How Do You Prevent Shrinkage in Injection Molding: Key Strategies

Shrinkage is one of the most common challenges in injection molding, and it directly affects the accuracy and quality of injection molded parts. The most effective way to prevent shrinkage is to control material choice, mold design, processing settings, and cooling conditions.

Those who manage these factors well not only achieve better dimensional accuracy but also reduce waste and production costs. Today at Moldie.com, you will explore the strategies about preventing shrinkage in injection molding.

Understanding Shrinkage in Injection Molding

Shrinkage in injection molding happens when molten plastic cools and contracts, reducing the final size of the molded part. This change affects dimensional stability, surface finish, and overall product performance. Managing shrinkage requires understanding why it occurs, how it influences quality, and the types of defects it creates.

What Is Shrinkage and Why It Occurs

Shrinkage in injection molding is the reduction in volume that occurs as molten polymer cools and solidifies in the mold. All plastics contract to some degree, but the amount depends on the polymer type, mold design, and processing conditions.

Thermoplastics with higher crystallinity, such as nylon or polypropylene, usually show greater shrinkage than amorphous materials like ABS. Crystalline structures pack more tightly as they cool, leading to higher contraction rates.

Uneven or rapid cooling can also create localized shrinkage, which affects dimensional stability. Holding pressure and packing time influence shrinkage as well, since they control how much material fills the mold before solidification.

Material properties, mold temperature, and wall thickness interact in complex ways. For example, thicker walls cool more slowly, often resulting in more shrinkage compared to thinner sections.

Impact of Shrinkage on Product Quality

Shrinkage directly affects the accuracy and performance of molded parts. When contraction is not uniform, parts may not meet specifications, leading to assembly issues or poor fit.

Inconsistent shrinkage can also create surface defects. A part may appear warped or uneven, which reduces its visual appeal and makes it harder to use in precision applications.

Dimensional stability is especially important in industries like automotive and medical devices, where even small deviations can cause functional problems. Shrinkage that is not controlled may also reduce mechanical strength, since internal stresses can weaken the material.

Manufacturers often adjust molding parameters such as packing pressure, cooling time, and mold temperature to improve consistency. Proper material selection also plays a critical role in reducing variability and maintaining product quality.

Material Selection and Its Role in Shrinkage

The choice of resin, its molecular structure, and the use of additives all influence how much a molded part shrinks as it cools. Proper drying and handling of materials also play a direct role in maintaining accuracy and reducing defects.

Material Shrinkage Rates

Every plastic has a specific shrinkage rate, usually listed by material suppliers. This rate indicates how much the part will contract after molding.

Semi-crystalline plastics such as polypropylene (PP) and polyamide (PA/nylon) generally shrink more than amorphous plastics like polycarbonate (PC) or ABS. For example, PP can shrink up to 2%, while PC often stays below 0.7%.

Selecting materials with lower shrinkage rates helps maintain tighter tolerances and reduces the need for mold and part design adjustments.

Effect of Molecular Structure

The internal structure of a polymer strongly affects shrinkage. Semi-crystalline plastics form ordered regions as they cool, which leads to higher contraction. Amorphous plastics lack this ordered structure, so they shrink less and more uniformly.

For example, nylon (PA) has a semi-crystalline structure, which makes it shrink more than ABS. This difference is why ABS is often chosen for parts requiring dimensional stability.

Molecular weight also matters. Higher molecular weight polymers may resist flow, creating uneven packing in the mold, which can increase shrinkage in thicker areas. Understanding these structural differences helps engineers match materials to part geometry and tolerance needs.

Influence of Additives

Additives and fillers can significantly reduce shrinkage. Glass fibers, talc, or calcium carbonate limit how much the polymer matrix contracts during cooling.

• Glass-filled nylon shrinks less than unfilled nylon and provides better dimensional stability.
• Talc-filled polypropylene reduces shrinkage and adds stiffness, which is useful in automotive panels.

The percentage of filler also affects results. Higher filler content usually means lower shrinkage but can make the material more brittle. Engineers must balance shrinkage control with mechanical performance when selecting filled grades.

Importance of Drying and Handling

Moisture content in materials, especially hygroscopic resins like nylon (PA), directly affects shrinkage. If not dried properly, absorbed water can cause hydrolysis during molding, leading to inconsistent shrinkage and weaker parts.

Proper storage in sealed containers and the use of desiccant dryers help maintain stable material properties. For PC and ABS, even small amounts of moisture can create bubbles or surface defects that affect accuracy.

Handling also matters. Contaminated or degraded pellets may change flow behavior, leading to uneven packing and shrinkage. Consistent drying and clean handling practices ensure the material performs as expected during molding.

Optimizing Mold Design to Reduce Shrinkage

Shrinkage in injection molding often comes from how the mold is designed and how the plastic cools inside it. Careful planning of mold dimensions, wall thickness, and cooling systems helps control material flow and solidification, which reduces uneven shrinkage and improves accuracy.

