and the depth of the hue, to determine the proportion of black and white in the composition; or for colors other than black and white, determine the color toner concentration or the proportion of the fluorescent toner in the color. The tinting strength of the toner can be identified by using various toners on the same plastic substrate. For instance, the PP material is added with 20g, 50g, 100g, or 300g, etc., to sufficiently determine its coloring degree. At the same time, we must understand the color change and concentration change of each color pigment after a certain proportion of titanium dioxide is added. In the process of coloring plastics, it needs to be clear whether the color is deep or dark in the depth direction. If it is deep, the hue can only be added with the toner. If it is dark, the toner can be added with the black color. Of course, a small amount of black can be added to increase the depth of a dark phase. In the shallow direction, the amount of titanium dioxide should be determined according to the degree of solid color. If it is not solid enough, it needs to be added with more titanium dioxide. In the meantime, other toners should be added in proportion, then estimate the proportion of colorants according to the shade of the hue and the tinting strength of various colorants.

Adjust the Hue

Theoretically, most of the colors can be matched with the mixture of the three primary colors of red, yellow and blue, but in fact, the colors of the various colorants are not simple colors but in between them, with the shade of the adjacent color. For example, the red toner contains yellowish red and blueish red, the blue toner contains reddish blue and greenish blue, and the yellow toner contains greenish yellow and reddish yellow. In the process of toning, pay attention to the complementarity of shades. For example, when the color is bright green, you can use the indigo green directly. If you need a darker green, you should choose greenish blue and greenish yellow to match the color, but not with the complementary reddish blue and greenish yellow.

Adjust the Difference

After estimating the depth and hue, the basic formula can be determined. After sampling, check the sample against the standard color, and then perform color difference correction. The color difference is related to the deviation of the depth, vividness, brightness, and hue. First of all, we need to determine what has caused the color difference – use the black and white toner to adjust the shade, and add a proper amount of toner, fluorescent power or whitening agent to adjust the vividness and brightness, while the hue can be adjusted by increasing or decreasing the amount of the toner or adjustment of the complementary colors, but be aware that the complementary colors may darken the color.

Pantone Matching System Color Chart

by

The Pantone Color Matching System is largely a standardized color reproduction system. By standardizing the colors, different manufacturers in different locations can all refer to the Pantone system to make sure colors match without direct contact with one another.used in a variety of industries notably graphic design, fashion design, product design, printing and manufacturing and supporting the management of color from design to production, in physical and digital formats, among coated and uncoated materials, cotton, polyester, nylon and plastics.

Please click below link for Pantone color chart

PANTONE-color chart

Pantone color chart

RAL Color Chart

by

RAL COLOR CHART

RAL is an European color matching system which defines colors for paint, coatings and plastics. The RAL color standard is administrated by the RAL Deutsches Institut für Gütesicherung und Kennzeichnung. ‘RAL’ is the abbreviation of ‘Reichs-Ausschuß für Lieferbedingungen und Gütesicherung’.

Please click the link as below for RAL Color reference

RAL Color Chart

RAL COLOR K7

PPS injection molding plastic resin

by

Poly phenylene sulfide, abbreviated as PPS, is a new type of engineering plastic, which is mainly divided into two categories: one is a branched thermoplastic polymer with a high viscosity, and the other is a thermosetting polymer which has a linear molecular structure before curing, and can be softened to a certain extent if sufficiently heated after curing. In the following, we will mainly introduce the thermosetting PPS.

PPS plastic injection molding

The Properties of PPS

  1. Physical Properties

PPS is a white, highly crystalline polymer with a density of 1.34. It is characterized by excellent mechanical properties, with tensile strength and flexural strength superior to that of PA, PC, and PBT, etc. It possesses extremely high rigidity and resistance to creep, but high brittleness and low notched impact strength that is even lower than that of PA, PC and PBT, but higher than that of POM. After being reinforced by glass fiber, better mechanical properties can be obtained. PPS is an inert and non-toxic substance.

