Nowadays there are mechanical engineering softwares that help the mold makers and molding professionals to understand to evaluate the part design for plastic injection molding and also check the tool design before cutting the steel. Although the simulation software’s results are not 100% reliable, it saves time and cost by predicting the week area of the design.
Although the results are not reflecting 100% accurate results, it gives an idea to the designers to observe the tool functionality and evaluate their design before cutting the tool. Also, it will reduce development time and save money for companies. Moldflow and Moldex are two examples of software that are very useful to simulate injection molding of plastic.
Below you can see two graphs produced by Mold Flow analysis.
Plastic injection molding presses are classified or rated based on tonnage, or more specifically, the clamping pressure or force. Presses can run in size from less than 5 tons of clamping pressure to over 6000 tons. The higher the press ton rating, the larger the machine.
A machine rated for 60 tons can deliver 60 tons of clamping pressure. This pressure keeps the mold closed during the injection process. Too much or too little pressure can cause quality issues. Too much or too little pressure can also cause flashing, where excess material appears on the part edge.
How to know the required press tonnage?
There are many factors that are taken into consideration when determining the size of the press. The size of the part, the polymer being used and something called the safety factor. The safety factor is an additional numerical percentage buffer that is added to the calculation to help avoid defects in the final part. Some recommend adding 10% to allow for the safety factor. As mentioned earlier, the MFI (Melt Flow Index) of the plastic compound will also impact the pressure needed to produce the part. Many calculations include the platen size as well as the mold and part size, however, to get an estimate of the press size your project will need, we have simplified it even further.
The first purpose of the injection unit for molding a crystalline material is to deliver to the mold the necessary amount of a homogeneous melt ( with no unmelt and no degraded material ). The rules of construction of the injection unit are then dependent on the molding material requirements in term of thermal behavior and heat needed. The first point to take into account for a crystalline material is the thermal stability at melt temperature, to avoid degradation. Then, screw, nozzle, backflow valve, adaptor should be designed to provide efficient melting of crystalline material and delivery of molten polymer to the mold.
Melting behavior is different between plastics are different. The amorphous polymer starts softening just after Tg and presents a continuous change in viscosity. This gives a very large temperature range to operate ( but a large variation of viscosity with temperature). In contrast, the crystalline polymer stays solid up to the melting point and suddenly melts to the liquid phase at a high temperature. This limits the processing range of temperature between unmelt and thermal degradation.
The second factor is the time the material stays at that temperature. For all polymers, the molecules can withstand a certain temperature before degradation can start. Obviously, the acceptable time limit becomes shorter when the temperature is higher. Degradation will result in the generation of gases which cause bubbles in the melt, splays on parts, mold deposit, yellow and brown marks on the parts.
The average residence time (or Hold-Up Time, HUT) in the injection unit is linked to the amount of polymer in the cylinder, the shot weight and the cycle time and can be calculated with the following:
HUT= (Weight of Resin in Cylinder) x Cycle Time / Shot Weight
or
HUT= (Maximum Screw Stroke x 2) x Cycle Time / Current Screw Stroke
Note: Effective screw stroke = distance the screw travels during rotation only
There are over 200 different parameters that must be established and controlled to achieve proper action molding of a plastic part. These parameters fall within four major areas :
4 parameter areas [1] Reference : A guide for injection molders , by D.M. Bryce
To the troubleshooter, all the 4 areas are important, but the pressure and temperature areas are the ones most commonly considered during the troubleshooting process. Based on the requirements of any particular plastic material, the must be sufficient to inject the plastic material and to hold the mold closed. In addition, the temperature of the injected plastic and mold must be correctly maintained.
1 . PRESSURE
Pressure is found primarily in the injection area, but there is also pressure found in the clamp unit of the molding machine.
1.1 Back Pressure
The first pressure to consider is back pressure. This is pressure that is created during t he return action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of screw. This pressure is used for better mixing of the plastic (especially if colors are added to the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of tthe plastic and result in thermally degraded plastic.
The first pressure to consider is back pressure. This is pressure that is created during the returns action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of screw. This pressure is used for better mixing of the plastic (especially if colors are added to the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of the plastic and result in thermally degraded plastic.
1.2 Injection Pressure
The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and the fastest fill rate are the best conditions. However, high pressure will increase molded-in stress. And the stress will be released at some time. There is no question as to its being released, only as to when it will be released. And remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.
The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and the fastest fill rate are the best conditions. However, high pressure will increase molded-in stress. And the stress will be released at some time. There is no question as to its being released, only as to when it will be released. And remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.
The design of the mold will be divided into 2 stages:
1) Preliminary Drawings
The preliminary design is to be submitted within 1 week after receipt of purchase order. The purpose of this design is to determine the general layout and working of the tool to a degree that will allow steel to be purchased.
This stage should include :
Runner system
Basic position of water lines
Clamp slots
Gate locations
Gate size
Support/Pillar locations
Overall size of the mold
and any conditions special to the mold.
Project engineer approval is necessary for this stage.
2) Final Design
Drawings are to be fully detailed as follows:
Details of final gate designs, runner style, layout and style of sprue puller must be shown.
Complete CAD files of the tools to be submitted. ( 2D and 3D)
Cores, cavities, inserts or any other part of the mold which are not standard are to be fully detailed.
Assembly view should be stated the approximate weight of both mold halves.
A material list is required.
Detailed drawings are required for all inserts, slides, lifters, etc., with steel hardness specified.
To summarize, in the mold design process some general requirements have to be considered:
