Injection Mold Cooling Design
The design of the injection mold cooling system is very important. The cooling time takes up 70% to 80% of injection molding cycle, a well-designed cooling system can shorten the molding time and improve the productivity magnificently. Poor design of cooling system will extend molding time, increase production cost, and the injection mold temperature has great influence to the mold shrinkage, dimensional stability, deformation, internal stress and surface quality.
So what are the factors that matter to the cooling effective?
Part with thicker wall would need longer cooling time. Generally, the cooling time is approximately proportional to the square and the thickness of plastic parts. If possible, propose to the part designer to minimum the wall thickness.
The higher the thermal conductivity of the injection mold steel, the better for heat transferring, the shorter cooling time needs. In practice, injection mold shop usually copper to replace steel on where the cooling line is not possible to do.
Cooling line layout
The closer the mold cavity goes to the cooling pipes, the greater the diameter of cooling pipes, the better the cooling effect, the shorter the cooling time is going to be. The designer should look out for all the possible to get maximum cooling effect.
The nature of the coolant could be different, usually used coolant are oil and water. Viscosity and thermal conductivity of the coolant also affect the heat conduction effect of the plastic injection mold. The lower cooling fluid viscosity, the higher the thermal conductivity, the lower the temperature is, the better cooling effect.
Cooling system design rules:
• Ensure cooling efficiency, achieve shortest the cooling line meanwhile get quality parts.
• Ensure uniform cooling to avoid deformation.
• Ease of manufacturing.
Some examples of injection mold cooling design
If possible, the number of cooling channels should be as many as possible, diameter of the cooling channel should be design as large as possible, cooling speed of A is faster than B as figure below. Diameter of cooling channel usually are 6-12mm.
Cooling channels layout must be reasonable. When the wall thickness of part is uniform, the distance of each channel to the surface of parts should be even, which means the layout of channels should follow the actual geometry of the part, see figure A. When the thickness of the part is un-uniform, then thicker wall area need more cooling, see figure B, the injection mold cooling channel can be closer to the part to enhance the cooling effect.
Usually temperature of the sprue gate area are highest, so the cooling start from there would achieve the best cooling effective, see figure below.
Re-think cooling, that is heat removal, as "Thermal Management" and not simply "Cooling". The engineering of the Thermal System is not trivial but is often over looked. This phase of the process cycle is the longest accounting for 70-80% or more of the time. This means there is a large financial incentive to reduce this and increase profit. However, the traditional way of treating cooling as less critical discounts that dimensional stability and quality is driven during this phase. Removing parts early leads to many issues so the balance is finding the best time and the best method to optimize cooling.
Look at cooling as "The goal of cooling is to remove heat from the part in an advantageous way to result in a low stressed part with the best dimensional and physical properties". Often this is treated as an even rate while in truth the cooling may need to be differential to pull the heat from difficult areas of the part as to allow for a steady state result. This requires a more thorough engineering approach including thermocouples to measure the thermal performance of the injection mold, using water circuits and manifolds to control temperatures and, perhaps, tool materials with tailored thermal properties. This list is not inclusive of a fully engineered thermal system but is a thought starter. Using this approach along with good engineering practice will go a long way of moving from designing injection molds (rule of thumb) and engineering tools (using solid practices) to getting efficient tools, improving processing margins and getting better profits.
Venting is a process that is used to remove trapped air from the closed mold and volatile gases from the processed molten plastic. Without venting, the trapped air will compress as plastic tries to force it out of the mold and the air will ignite, burning the surrounding plastic and causing charred areas on the molded part. Trapped air also keeps the plastic from filling in those areas of the cavity where the air is trapped so a non-filled (or short) part is molded. Volatile gases will be absorbed by the plastic and will cause voids, blisters, bubbles, and a variety of other defects.
The concept of venting is simple: provide many pathways to allow trapped air and volatile gases to escape from the mold quickly and cleanly. The pathways should lead directly from the edge of the cavity image of the mold, or through ejector and/or core pin clearance holes, to the outside atmosphere surrounding the mold. These pathways need to be deep enough to let air and gases out easily, but not deep enough to allow the molten plastic to escape through them.
Venting should be present on every mold and the vents should be inspected periodically to make sure they are not blocked due to scale buildup. The scale can build up especially if the vents are not properly designed. The correct design is discussed in the next section.
VENT DIMENSIONS FOR COMMON MATERIALS
Depending on the type of plastic being molded, and whether it is stiff or easy flow, the vent depths should range from 0.0005'' to 0.0020'' in depth. Table 1 gives average depths for some common materials.
MATERIALVENT DEPTH (in.)ABS0.002Acetal0.0007Acrylic0.002Cellulose Acetate0.001Cell. Ace. Butyrate0.001Ionomer0.0007Nylon 6/60.0005Polycarbonate0.002Polyethylene0.001Polypropylene0.001Polyphenylene Oxide0.002Polyphenylene Sulfide0.0005Polysulfone0.001Polystyrene0.001PVC Rigid0.002Pvc Flexible0.0015
Table 1 Recommended Vent Depths
The vent acts like a window in a wall. When the mold is closed the vent appears as a small tunnel going from the cavity to the edge of the mold. The dimension nomenclatures for the vent are shown in Figure 1.
Figure 1 Typical Vent Dimensions.
There are three major dimensions for the vent. First is ``D'', the vent depth, already determined by the chart shown in Table 1.
Then, comes ``W'', the vent width. This can be anywhere from 1/8 inch wide or more, and a common width is 1/4 inch. There is no maximum width to a vent, but to be practical it should be somewhere between 1/8 and 1/2 inch in width.
