Injection Molding Material Selection Guide: 7 Key Factors

The right injection molding material directly shapes an end product’s quality, production cost and long-term durability. Picking the ideal option requires careful analysis of multiple critical influencing factors to avoid poor performance or unnecessary resource waste.

To steer clear of material mismatches, our Injection Molding Material Selection Guide sorts out 7 kinds of core considerations, helping you make efficient and reliable choices.

Shrinkage Rate

Factors affecting the molding shrinkage of thermoplastics are as follows:

Plastic Type

During molding, thermoplastics undergo volume changes due to crystallization, along with high internal stress, large residual stress frozen in molded parts, and strong molecular orientation. Compared with Injection Molding Thermoset Materials, thermoplastics have higher shrinkage rates, wider shrinkage ranges, and obvious directionality.Additionally, shrinkage after molding, annealing, or humidity conditioning is generally higher than that of thermosets materials.

Molded Part Characteristics

When molten plastic contacts the mold cavity surface, the outer layer cools rapidly to form a low-density solid shell. Due to poor thermal conductivity of plastics, the inner layer cools slowly, forming a high-density solid layer with large shrinkage.

Therefore, thicker-walled parts with slow cooling and thick high-density layers exhibit greater shrinkage. Moreover, the presence, layout, and quantity of inserts directly affect melt flow direction, density distribution, and shrinkage resistance, making part characteristics a major factor in shrinkage magnitude and directionality.

Gate Type, Size and Distribution

These factors directly impact melt flow direction, density distribution, packing compensation, and molding cycle time. Direct gates with large cross-sections (especially thick ones) reduce shrinkage but increase directionality; wide, short gates minimize directionality.

Areas close to the gate or parallel to the melt flow direction show higher shrinkage.

Molding Conditions

Higher mold temperatures slow melt cooling, increase density, and raise shrinkage—especially for crystalline plastics, where high crystallinity causes significant volume changes and greater shrinkage. Mold temperature distribution affects internal and external cooling of parts and density uniformity, directly influencing shrinkage magnitude and directionality across different part sections.

In addition, packing pressure and holding time exert a notable impact: higher pressure and longer holding times reduce shrinkage but enhance directionality. Higher injection pressure lowers melt viscosity, reduces interlayer shear stress, and increases elastic recovery after demolding, thus moderately reducing shrinkage. Higher melt temperatures increase shrinkage but reduce directionality. Therefore, adjusting mold temperature, pressure, injection speed, and cooling time during molding can properly control part shrinkage.

When designing molds, determine the shrinkage rate for each part section based on the shrinkage range of the selected plastic, part wall thickness, shape, gate type, size, and distribution, and then calculate the mold cavity dimensions. For high-precision parts or when shrinkage rates are hard to predict, mold design should follow these steps:

1. Adopt a smaller shrinkage rate for the outer diameter and a larger one for the inner diameter of parts to allow for post-trial mold adjustments.
2. Determine the gating system type, size, and molding conditions through mold trials.
3. Measure dimensional changes of parts requiring post-treatment (measurements must be taken 24 hours after demolding).
4. Modify the mold according to actual shrinkage data.
5. Conduct a second mold trial and slightly adjust process conditions to fine-tune shrinkage and meet part requirements.

Fluidity

Fluidity is an important factor in the selection of injection molding materials. The fluidity of thermoplastics can be evaluated by a series of indices, including molecular weight, melt flow index (MFI), Archimedean spiral flow length, apparent viscosity, and flow ratio (flow length/part wall thickness).

Better fluidity is indicated by lower molecular weight, wider molecular weight distribution, poor molecular structural regularity, higher MFI, longer spiral flow length, lower apparent viscosity, and higher flow ratio. For plastics of the same grade, always refer to the product specification sheet to verify if their fluidity is suitable for injection molding. Based on mold design requirements, common injection molding plastic material can be roughly classified into three fluidity categories:

• Good fluidity: PA, PE, PS, PP, CA, Polymethylpentene
• Medium fluidity: Polystyrene series resins (e.g., ABS, AS), PMMA, POM, Polyphenylene oxide
• Poor fluidity: PC, rigid PVC, Polyphenylene oxide, Polysulfone, Polyarylsulfone, Fluoroplastics