  1. Thermal Properties

Since PPS is a crystalline polymer, the highest crystallinity can reach up to 65%, its crystallization temperature is 127°C, melting point is 286°C, and the heat distortion temperature is 260°C. With a thermal stability far exceeding that of such engineering plastics as PA, PBT and POM, it only decomposes when the air temperature reaches above 430-460°C, so its long-term application temperature is the highest among all thermoplastics, reaching up to 220-240°C. PPS also has great thermal insulation and flame retardancy performances. Its critical oxygen index is equivalent to that of PVC, up to 47%, so there is no need to add flame retardant, because PPS is able to reach the UL94 V-0 flammability rating.

  1. Electrical Properties

PPS has a symmetrical molecular structure, no polarity and a low water absorption rate, so it boasts an excellent electrical insulation performance. Compared with other engineering plastics, its dielectric constant is low, and its arc resistance is equivalent to that of thermosetting plastics. Under such conditions as high temperature, high humidity and frequency conversion, PPS can still maintain outstanding electrical insulation. Conductive PPS composites can be obtained by adding conductive fillers for antistatic and electromagnetic shielding effects.

  1. Chemical Resistance

It is insoluble in any organic solvent below 200°C. It is able to withstand the erosion of all kinds of acids, alkalis and salts, except the strong oxidizing acid. It still maintains a high strength after long-term immersion in various chemicals under high temperature conditions. PPS also boasts great weatherability and resistance to radiation.

PPS Injection Molding Processability

1). Water Absorption

PPS features a low water absorption of only 0.02%.

2). Fluidity

With great fluidity, PPS can be used to make thin-walled products. However, if the temperature is too high or the material stays in the barrel for too long, the material will partially crosslink, resulting in a low fluidity.

3). Crystallinity

PPS is a crystalline polymer, of which the crystallinity varies depending on the cooling temperature and rate during the plastic injection molding process. The faster the cooling rate, the lower the crystallinity. The degree of crystallinity has a great influence on its strength, thermal resistance, weatherability and dimensional stability. As the degree of crystallinity increases, the heat deflection temperature (HDT) of the product rises, with increased rigidity, surface glossiness, surface shrinkage, and dimensional stability.

4). Thermal Stability

PPS will undergo partial oxidation and crosslinking reaction if exposed to high temperature conditions for a long time, resulting in a decreased fluidity and a deeper color of the material, which will affect the quality and performance of the product. In addition, PPS is strongly adhesive to metals, so it is necessary to prevent the material from solidifying in the barrel.

5). Shrinkage

For the crystalline plastics, the crystallinity increases as the mold temperature increases during the plastic injection molding process, while the shrinkage rate increases with the increase of crystallinity. Therefore, the shrinkage increases with the rise of mold temperature. Generally speaking, the shrinkage rate of PPS is low, but the shrinkage perpendicular to the flow direction is 2-4 times higher than that in the flow direction. Product thickness, shape, and injection speed also the factors that influence the shrinkage rate.

6). Secondary Processing

PPS products can be processed by machining, ultrasonic welding, adhesive bonding, etc., such as cutting and tapping.

Molding Preparation: Conventional injection molding machines can all be used for PPS processing. It is recommended to use wear-resistant barrels, screws and molds that are suitable for such fillers as fiberglass and minerals.

Drying Preparation: Although PPS absorbs very little water in a humid environment, drying is necessary if you want to get high quality products. Please refer to the following drying conditions:

120℃: 4-6 hours

130℃: 3-5 hours

140℃: 2-3 hours

If the drying temperature is too high or the drying time is too long, the color or fluidity of the raw material may change.  The melting point of PPS is 280°C, so the barrel temperature of 300-340°C is the most commonly used for most applications.

Mold Temperature

The mold surface temperature should be no lower than 120°C. Usually, 130-150°C is recommended. Higher mold temperatures ensure a high degree of crystallization, a smooth product surface and reduced shrinkage after molding. If a lower mold temperature is required in special circumstances, you should try to avoid the temperature range of 80-100°C, which may cause the vitrification of PPS, thus making it hard for ejection.

