Tips for Injection Moulds and Product Design
The mould determines the shape of the part, carries the hot plasticized material from the runner to the mould cavity, vents the trapped air or gas, acts as a cooler to cool the part, and removes the finished part without leaving any marks or damage. The moulds are made of special molding steel, beryllium copper, stainless steel, aluminum brass, zamak materials.
Finished mould surfaces are often polished and coated to prevent wear and facilitate part injection. Moulds are produced by machining, EDM or casting. Mould design, production and workmanship greatly affect part quality and cost.
The injection mould is generally defined by the following criteria.
-Number of observations in the mould
-Construction material (Steel, stainless steel, hardened steel, beryllium copper, chrome plated aluminum and epoxy steel)
– Die line (regular, irregular, two-plate mould, three-plate mould)
-Production method (Machining, hobbing, casting, pressure casting, electro-molding and spark erosion)
-Runner system (cold, hot runner and insulated runner)
-Moulding type eg; edge, limited (Needle tip), submarine, pour hole, ring, aperture, tab, flash, fan and multi.
-Injection method (push pins, stripping pins, stripping layer, screw casting wedge, removable , Hydraulic core arm, pneumatic core arm)
Types of Injection Moulds
There are two types of injection moulds. These are hot runner and cold runner moulds. The runner is a channel that allows the plastic to be transferred from the furnace of the plastic injection machine to the part in the mould. The runner channel is in the hot or cold state in the above mould types.
Cold Runner Mould
The runner is part of the output product and is produced with each press. It is ground and scrapped for reuse. The moulds are simple and less expensive than hot runner moulds. Less maintenance and less skill are required to set up and operate the mould. Also, color change is quite easy. Because all the plastic in the mould is removed in each cycle. Common cold runner mould types are 2- and 3-plate moulds.
The two-plate mould has a parting plane and the mould is split in two. The gating system must be located at these separation planes. Thus, the part can only be molded on its circumference. The three-plate mould has two separation planes, and the mould is divided into three parts. Thus, flexibility is provided in the molding location.
Hot Runner Mould
The runner is located in the mould and is heated to keep the temperature above the melting point of the plastic. Thus, little or no scrap is produced, so no waste and burrs are formed. Hot runner moulds are commonly used in very high mesh moulds. The most important disadvantage of the hot runner is that it is much more expensive and requires costly maintenance and skill.
Types of Hot Runner Dies
In the externally heated type, molten plastic is transferred through a solid manifold and nozzle. Internally heated type (no longer used) plastic flows directly over thin heaters into oversized runners. Externally heated hot runner ducts have the lowest pressure drop of any runner system. This is more amenable to color changes and provides consistent molten plastic in the gating system. There is no point where the material will snag and deteriorate. Therefore, it is suitable for heat sensitive materials. Hot runners are generally more expensive to manufacture and operate, but save money by reducing plastic residue and cycle time.
Tips and Guidelines for Product Design
Wall thickness: Use equal wall thickness on the whole piece. This will improve mold injection and draft times by minimizing shrinkage, shrinkage and residual stresses. If there is no degradation in strength, design the part with minimal wall thickness to ensure rapid cooling, short cycle times and minimal weight. All this will ensure the lowest possible part cost.
Molded hinges: The hinge is the element that allows one to rotate while joining two parts. The concept here is called live hinge, integral hinge or molded hinge. It also consists of a thin-walled section that connects the lid and dry sections integrally injection molded with them in a single operation. The preferred material is polypropylene, which has the best resistance to flexural fatigue.
-Avoid sharp corners.
– Release the back surface of the hinge.
-The usual thickness for a polypropylene hinge is 0.25-0.50 mm.
-Flow should be provided through the hinge and the door position is important.
-Beware of snagging effects, weld lines and overfilling.
-Provide a separate hinged cooling circuit.
– Flex the hinge immediately after removal
Corners: Thoroughly round all corners to reduce stress concentrations and breakage. The inner diameter should be at least as much as the wall thickness.
Taper angle: Design parts to facilitate demoulding by providing a taper in the direction of the eye or core exit. Parts with a textured surface require a larger taper angle to prevent the formation of scratches on the textured surface when the part is removed from the mould.
It should be applied 1-3 degrees for medium matte or sandblasted surfaces and 3-5 degrees for rough surfaces.
Bearings: The wall thickness of the bearings should not exceed 60 percent of the main wall thickness. The base radius should be at least 25 percent of the main wall thickness. It should be supported by ribs attached to adjacent walls or brackets at the base. If the support is to be placed near a corner, it should be insulated using ribs.
Diameter: Outer diameter/inner diameter= 2-3
Thickness: 1/2 – 2/3 nominal wall thickness
Angle height: 2/3 height
Height: Retainer minimum requirements
Taper: 1 degree all over
Diameter ratio should be at least 2. This ratio will reduce the risk of failure.
Feds (Registers): To improve part stiffness, use ribs or angles instead of increasing wall thickness. The application of Feder will help improve the rigidity and geometric integrity of the molded part. This will save part weight, material cost and cycle time cost.
Tips: The thickness of the ribs should be 50-60 percent of the thickness of the walls to which they are attached. The height of the ribs should be at least 3 times the wall thickness. The port should be rounded. Apply a taper angle of at least 0.25 degrees.
Bottom gouges: Minimize the number of external bottom gouges. External bottom gouges require the use of side pieces. This increases the tooling cost. Some simple external undercuts can be molded by repositioning the parting line.
Weld lines: Weld lines are formed when plastic flows in opposite directions and eventually comes together again. Weld lines are formed at the junction of the flow surfaces of the plastic during molding. The weld line region is more prone to fractures and stress cracking.
