How to Minimize Ejector Pin Marks

What Are Ejector Marks In Injection Molding?

Ejector pins are used to eject the moulded part from the mould. In some cases, they can leave marks on the part surface. Ejector pin marks often occur because the ejector pin applies force to the part during the ejecting process, which will cause localised indentations or slight damage. This phenomenon can be the uneven surface or dull, white, shadow marks at the pin locations and their opposite side areas.

Shallow Indentation Ejector Pin Marks: 

These marks usually appear as pin-shaped indentations on the surface where the plastic part contacts with ejector pin. But this kind of ejector pin mark usually has no influence, because they often show on the obscured areas when the parts are assembled.

Whitening Ejector Pin Marks: 

In this situation, the color of ejector pin area may lighten, forming white or halo-like marks. These marks can show at the contact surface of the ejector pin and part, it can show at the opposite surface as well. The main reason for these marks is that there is localised ejection pressure or temperature differentials between the area and the surrounding material.

Impact on Product Quality

Product appearance: ejector pin marks will cause uneven surfaces or white spots, or indentations.

Product performance: the internal stresses in the area affected by ejector pins will increase and improve the risk of material fracture. The fracture could appear during the use phase of the product.

Causes and Solutions for Ejector Pin Marks

First, check if the part’s wall thickness is too thin, can’t withstand the ejection force. Confirm this defect occurs continuously or regularly, then adjust the injection molding parameter accordingly. The main influencing factor is the ejection velocity, if the ejection speed is too fast, it will increase the adhesion between the molded part and the mold. Additionally, we need to verify if there is whitening still occurs when there is no ejector action. If whitening still happens, it shows that the reason is that marks are caused by the adhesion of the part’s leading edge and the mold surface.

1 Product Design Factor

• Improper design, like too many ribs or too thin wall thickness.
• Draft angle too small.

Solutions:

• Maintain uniform and consistent wall thickness according to the industry standards.
• Depending on the raw material and product structure, products with simple structure should keep the wall thickness at around 2.5mm, for products with complex structure which could have many ribs, their wall thickness should be at around 2.8mm.
• Increase the demolding angle.

2 Material Factor

Inappropriate selection of raw material, or lack of necessary additives like lubricants.

Solutions:

Change the material or add a proper amount of lubricant to improve the injection molding performance.

3 Mold Factor

3.1 Runner

Narrow flow channels, excessively long gates, or sharp bends within the runner all increase the flow resistance of the molten material and affect the adjustment of process parameters. This is one of the typical causes of ejector pin marks.

Solutions:

Optimise the runner layout or appropriately increase the runner dimensions.

3.2 Gate

• Improper design of gate size, shape, position or amount will affect injection molding performance. If the gate is too small, the melt flow resistance will be too large, causing orientation stresses. If the ejector pin location is too close to gate, the injection pressure of this area will be inclusively high. Adjusting the gate size properly will be helpful to decrease the localised pressure and prevent related defects.
• When ejector pin marks appear on the product, appropriately increase the gate and runner size to alleviate excessive injection pressure and other parameters caused by filling difficulties (Note: Gate and runner dimensions should only be enlarged when injection parameters are excessive, otherwise may cause other serious issues; venting should be provided at the runner end).
• Internal stresses are highest near the gate. Therefore, gates should be positioned where filling is easy to acheive and as far as possible from ejector pins. Furthermore, ensure uniform material flow to achieve balanced stress distribution.

Solutions:

• Avoid using excessively small gates or overly long runners to minimise pressure loss in the melt. 
• Attention must be paid to the distance between the ejector pins and the gate to prevent localised overpressure that could cause defects in the ejector pin area.

3.3 Ejector Pin

If the ejector pin is not designed properly, such as the ejector type, arrangement, dimensions, position, or quantity is not correct, it can cause defects in molded parts. If the ejector pin is located in the weak structure area, it is more likely to have deformation during the ejecting process.

Solutions:

Add quantity of ejector pins or change the ejector pin to larger ones, adjust the location when it is necessary.

The design principle is to make sure ejector pin is even stressed during the ejection and ejector pin size is proper. Enhance the ejection for areas where parts are hard to demold, which are the areas with deep ribs, and the ejector should be designed at the rib location in most cases. The parts are easy to deform due to the high injection molding pressure when ejector pins are too small or ejector pins are too close to the gate, the ejector pins are spring back when the mold opens and leading to ejector pin marks.

