Molding Process
Plastic ABS can also be considered a modification of polystyrene, with higher impact resistance and better mechanical strength compared to HIPS. It has good processability and can be molded using plastic forming equipment such as injection molding machines, extruders, and the like for injection molding, extrusion, blow molding, calendering, lamination, foaming, and thermoforming. Additionally, it can be welded, coated, electroplated, and machined. ABS has a relatively high water absorption and requires drying before processing. The drying temperature should be between 70-85°C, with a drying time of 2-6 hours. ABS products are prone to internal stress during processing, which can lead to cracking if the stress is too great. To mitigate this, annealing is recommended by placing the parts in a hot air circulating oven at 70-80°C for 2-4 hours, then cooling to room temperature.
Extrusion Process
Extrusion. Plastic ABS is used to produce pipes, sheets, films, and profiles. The pipes can be used for various water pipes, gas pipes, lubricating oil and fuel oil transport pipes; sheets and films can be used for flooring, furniture, tanks, filters, wall partitions, and for thermoforming or vacuum forming. The screw length-to-diameter ratio (L/D) of the extruder is usually relatively high, between 18 and 22, with a compression ratio of (2.5 to 3.0):1. It is suitable to use a gradually tapered screw with a torpedo head. The temperatures of the barrel are as follows: hopper section at 150 to 160°C, front part of the barrel at 180 to 190°C, die head temperature at 185 to 195°C, and mold temperature at 180 to 200°C. Subsequently, the blow molding temperature can be controlled between 140 to 180°C.
Injection Molding Process
ABS resin is a terpolymer developed on the basis of modified polystyrene resin. In ABS, A stands for acrylonitrile, B represents butadiene, and S signifies styrene. ABS resin exhibits the combined properties of its three components: A enhances resistance to oil and chemical corrosion, thereby providing a certain degree of surface hardness; B imparts the toughness of a rubbery state to ABS, improving its impact resistance; S endows ABS plastic with good fluidity, allowing it to have excellent performance in thermoplastic molding processes.
In China, ABS plastic is primarily used for manufacturing the casings of instruments, household appliances, telephones, televisions, and for electroplated ABS plastic which is given a metallic sheen, allowing ABS to be used as a substitute for metal. The inner liners of various models of refrigerators produced by our factory, as well as a variety of other plastic products, are made of ABS injection-molded products, which account for more than 88% of the total number of plastic products used in refrigerators.
Craftsmanship
ABS belongs to the category of amorphous polymers and does not have a distinct melting point. Due to the wide variety of grades available, appropriate processing parameters should be established according to the different grades during the injection molding process. Generally, molding can be performed at temperatures above 160°C and below 240°C. This is because at excessively high temperatures, there is a tendency to damage the rubber phase in ABS, and decomposition begins to occur above 250°C. During the molding process, ABS exhibits good thermal stability, offering a broad processing window and is less prone to degradation or decomposition. Moreover, ABS has a moderate melt viscosity, and its flowability is better compared to polymers like polyvinyl chloride (PVC) and polycarbonate (PC). Additionally, the melt cools and solidifies fairly quickly, typically within 5 to 15 seconds.
The flowability of ABS is related to both injection temperature and injection pressure, with the latter being slightly more sensitive. Therefore, during the molding process, adjusting the injection pressure can help reduce melt viscosity and improve mold filling performance. Due to differences in component composition, ABS exhibits varying levels of water absorption and adhesion properties. The rate of surface water adhesion and absorption can range between 0.2 to 0.5%, and sometimes it may reach between 0.3 to 0.8%. To achieve more ideal products, drying treatment should be conducted before molding to reduce the moisture content to below 0.1%. Otherwise, defects such as bubbles and silver streaks may appear on the surface of the molded parts.
