Injection molded products have the characteristics of high precision, good consistency, and high surface quality. Through injection molding, products with smooth appearance and precise dimensions can be produced. The internal structure of the finished product is uniform, and the quality is stable and reliable. These advantages make injection molding widely used in fields such as electronics, healthcare, and automotive, and it is considered a high-quality and efficient production method.

The dimensional accuracy of molds directly affects the dimensional accuracy of plastic products. In the process of mold manufacturing, it is necessary to strictly control the dimensional tolerances, geometric tolerances, etc. of the mold cavity. For example, high-precision machining equipment such as CNC machining centers and electrical discharge machining machines are used to manufacture molds, and precise measurement and debugging are carried out after the mold processing is completed.

What are the reasons for uneven material shrinkage during injection molding in Dalian?

1、 Material characteristics

Differences in shrinkage rates of different plastics: Different types of plastics have different shrinkage rates. For example, crystalline plastics (such as polyethylene, polypropylene, etc.) typically have a higher shrinkage rate than amorphous plastics (such as polystyrene, polycarbonate, etc.). This is because crystalline plastics undergo crystallization during the cooling process, resulting in tighter molecular arrangement and greater volume shrinkage. Amorphous plastics, on the other hand, have relatively disordered molecular arrangements and lower shrinkage rates during the cooling process.

The influence of fillers and additives in plastics: The fillers (such as glass fiber, talcum powder, etc.) and additives (such as plasticizers, stabilizers, etc.) added to plastics can affect the shrinkage performance of the material. Fillers usually reduce the shrinkage rate of plastics because they increase the rigidity and strength of the material, reducing the mobility of molecules. However, if the distribution of fillers is uneven, it can also lead to uneven shrinkage. The type and amount of additives can also affect the shrinkage rate. For example, plasticizers can increase the flexibility of plastics and reduce their shrinkage rate; Stabilizers can improve the thermal stability of plastics, reduce decomposition during processing, and thus affect shrinkage performance.

2、 Process parameters

Injection temperature and pressure: Injection temperature and pressure have a significant impact on the shrinkage of materials. If the injection temperature is too high, the flowability of the plastic melt will be good, but the shrinkage rate will also increase during the cooling process. On the contrary, if the injection temperature is too low, the flowability of the plastic melt may be poor, which may lead to incomplete filling and also affect the uniformity of shrinkage. Excessive injection pressure can cause significant compression force on the plastic melt inside the mold cavity, leading to an increase in shrinkage rate after demolding; If the injection pressure is too low, it may lead to insufficient filling and uneven shrinkage.

Pressure holding time and pressure: The purpose of the pressure holding stage is to continue applying pressure to the mold cavity during the cooling and shrinkage process of the plastic, in order to compensate for the shrinkage of the material. If the holding time is too short or the pressure is insufficient, the plastic will undergo significant shrinkage during the cooling process, resulting in uneven shrinkage. On the contrary, if the holding time is too long or the pressure is too high, it will increase the internal stress of plastic products and may also lead to problems such as warping and deformation.

Cooling rate: Cooling rate also has a significant impact on the shrinkage of materials. If the cooling rate is too fast, the uneven cooling of the plastic in the mold cavity can lead to uneven shrinkage. Especially between thick walled and thin-walled sections, there will be significant differences in shrinkage rates due to differences in cooling rates. If the cooling speed is too slow, it will prolong the production cycle and increase costs.

3、 Mold design

Gate position and quantity: The position and quantity of gates will affect the flow and filling method of plastic melt in the mold cavity, thereby affecting the uniformity of shrinkage. If the gate position is not reasonable, such as being set at the edge or corner of plastic products, it may cause uneven filling and uneven shrinkage. Increasing the number of gates can improve the flow and filling of plastic melt, but if the distance between gates is not reasonable, it may also cause uneven shrinkage.

Mold temperature control: Mold temperature has a significant impact on material shrinkage. If the mold temperature is uneven, it will cause different cooling rates of plastic products in different parts, resulting in uneven shrinkage. For example, the thick walled part of the mold has a lower temperature, faster cooling rate, and smaller shrinkage rate; The thin-walled part has a higher temperature, slower cooling rate, and greater shrinkage rate. Therefore, it is necessary to design the cooling system of the mold reasonably to ensure uniform temperature of the mold.

Mold structure design: The structure design of the mold can also affect the shrinkage of the material. For example, if the demolding slope of the mold is not reasonable, it may cause uneven stress on the plastic product during the demolding process, resulting in warping deformation and uneven shrinkage. The design of the parting surface of the mold also needs to consider the shrinkage characteristics of the plastic to avoid defects such as burrs or shrinkage marks on the parting surface.

The process flow of injection molding generally includes the following steps: raw material pretreatment, injection molding processing, cooling and solidification, demolding and part taking, etc. Firstly, the pre-treatment of the raw materials involves heating and melting the plastic particles. Then the molten plastic is injected into the mold, which is made according to the design and shape of the product. After injection, wait for a certain period of time to cool and solidify the plastic completely in the mold. The next step is to demold and remove the molded product from the mold, completing the entire injection molding process.

Injection molding process parameters (such as injection pressure, holding pressure, temperature, etc.) can also affect the size of plastic products. By optimizing process parameters, the shrinkage rate of plastic products can be kept stable, thereby controlling dimensional accuracy. Statistical Process Control (SPC) methods can be used to monitor and adjust process parameters.