The fluidity of all plastics varies with molding factors, with the key influences being:

• Temperature: Higher melt temperatures increase fluidity, but the degree of variation differs by plastic type. Fluidity of PS (especially impact-resistant grades and those with high MFI), PP, PA, PMMA, modified polystyrene (e.g., ABS, AS), PC, and CA is highly sensitive to temperature changes, so temperature adjustment is the preferred method to control their fluidity during molding.In contrast, temperature has a minimal effect on the fluidity of PE and POM.
• Pressure: Higher injection pressure intensifies shear action on the melt, increasing fluidity. This effect is particularly pronounced for PE and POM, making pressure adjustment the optimal way to control their fluidity during molding.
• Mold Structure: The gating system design (type, size, layout), cooling system design, and melt flow resistance (e.g., mold surface finish, runner cross-section thickness, cavity shape, and venting system) directly impact the actual fluidity of the melt inside the cavity.

Any factor that lowers melt temperature or increases flow resistance will reduce fluidity. Mold design should adopt appropriate structures based on the fluidity of the selected plastic. During molding, controlling melt temperature, mold temperature, injection pressure, and injection speed can also properly regulate melt filling to meet molding requirements.

Crystallinity

Thermoplastics are divided into two categories based on whether crystallization occurs during solidification: crystalline plastics and amorphous plastics. Crystallization refers to the process where molecules transition from a free-moving, disordered state in the molten phase to a state where molecular motion stops, molecules settle in relatively fixed positions, and tend to arrange into a regular pattern upon cooling.

A visual criterion for distinguishing the two types is the transparency of thick-walled parts: crystalline plastics are generally opaque or translucent (e.g., POM), while amorphous plastics are transparent (e.g., PMMA). However, there are exceptions: Polymethylpentene is a crystalline plastic with high transparency, and ABS is an amorphous plastic that is opaque. When designing molds and selecting injection molding machines, the following requirements and considerations apply to crystalline plastics:

• More heat is required to raise the melt to the molding temperature, so equipment with high plasticizing capacity is needed.
• Significant heat is released during cooling and crystallization, requiring adequate cooling.
• The large density difference between the molten and solid states leads to high molding shrinkage, which easily causes shrinkage cavities and air bubbles.
• Fast cooling results in low crystallinity, low shrinkage, and high transparency. Crystallinity is related to part wall thickness: thicker walls cool more slowly, leading to higher crystallinity, greater shrinkage, and better mechanical properties.Therefore, mold temperature must be strictly controlled for crystalline plastics as required.
• Obvious anisotropy and high internal stress are common. After demolding, uncrystallized molecules tend to continue crystallizing, leaving the part in an energy-unbalanced state that easily causes deformation and warpage.
• The narrow crystallization temperature range may lead to incomplete melting of the plastic or runner blockage.

Heat Sensitivity and Hydrolysis

Heat Sensitivity

Heat sensitivity refers to the tendency of certain plastics to discolor, degrade, or decompose when exposed to high temperatures for extended periods, or when passing through small cross-sections that generate high shear stress and increase melt temperature. Plastics with this property are called heat-sensitive plastics, such as rigid PVC, Polyvinylidene chloride, Vinyl acetate copolymers, POM, and Polychlorotrifluoroethylene.

Decomposition of heat-sensitive plastics produces by-products like monomers, gases, and solids; some of these decomposition gases are irritating, corrosive, or toxic to humans, equipment, and molds. Therefore, the following precautions should be taken for mold design, injection machine selection, injection molding material selection, and molding:

• Use screw-type injection molding machines.
• Adopt gating systems with large cross-sections.
• Use chrome-plated molds and barrels to avoid dead spots where material can stagnate.
• Strictly control molding temperatures.
• Add stabilizers to the plastic to reduce heat sensitivity.

Hydrolysis Susceptibility

Some plastics (e.g., PC) will decompose under high temperature and pressure even if they contain trace amounts of moisture—a property known as hydrolysis susceptibility.

These plastics must be preheated and dried using appropriate methods and specifications before processing, and reabsorption of moisture during use should be prevented.

