The molding process involves the following steps: placement and preheating of the inserts, feeding of materials, closing the mold, venting, holding pressure, curing, demolding, and cleaning the mold. Now, I will introduce each step in detail for you.
1. Insert placement
The inserts are usually made of metal and can enhance the performance of the product. Products with inserts generally have a significant improvement in mechanical properties. Some inserts are added to give the product electrical conductivity, thermal conductivity or other functional properties. It is advisable to preheat before placing the inserts.
Usually, the inserts are placed by hand. The placement position should be accurate and stable. For small inserts, pliers or forceps can also be used for installation. One piece of product can use one insert, or several different inserts can be placed. The position must not be wrong or skewed. The inserts must be stable. If necessary, they should be fixed to prevent displacement or detachment. Otherwise, the purpose of using the inserts cannot be achieved, and the product may be scrapped or even the mold may be damaged.
2. Adding Materials
The accuracy of the amount of added materials will directly affect the size and density of the product. Therefore, it is necessary to strictly measure the quantity and evenly add the materials into the mold cavities. The methods of quantitative addition include: weight method, counting method and volume method.
The gravimetric method is accurate but rather cumbersome. It is mostly used for materials with precise size requirements and those that are difficult to be dosed using volumetric methods, such as powdery or fibrous substances.
The volumetric method is less accurate than the gravimetric method, but it is more convenient to operate and is generally used for powder material measurement.
The counting method is only used for loading pre-pressed materials.
Notes: Before adding the material, it is necessary to first check if there are any oil stains, burrs, debris or other foreign objects in the cavity. Add the accurately measured material according to the shape of the cavity, and fill as much as possible in some areas with high flow resistance. Pay attention to the parts that are difficult to fill (such as protrusions, small holes, narrow slots and areas near openings), and add more material to these parts. To facilitate排气, it is best to make the material protrude in the middle and place the material around the core first and press it firmly. This can reduce the impact force of the material flow on the core, and there will be no "material escape" phenomenon in the holes of the core. If it is more convenient to add the material in the shape of the pre-pressed finished product, it is recommended to do so.
3. Mold Closure
Mold closure is divided into two steps. Before the punch is in contact with the material, it needs to close at a low pressure (1.5 - 3.0 MPa) quickly. This can shorten the cycle and prevent the plastic from changing. After the punch comes into contact with the material, the closing speed should be gradually slowed down and a high pressure (15 - 30 MPa) used for a slower closing. This is to avoid damaging the inserts and to expel the air inside the mold.
4. Ventilation
To remove air, moisture and volatile substances from the mold, after the mold is closed, in some cases, it is necessary to open the mold for a period of time. This process is called ventilation. The ventilation operation should be carried out as quickly as possible and must be completed before the material has been plasticized. Otherwise, the material will harden and lose its plasticity. At this point, even if the mold is opened, the air cannot be vented, and even if the temperature and pressure are increased, an ideal product cannot be obtained. Ventilation can shorten the curing time and improve the mechanical and electrical properties of the product. To avoid the phenomenon of layering in the product, it is not good to ventilate too early or too late. Ventilating too early will not achieve the purpose, and ventilating too late, the surface of the material has already solidified, and the gas cannot be vented.
5. Curing
The process by which materials change from a flowing state to a hard, non-melting and non-dissolving state is called the curing of thermosetting resins. What is referred to as curing here essentially refers to the curing rate, that is, the rate of stabilization. It is the speed at which the plastic changes to a non-melting and non-dissolving state within the mold during the standard molding of the sample. It is usually expressed in (s/cm thickness). The curing rate is closely related to the properties of the plastic, pre-pressing, pre-heating, molding temperature and pressure, etc.
The speed of curing depends on the rate at which the low-molecular-weight components in the resin transform into high-molecular-weight products. That is, the curing speed is related to the molecular structure of the resin. For example, thermosetting phenolic resins have a lower relative molecular weight and fewer branches, so the curing agent can easily react with the active groups, resulting in a fast curing speed. With a higher relative molecular weight and greater viscosity, it is less conducive to the condensation of the active groups (hydroxymethyl), thus the curing speed is slow. The speed of curing directly affects the production efficiency. To accelerate the curing of thermosetting plastics, sometimes some curing agents are added during the molding process. For example, hexamethylenetetramine can be added to thermosetting phenolic molding powder; oxalic acid can be added to urea-formaldehyde molding powder. Certain inorganic fillers also have an impact on the curing speed of molding powder. For instance, chlorides or hydroxides of magnesium can accelerate the hardening of phenolic molding powder.
6. Holding Pressure Time
During the resin curing process within the mold, it is always under high temperature and high pressure. The time required from the start of heating, pressurization, to the completion of curing and the subsequent cooling and depressurization is called the holding pressure time. Essentially, the holding pressure time is the duration for maintaining the temperature and pressure. It is completely consistent with the curing speed. If the holding pressure time is too short, that is, if the cooling and depressurization occurs too early, it will result in incomplete resin curing, reducing the mechanical properties, electrical properties, and heat resistance of the product. At the same time, after the product is demolded, it will continue to shrink and cause warping. If the holding pressure time is too long, it not only prolongs the production cycle but also causes excessive cross-linking of the resin, resulting in excessive material shrinkage, increased density, and internal stress between the resin and the fillers. In severe cases, the product may crack. Therefore, an appropriate holding pressure time must be determined based on the plastic properties. Neither too long nor too short is suitable. Usually, during the molding process, the curing time is adjusted to be between 30 seconds and several minutes.
7. Demolding
Demolding is usually accomplished by using a push (out) rod. For products with forming rods or certain inserts, the forming rods and other components should be unscrewed with specialized tools before demolding can be carried out.
8. Cleaning the mold
Since there may be some residual materials and burrs left in the mold during the molding process, the mold must be cleaned thoroughly after each molding. If the adhering substances are too firmly attached, they can be removed using copper sheets or by wiping with polishing agents. After cleaning, apply a release agent to facilitate the next molding process.
9. Post-treatment
To further enhance the quality of the products, after the products are demolded, they often need to be treated at a higher temperature. The temperature for post-treatment varies depending on the type of plastic. The purpose of post-treatment is:
Ensure that the plastic products have fully cured.
② Reduce the moisture and volatile substances in the product to enhance its electrical properties.
③ Eliminate the internal stresses of the product, etc.
During the post-treatment drying process, due to the further evaporation of volatile substances, the products will also undergo shrinkage and dimensional changes. Sometimes, warping and cracks may even occur. Therefore, the post-treatment conditions must be strictly controlled.