The final dimension is ``L,'' which stands for land. This dimension should be a minimum of 0.030 inch and a maximum of 1/8 inch. If it is too short, the remaining steel is too weak and will not hold up. If it is too long, the tunnel shape of the vent is too great and the hot gases and trapped air will condense in the tunnel as they travel through the opening, and block the venting action. This will not occur in the first 1/8 inch of travel. Therefore, we make the ``L'' dimension a maximum of 1/8 inch and then open up the depth and/or width of the vent to allow the condensed gases to deposit beyond the vent itself.
HOW MANY VENTS ARE NEEDED?
The simple answer is that you cannot have too many vents. The more detailed answer is to have at least 30% of the perimeter of the cavity image in vents, equally spaced around that perimeter, as shown in Figure 2.
Figure 2 How many vents?
Note that the part is small. It measures 2-1/2'' per side. By measuring the perimeter of the part, we add up the sides to get a total of 10 inches. If 30% of that should be in vents, we need a total of three inches worth of vents.
If we use 1/4 inch wide vents, we end up with a total of 12 vents for this part. They can be equally spaced as shown, or begin by placing a vent in each of the four corners, one directly opposite the gate, and the others equally spaced in the remaining areas.
Another, more conservative approach to venting states that there should be a vent at least every inch along the perimeter of the part cavity. In that case, there would be at least 10 vents for the part in Figure 2.
Do not forget to vent the runner. Any air or gas that is trapped in the runner gets pushed into the cavity and must be removed from there. One of the big advantages of venting is that, if properly done, it allows gas and air to get out quickly and helps the plastic to inject faster and at lower injection pressures, thereby reducing stress and speeding up cycle times.
The function of the mold feed system is to make the high temperature melt under high pressure go into the mold cavity to fulfill that. The style and quantity of gate and feed style often determine the specifications of plastic mold base. Whether the design of the gate system is reasonable will directly affect the appearance, internal quality, dimension precision and molding cycle of the molded plastic parts.
Concept and classification of feed system
The feed system of the mold is a section of the melt flowing passage from the nozzle of the injection molding machine to the cavity; it can be divided into two main types: cold runner system and hot runner system. Ordinary the cold runner system is also divided into the side gate system and point gate system.
The gate system is composed of the main runner, sub-branch runner, the cold material well and the gate.
The hot runner system has no main runner and sub-branch runner. The melt passes through the manifold plate and the hot nozzle directly from the gate into the cavity.
Design principle of feed system
The design of feed system should follow the principles.
(1) Ensure the appearance quality of the products.Any gate will leave traces on the surface of the product, thus it will affect the surface quality. In order to not affect the appearance of the product, design should try to set the gate in the invisible parts of the product. If it is impossible, the gate should be easily removed, and after the removal, the left traces should be the minimum.
(2) Ensure the quality of the product.The style and quantity of the gate should be reasonable, which need ensure that the plastic melt quickly fill the cavity and the pressure and heat loss are reduced. The internal organization of products is fine.
The design of the gate system should prevent the defects of the products, like the shrinkage mark, short filling, flash, poor weld line, residual stress, deformation, uneven shrinkage, jetting, drooling, resin degradation and other defects.
The gate system should be able to smoothly guide the molten plastic to fill cavity in every corner, so that the gas can be discharged, avoiding the bubbles by trapped air.
The feed system should collect the melt material of the low temperature, prevent its entering the cavity and affect the quality of the products.
The gate and runner system are arranged balanced possibly. Melt plastic can be evenly filled in each cavity, so that the shrinkage rate of each cavity is uniform. The good feed system design could improve the size accuracy of the plastic parts, and ensure the injection part to be interchangeable.
(3) The minimum flow resistance.Flow path of the melt plastic should be as short as possible, turning should be reduced and the cross-sectional area of the runner should be reasonably. the appropriate cross-sectional area should be small. The reason is; increasing the small to the big is simple, but the contrary is hard;
The small runner reduces the proportion of waste material.;
The circular melt can fill the cavity in the shortest time to shorten the molding cycle and improve the productivity of the mold;
The less air in the feed system would reduce the venting burden of the plastic mold
If The temperature and pressure loss of the melt in the flow channel are small, the molding quality of the product is easy to guarantee.
(4)Not affect the automatic production.If the mold is designed for the automatic production, the feed system (runner and gate) should be able to fall off automatically.
Contents and steps of the feed system design
The design procedure and content are as follows.
(1) The type of the gate system. According to the structure, shape and size of the product, they would decide its filling process, like the side gate system, the point gate system or the system without runner, and then determine the adoption of mold base.
(2) The design of the gate.According to the structure, size and appearance of the product, determine the form, location, quantity and the size of the gate.
(3) The dimensions and locations of the main runner
(4) Design of branch runner.According to the structure, size and variety of the products, the shape and cross-section of the branch runner are determined.
(5) Design of assistant runner.According to the follow-up process or the structure of the product, determine whether to design the assistant runner, and its Shape and size:
(6) Design of cold material well.According to the length of the sub runner and the structure of the product, determine the location and size of the cold material well.
Guiding system of injection mold works as guides to ensure precise mechanic movements. Generally the standard mold base all have guiding design, like guide pins between cavity side and core side, mold designers can just choose the proper mold base. Typical mold guiding system include, guiding between A/B plates, guiding for stripper plates, guiding for runner plates, guiding for ejector pins. All this guiding system can be customized when you order the mold base.
But when the parts have particular structure and require high precision, this customized guiding system might not be enough, the guiding pins could not perform as well as before after a long time running, so injection mold designers should consider special guiding design besides these customized guiding design on the mold base.
Precision guiding components have been standardized, such as tapered dowel pins, side lock, etc. some precision guided positioning device must be designed according to the specific structure of the injection mold.