Plastics - Characteristics, Processes and Common Defects

Plastics - Characteristics, Processes and Common Defects

1. Material Understanding

Plastic raw materials are processed from petroleum into mineral spirits and then decomposed into dyes or fillers to obtain various plastic raw materials, which are usually in granular form for ease of use. Production of raw materials such as ethylene and propylene. After polymerization and purification, plasticizers, lubricants, colorants or fillers are added to produce various plastic raw materials, which are usually in granular form for ease of use. It is an artificial resin material, consisting of organic synthetic resin as main raw material, with or without addition of other materials. Under certain conditions (such as temperature and pressure), it can be shaped into a particular shape through physical state transformation or cross-linking and curing.

Plastic classification:

(1) General purpose plastic: low polymer content, good fluidity, easy molding conditions, and suitable for various shapes such as ABS/SAN/PP/PS/PE/PVC, etc.

(2) Engineering plastics: their polymer has a high molecular weight, poor flowability and difficult molding conditions, but has high mechanical properties and is suitable for making machine parts that can replace metal (or glass). For example: PA/POM/PC/PET/PPO/PBT etc.

(3) Mixed plastics: It is a mixture of above two raw materials, which can improve mechanical engineering properties of conventional plastics or improve molding conditions of engineering plastic raw materials. For example: ABS+PC/PC+PBT etc.

Second, main characteristics of plastic

A: Easy processing, free coloring, light and strong, non-absorbent, non-conductive, non-thermal conductive, corrosion resistant.

B: But biggest disadvantage of plastic is that it is highly flammable, because plastic is recycled from oil, so it must be combustible. However, some compounds can be added to make it flame retardant. Commonly, addition of chlorine is used to make it a fire retardant plastic, and fire retardant grade is also divided into high and low, such as: V0, V2, and so on. V0: When plastic is ignited by fire, it will go out immediately if it leaves fire. V2: When plastic is ignited by fire, it will automatically go out after two seconds if it comes out of fire.

Third, performance and application of thermoplastics

Plastics - Characteristics, Processes and Common Defects

Properties and applications of thermoplastics

Fourthly, three main conditions of injection molding process: temperature, pressure, time

(1) Temperature: The temperature to be controlled in injection molding process includes cylinder temperature, nozzle temperature, etc. The temperature of material being plasticized (plasticizing temperature) and temperature of melt injected from nozzle (injection temperature) are mainly dependent on temperature of barrel and nozzle. In order to ensure that product has high shape and size accuracy, large warpage deformation after product is taken out of mold should be avoided, and mold temperature should be lower than thermal deformation temperature of plastic.

(2) Pressure: The pressure in injection molding process includes injection pressure, holding pressure and back pressure.

Injection pressure - Injection pressure is used to overcome resistance of melt flow from barrel into cavity, ensure filling rate and seal melt, etc.

Holding pressure and holding time. The holding pressure value depends on hydrostatic pressure of mold on melt and is related to shape and wall thickness of product. Generally speaking, for complex shapes and thin products, due to high injection pressure, holding pressure may be slightly lower than injection pressure. Holding pressure selection for thick-walled products is more difficult. When holding pressure is high, it is easy to increase molecular orientation, which makes product more clearly anisotropic. When holding pressure is equal to injection force, shrinkage rate of product can be reduced and batch size fluctuation of product is small, but it will cause more pressure on product.

Backpressure and screw speed - Backpressure refers to pressure on melt at top of injection screw as screw rotates and recedes, called backpressure. Back pressure mainly reflects plasticizing effect and plasticizing ability of materials, so it is also called plasticizing pressure. Increasing back pressure can not only expel air from material and improve density of melt, but also increase internal pressure of melt, reduce retraction speed of screw, enhance plasticizing shear effect, and increase heat of friction. The plasticizing effect is improved. However, if screw speed is not increased after increasing back pressure, melt will have a large backflow and leakage in screw groove, which will reduce plasticizing ability.

(3) Time: The time required to complete injection molding process is called injection molding cycle. It includes the following parts

Plastics - Characteristics, Processes and Common Defects

Injection molding cycle

In order to ensure quality of plastic products, each period of molding cycle should be as short as possible to improve productivity. Among them, most important are injection time and cooling time, which have a decisive influence on product quality.

5. Common defects in plastic products

(1) Gluing (also known as beer dissatisfaction): product is not ready due to insufficient filling (or insufficient injection pressure, mold nozzle too small).

Plastics - Characteristics, Processes and Common Defects

(2) White top: product formed due to poor demoulding or excessive pressure on ejector pin. White marks and tubercles appear on surface of the product.

Plastics - Characteristics, Processes and Common Defects

(3) Surface scratches: The product has scratches due to improper demoulding or artificial roughness and scratches, and corresponding scratches also occur on product due to cavity surface scars.

Plastics - Characteristics, Processes and Common Defects

(4) Deformation: Deformation of product is usually caused by insufficient cooling time of product or abnormal mold temperature.

Plastics - Characteristics, Processes and Common Defects

(5) Mixing colors: color and gloss of surface is different, marble-like color streaks appear, which easily appear when using beer dye powder or color inoculum.

Plastics - Characteristics, Processes and Common Defects

(6) Weld line: A line mark on surface of a product caused by incomplete fusion at an interface when two streams meet during molding.

Plastics - Characteristics, Processes and Common Defects

(7) Thickening: Adhesive flows out of gap of pressing surface of mold during beer molding process, and remaining material remains on edge of product, hair is pulled. This is called rough edge and flare edge.

Plastics - Characteristics, Processes and Common Defects

(8) Flower material: The surface of product has radial silver streaks. This is because plastic material is not completely dry and contains moisture. The temperature of plastic material will be too high.

Plastics - Characteristics, Processes and Common Defects

(9) Shrinkage: A local slump defect caused by shrinkage of product in mould. The surface of strut or thick rubber of general purpose product forms a concave seal.