Accounting for Shrinkage in Mold Dimensions

Every plastic material has a known shrinkage rate that must be factored into mold design. If ignored, the final part will not match the intended dimensions. Designers use shrinkage data from material suppliers to scale mold cavities so the cooled part meets the required size.

This process often involves adjusting cavity dimensions by a small percentage. For example, a material with a 1.5% shrink rate requires the mold to be slightly larger than the target part.

In precision applications, engineers also consider directional shrinkage. Some plastics shrink more along the flow direction than across it. Accounting for this anisotropy ensures parts fit correctly in assemblies.

Key practices include:

• Using supplier shrinkage data
• Scaling cavity dimensions appropriately
• Considering directional shrinkage effects

Ensuring Uniform Wall Thickness

Uneven wall thickness is one of the most common causes of shrinkage variation. Thick sections cool slower than thin ones, leading to errors like warpage.

Designers aim for uniform wall thickness throughout the part to promote even cooling and consistent shrinkage. When thickness changes are unavoidable, smooth transitions such as fillets or tapers help reduce stress and distortion.

A simple guideline is to keep thickness within a narrow range, such as 2–4 mm for many thermoplastics. If ribs or bosses are needed for strength, they should be no more than 50–70% of the adjoining wall thickness to avoid excess shrinkage.

Best practices for wall thickness:

• Maintain consistent thickness
• Use gradual transitions between sections
• Limit rib and boss dimensions relative to walls

Designing Effective Cooling Channels

The cooling system in a mold directly affects shrinkage. Uneven cooling creates temperature differences that cause non-uniform contraction. Properly placed cooling channels allow heat to be removed evenly, reducing distortion and improving cycle times.

Channels should be positioned close to mold surfaces without weakening the mold structure. Their spacing and diameter must be optimized to balance efficiency and mold strength.

Advanced designs may use conformal cooling, where channels follow the shape of the cavity. This method improves temperature control in complex parts and reduces localized shrinkage.

Design considerations:

• Place channels evenly around critical areas
• Maintain consistent channel depth and spacing
• Use conformal cooling when part geometry is complex

Controlling Processing Parameters

Precise control of processing parameters reduces dimensional errors and helps maintain consistent part quality. Key factors include pressure, speed, temperature, and time settings, each of which directly affects shrinkage behavior in molded parts.

Injection Pressure and Holding Pressure

Injection pressure determines how well the molten plastic fills the mold cavity. If the pressure is too low, the material may not fully pack, leading to voids and higher shrinkage. Excessive pressure, however, can cause stress and warping.

Holding pressure is equally important. After the mold is filled, applying steady pressure keeps material packed as it cools. This reduces volumetric shrinkage by compensating for material contraction.

Key considerations for pressure control:

• Maintain enough injection pressure to fully fill the mold.
• Use holding pressure to counteract shrinkage.
• Avoid excessive pressure that may create stress in the part.

Balancing both injection and holding pressure ensures the part retains its intended dimensions without overpacking.

Injection Speed Optimization

Injection speed affects how the molten polymer flows into the mold. A very fast fill can cause uneven flow, trapped air, or weld lines. A very slow fill may allow premature cooling, increasing shrinkage and defects.

Moderate and consistent injection speed usually produces better dimensional stability. Adjustments should be based on part geometry and material type. For example, thin-walled parts often need higher speeds to avoid incomplete filling.

Best practices for speed control:

• Use slower speeds for thick parts to prevent internal stress.
• Apply faster speeds for thin sections to avoid short shots.
• Monitor flow balance across multiple cavities.

Barrel and Mold Temperature

Barrel temperature controls how well the polymer melts. If the melt temperature is too low, it may not pack properly, leading to shrinkage. If it is too hot, the polymer may degrade or shrink more as its temperature drops.

Mold temperature also plays a critical role. A cooler mold typically reduces cycle time but may increase shrinkage due to rapid contraction. A warmer mold allows more uniform cooling, which can improve dimensional accuracy.

Temperature control factors:

• Set barrel zones to maintain consistent melt viscosity.
• Keep temperature within the recommended range for each polymer.
• Use temperature control units to avoid hot or cold spots.

Holding and Cooling Time

Holding time allows pressure to remain on the material while it solidifies. Too short a holding time can leave voids or sink marks. Too long can increase cycle time without significant improvement.

Cooling time is equally important. Uneven or insufficient cooling leads to internal stress and inconsistent shrinkage. Ensuring the whole molding process takes place under controlled conditions can guarantee the part solidifies evenly across its thickness.

Guidelines for time management:

• Match holding time to the part’s wall thickness.
• Extend cooling time for thicker parts to prevent warping.
• Use balanced cooling channels to maintain even temperature distribution.

Optimizing holding and cooling times reduces shrinkage while keeping cycle times efficient. Proper timing ensures that parts are dimensionally stable and free from common molding defects.