Injection Mould Construction

A molding according to the function of each of its parts can be divided into four sections 1. Introductory part of the plastic nozzle into the cavity cavity 2. penunjuang system (support system) 3. demolding system 4. heat transfer system

Mold Base Standard Part

In the manufacture of injection mold, mold base is an integral part, mold maker can make your own mold base or buy a standard mold base, the system of the mold base can be adjusted with the construction standards required, both for the two plate and three plate, stripper plate ejectors, hot runner and mold base for a screw, when the entire standard mold base is not there to meet the new final step is to make a special mold base.

Injection Mould Classification

classification or types of mold injection very depend on what we need to make the plastic parts, because every parts have specific and unique design. when design molds we must see what the influencing factor like geometry, number of cavities, ejection principle, plastic material and shape of part.

Injection Mold Cooling

This section is the most important part of the overall mold cylcle time, because in one cycle time, the process of heat exchange to spend about 70-80% of the total cycle time, thus setting the optimal heat transfer system will greatly affect the quality and cycle time of a product.

Undercut System : Cam, Angular, Lift Cavity, Loose core

cavity and core, and its derivatives when there are undercut on product, design and construction of a good core cavity and in accordance with the requirement could increase the life of tooling itself, reduce material consumption, reduce dependence on maintenance inserts, and can reduce the cost of making the mold so the mold cost per products also declined.

Wednesday, March 24, 2010

Troubleshooting weld lines and meld lines on injection molds

In manufacturing, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "knit" together, or "weld", during the molding process. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part.

A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. Weld lines can be caused by material flowing around holes or inserts in the part, multiple injection gates or variable wall thickness where hesitation or "race tracking" can occur. If the different flow fronts have cooled before meeting, they don't interfuse well and can cause a weakness in the molded part. A line, notch and/or color change can appear.

What Caused Weld or Melt Line?
A meld line occurs when two moving melt fronts converge and flow parallel to each other. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and race track effects.

explanation from http://www.imtechdesign.com/ give brief reason why and when weld line, and melt line will occurs.

a meeting angle, smaller than 135º produces a weld line; greater than 135º, a meld line. Note that the weld line surface mark disappears when the meeting angle reaches 120º to 150º. Normally, weld lines are considered to be of lower quality than meld lines, since relatively less molecular diffusion occurs across a weld line after it is formed. However, there is always an exception to the rule and some materials by nature tend to exhibit more visible weld lines irrespective of the angle. In addition many fillers and additives also highlight weld & meld lines. Glass fibres and metallic pigments are good examples.

Weld lines, melt lines or Knit lines could be caused by different causes:

- Low temperature of molding machine barrel
- Inadequate back pressure
- Injection pressure or injection speed is too low
- Low mold temperature
- Small injection gates and/or runners
- Improper location of injection gate
- Excessive gate land length
- Improper flow rate of injected materials
- Inconsistent process cycle

Weld Line and Physical Properties
A study of the effect that weld lines and orientation have on physical properties of molded parts was undertaken to establish general guidelines for use in part design. an experiment by dupont show that between polymer orientation, Weld Lines. And tensile Strength have Relationship, see graph below.



These results show the importance of gate location and the need for the product
designer to coordinate his efforts with the tool builder. The gate should be located
both to avoid weld lines in high stress areas and to take full advantage of glass fiber reinforcement. Techniques the designer might use to reduce stress level at a weld line include increasing the wall thickness or adding ribs.

Solution
- This can be done by changing the polymer injection location or altering wall thicknesses to set up a different fill time.
- increasing mold temperature, melt temperature, injection rate and pressure.
- Increased mold venting or addition of flow tabs also can help turn a bad weld into
a good weld
- Change the gate positions.
- Change the part thickness.
- Optimize runner system design. Reduce runner dimensions and maintain the same flow rate to use shear heating to increase the melt temperature at the flow front
- Instead, it often helps to increase the injection velocity, decrease the fill time, and increase the shear rate, which can lower the viscosity of the polymer during fill and thereby allow for better chain entanglement and better packing. Increasing pack or hold pressure also helps, as well as having longer pack and hold times. Increasing pack or hold pressure helps to eliminate low-pressure conditions at the weld line. Another tactic to promote more chain entangling at the weld line is to raise the mold temperature 10° C (20° F).

Wednesday, March 3, 2010

Troubleshooting short shot problems on injection mold

The term "short shot" or "short mold" is used to refer to a phenomenon where plastic being injection molded does not reach certain portions of the inside of the die before solidifying. or it means that not enough resin has flowed into the mold to fill it adequately. The main causes for short shot are the mold condition or flowability problems with the resin. The problem is alleviated when a greater volume of resin flows more easily.
To ensure the finished part is of good quality, the part must also be adequately packed with plastic. Therefore the question to ask is not only, ""Will the part fill?"" but also, ""Can a good quality part be made?""


Some of the typical factors affecting this type of problem are as follows:
- Fluidity and viscosity characteristics of the plastic
- mold design (Flow restrictions. Due to channels freezing or inadequate runner design. gate design, bushing construction for ribs and bosses, the presence of venting, .)
- Hesitation and long or complex flow paths.
- Inadequate venting. Back pressure due to unvented air traps can cause a short shot.
- Molding machine performance (Including an empty hopper, blocked feed throat, or a worn non-return (check) valve that causes loss of pressure or volume leakage.)


picture above i copy from UMgabs.com, those check point is very important for troubleshooting short shot problems

Solution
- Eliminate air traps. If air traps do exist, they should be positioned in areas that can be easily vented or ejection pins added so that air can be removed.
- if Resin flows too slowly, Increase temperature in the cylinder. Increase injection speed. If no change in the filling time is noticed after setting of a high injection rate, there is a possibility that the performance of the molding machine is insufficient or that there is excessive pressure loss in the sprues, runners, and gates
- check resin viscosity, Increase the mold temperature.This will decrease the viscosity of the melt, making it easier for the plastic to flow through the part.
- Increase ram speed. This can cause greater shear heating, which decreases the viscosity of the melt, making it easier for the plastic to flow through the part.
- Increase the sectional area of the gate, sprue and runner, and shorten the length. Make the sprue, runner and gate surfaces more slippery.
- Change the gate position, increase the thickness of the mold cavity, modify design to improve flow.
- Change the part geometry. Balance flow paths so they fill in an equal time and an equal pressure. You may need to thicken thin sections, or reduce the complexity of a flow path.
- Use a different material. Select a less viscous material (higher melt flow rate). By choosing a material with a higher melt flow rate, less injection pressure will be required to fill the part.