What are the factors that affect the gloss of plastics? How to improve plastic gloss?

Gloss is an important appearance attribute of plastics, and it is an important indicator to test the appearance quality of plastics. However, in the production process, due to the influence of raw materials, injection molds, injection molding processes, etc., it is very easy to have inconsistent gloss between batches. In order to effectively control the appearance quality of plastics and improve the gloss of plastics, it is necessary to understand the factors affecting the gloss of plastics.

The glossiness of plastics usually refers to the specular reflection ability of the plastic surface to light. It mainly refers to the physical quantity that evaluates the ability of the material surface to reflect light under a set of geometrically specified conditions, and also has the reflection property of direction selection. The specific gloss of plastics is often detected by a professional gloss meter. Instruments are of great use in the plastics industry. Under normal circumstances, the refractive index of plastics ranges from 1.4 to 1.6, and the gloss is generally within 70 to 110 gloss units. However, the reflection of transparent plastics and translucent plastics is different and cannot be generalized. The purpose of testing the gloss of plastic is to check and evaluate the quality of the product based on the data, and to indicate whether the production conditions are related to the gloss of the material surface.

Factors Affecting Plastic Gloss

In the production process of plastics, the main factors that cause poor plastic gloss are: raw materials, injection molds, injection molding processes, etc.

1 Raw materials

(1) The difference in particle size of raw materials is large, making it difficult to plasticize uniformly, and the gloss is poor.

(2) Too much recycled material or nozzle material is added to the raw material, which affects the uniform plasticization of the melt and causes poor gloss.

(3) Some raw materials will decompose and discolor during temperature adjustment, resulting in poor gloss.

(4) The content of moisture or volatile matter in the raw material is too high, which volatilizes into gas when heated, and condenses in the cavity and melt, resulting in poor gloss of plastic parts.

(5) The dispersibility of some additives is too poor and the gloss of the plastic parts is poor.

(6) The raw materials are mixed with foreign matter, miscellaneous materials or incompatible materials, which cannot be evenly mixed with the raw materials, resulting in poor gloss.

(7) If the amount of lubricant is too small, the fluidity of the melt is poor, and the surface of the plastic part is not dense, resulting in poor gloss.

2 Injection molds

(1) If the mold cavity is not processed well, such as scars, micro-holes, wear, roughness, etc., it will inevitably reflect on the plastic parts, making the plastic parts poor gloss.

(2) If there are oil stains, water stains on the surface of the cavity, or too much release agent, the surface of the plastic part will be dark and dull.

(3) If the demoulding slope of the plastic part is too small, it is difficult to demold, or the force during demolding is too large, so that the surface gloss of the plastic part is not good.

(4) If the mold is poorly vented, too much gas stays in the mold, which also leads to poor gloss.

(5) If the cross-sectional area of the gate or runner is too small or changes suddenly, the melt will be subjected to too much shear force when it flows in it, and it will flow in a turbulent dynamic manner, resulting in poor gloss.

3 Injection molding processes

(1) If the injection speed is too small, the surface of the plastic part is not compact and the gloss is poor.

(2) For thick-walled plastic parts, if the cooling is not sufficient, the surface will be hairy and the gloss will be darker.

(3) If the holding pressure is insufficient, the holding time is too short, so that the density of the plastic parts is not enough and the gloss is poor.

(4) If the melt temperature is too low, the fluidity is poor, and it is easy to cause poor gloss.

(5) For plastic parts made of crystalline resins, such as PE, PP, POM, etc., uneven cooling will lead to poor gloss.

(6) If the injection speed is too large and the gate cross-sectional area is too small, the vicinity of the poured product will be dark and the gloss will be poor.

Plastic gloss measurement method

Gloss is a property of the surface of an object, which can be expressed by the ability of the surface to produce specular reflections. Due to different raw materials and processing methods, plastic parts will cause great differences in appearance and gloss. If the surface gloss of plastic parts is uneven or the gloss of different batches of plastic parts is significantly different, it will affect the overall appearance.

For the detection of plastic gloss, the commonly used methods are visual evaluation and instrument detection. However, visual evaluation is easily affected by factors such as light and viewing angle. Therefore, for manufacturers with high gloss requirements for plastic parts, the gloss meter will be used directly.

Glossmeters basically consist of optics. It houses an incandescent light source, a condenser and a projector or source lens. The incident beam generated by these devices strikes the sample directly. A sensitive photodetector collects the reflected light and generates an electrical signal, which is amplified to excite an analog meter or digital display meter to show the gloss value. At present, many digital gloss meters on the market support automatic calibration, and users can measure the gloss of plastic samples using the calibrated instrument. When measuring, keep the measuring aperture and the plastic sample in close contact with no gaps, so as to avoid external light from affecting the accuracy of the measurement results. Since the gloss of different areas of the plastic sample may be different, in order to better represent the gloss of the sample, you can select multiple points on the surface of the plastic sample to measure, and then express it as an arithmetic mean. However, it should be noted here that different measurement angles are used to measure the gloss of plastics. When expressing the gloss of plastics, it is necessary to note the corresponding measurement angles.

How to improve plastic gloss?

Gloss is a very important indicator in the plastics industry. If the gloss meter is used to detect the lack of gloss on the surface of the product, it is necessary to take certain measures to improve the gloss of the plastic. You can start from resin and filler.

(1) Use resin with relatively high gloss

For the same resin, different polymerization processes and structures will result in different glossiness of the corresponding products. For PP, the order of gloss is as follows: PP-R>homopolymer PP>block copolymer PP. For PE, the order of gloss is as follows: LDPE>LLDPE>HDPE. For PVC, emulsion-process PVC resin has higher gloss than suspension-process PVC resin. For PS, the gloss of high-impact polystyrene (HIPS) is lower than that of general-purpose polystyrene (GPPS), but the gloss can be improved by adjusting the rubber particle size and morphology.

In addition, the same resin has different properties and different gloss levels: The higher the melt flow rate (MFR), the higher the gloss of the corresponding product; The molecular weight distribution is broadened, and the gloss of its corresponding products is reduced; Water absorption has a great influence on the gloss of corresponding products, such as PA, PI, PSF and PC, etc. If the drying is not carried out or the drying is incomplete, water ripples, bubbles, silver threads, spots, blemishes, etc. will appear on the outer surface of the product, which will greatly reduce the gloss of the outer surface.

(2) Select the appropriate filler shape and particle size

Different microscopic shapes of filler particles have different effects on the gloss of the product. The degree of influence is from small to large: spherical < granular < needle < flake.

The smaller the particle size of the filler, the smaller the reduction in gloss of the product. In addition, the distribution of filler particle size is different, and the impact on the gloss of the product is also different. The influence rule is: the wider the particle size distribution of the filler, the lower the gloss of the product. This is mainly because the larger the particle size difference of the filler, the more uneven the outer surface of the product is, and the more prone to diffuse reflection of incident light.

In addition, you can choose to add some plastic brighteners. The basic principle is to form a thin film layer on the surface of the product. By brightening the surface coating of the plastic product, the smoothness and gloss of the plastic product can be greatly improved.