Stress Cracking and Melt Fracture

Stress Cracking

Certain plastics are sensitive to stress, easily developing internal stress during molding and becoming brittle and prone to cracking. Cracking can occur when the part is subjected to external forces or exposed to solvents. To address this issue:

• Add anti-cracking additives to the raw material to improve crack resistance.
• Ensure proper drying of the raw material.
• Select optimal molding conditions to reduce internal stress and enhance crack resistance.
• Design parts with reasonable shapes and avoid unnecessary inserts to minimize stress concentration.
• Increase mold draft angles during mold design, select appropriate gating systems and ejection mechanisms.During molding, properly adjust melt temperature, mold temperature, injection pressure, and cooling time to avoid demolding parts while they are too cold and brittle. Conduct post-molding treatment to improve crack resistance, eliminate internal stress, and avoid contact with solvents.

Melt Fracture

Melt fracture occurs when a polymer melt with a specific melt flow rate flows through a nozzle orifice at a constant temperature, and its flow rate exceeds a critical value, causing obvious transverse cracks on the melt surface. This defect impairs the appearance and mechanical properties of molded parts. To prevent melt fracture:

• For polymers with high melt flow rates, increase the cross-section of nozzles, runners, and gates.
• Reduce injection speed.
• Increase melt temperature.

Thermal Properties and Cooling Rate

Thermal Properties

Different plastics have distinct thermal properties such as specific heat capacity, thermal conductivity, and heat deflection temperature:

• Plastics with high specific heat capacity require more heat for plasticization, so injection molding machines with strong plasticizing capacity should be selected.
• Plastics with high heat deflection temperature can be demolded earlier with shorter cooling times, but post-demolding cooling should be controlled to prevent deformation.
• Plastics with low thermal conductivity cool slowly (e.g., ionomers have extremely slow cooling rates), necessitating enhanced mold cooling efficiency.
• Hot runner molds are suitable for plastics with low specific heat capacity and high thermal conductivity.Plastics with high specific heat capacity, low thermal conductivity, low heat deflection temperature, and slow cooling rates are not conducive to high-speed molding. For these materials, appropriate injection molding machines must be selected, and mold cooling systems must be optimized.

Cooling Rate Control

For all plastics, the cooling rate must be properly controlled based on their material characteristics and part shape. Molds must be equipped with heating and cooling systems to maintain a stable mold temperature as required:

• When melt temperature raises the mold temperature, cooling is required to prevent part deformation after demolding, shorten molding cycles, and reduce crystallinity.
• When the residual heat of the melt is insufficient to maintain the required mold temperature, mold heating systems should be used to control cooling rates, ensure melt fluidity, improve filling conditions, or enable slow cooling of parts to prevent uneven cooling in thick-walled sections and increase crystallinity.For plastics with good fluidity, large molding areas, or uneven melt temperature distribution, molds may require alternating heating and cooling, or localized heating/cooling, depending on part molding conditions. Molds should be equipped with corresponding cooling or heating systems to meet these needs.

Hygroscopicity

Due to the presence of various additives, plastics exhibit different levels of moisture affinity, and can be broadly classified into two types: hygroscopic plastics (which absorb and retain moisture) and non-hygroscopic plastics (which do not absorb moisture and are not prone to moisture adhesion).

The moisture content of plastic raw materials must be controlled within allowable limits. Excess moisture will vaporize into gas or cause hydrolysis under high temperature and pressure during molding, leading to defects such as bubbles, reduced fluidity, poor surface finish, and degraded mechanical properties. Therefore, hygroscopic plastics must be preheated and dried using appropriate methods and specifications before processing, and reabsorption of moisture during storage and use should be prevented.

Conclusion

The appropriate Injection Molding Material Selection is a crucial step in achieving stable part quality, consistent production, and cost-effectiveness. Understanding key factors such as shrinkage, flowability, crystallinity, heat sensitivity, stress behavior, thermal properties, cooling rate, and hygroscopicity can help you better understand the match between design requirements and materials.

LVMA specializes in producing complex injection molded parts and regularly updates its knowledge on injection molding. For more information, please contact us.