Introduction
Are you dealing with deformed or weak holes in your plastic parts? Improper hole design can result in reduced part functionality and durability as well as costly production mold modification and overrun costs.
In plastic injection molding, accurate hole design is critical to achieving part functionality, structural strength, and reducing production defects like sink marks or surface deformation.
Poor hole design can result in weakening or deformation of parts during molding, leading to a decrease in quality and an increase in scrap rates.
The Role of Holes in Plastic Parts
There are many holes with different shapes and sizes on plastic products, and the purpose of these holes is primarily to assemble and connect with other parts, or decorative holes and functional holes of products.
Types of Plastic Injection Molding Holes
Different types of holes (through holes, blind holes, shaped holes) are used for specific purposes. Precise design and molding techniques are needed for each type of hole, since the ultimate goal is to create a hole which has the required functionality and strength.
Selecting the correct type of hole lessens post-molding manufacturing steps, thus reducing the overall production costs and increasing product quality.
Through Hole
A through hole is a hole that goes through the whole part, from one side to the other side. This is the most common way to create holes, especially when the holes are used for screws, bolts, or assembly pins that need to be used to join parts together.
Design tip:
In order to make sure that the molded part is strong, keep a uniform wall thickness around the hole and stay away from the edges. This is to prevent part cracking or weaknesses in material during assembly.
Through holes are to be used in applications that need to be fastened parts together, as they offer an easy way to secure parts to other parts, without drilling holes through the part.
Injection Molding For Through Hole:
Through holes in injection molding are created by placing core pins into the mold. The role of these core pins is to hold space in the part, as plastic flows around them. When the part is cooled and solidified, the core pin is removed, and a hole is formed.
Core Pin Design:Core pins should be made of tough, heat resistant materials, as they are subjected to high temperatures during injection molding. They must also have precise size and shape, dictated by the dimensions of the finished part.
Mold Design: The location of the hole area in the mold is extremely important. Design engineers must ensure that the holes are not in the line of two mold pieces (parting line). They must also make sure that the plastic flows evenly around the core pin and that no voids are left around the core pin.
Possible problems
1. There is a flash that is not easy to trim at one end of the hole;
2. Because one end of the core is fixed, the core is shaped when the hole is deep
It is also long, and the core is easy to bend and deform.
3. The coaxiality of the two holes is not easy to ensure, and the size of one core should be designed to be 0.5~1mm larger than the other in the design, so that even if the coaxiality between the two axes is slightly different, it will not cause difficulties in installation and use.
4. After a long time of use, the guide part is caused by wear and tear, resulting in overflow at the core and guide hole.
Blind Hole
A blind hole is a hole that doesn’t travel through the entire part, it’s like a tube whose one end is sealed. Blind holes are often were aesthetic reasons limit the design, or in areas where for strength reasons we need to add extra material on one side.
Design tip:
Ensure that the depth of the blind hole is optimized to prevent sink marks or voids. The surrounding material should be thick enough to support the hole without causing deformation.
Blind holes are often found in products that require hidden attachments or recessed features, such as electronics housing or aesthetic covers.
Injection Molding For Blind Hole:
The blind hole cannot be penetrated through the product, and the bottom surface of the hole is flat. The structure can only be formed with a fixed core at one end, and the core is a cantilever structure.
If the core is long, it is easy to bend or break, so the depth of the blind hole should not be too deep.
Based on experience:
• For injection mold, the hole depth shall not be greater than 4 times the aperture;
• During compression molding, the hole depth perpendicular to the compression direction is 2 times the aperture, and the hole depth parallel to the compression direction is generally not more than 2.5 times the aperture.
If a deep blind hole is needed on the plastic part, and the position of the hole is perpendicular to the compression direction, a support column can be arranged under the core to prevent the core from bending.
Shaped Hole
A shaped hole has a profile that is not circular, for example, square, oval, hexagonal. They are used when we insert in the hole a non-circular part, or to prevent a part inserted in the hole to rotate (a nut, for example).
Design tip:
Shaped holes require precise tooling to maintain their profile, especially in high-tolerance applications. Ensure that the mold design compensates for any material shrinkage or deformation that might distort the hole’s shape.
Shaped holes are often used in mechanical parts where alignment or anti-rotation features are required, such as for gears, fasteners, or custom locking mechanisms.
Injection Molding For Shaped Hole:
For inclined holes or complex special-shaped holes, a split core can be used to form the mold to avoid side core pulling and simplify the mold structure.
What is the Best Practice for Hole Placement in Molding?
Not sure where to put holes for maximum strength and moldability? Incorrect placement of holes can lead to an overall weaker part structure or increase the chance for other molding defects.
The best practices for hole placement while designing parts for injection molding involve avoiding the design edges with the hole, keeping the hole from being too close from the other holes and aligning it with the direction of the plastic flow of the mold. This way the mold cavity is filled in the most efficient way and ensures that there is no significant internal stress after the cavity is filled.
By optimizing the placement of holes in your part you can be sure that you end up with a strong and reliable part that meets your specifications.
Strengthening of holes in plastic parts
To strengthen holes in plastic injection molded parts, designers can add ribs, bosses, or thicker walls around the holes. This additional material provides extra support, reducing the likelihood of stress fractures and improving overall part durability.
The design method is shown in the figure below
Plastic hole placement principle:
① Located in a place that is not easy to weaken the strength of plastic parts;
② There should be enough distance between the hole and the hole, and between the hole and the side wall.