3.4 The Smoothness of the Mold Surface

The mold core is not polished enough.

Solutions:

Improve the polish precision of the mold core.

3.5 Draft Angle

If the draft angle is not enough, the demolding will be difficult.

Solutions:

Increase the draft angle, to decrease the ejection pressure needed, and parts will be easier to demold.

4 Cavity Cooling

The cooling is not average inside the mold cavity. Especially in the area where the parts is thick, if it is ejected before the material is not all cooled and solidified, the part will be very easy to deform.

Solutions:

Optimize the cooling channels, so that the temperature of all mold areas will be uniform.

Make the part fully and uniformly cooled by controlling the mold temperature and extending the cooling time properly.

5 Uncooled Ejector Pin

Due to the lack of cooling channels and the assembling gap being too large caused by the lack of manufacturing precision, the cooling effect will not be enough. The temperature differentials between the ejector pins and the mold core surrounding areas, will lead to the ejector pins on the molded part surface.

Solutions:

Optimize the cooling time to ensure enough cooling effect.

6 Vacuum

The part is in a nearly vacuum mold cavity. 

Solutions:

The venting system design is an important part of injection mold design, if the venting is unimpeded, the plastic material filling will be smooth, and relatively, the injection molding parameter will be easier to control.

7 Excessive Injection and Holding Pressure

The ejector pin will move downward or have deformation due to too much injection pressure. Additionally, too much injection pressure will increase the adhesion between the molded part and mold, leading to a greater demand for ejector force.

Solutions:

Reduce the injection pressure and shorten the holding pressure time.

Adjust the runner, gate size, or ejector size.

Another effective way is to replace the ejector pin with an ejector block (such as a lifter-type ejection mechanism). To shorten the length of the ejector pin, thereby reducing deformation caused by ejection force.

8 Thermal Conductivities of Ejector Pin and Mold Steel

This factor has an obvious influence on high-gross parts. The part surface has different brightness due to the temperature difference. Mainly caused by the ejector pin material and mold core material different thermal conductivities. Also, the necessary gaps and lubricant between the ejector and the mold core contribute to this phenomenon.

Solutions:

• Add material (0.05–0.20 mm) at the ejector-pin location. It can create slight turbulence as the melt flows past the ejector-pin area, which reduces molecular alignment and tensile stress, thereby lowering the stress concentration that appears after cooling.
• Apply a textured or etched finish on the ejector-pin surface. Help reduce visible stress marks on high-gloss parts, using the same principle as above — disrupting flow orientation and reducing localized stress contrast.

9 More Tips to Reduce Ejector Pin Marks

• Use mold flow analysis to predict the injection molding pressure and optimize gate design. However, the specific ejector pin layout location mainly relies on designer’s experience.
• The gap between ejector pin hole and ejector pin should be around 0.03mm, and the straightness is strictly required. This is to make sure good thermal conductivity between ejector pin and mold core, and to reduce the elastic deformation when the ejector is under pressure.
• Control the quantity of melt inside the mold cavity.
• Optimize the design of the feeding system, make sure it has a good filling effect, reduce the injection spped meanwhile.
• Control the ribs and bosses’ height and quantity, to reduce the difficulty of demolding and ejection.
• The mold cavity surface should be as smooth as possible, to improve demolding performance and reduce ejection pin marks.
• Injection moulding process conditions: Due to the inherent characteristics of the molding process, ejector marks are unavoidable in injection-molded products. However, with appropriate control of process parameters, their occurrence can be minimised. Factors influencing ejector pin marks include mold temperature, injection speed, injection pressure, holding pressure, injection time, holding time, and cooling time. Among these, temperature, pressure, speed, and time are primary factors in the plastic injection molding process.
• The holding pressure, holding time, and cycle transition time need to be well controlled, as they significantly influence the formation of ejector pin marks. Reducing the holding pressure and shortening the holding time can help decrease orientation stress.
• Hiring an experienced injection molding manufacturer can help deal with these challenges. They can offer optimized advice and use high quality material and machines.

Project Evidence: Ejection Review Before Tooling

A de-identified magnetic-holder DFM file records proposed ejection locations together with draft and wall geometry. Later sample feedback was used to review visible imperfections and correction priorities.

The image is from an actual project file. Customer identity and commercial details are removed.