Injection Molding Equipment
Using a screw-type injection molding machine allows the plastic to be heated by electric heating rings and the frictional heat generated by the rotation of the screw within the barrel, pre-plasticizing and melting the material before injecting it into the mold cavity. After cooling, the mold opens and the product is ejected. This method achieves good plasticization, and the molding temperature can be slightly lower than that of other types of equipment (such as plunger-type machines), thus avoiding the damage high temperatures can cause to the rubber phase. It is important to note the following:
1) Each injection volume should be 50-75% of the machine's maximum injection capacity.
2) The screw should be single-headed, with equal pitch, a gradual profile, full threading, and equipped with a non-return ring. The screw's length-to-diameter ratio (L/D) should be 20:1, with a compression ratio of either 2:1 or 2.5:1.
3) The nozzle can be a general-purpose open type or an extended nozzle (with the extension not exceeding 150mm) to avoid using a self-locking nozzle, which could reduce the injection process efficiency or cause discoloration of the material. Additionally, a heating and temperature control device should be installed on the nozzle.
Product and Mold Design
Wall Thickness of the Product: The wall thickness of the product is related to the flow length of the melt, production efficiency, usage requirements, and other factors. The maximum flow length of the ABS melt to wall thickness ratio is approximately 190:1. This value can vary depending on the grade of the material. Therefore, the wall thickness of ABS products should not be too thin. For products that require electroplating, the wall thickness should be slightly thicker to increase the adhesion between the plating layer and the product surface. Consequently, it is appropriate to select a wall thickness between 1.5 and 4.5mm.
When considering the wall thickness of the product, attention should also be paid to the uniformity of the wall thickness; it should not vary too much. For products that require electroplating, their surfaces should be flat and free of bumps because these areas are prone to attract dust due to static electricity, which is difficult to remove and can lead to poor adhesion of the plating layer. Additionally, sharp corners should be avoided to prevent stress concentration. Therefore, it is advisable to use rounded arcs for transitions at corners, junctions of varying thicknesses, and other such areas.
2. Draft Angle for Demolding: The draft angle for demolding of a product is directly related to its shrinkage rate. Due to different grades, various product shapes, and molding conditions, the molding shrinkage rate can vary, typically ranging from 0.3% to 0.6%, and sometimes reaching 0.4% to 0.8%. Therefore, the dimensional accuracy of the product is relatively high. For ABS products, the draft angle should be considered as follows: for the core part, it should be 31° along the demolding direction, and for the cavity part, it should be 4° to 1°20' along the demolding direction. For products with complex shapes or those with letters or patterns, the draft angle should be appropriately increased.
3. Ejection Requirements: Since the apparent smoothness of the product greatly affects the plating performance, any minor scratches on the surface will become conspicuously visible after plating. Therefore, in addition to requiring that there be no scratches on the mold cavity, it is also necessary to have a large effective area for ejection. The synchronization of multiple ejector pins during the ejection process must be good, and the ejection force should be uniform.
4. Venting: To prevent issues such as poor venting during the molding process that can cause the molten material to burn, and prominent seam lines, it is required to create vent holes or slots with a depth not exceeding 0.04mm to facilitate the escape of gases generated by the molten material.
5. Runners and Gates: To ensure that the ABS melt can quickly fill all parts of the mold cavity, the diameter of the runners must be no less than 5mm, and the thickness of the gates should be at least 30% of the product's thickness. The straight part (the section that will enter the mold cavity) should be approximately 1mm in length. The location of the gates should be determined based on the product requirements and the direction of the material flow. For products that require electroplating treatment, gates are generally not allowed on the surface where the plating will adhere.
Preparation of Ingredients
The ABS resin used for injection molding, except for special grades or those that have been colored, is mostly in the form of light ivory or porcelain white opaque granules. The resin does not have very high water absorption; if it is below the allowable processing value of 0.1 to 0.2%, and the circumstances involve tightly packed, properly stored material, and the product requirements are not too demanding, it may be possible to proceed with molding without drying. However, if the moisture content in the granules exceeds the specified value, they must be dried before molding can take place. For special grades of granules or products with higher requirements (such as those for electroplating), drying must also be performed before the molding process.