Injection Speed

To prevent melt cooling / curing during the injection process, a higher injection speed is necessary. The optimal product appearance is usually obtained at an appropriate speed. The filling time is usually 0.5-1.5 seconds, but excessively high speed may cause warpage and burn marks.

Injection Pressure

Since a higher injection speed is recommended, a higher injection pressure is often adopted to keep the pressure at 50MPa or higher.

Screw Speed & Back Pressure

The plasticization process requires a low to medium screw speed of 40-150rpm, usually with a low back pressure of 1-2MPa. If the back pressure is not stable, 3-4MPa is recommended. If both the screw speed and the back pressure are too high, the glass fiber will be destroyed, which decreases the physical properties of the final products.

Flash plastic injection molding defects

by

Flash, also known as flashing or burrs, mostly occurs between the matching parts of a mold, such as the parting surface of the mold, the sliding part of the slider, the clearance of the insert, and the gap of the ejector, etc. If the flashing is not resolved in a timely manner, it will be further worsened, so that the mold will be pressed to cause local collapses or other permanent damages. The flashing occurring in the clearance of the insert and the gap of the ejector will also cause the product to stick on the mold, thus affecting ejection.

Flash injection molding defects

Flash is actually the excessive plastic material which enters the matching gaps of the mold and is attached to the final product after cooling. It is very easy to solve the problem of flashing, i.e., keeping the melt out of the matching gaps in a mold. The plastic melt usually enters the mold matching gaps in two scenarios: one is that the mold matching gap is large, making it easy for the melt to get in; the other is that the mold matching gaps are not large at all, but the melt forces its way in due to high pressure. Seemingly, flashing can be completely solved by enhancing the manufacturing precision and the strength of the mold. Surely, it is necessary to improve the manufacturing precision, and reduce the matching clearance of the mold, so as to prevent the molten plastic from entering the gaps. However, the strength of the mold, in many cases, cannot be infinitely strengthened to keep the melt out under any pressure conditions.

The flash problem lies not only in production, but also in the process. To check the process, first check whether the clamping force is sufficient, and only check the mold itself if flashing still occurs after sufficient clamping force is ensured.

Ways to check if the clamping force is sufficient:

1). Gradually increase the injection pressure.  If flashing increases as the pressure goes higher, and the flashing mainly occurs on the mold parting surface, it indicates that the clamping force is insufficient.

2). Gradually increase the clamping force of the injection molding machine. If the flash on the parting surface disappears when the clamping force reaches a certain value, or the flashing on the parting surface no longer worsens when the injection pressure is increased, it is considered that the clamping force is sufficient.

Ways to check if flash is caused by problems lying in mold precision:

Fill the mold cavity just right with a low material temperature, a low injection speed, and a low injection pressure (the product may shrink slightly). At this point of time, it can be considered that the melt is not very likely to enter the mold matching gaps. So, if flashing occurs, it can be determined that the mold manufacturing precision is problematic, thus mold repair is needed. You can give up the idea of solving the flashing issue by looking into the process.

It should be noted that the above-mentioned three “low” conditions are indispensable, as high melt temperature, fast injection speed, and high injection pressure will lead to partial pressure increase in the mold cavity, which may help the melt enter the mold matching gaps. As a result, the mold is forced open, though the cavity is not fully filled at the moment.

The causes of flash are to be analyzed on condition that the clamping force is high enough. If the clamping force is insufficient, it is difficult to analyze the reason for flashing. So, please be aware that the following analysis is based on the assumption that the clamping force is sufficient.

How to improve the life of plastic injection mold?

by

A plastic injection mold is one of the most important tooling for injection molding, whose quality directly determines the quality of the final product. In the meantime, the mold cost takes up a great proportion in the production cost of an injection molder. If the injection mold life is short and the precision is not well kept, not only will the product quality be affected, but also huge waste will be caused, including material waste and labor waste during plastic injection molding. Therefore, increasing the service life of injection mold has a significant impact on reducing the cost of plastic products, as well as improving tooling productivity and company competitiveness.

injection mold life

During the plastic injection molding process, some plastics will decompose and thus release some corrosive gases under high temperature and high pressure conditions, which causes corrosion and damage to the mold surface. When the mold is damaged, or the shape, dimensional accuracy and surface finish of the plastic part are not up to standard due to the excessive wear of the mold, severe flashing will occur, and if the mold is beyond repair, mold failure will be inevitable. The number of molding cycles or the total number of parts molded before the mold fails is referred to as the service life of a mold. Usually, an injection mold has a service life of more than 300,000 cycles.

