Optimizing an injection molding process involves considering various factors to ensure efficiency, product quality, and cost-effectiveness in manufacturing.

Key factors in injection molding include mold design, material selection, machine settings, and process parameters, all crucial for product quality, cycle time, and production efficiency.

Understanding the critical factors in injection molding setup is essential for maximizing efficiency and product quality. Delve deeper into these aspects to improve your manufacturing outcomes and achieve optimal production results.

What Role Does Shrinkage Play in Successful Injection Molding?

Understanding shrinkage is vital in injection molding, as it affects the final dimensions and quality of molded parts. Proper management ensures product consistency and reduces defects.

Shrinkage in injection molding is the size reduction of plastic parts during cooling. Effective control is essential to meet specifications, reduce warping, and enhance quality.

Factors Affecting Shrinkage in Thermoplastic Molding

Plastics have different shrinkage rates when they are molded. This is because of things like crystallization, which changes the shape of the plastic, and internal stress. When the plastic cools, it keeps some of that stress. It also has a lot of stress from the way the molecules are lined up. All of this means that plastics shrink more than other materials. They can shrink a lot, or just a little. They can shrink more in one direction than in another. Even after they are molded, they can still shrink more if they are heated or if they get wet.

When the molten material contacts the surface of the mold cavity, the plastic part forms a low-density solid shell that cools immediately. Because plastic has poor heat conductivity, the inner layer of the molded part cools slowly and forms a high-density solid layer that shrinks a lot. So, the thicker the wall, the slower the cooling, and the thicker the high-density layer that shrinks. Also, whether there are inserts and how they are arranged directly affect the direction of material flow, the distribution of density, and the resistance to shrinkage. Therefore, the characteristics of the molded part have a greater impact on the size and direction of shrinkage.

The direction of material flow, density distribution, presión de mantenimiento¹ to make up for shrinkage and molding time are directly affected by the inlet form, size, and distribution of these factors. The direct inlet, the larger the cross-section of the inlet (especially the thicker cross-section), the smaller the shrinkage but the larger the directionality; the shorter the width and length of the inlet, the smaller the directionality. The closer to the inlet or parallel to the direction of the material flow, the larger the shrinkage.

When the molding conditions are such that the mold temperature is high, the molten material cools slowly, resulting in a high density and a large shrinkage. Especially for crystalline materials, the shrinkage is greater due to the high crystallinity and the volume change. The temperatura del molde² distribution and the uniformity of cooling inside and outside the molded part are also related to the density, directly affecting the size and direction of the contraction of each part.

Furthermore, the duration of the holding pressure also affects the shrinkage. If the pressure is high and the duration is long, the shrinkage will be small but the directionality will be large. If the injection pressure is high, the difference in viscosity of the molten material will be small, the interlayer shear stress will be small, and the elasticity after demolding will be large.

Therefore, the shrinkage can be moderately reduced. If the material temperature is high, the shrinkage will be large but the directionality will be small. Therefore, adjusting the mold temperature, pressure, injection speed, cooling time, and other factors during molding can also appropriately change the shrinkage of plastic parts.

Mold Design According to Experience to Determine the Shrinkage Rate of Each Part of the Plastic Parts

When designing a mold, you need to consider the shrinkage range of the plastic, the thickness of the plastic wall, the shape of the plastic part, the size of the gate, and the distribution of the gate. Based on experience, you can determine the shrinkage rate of each part of the plastic part and then calculate the size of the cavity.

For high-precision plastic parts or plastic parts with a difficult-to-control shrinkage rate, you can generally use the following methods to design the mold: take a smaller shrinkage rate³ for the outer diameter of the plastic part and take a larger shrinkage rate for the inner diameter, so that there is room for correction after the trial mold.

When you design a mold, you need to consider the shrinkage range of the plastic, the thickness of the plastic wall, the shape of the plastic part, the size of the gate, and the distribution of the gate. Based on experience, you can determine the shrinkage rate of each part of the plastic part and then calculate the size of the cavity.

For high-precision plastic parts or plastic parts with a difficult-to-control shrinkage rate, you can generally use the following methods to design the mold: take a smaller shrinkage rate for the outer diameter of the plastic part and take a larger shrinkage rate for the inner diameter, so that there is room for correction after the trial mold.

Why is Flowability Important in Injection Molding?

Flowability in injection molding ensures the plastic material fills the mold properly, leading to high-quality parts and reducing defects.

Flowability is essential for complete mold filling and minimizing defects, impacting processing, material selection, and part quality across industries such as automotive, electronics, and consumer goods.

Thermoplastic Flow Size

The flowability of thermoplastics can be generally analyzed from a series of indices such as molecular weight, melt index, Archimedes spiral flow length, performance viscosity and flow ratio (flow length/wall thickness of plastic part).

Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, screw flow length, performance viscosity is small, flow ratio of the flow is good, the same name of the plastic must be examined in the manual to deduce whether its flow is suitable for injection molding.

