What causes part sticking (Mold adhesion) or sprue sticking during injection molding

what causes part sticking (mold adhesion)

Table of Contents

Ejection of product from the mold is often overlooked in injection molding. In the ideal case project ejection happens in an instance. It is also the final part of the process. This stage aims to get the product out of the mold. Seems simple enough. To ensure fast and effective product ejection, the ejectors must be well designed. Quite a lot goes into the design of the ejectors. The ejection of products gets considered in the mold design. Also in material selection and product design. The processing conditions also impact the effectiveness of the product release. When product ejection goes wrong a significant amount of time is lost in troubleshooting. While the product and sprue remain stuck not much else can get done in injection molding. The mold gets used in the next cycle to follow so the whole process gets held. This leads to an idle time where the machine is not working and potential profit is getting lost. This article looks at the problem of product and sprue sticking and how to address or avoid it.

Mold Surface

For effective product removal, the mold surface needs to be smooth and well polished. The surface finish of the mold reflects on the product. A rough surface increases the surface area of contact between the mold and the product. At the microscopic level, these layers interlock. The melt fills the gaps between the roughness of the mold surface. This becomes part of the product. So when it comes to pushing the product out of the mold the interlocked protrusions need to get cut. The rougher the surface the more difficult this is. It is the same for the sprue bushing. A sprue bushing with a rough surface will make removal more difficult. 


Use a mold release agent where the plastic-type allows. Ensure the mold surface is well polished. Have a system to polish the mold surface after a specific number of cycles. This adds to process time but well worth the headache it saves down the road. Use chrome plating on the mold surface and polish in the direction of melt flow into the mold.

Image illustration of the smooth and rough mold surface. This is of course an exaggerated illustration. The aim is to explain what attains at the microscopic level.

Wears and Abrasion on Mold or sprue

When processing materials like glass-filled plastics. They tend to cause abrasion on the mold and sprue walls. Composites of plastics with filler particles or fibers also have similar effects. The wearing and abrasions of the mold can also occur as a result of insert clearance being too large. This causes the insert to scratch the surface of the mold. 


Allow only need clearance for inserts. When processing materials that cause abrasion. uUse high-quality tool steel and good polishing. The regular surface repair might be necessary. 

Mold Rigidity

This is where using a strong, high-quality metal for the mold pays off. But if you go for a metal that does not have the strength to handle the clamping pressure required. This is where you might realize the fault in that decision. The mold must have the rigidity to withstand the forces exerted during the opening of the mold. The yield point of the mold must exceed the stress exerted. If otherwise, the mold loses its shape and opening becomes difficult. As the mold loses its shape, the fit with the product gets altered. This deviates from the original design. The alignment ensures easy removal and factors like draft angles get altered. This results in the sticking of the product hence mold adhesion.


Use high-quality tool steel in particular when doing high volume production. Where it is for small volume or prototypes and lower quality metals like aluminum gets used. Then this calls for optimal design to get the most rigidity from the material. A dial that indicates if there has been deformation of the mold. While doing mold testing prevents this from occurring. Lowering the injection pressure also helps. You need enough pressure to dimensional stability and even shrinkage. But not so high that it damages the mold. Another option is to use a frame to support the mold. The frame gives enough rigidity to the mold.


The previous article on www.cavitymold.com discusses the draft angle in detail. The draft angle is an important factor in mold design. It is an inclination incorporated into the mold and hence product design. It plays an important role in aiding product release from the mold. Where there is no draft angle or it is not enough, mold adhesion tends to occur. If there is an attempt to remove the product with no draft angle. The product tends to warp or get damaged in other ways. A forceful product removal also costs more time. Improper drafting can also lead to a product getting scratched or damaged. For hard or textured parts, the scratch can alter the surface texture or scratch the mold surface.


Always use enough draft angle in product and hence mold design. The draft angle should get incorporated even at the early design stage.


When the plastic heats to excessive temperature there is a high chance of degradation. Also if there is plastic debris from previous runs with plastics of lower melting point. This can cause degradation in parts near the walls. The degraded plastics tend to stick to the wall. This causes difficulty with part release from the mold. Local hot spots also lead to overheating. This can be a result of bubble formation or improper mixing of melt.


Ensure mold surfaces get proper cleaning and polishing regularly. In particular where different types of plastic get to run in the same machine. Ensure proper mixing and temperature control. 