Molding Process Specification
Injection Temperature: This includes the barrel temperature (which can be divided into rear, middle, and front sections), nozzle temperature, and mold temperature. Although the effect of temperature on the melt viscosity of ABS is not as significant as that of injection pressure, higher temperatures are advantageous for the molding of thin-walled products. The theoretical decomposition temperature of ABS can reach above 270°C; however, in actual injection molding processes, due to the influence of time and other processing conditions, the resin often begins to discolor at around 250°C. Additionally, the rubber phase contained in ABS is not suited to excessively high temperatures, as this can affect the performance of the product. Besides heat-resistant and electroplating grades of ABS resin, which require slightly higher temperatures (between 210-250°C) to alleviate difficulties in melt filling or to enhance electroplating performance, other grades such as general-purpose, flame-retardant, and impact-resistant ABS resins prefer lower temperatures to prevent decomposition or adverse effects on their physical and mechanical properties. Plunger injection machines generally select slightly higher temperatures than screw-type injection machines. For general products, plunger machines choose a temperature range between 180-230°C, while screw-type injection machines can mold at 160-220°C. During the molding process, the typical barrel temperature is (rear section 150-170°C, middle section 170-180°C, front section 180-210°C). The nozzle temperature is generally set at 170-180°C. It is particularly important to note that any changes in the homogenization section and nozzle temperature will be reflected in the product, causing defects such as flash, silver streaks, discoloration, poor gloss, and prominent weld lines.
2. Mold Temperature: Mold temperature plays a significant role in the surface roughness of ABS products and in reducing internal stresses within the products. Higher mold temperatures make it easier for the melt to fill the mold, resulting in a better appearance of the product, less internal stress, and also improved electroplating compatibility. However, this can also lead to issues such as higher shrinkage rates of the molded product, longer molding cycles, and a tendency for the product to warp after demolding. For products with general requirements, the mold temperature can be controlled between 40-50°C; for products with high demands on appearance and performance, the mold temperature can be controlled between 60-70°C. Additionally, the mold temperature must be uniform, and the temperature difference between the mold cavity and the core should not exceed 10°C. For products with deep holes or more complex shapes, the temperature of the mold cavity should be slightly higher than that of the core to facilitate smooth demolding.
3. Injection Pressure: Compared to plastics such as polyethylene, polystyrene, and nylon, ABS has slightly poorer flowability, hence it requires higher injection pressure. However, excessively high injection pressure can lead to difficulties in demolding or damage during demolding, and may also introduce significant internal stresses into the product. The injection pressure for ABS is not only related to the wall thickness of the product and the type of equipment used but also to the grade of the resin. Products with thin walls, long flow paths, and small gates require higher injection pressures, reaching up to 130-150 MPa, whereas thick-walled products with large gates can be produced with pressures of around 70-100 MPa. In actual production, screw injection machines commonly use injection pressures below 100 MPa (we use 50-70 MPa), while plunger injection machines generally operate at pressures above 100 MPa. The holding pressure should not be too high; for screw injection machines, a pressure of 30-50 MPa is typically used, while plunger machines require over 60-70 MPa. If the holding pressure is too high, it will increase the internal stress of the product.
4. Injection Speed. The injection speed plays a certain role in altering the flowability of the ABS substrate. If the injection speed is slow, the appearance of the product may exhibit ripples and poor welding effects. If the injection speed is fast, it can facilitate rapid mold filling, but it may lead to poor venting and rough surface finish, as well as a decrease in the tensile strength and elongation of the product. Additionally, the adhesion of the plating layer can be reduced due to excessively high injection speeds. Therefore, in the production process, except when high injection speeds are necessary due to difficulties in mold filling, it is generally advisable to use medium or low injection speeds.