Injection mold cooling system design

Definition of injection mold cooling system:

Injection mold cooling system: AKA. injection mold temperature control system

Heat or cool the mold to keep its temperature within a reasonable range.

-Mold cooling medium: water, oil, beryllium copper (BeCu) and air, etc.;

-Mold heating methods: hot water, steam, hot oil and heating rod, etc.

injection mold cooling system

Influence of Mold Temperature on Different Plastics

  • For plastic materials with great fluidity (PE, PP, HIPS, and ABS, etc.), lowering the mold temperature helps reduce stress cracking (the mold temperature is usually around 60°C);
  • For plastic materials with poor fluidity (PC, PPO and PSF, etc.), raising the mold temperature helps reduce the internal stress of the plastic product (the mold temperature usually ranges between 80°C and 120°C).

Influence of mold temperature on quality of molded plastic products

  • Temperature too high: The plastic products deform greatly after mold release, and it is easy to cause flashing and sticking;
  • Temperature too low: Leading to poor melt fluidity, as well as such surface defects as silver streaks, flow lines, and underfilling;
  • Uneven temperature: The plastic products shrink unevenly, resulting in warping deformation.

Mold temperature directly affects the injection cycle  

  • Injection mold cooling time takes up about 80% of the injection cycle.

injection mold cooling line

Ways to improve mold temperature control

  • Appropriate size of cooling lines: diameter 5-13mm (3/16″-1/2″).
  • Select mold materials with high thermal conductivity.
  • Reasonable plastic product design.
  • Proper cooling circuit.
  • Enhance the cooling of the thick areas of a plastic product.
  • Fast and slow cooling.
  • Strictly control the temperature difference between the coolant outlet and inlet.

Key considerations for injection mold cooling channel design

1.Which is more important, cooling or ejection?

2.Try to keep the thermal balance of the mold, so that the temperature is uniform in each part of the mold.

cooling in plastic injection mold

3.A parallel cooling channel is not preferred

injection mold cooling in types of parallel and series

 Location of cooling lines:

1.Try to keep a consistent distance between the coolant and the filled plastic in the cavity, 10-15mm is preferred. The center distance of the coolant is about 5D.mold cooling location

2.The cooling lines should not be close to locations where the melt flows finally meet;

3.Prevent the cooling lines from interfering with other mechanisms in the mold, and maintain a steel part of 3mm in the middle;

4.The coolant for cavity insert should be as close as possible to the filled plastic, and that of the core insert should be set as far as possible to the outer edge. When the mold cavity / core is too big, the coolant must be in contact with it.

5.For the BeCu mold, coolant may go straightly between plate A and plate B.

Coolant channel length design 

The longer the coolant channel, the more difficult it is to process and the worse the cooling effect. The number of cooling line elbows should not exceed five.

The distance between the hoses should not be less than 30mm

injection mold cooling chanel location

O-ring design

1.When the cooling channel passes through two inserts, a O-ring is added in the middle;

2.Try to avoid wear and shear on O-rings during assembly.

injection mold cooling and o-ring

Empirical identification of injection mold cooling line diameter:

Mold width < 200mm: diameter 5-6mm (or φ3/16″-1/4″);

Mold width between 200 to 400mm: diameter 6-8mm (or 1/4″ – 5/16″);

Mold width between 400 to 500mm: diameter 8-10mm (or 5/16″-3/8″)

Mold width > 500mm: diameter 10-13mm (or 3/8″-1/2″)

injection mold cooling channel diameter

Mold core cooling

1.Core diameter ≤ 10MM: natural cooling;

2.Core diameter between 10 to 15mm: cooling with inlaid BeCu;

3.Core diameter between 15 to 25mm: jet cooling system;

4.Core diameter 25-40MM: cooling bladder + spacer

injection mold cooling special types

5.When mold core diameter is greater than 40mm and the height is less than 40mm, it is inconvenient to guide the coolant through to the center, so cooling from the lower surface is recommended.

6.When mold core diameter is greater than 25mm, cooling from the outer side can be adopted when it is inconvenient to guide the coolant through to the center.

injection mold cooling design

 

 

The texture for plastic injection mold

While our everyday life is filled with more and more plastic products, people start to be aware that they do not want them to look like “plastic” products. Therefore, the plastic injection molds decorated with the texture process are more welcomed because they cater to people’s interests.

texture plastic cover

Purpose of Texture

(1)Improve product appearance. The texture process is able to camouflage part of the shrinkage, welding line, parting line and steps of slider, etc.

(2)Product surface strength can be improved via texturing and sandblasting.

(3)Improve the look and feel of plastic products, to allow the creation of diversified and/or brand-new product designs.

 

Principle of Texturing
Allow chemical agents (e.g. sulfuric acid and nitric acid, etc.) to chemically react with mold steels, and at the same time control the reaction process to obtain various desired effects.