Factors that affects the service life of injection molds, and the basic ways to improve the service life

Different injection temperatures and pressures are required for different raw plastic materials, while different working conditions have different effects on the service life of a mold, and different plastic parts also cause different level of tear and wear to the mold. Therefore, under the prerequisite of meeting the product performance and quality requirements, it is usually necessary to select a raw plastic material with great processability to produce the molded product. This is beneficial to both the product molding process and the improvement of the mold service life.

Speaking of mold structural design, the cavity and the core are the main molding parts to shape the plastic products – the cavity shapes the exterior surface of a plastic part, and the core shapes its inner surface. Cavities and cores of different structural designs have different levels of strength and rigidity, thereby different levels of convenience with regard to the repair and replacement of wear parts. Therefore, from the perspective of the service life of a mold, the structural form that is relatively good in strength and rigidity, while being easy to repair is able to extend the service life of a mold.

In addition to the molding parts, other structural parts in a mold should have sufficient strength and rigidity to withstand the mold opening & clamping forces, injection pressure and thermal stress during operation, so as to avoid excessive deformation.

The structure and precision of the guiding mechanism have a direct influence on the clamping of the core and the cavity, which affects the accuracy of the plastic part and the service life of the mold. Therefore, it is necessary to select an appropriate guide form and guide accuracy. Common injection molds mainly rely on the guide pin mechanism to ensure the matching precision. In high-precision injection molds, in order to ensure the matching precision of the mold core and the mold cavity, the tapered precision locating mechanism, or the cylindrical pin locating mechanism and the guide pin can be adopted.

During mold structural design, attention should also be paid to keeping the thermal balance of the mold. The appropriate design of the gating system, the temperature control system and the venting system is able to reduce the hot tearing tendency of the mold, thus improving its service life.

The mold material and the heat-treated injection mold cavity are usually more complicated, with relatively higher requirements for precision and surface roughness. The quality of the mold material influences the quality and service life of the mold. The selection of the mold material must meet the customer’s requirements for the product quality, taking the cost of the material and the strength during the preset period into consideration. During material selection, certain working conditions must be met, such as wear resistance, toughness, thermal resistance, cold / heat fatigue resistance and corrosion resistance.

Whether the heat treatment process is appropriate or not also greatly affects the service life of the mold. The factors affecting the quality of heat treatment include heating rate, quenching temperature, quenching cooling rate and tempering temperature. The heat treatment process should be reasonably carried out during the mold making process, with process conditions strictly controlled.

During the mold making process, the method and accuracy adopted for mold machining and surface finishing have a direct influence on the quality and service life of the mold. If the mold is not appropriately cut, sharp corners or excessively small radius of the fillet will be caused, which may lead to severe stress concentration during mold operation.

In order to ensure the precision of each part in a mold, high-precision machining methods, such as EDM, wire cutting and CNC should be appropriately selected in the mold making process, in a bid to improve the mold quality and service life.

Whether the mold is appropriately used and maintained is also a major factor affecting the service life. Whether the mold is appropriately set up and commissioned, whether the parameters of the injection molding machine are set in conformity with the design requirements during production, and whether the mold is regularly maintained as planned can all improve the service life of the mold to a certain extent.

Conclusion: The factors that influence the service life of the plastic injection mold are multi-faceted, so comprehensive measures should be taken to improve the service life of them. In the design, manufacturing and application process of the molds, it will help improve the mold quality and service life if you can select the appropriate raw plastic material, reasonably design the mold structure, select the appropriate mold material / heat treatment process / mold machining method, and properly use and maintain the mold.