Based on the requirements of the mold design, plastics can be divided into three categories according to their flowability: plastics with good flowability include PA, PE, PS, PP, CA, and poly (4) methylpentene; plastics with moderate flowability include polystyrene series resins (such as ABS, AS), PMMA, POM, and polyphenylene ether; plastics with poor flowability⁴ include PC, rigid PVC, polyphenylene ether, polysulfone, polysulfone, and fluoroplastics.

The Main Factors Affecting the Flowability of Plastics

When the temperature of the material is high, the flowability increases. However, different plastics also have their own differences. PS (especially the impact-resistant type and the higher the MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics, the flowability of plastics with the temperature change is greater. For PE, POM, the temperature increase or decrease has less impact on its flowability. So the former in the molding temperature is appropriate to adjust to master the flow.

The injection pressure increases the molten material by the shear effect, and the flow also increases. Especially for PE and POM, they are more sensitive, so it is appropriate to adjust the injection pressure to control the flow of molding.

The mold structure, pouring system form, size, arrangement, cooling system design, molten material flow resistance (such as type surface smoothness, channel cross-section thickness, cavity shape, exhaust system), and other factors have a direct impact on the actual flow of the molten material in the cavity. Wherever the molten material is promoted to reduce the temperature, increase the resistance to flow, the flow of the molten material will be reduced.

When designing a mold, you should consider the flow of the plastic and choose a reasonable structure. You can also adjust the filling conditions, such as material temperature, mold temperature, injection pressure, and injection speed, to meet the requirements of the molding process.

How Does Crystallinity Affect the Injection Molding Process?

Crystallinity plays a crucial role in the injection molding process, influencing the mechanical properties and dimensional stability of molded parts.

Crystallinity influences injection molding by determining plastics’ thermal and mechanical properties. Higher crystallinity increases strength and rigidity but requires longer cooling, affecting processing in sectors like packaging, automotive, and aerospace.

Crystalline Plastics and Non-Crystalline (Also Called Amorphous) Plastics

Thermoplastics can be divided into two categories: crystalline and non-crystalline (also called amorphous) plastics. This is determined by whether or not they crystallize when they cool.

The so-called crystallization phenomenon is the plastic from the molten state to the condensation, the molecules from the independent movement, completely in an unordered state, the molecules stop moving freely, according to the slightly fixed position, and there is a tendency to make the molecular arrangement of a regular model of a phenomenon.

The appearance of these two types of plastic standards depends on the transparency of the plastic thick-walled molded parts. Generally, crystalline materials are opaque or translucent (such as POM, etc.), while amorphous materials are transparent (such as PMMA, etc.). However, there are exceptions. For example, poly (4) methyl garbage is a crystalline plastic but highly transparent, and ABS is an amorphous material but not transparent.

Requirements and Precautions for Crystalline Plastics

The heat needed to raise the temperature of the material to the molding temperature is high, so you need to use equipment with a large plasticizing capacity. The heat released during cooling is large, so it should be cooled sufficiently. The specific gravity difference between the molten state and the solid state is large, the molding contraction is large, and shrinkage and porosity are easy to occur. Fast cooling, low crystallinity, small shrinkage, high transparency.

The crystallinity and wall thickness of plastic parts are related. The wall thickness is slow to cool, the crystallinity is high, the shrinkage is large, and the physical properties are good.

Therefore, the crystalline material should pay attention to the control of the temperatura del molde⁵. The crystalline material has obvious anisotropy and high internal stress. After demolding, the uncrystallized molecules have a tendency to continue to crystallize, which is in a state of energy imbalance and is prone to deformation and warping. The crystallization temperature range is narrow, and it is easy to occur that the end of the unmelted material is injected into the mold or blocks the feed port.

How Do Heat-Sensitive Plastics and Hydrolyzable Plastics Affect the Injection Molding Process?

Heat-sensitive and hydrolyzable plastics play a crucial role in the injection molding process, influencing factors such as molding temperatures and material degradation.

Proper temperature control is crucial for heat-sensitive plastics in injection molding to prevent degradation, while moisture management is essential for hydrolyzable plastics to preserve material integrity, ensuring consistent, high-quality products.

Heat-sensitive plastics are those that are more sensitive to heat. When exposed to high temperatures for an extended period of time or when the feed port cross-section is too small and the shear effect is significant, these plastics tend to change color, degrade, and decompose. This characteristic is what makes them heat-sensitive plastics.

Like hard PVC, polyvinyl chloride, vinyl acetate copolymer, POM, polytrifluoroethylene, and so on. When heat-sensitive plastics⁶ decompose, they produce monomers, gases, solids, and other by-products. The gases produced during decomposition can be irritating, corrosive, or toxic to humans, equipment, and molds.

So, you need to pay attention to the mold design, the choice of injection molding machine, and the molding process. You should choose a screw injection molding machine, the runner cross-section should be large, the mold and the barrel should be chrome-plated, and there should be no dead corners for the material. You need to strictly control the molding temperature, and add stabilizers to the plastic to weaken its heat sensitivity.

Some plastics, like PC, can decompose at high temperatures and high pressures, even if there is only a little bit of water in them. This is called being hydrolyzable, and you have to heat them up and dry them out before you use them.

How Do Stress Cracking and Melt Cracking Affect the Injection Molding Process?