 

Process of Texturing
Mold Preparation- Filmmaking – Film Application – Exposure to Light – Corrosion – Cleansing – Post Treatment

 

Categories of Texturing
Sand pattern, satin pattern, leather pattern, stone pattern, geometric pattern, HANDS and HN3D, etc.

 mold texture pattern

Procedure of Texturing

  1. Cleansing: Clean the mold cavity surface, to remove surface oil/grease.
  2. Sealing: Apply adhesive paper or corrosion resistant coating to the cavity surface that does not need to be textured, so as to prevent corrosion. This is the most time consuming step, during which the 3 commonly used sealing materials include: Thick adhesive paper, to cover the majority part of the cavity surface; thin adhesive paper, to seal the details; and corrosion resistant coating, to cover the area that adhesive paper fails to cover, e.g. complicated curvy surfaces.
  3. Drying: Dry the anti-corrosion coating.
  4. Surface treatment: Carefully wipe the cavity surface to be textured using absorbent cotton, to make it free from any dirt, thus ensuring the texturing effect.
  5. Texturing: Apply a coating to the cavity surface to be textured and then soak it in the corrosive fluid. During this process, attention should be paid to the texturing status. Repeated soaking is required to get the desired textures.
  6. Sandblasting: Sandblasting serves 2 purposes: A). To remove the residue liquid on the cavity surface after cleansing, with ammonia and pressure washer; B). To tune the gloss of the texture; different levels of gloss can be achieved by using different sands and different pressure levels.
  7. Post treatment: Cleanse the cavity surface and apply rust protection agent before delivering the mold parts back to the mold manufacturer.
  8. mold texturing process

Pre-texturing Requirements on Molds

The pre-texturing treatment of a mold plays an important role in defining the final texturing effects. As a result, every detail should carefully considered:

  1. Requirement on the draft angle (lower than 500mm): At least 1 degree for each 13μm of texture finish depth (excluding special textures).
  1. Polishing Requirements:
  • Apply 1,200+ sand paper for depth of around 5μm
  • Apply 1,000 sand paper for depth of around 10μm
  • Apply 800 sand paper for depth of around 25μm
  • Apply 600 sand paper for depth of around 50μm
  1. Parting line treatment: A 0.2 – 1mm margin is suggested; chrome plating is required after texturing, and it is also suggested to deepen the texture by 10μm.
  1. Pre-texturing requirements on mold surface:
  • No machining marks
  • No welding marks
  • No polishing marks
  • No EDM marks
  • Smooth mold surface
  • Mold surface allows texturing

 

Common Post-texturing Problems

  • Due to the fact that the mold cavity surface is roughened after texturing, the most common problems like scratches and stickiness to the cavity may arise. In some areas, the originally small draft angle will be made smaller after texturing, or even resulting in a undercut sometimes, so scratches are often caused. During the ejection process, ejector marks tend to appear due to unfavorable mold release, thus greatly affecting the part appearance.
  • To resolve the problem of scratches and ensure smooth mold release, the textured surface usually needs to be sandblasted to reduce the texture depth and at the same time eliminate the acute angles caused by texturing. In the practical production scenario, it is very difficult to resolve the mold release problem by adjusting injection parameters, so release agent is usually applied to the textured surface to facilitate production. From the perspective of mold, the situation may be improved by increasing the draft angle in the scratched surface area/increasing the number of ejector pins.

 

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Why the venting is so important for a plastic injection mold?

The injection mold venting slot serves two purposes: 1. Expel air from the plastic injection mold cavity during the injection process of the molten plastic material; 2. Get rid of the various gases produced during injection molding processing. The setup of venting slots is of great importance, especially for thin-walled products or the locations far away from the gate. In addition, close attention should also be paid to venting slots for the production of small-size or precision parts, because they are able to protect the products from surface burns, insufficient filling, as well as other defects.

mold venting systemmold venting system

So, what is sufficient air venting for plastic injection molding? Generally speaking, if no burn marks are left on the product at the highest molten plastic injection speed, then the venting effect of the mold cavity will be considered sufficient.

 

Venting Methods:

There are lots of ways to vent the mold cavity, but each of them has to guarantee that: while performing venting functions, the size of the vent slot needs to be able to prevent material from entering the slot, as well as clogging at the same time. However, if there are too many slots, it will do more harm than good, because if the clamping force against the mold cavity parting surface area without vent slots is too high, the cavity material will be prone to cracks, which is very dangerous. In addition to being designed on the parting surface of the mold cavity, the vent slot can also be machined in the end section of the runner system. The clearance around the ejector can also act as a way to let the trapped air out. If the height, width and position of the vent are not appropriately defined, flash will be caused, thus affecting product aesthetics and precision. As a result, the clearance design should be able to prevent flash from occurring around the ejector pin. In particular, it is worth noting that molded parts like gears expect no flash at all. Therefore, the following venting methods should be employed:

  1. Completely eliminate the air in the runner;
  2. Apply peening treatment to the parting surface with the 200# silicon carbide abrasive, and open vent slots in the end section of the runner system, mainly referring to machining slots in the end section of the sub-channel, of which the width should be equal to that of the sub-channel while the height may vary from material from material.

mold venting system

Design Approach:

Based on the years of injection mold design and mold trial experience, this article is aimed to generally explain the design principles of several mold venting types. For parts with complicated geometric shapes, the vent slot positions should be identified after several mold trials. If a mold design adopts the integrated structure, poor ventilation will be its biggest disadvantage. So, for molds with integrated cavity and core, the following venting methods can be adopted:

mold venting system

(1) Make use of the slot or insert location in the mold cavity;

(2) Make use of the lateral insert crevice;

(3) Machine the local part into the spiral shape;

(4) When it is extremely difficult to expel the air out of the mold, an insert should be adopted. If it is not easy to machine a vent slot in some locations of a mold, such as in the corners, the insert molding process may be appropriately applied on condition that product appearance and precision are not affected. This method not only helps with venting, but is also able to lower the difficulty level for machining, and convenient for maintenance, too.

mold venting system

Vent Slot Design Dimensions:

The width of the vent slot ranges from 1.5 to 6mm, while the depth design should be able guarantee that the plastic material will not get into the slot to cause flash. Its value is dependent on the viscosity of the molten plastic, but usually its applicable range is from 0.013 to 0.05mm.

mold venting depth mold venting depth for common plastics

 

Conclusion:

Appropriately designed venting slots are able to drastically reduce injection pressure, injection time, pressure holding time and clamping force, thus making the plastic injection molding process much easier by improving production efficiency, lowering production costs and saving the energy consumed by machine.

 

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Latch lock

The Functions of a Latch Lock:

 

In the two-step parting, two-step ejection or cavity ejection mechanism, a latch lock unit is always employed to control the opening sequence and stroke of each mold plate.

latch locks for plastic injection mold

Though latch locks perform similar functions, they have a diversity of different structures. So, it is necessary to select the right latch lock for the right circumstance according the characteristics of the latch lock. The several commonly seen latch lock structures are explained as below.

 

1,Daido Latch Lock: 

  1. Pros: The Daido latch lock features a simple and concise structure, and requires little machining of the mold base. It takes up a little space and can be mounted on the mold freely, so the chances of interfering with the other parts on the mold base are minimized. Daido latch lock is able to control the mold opening stroke flexibly – a stop block can be mounted on the mold base to realize opening stroke control. In addition, it is inexpensive.
  2. Cons: Daido latch lock uses a spring to press the steel ball into contact with the control lever to realize mold closing. As a result, it is not easy to control the clamping force. There are cases of unhooking before reaching the required stroke. It is not so reliable, nor is it easy to adjust.
  3. Applicable Scenario: This type of latch lock is suitable for medium-/small-sized plastic injection molds, which do not require a large clamping force or a high reliability. Usually, it is not selected due to its poor reliability. See figure I: Used for the 2-step ejection of the ejector plate.

Daido Latch Lock

Note: The Daido latch lock uses a spring to press the steel ball into contact with the control lever to realize mold closing. As a result, it is not easy to control the clamping force. It is only applicable for medium-/small-sized molds, which do not require a large clamping force.

2,HASCO Latch Lock: 

  1. Pros: Sold at a moderate price, the HASCO latch lock boasts diversified specifications and a wide range of available options; the mechanical closing mechanism is secure, reliable, convenient to mount and requires little machining of the mold base. The lengths of its control lever and hook can be adjusted as per actual conditions.
  2. Cons: When mounting, machining is needed after the location of pin holes and screw holes are determined. It is not easy to change the mold opening stroke after it is designed. Also, the stroke range is limited by the length of the control lever.
  3. Applicable Scenario: The mechanical closing mechanism is secure and reliable, so it is suitable for various large-/medium-/small-sized molds.

 

HASCO Latch Lock

Note: It is not easy to change the mold opening stroke after the stroke is designed, and the pin hole and screw hole locations are machined. Also, the stroke range is limited by the length of the control lever.

3,STRACK Latch Lock:

 

  1. Pros: The mechanical closing mechanism is secure and reliable. A wider stroke range can be covered by changing the relative location between the control lever and the hook. After the latch lock is mounted, the mold opening stroke is still able to be adjusted by moving the adjusting slider on the control lever.
  2. Cons: Occupy a larger space; complicated mounting process; require a lot of machining of the mold base; high price.
  3. Applicable Scenario: Suitable for large-size molds, and able to cover a wide opening stroke range. Usually, it is not selected due to its high price, unless specified by the client.

STRACK Latch Lock

Note: After the latch lock is mounted, the mold opening stroke is still able to be adjusted by moving the adjusting slider on the control lever.

4,RABOURDIN Latch Lock

  1. Pros: With a large clamping force, the mechanical closing mechanism is secure and reliable. Its biggest benefit is that the mold opening stroke is able to be adjusted by moving the adjusting slider after the latch lock is mounted, so a wider stroke range can be covered.
  2. Cons: Occupy a larger space; complicated mounting process; require a lot of machining of the mold base; high price.

3. Applicable Scenario: Suitable for large-size plastic injection molds, and able to cover a wide opening stroke range. Usually, it is not selected due to its high price, unless specified by the client.

 

RABOURDIN Latch Lock

Note: After the latch lock is mounted, the mold opening stroke is still able to be adjusted by moving the adjusting slider on the control lever.

 

 

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