Gate and Runner Systems

There are quite many things that can go wrong with the gate and runner design and construction. The factors considered when designing a gate includes the location, type, and size of the gate. For runners, the width and length of the runner affect the mold filling. There also needs to be a good match between the parent runner the runners branching off. These go on to fill the different cavities. Where the gates and runners are not well designed this leads to problems. Such problems include flashing or cold sludge and difficulty in a product release.


Balance the rate of filling of the different cavities. This is by carrying out analysis and/or simulations of melt flow within these channels. There should be a diameter difference of at least 0.5mm between the nozzle and the runner. The nozzle is smaller than the runner. The gate should be at least 1mm wider than the nozzle. This ensures the right pressure difference that allows the flow of the melt. The flow pattern that achieves the property filling to avoid mod adhesion.

Excessive Clamping and compacting pressure

The product needs to form under enough pressure. Enough to allow dimensional stability and good filling. But where the pressure gets too much then there isn’t much room between the product and the mold cavity wall. Shrinkage contributes to mold release. But where the pressure is so high this defeats the purpose. The product shrinks but doesn’t leave much gap allowance for release. Similarly, excessive back pressure isn’t good for product release. The product becomes too compacted. Excessive pressure can also alter the temperature. This then leads to not enough cooling. All of which contribute to the product sticking in the mold. 


Reduce back pressure and clamping pressure. Run tests to ensure the right pressure is set.

Ejector System Design

The design of the ejector should get enough consideration. The ejectors exert the push force that aids product release. The size of the individual ejector pin is important. So is their distribution, and total number affect product release. The direction of the push should align with the draft angle and the mold cavity. If the ejector pins cause a bending moment this can cause product sticking. When there is a part misalignment during ejection. This causes the product to get stuck in the mold. The rate of ejection should not be too fast. This can cause damage to the product or bending of the pins. If too slow then enough kinetic energy is not generated to allow product release.


Have as many pins as possible where there is the most resistance to product release. Use pins of the right width. Also, ensure that the ejector pins are in synchronized form. And push the product out without causing bending. Spread the pins out over the widest area as is possible.   

Overpolished Mold cavity

A rough mold surface can lead to product sticking but an over polished surface doesn’t help either. Where the surface is too polished. this can create a vacuum between the mold cavity walls and the product. This is also a result of the electrostatic attraction force getting generated. 


To get out the stuck part pump in the air into the gap using a pipette. To prevent further mold adhesion, reduce the level of polish to moderate. This gets done using surface treatment. Polishing should occur along the melt flow direction. This should occur in a back and forth manner rather than circular.

Finding out What Side of the mold gets Affected 

Often it is not both mold halves that get affected. The fault sometimes lies in only one half of the mold. This can save a fair amount of time. One approach is to open the mold at a slow pace when you detect or suspect mold adhesion. When you find which half the mold sticks to you then want to know what part of that half is sticky. To do this, slowly eject the product and observe. You should see as the mold tries to eject or partly gets ejected what part remains stuck. This lets you know if it’s a vacuum or the plastic gets attached to the wall and well. This makes solving the problem that much quicker.

Finally, check your equipment

The injection molding process relies on sensors and control systems. These help to keep track of the operating conditions. Temperature, pressure, feed rate, and flow rate are some of the measured parameters. Where any of this malfunction, could result in false readings. That or processes deviating from the set point. The sensors and auxiliary devices should get regular checks. These include the heating and water circulation systems. The systems should get calibrated and tested to ensure that the readings are right. Where for example the wrong mold temperature gets measured. This results in the mold opening before complete cooling. This causes the sticking of the product to the mold.  

Some quick hacks

Here we give a couple of quick hacks that can give you a quick diagnosis or solution. When you press a cardboard paper over the sprue. There should be a smooth imprint of the sprue on the cardboard paper. Otherwise where there is unevenness in the outline the nozzle needs changing. Use a different size. Another tip is to use a  “Z” puller that can help with products having undercuts. 


Part sticking occurs for different reasons. It can cause delays in the process or even lead to damage. You can avoid this by taking precautions. From proper process settings to using a well-designed mold. These and several others we have discussed within the article above. For further consultation and services in mold making contact www.cavitymold.com. 

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