Stress cracking and melt cracking are critical issues in injection molding, impacting product durability and manufacturing efficiency across industries.

Stress and melt cracking in molded parts cause weakness and flaws. Both can be mitigated by optimizing processing temperature and material selection to enhance product integrity and performance.

Some plastics are stress-sensitive, meaning they can easily develop internal stress during the molding process. As a result, these plastics can become brittle and prone to cracking. When exposed to external forces or solvents, plastic parts made from these materials may crack.

So, in addition to adding additives to the raw material to improve the crack resistance, you should pay attention to the drying of the raw material, choose a reasonable molding condition, reduce the internal stress, and improve the crack resistance. And you should choose a reasonable shape for the plastic parts, not set up inserts and other measures to minimize stress concentration.

When designing a mold, you should increase the mold slope, choose a good inlet and ejection mechanism, and adjust the material temperature, mold temperature, injection pressure, and cooling time appropriately during molding. Try to avoid the plastic parts being too cold and brittle when demolding. It is also good to do some post-processing on the molded plastic parts to improve their crack resistance, reduce internal stress, and avoid contact with solvents.

Cuando el caudal de fusión⁷ of a certain polymer reaches a certain value, if the flow rate exceeds a certain value, obvious transverse cracks will appear on the melt surface, which is called melt cracking. It is harmful to the appearance and physical properties of plastic parts.

Therefore, when selecting a polymer with a high melt flow rate, etc., the nozzle, runner, and gate cross-sectional area should be increased, the injection speed should be reduced, and the material temperature should be increased.

How Do Thermal Properties and Cooling Rate Affect the Injection Molding Process?

The thermal properties of materials and the cooling rate significantly impact the injection molding process, affecting the final product’s quality and production efficiency.

In injection molding, thermal properties influence material behavior under heat, impacting flow and cooling. Controlled cooling rate is crucial for structural integrity and stability, optimizing part quality and reducing warping and cycle times.

Different plastics have different thermal properties, such as specific heat, thermal conductivity, and heat distortion temperature. Plastics with high specific heat require a lot of heat to melt, so you need to use an injection molding machine with a large melting capacity. Plastics with a high heat distortion temperature can be cooled quickly and can be demolded quickly, but after demolding, you need to prevent them from cooling and deforming.

Plastics with low thermal conductivity have a slow cooling rate (e.g. ionic polymers, etc. have a very slow cooling rate), so they need to be cooled sufficiently and the cooling effect of the mold should be strengthened.

Hot runner molds are suitable for plastics with low specific heat and high thermal conductivity. Specific heat is large, low thermal conductivity, low heat distortion temperature, slow cooling rate of the plastic is not conducive to high-speed molding, the need to choose the appropriate injection molding machine and strengthen the mold cooling.

Different types of plastics, based on their properties and the shape of the molded part, require a specific cooling rate. Therefore, the mold must be designed with a heating and cooling system to maintain a certain mold temperature, according to the molding requirements. When the material temperature is higher than the mold temperature, the mold should be cooled in order to prevent the plastic part from deforming after it is ejected, to shorten the molding cycle, and to reduce the degree of crystallinity.

When the heat from the plastic waste isn’t enough to keep the mold at a certain temperature, the mold should have a heating system to keep the mold at a certain temperature, so you can control the cooling rate, make sure the flow is good, improve the filling conditions or use it to control the plastic part so it cools slowly, so you don’t have thick-walled parts that cool unevenly on the inside and outside and improve the crystallinity, and so on.

To ensure smooth production and the quality of the molded parts, sometimes it is necessary to adjust the temperature of the mold during the plastic molding process. This can be done by heating or cooling the entire mold or by heating or cooling specific areas of the mold. Therefore, the mold should be equipped with a corresponding cooling or heating system.

How Does Hygroscopicity Affect the Injection Molding Process?

Understanding hygroscopicity is vital for maintaining the quality and efficiency of the injection molding process, as it directly affects material properties and final product integrity.

Hygroscopicity in plastics influences moisture absorption, affecting viscosity and molding processes. Proper drying and handling are crucial to prevent defects like warping, bubbles, and reduced strength.

Plastics have different levels of water absorption⁸ due to various additives. Therefore, plastics can be divided into three categories: hygroscopic, water-absorbing, and non-hygroscopic. The water content of the material must be controlled within a certain range. Otherwise, when the temperature and pressure are high, the water will turn into gas or undergo hydrolysis.

So, if the resin is blistered, the fluidity is reduced, and the appearance and mechanical properties are poor. Therefore, when using hygroscopic plastics, it is necessary to preheat them according to the requirements of the appropriate heating method and specifications to prevent moisture absorption.

Conclusión

Moldeo por inyección⁹ is a complex process that requires careful consideration of many factors to ensure smooth production and consistent product quality. These factors include the shrinkage, flowability, crystallinity, heat sensitivity, and hydrolysis of the plastic, as well as its resistance to stress cracking and melt cracking, and its thermal properties, cooling rate, and moisture absorption.

Different types of plastics have different flow behaviors and shrinkage characteristics during the injection molding process, and these characteristics are influenced by temperature, pressure, and mold design.