③ Holes should be symmetrically distributed and placed away from high-stress areas or areas with rapid material flow changes.
④ Avoiding placing holes near parting lines reduces the risk of flash or deformation.
Common Defects in Injection Molding Holes
Struggling with defects in your injection-molded holes? Defects such as sink marks, warping, or misalignment can lead to part rejection, increased production costs, and poor product performance.
Injection molding guide holes—whether through holes, blind holes, or shaped holes—are prone to various defects if the mold design, material selection, or processing parameters aren’t optimized. These defects can compromise the functionality, aesthetics, and durability of the molded parts.
Understanding these common defects and their causes is essential for producing high-quality parts with precise, defect-free holes.
1. Sink Marks
Problem: Sink marks appear as depressions or indentations around the hole, especially in areas with thick sections. They are caused by uneven material shrinkage during cooling.
Causes: Insufficient cooling around the hole. Thick walls around the hole. High material shrinkage.
Solution: Maintain uniform wall thickness around the hole. Optimize cooling time and pressure to reduce shrinkage. Use lower-shrinkage materials or add structural ribs for support.
2. Deformation
Problem: Deformation of the hole occurs when the hole becomes misshapen or does not retain its original design dimensions. This often leads to poor fit or functionality.
Causes: Uneven cooling. Improper mold pressure. Weak core pin design.
Solution: Ensure even cooling around the core pin. Use robust core pins made of strong materials like hardened steel. Adjust injection pressure to prevent excess force on the hole.
3. Misalignment
Problem: Misalignment refers to the hole being offset from its intended position, which can lead to assembly issues, poor fitting, or compromised part strength.
Causes: Incorrect core pin positioning. Inconsistent mold clamping. Improper part ejection.
Solution: Ensure precise core pin placement during mold design. Maintain proper mold alignment and clamping force. Use a balanced ejection system to prevent part movement during demolding.
4. Incomplete Filling
Problem: Incomplete filling occurs when the plastic does not fully fill the hole cavity, leaving the hole shallow or unfinished.
Causes: Low injection pressure. Inadequate venting near the hole. Incorrect material flow.
Solution: Increase injection pressure or material flow rate. Add proper venting near the hole area. Ensure balanced material flow by adjusting runner and gate designs.
5. Warping or Deformation
Problem: Warping refers to the bending or twisting of the part, which can distort the shape of the hole or the surrounding areas. This affects both the hole’s precision and the overall part geometry.
Causes: Uneven cooling. Excessive internal stress. Thin walls or poor design support around the hole.
Solution: Balance cooling systems, particularly near the hole area. Optimize part design to include ribs or thicker sections for added strength. Reduce molding pressure to decrease internal stress during cooling.
6. Distortion of Hole Shape
Problem: Distortion refers to the hole losing its intended shape, which is common with shaped holes (e.g., square, hexagonal). This leads to misfits and functional failures.
Causes: Uneven cooling. Shrinkage due to excessive pressure. Weak core pin design.
Solution: Ensure balanced cooling and apply draft angles to aid demolding. Use reinforced core pins to retain the hole’s shape during injection. Lower injection pressure to minimize shrinkage and distortion.
7. Flash Around the Hole
Problem: Flash is the excess material that leaks out of the mold cavity and forms a thin layer or extra material around the hole. This defect requires additional trimming and can affect part appearance and function.
Causes: Improper mold fit. Low clamping force. Poor mold maintenance.
Solution: Tighten the mold fit to prevent material leakage. Increase clamping force to ensure the mold remains closed during injection. Regularly maintain the mold to prevent wear and gaps.
8. Cooling-Related Warping
Problem: Cooling-related warping occurs when parts cool unevenly, causing the hole or surrounding areas to warp or distort. This can result in misshapen holes and weakened part structure.
Causes: Poorly designed cooling channels. Rapid cooling rates in some areas while others cool slowly.
Solution: Optimize cooling channel placement near the hole. Use consistent cooling rates throughout the part to avoid uneven shrinkage and warping.
How to Prevent Deformation of Holes in Injection Molding?
To avoid the deformation of cylindrical holes during injection molding, make sure to design the core pin properly, maintain uniform wall thickness around the hole, optimize cooling times and adjust molding pressure. The use of suitable tooling and material also helps to ensure the integrity of the hole shape.
If you follow the guidelines outlined in this article, you should be able to produce consistently well-formed holes during injection molding, and avoid the significant cost associated with out-of-spec parts.
Use Rigid Core Pins
Core pins create the holes in the molded part, and their rigidity is key. Use high-strength materials for core pins to prevent bending or misalignment during molding. This ensures that the hole shape remains precise during the entire cycle.
Ensure Uniform Wall Thickness
Uneven wall thickness around the hole leads to differential shrinkage, causing warping or distortion. Designing parts with consistent wall thickness ensures even cooling, reducing the chance of hole deformation.
Optimize Cooling Time
Proper cooling helps stabilize the material around the hole, preventing shrinkage or deformation. Extend cooling times if necessary to allow the part to set properly before ejection, particularly in areas around the holes.
Adjust Molding Pressure
Excessive injection pressure can cause material to flow unevenly around the core pin, leading to hole distortion. Lowering the pressure, especially around delicate or complex hole designs, helps maintain shape integrity.
Add Sufficient Draft Angles
Incorporating draft angles around the hole helps in smooth part ejection from the mold. This prevents sticking or pulling that can cause deformation during demolding.