PMMA acrylic injection Molding Defect Causes and Solutions
The defects in the injection molding of acrylic PMMA usually include: short shot, silver streaks, sink marks, voids, jetting, weld marks, gray / black specks, cracking, warpage, and material impurity, while the preventive measures against them are usually: Short Shot This is caused because the resin does not fill the cavity fully. To prevent its occurrence, we can take the following measures: 1. Adjust the supply quantity of acrylic pellets 2. Increase the injection pressure 3. Increase the injection speed 4. Raise the barrel temperature 5. Increase the screw backpressure 6. Raise the mold temperature for molding of thin-walled products 7. Extend the pressure holding time during injection 8. Extend the molding cycle 9. Increase the cross-sectional area of the … Read more
Plastic annealing and Moisture conditioning process for plastic product
Plastic annealing process The purpose of plastic annealing is to eliminate internal stress. For example, in the area where the thin and thick parts meet, the cooling of the thick part is slower than that of the thin part, causing the joint to shrink unevenly. As a result, there is stress concentration in the area. This phenomenon is more obvious around metal inserts. If annealing is not performed, cracking or even deformation may occur in the stress concentration area after a period of time. Annealing method: Usually, the product is immersed in hot oil, hot water, or cycling hot air. The annealing temperature is adjusted according to the type of the plastic, but usually, it is 10-20°C lower than the … Read more
What causes ejector pin marks in injection molded parts?
Ejector pin marks usually refer to the glossy or white imprints on the surface of a injection molded product, as well as the different levels of glossiness – dark or shadow, without convex or concave – visible directly opposite to the ejector pin location. Factors that cause ejector pin mark 1. Product design: Inappropriate product design: 1). The thickness of a product is an important factor to consider during structural design. Increasing the thickness of a product is able to not only improve the filling performance of the resin, but also reduce the orientation stress as well as product deformation. On the contrary, the excessively thin wall will make it difficult to fill the cavity, and accordingly require the increase … Read more
What causes cracking in plastic injection molded parts
Cracking is a commonly seen defect in plastic injection molded products, of which the main causes are stress and deformation, mainly including residual stress, external stress and product deformation caused by external environment. (I) Cracking caused by residue stress The residual stress is mainly generated in the following three scenarios, i.e., overfilling, product ejection and metal inserts. The cracking generated in the case of overfilling can be solved from the following aspects: (1) Since the direct gate is able to minimize pressure loss, if cracking is mainly generated around the direct gate, you can opt for the multi-point gate, the side gate or the tab gate. (2) On condition that the resin material does not decompose or deteriorate, appropriately increasing … Read more
What causes black specks/black streaks in injection molding parts?
Black specks/streaks and impurities are the most important factors that lead to rejects in the normal production process. It mainly affects the appearance of the product and leads to scrap. Most of the impurities and black specks are usually caused by foreign substances, i.e., they have nothing to do with the raw material itself. Only a small portion of the black specks and impurities are caused by the raw material itself. Black specks and impurities are characterized by small dark brown particles, which generally do not reflect light. When the particles are large, the impurities are layered, brittle, fragile and porous after being broken. Some of them are irregularly scattered on the whole, some are scattered randomly in local … Read more
What causes parts sticking or sprue sticking during injection molding?
The causes of parts sticking and sprue sticking are multifaceted, among which mold failure is one of the main reasons. The reasons and corresponding solutions are described as follows: 1. The surface of the mold cavity and sprue bush is rough. If there are surface defects such as gouges, nicks, scars, and dents in the mold cavity and sprue bush, the plastic parts and sprue will easily stick to the mold, resulting in difficulty in ejection. Therefore, the surface finish of the cavity and sprue should be improved as much as possible. The inner surface of the cavity should preferably be chrome-plated. When polishing, the movement direction of the polishing tool should be the same as the direction of mold … Read more
What Causes Jetting in Plastic Injection Molding Product?
Jetting, also known as jet, worm track, or snake-like pattern, refers to the snake-like curves on a plastic injection molded part along the flow direction from the gate, as shown in Figure 1. Under normal circumstances, the molten plastic fills the mold cavity in a “fountain flow” manner, as shown in Figure 2, which depicts the “fountain flow” in detail. However, when the molten plastic flows at a high speed through narrow areas, such as the nozzle, the runner and the gate, and then suddenly enters an open and relatively wider area, the plastic melt will be injected from one end to the other of the cavity in the form of jet streams, creating folded strips. The melt that enters … Read more
What Causes Scratches in Injection Molded Products?
The scratches (scrape marks)on the appearance of an injection molded product usually occur on such deep-cavity products as chargers, routers, and chassis, etc. It is also a particularly common problem during the injection molding process, known as one of the 10 most challenging problems in the mold and injection molding industry. Glossy products, as well as products with textured sides are easily scratched. What on earth has caused it? When it comes to the scratches on the textured surface, the first thing that comes to the mind of an engineer is that the draft angle on the side of the product is not large enough, but why the scratch still exist after the draft angle is increased? So, solving the occurrence … Read more
What Causes Weld Lines in Plastic Injection Molding Parts?
The definition of weld lines: When multiple plastic flow fronts meet in the cavity after encountering inserts, holes, or areas with inconsistent flow rates or interrupted melt flows, or in the case of gate injection filling, the inability of two or more flow fronts to fully “knit” together will cause the linear weld marks. The appearance of weld lines greatly reduces the mechanical strength of the product. The solution to the weld lines is basically the same as the method for reducing product sink marks. I. Equipment To respond to poor plasticization and uneven melt temperature, the molding cycle can be extended to make plasticization more complete, and select the molding machine with a higher plasticizing capacity when necessary. II. … Read more
What Causes Burn Marks in Plastic Injection Molding?
Factor 1: Melt fracture causes burn marks When the melt is injected into a large-sized mold cavity under high-speed and high-pressure conditions, melt fracture will be easily caused. At this point of time, the surface of the melt is laterally fractured, with the fracture area coarsely mixed to form burn marks on the surface of the plastic part. In particular, when a small amount of molten plastic is directly injected into an oversized mold cavity, melt fracture will be more serious, leaving larger burn marks on the product surface. In essence, melt fracture is caused due to the elastic behavior of the polymer melt. When the melt flows into the barrel, the melt close to the barrel is rubbed by … Read more