Cold Sprue vs. Hot Runner Technology

With the advancements in feed system technology, when is it truly practical to design a cold sprue into a part or feed system? Before we answer this question, the purpose of the cold sprue must be considered.

The cold sprue is simply a means to get polymer from the injection molding machine to the cavity (or at least that’s how it was in ‘the beginning’).  However, cold sprues are typically large, which leads to waste, and are typically thick, which may contribute to long cycle times.

With the advancement of feed system technology, there are additional and potentially more efficient ways of transferring polymer from the machine to the cavity. However, costs will need to be considered.

Designing a Cold Sprue into a Part

Cold Sprue


Considerations when designing a part with a cold sprue include:

  1. Wall thickness: If the cold sprue is designed to drop directly onto the part, the nominal wall thickness of the part must be considered. The nominal part thickness will dictate the size the diameter sprue should be on the part (or at least it should) based on sink marks and freeze times.
  2. Properly sized: If the cold sprue drops onto a cold runner which then feeds the part, the cold sprue must be sized such that both the cold runner and cold sprue are not so large that they control cycle time. On the other side of the spectrum, they must not be too small as to create a severe pressure loss and also allow for sufficient pack times to allow the gate to freeze AFTER the part freezes.
  3. Flow Path: The longest flow path (distance) of the polymer as it flows through the cavity.  The flow path must be as short as possible and balanced within the part. It will also depend on the material being used. Some materials can flow much further than others.
  4. Sink Marks: Account for possible sink marks in the part design on the opposite side of the cold sprue. Sometimes a small mound may be required to be designed onto the part opposite the cold sprue to prevent excessive shear which may cause visual defects such as splay. However, a thick region on the part may lead to a sink mark. If there is potential for a sink mark on the part opposite the sprue, it may even be necessary to model in a small dimple opposite the sprue.
  5. Removal of Sprue: Removing a cold sprue after ejection is an additional process and has associated costs.
  6. Proper Design: The sprue will also need to be designed so that sharp edges at the base of the sprue where it meets the part are relieved with sufficient radii to prevent the polymer from shearing as it enters the part, which could cause surface defects on the part.
  7. Location of Sprue: The sprue location is typically dictated by the part as it lies in tool position.  However, as mentioned in line 3, the    sprue must be placed in a balanced location on the part. If the part does not fill in a balanced fashion, it could lead to potential flash, cantilevering of the tool, actions in the tool sticking and many other potential processing issues.

Shear zone for an injection molded part

Part Design

With the demand for thinner, more complicated parts, it may be necessary to have multiple gate locations on the part to fill the part completely.

There are ways to determine if one sprue is sufficient is to fill the part based on the flow length. 

  • Flow length, i.e. ‘distance – to- part thickness’ ratio which is material dependent. Some materials can flow much further than others.
  •  Identify the temperature loss across the part as it is being filled. It is important to maintain a certain temperature across the part.
  • The pressure to fill the part is also very important. High fill pressures can lead to stress within the part which can lead to warpage. High fill pressures can also place undo wear on the injection molding machine as well as high clamp force.

From a design perspective, it may seem best to simply place the cold sprue in the center of the part based on the parts location in tool position. This can potentially lead to many issues based on the aforementioned considerations.

Without the use of analytical tools, it’s difficult to prove that a part will not be optimally designed if a single, cold sprue is used. However, it’s often proved that it wasn’t the best design after the part was made. It’s old technology and only has a place in very specific conditions.

Hot Runner Technology

With the advances made in hot feed systems, it can sometimes be more cost effective to employ the use of hot runner technology.

sprue bushing


With the use of a hot sprue bushing, polymer can be taken directly from the machine nozzle and fed directly onto a part or a cold runner.  The sprue bushing both eliminates scrap and post-process trimming if applicable.  Naturally, eliminating scrap is an automatic cost savings. Many times, scrap cannot be re-used and it is not cost effective.

nozzel for hot drops

Hot drops (both thermal and valve gates) allow many drop locations to be considered for a part based on the requirements of the part design and material being considered.

There are quite a few options with regards to hot runner technology but price and application will need to be considered.

Hot runners can also employ the use of cold sprues. In some cases it is necessary to utilize a cold sprue on the tip of a hot drop because a hot drop may fall onto a part surface with a steep angle, therefore a cold sprue must be designed into the drop tip. Even so, technology now permits hot drops to be designed at an angle so that a cold sprue is no longer required.

In conclusion, sometimes it is practical to design a cold sprue onto a part but there are many factors to consider. Hot runner technology allows for additional options which may incorporate the use of cold sprues.  Typically, a flow analysis will help determine which application is best.

Contact Bozilla Corporation today and let’s discuss how we can successfully contribute to your project., 800-942-0742,

Bozilla Corporation’s Injection molding Team has over 20 years of experience analytically and on the floor. We specialize in optimization, consulting, engineering, troubleshooting and Autodesk Moldflow software training. Additionally, our plastics engineers have a full understanding of polymers and how they influence an injection molded part. Your success is our success. Our skilled Team is focused upon meeting the goals and timelines of our customers.

About the Author:

Chris Czeczuga is a Plastics Engineer, Injection molding expert, Military Veteran and the President of Bozilla Corporation. He has proven success with many Fortune 500 companies throughout the injection molding industry. A graduate from UMass Lowell, he is Expert Certified with Autodesk, has 20+ years of field experience, intimate knowledge of injection molding part, tool and feed system design. Bozilla Corporation’s success is built on providing the highest level of injection molding simulation and consulting advice to businesses who have short lead times, require an efficient, cost-effective molding process, and desire to produce a correct part the first time.