Three cautions for injection molding
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Upfront costs tend to be very high due to design, testing, and tooling requirements. If you are going to produce parts in high volumes you want to make sure you get the design right the first time. That is more complicated than you might think. Getting the design right includes:
- Initial prototype development is typically completed on a 3D printer and often in a different material (such as ABS plastic) than the final part will be constructed in
- Designing an injection mold tool for an initial production round
- Refining any and all details in the injection mold tool prior to mass-production in an injection mold manufacturing plant.
However, APSX-PIM offers the flexibility and opportunity to perform a test quickly and very close to actual production setting. That may include using the actual material and mold design during the initial tests. Plus for low volume production industries such as medical devices, aerospace and jevelry, the APSX-PIM itself an ultimate solution and a solid replacement of conventional injection molding approach.
1 - High tooling costs and long lead times
Tooling is a huge project and only one phase of the entire injection molding process. Before you can produce an injection molded part you first have to design and prototype a part (probably via CNC or 3D printing), then you have to design and prototype a mold tool that can produce replicas of the part in volume. As you can imagine, all of the iteration required to get the tool correct prior to mass production requires both time and money.
2 - Difficult to make changes on tool
If you want to add plastic to the part you can always make the tool cavity larger by cutting away steel or aluminum. But if you are trying to take away plastic you need to decrease the size of the tool cavity by adding aluminum or metal to it.
3 - Uniform wall thickness requirement
While there are no wall thickness restrictions, the goal is usually to choose the thinnest wall possible. Thinner walls use less material which reduces cost and take less time to cool, reducing cycle time. Keeping walls from being too thick is important to prevent inconsistencies in the cooling process resulting in defects like sink marks. A good rule of thumb is to keep walls less than or equal to 4mm thick. The thicker the walls the more material you will use, the longer the cycle time will be and the higher the cost per part will be. Conversely, if wall thickness is any thinner than 1mm or so you might experience trouble filling the mold tool. Designers can compensate for this potentiality by using a material with a higher melt flow index like Nylon which is often suitable for walls as thin as 0.5mm.
4 - Financial Considerations
Entry Cost: Preparing a product for injection molded manufacturing requires a large initial investment.
Production Quantity: Determine the number of parts produced at which injection molding becomes the most cost effective and the number of parts produced at which break even on investment
5 - Design Considerations
Part Design: You want to design the part from day one with injection molding in mind. Simplifying geometry and minimizing the number of parts early on will pay dividends down the road.
Tool Design: Make sure to design the mold tool to prevent defects during production.
The main enemy of any injection molded plastic part is stress. When a plastic resin is melted in preparation for molding, the molecular bonds are temporarily broken due to the heat and force. As the molecules are pushed through each feature, they are forced to bend, turn and distort to form the shape of the part. As the material cools and the molecular bonds re-link the resin into its rigid form, these stresses are in effect locked into the part. Part stresses can cause warpage, sink marks, cracking, premature failure and other problems. You should design your parts with as much consideration for stress reduction as possible. Some ways to do this are by adding smooth transitions between features and using rounds and fillets in possible high stress areas.
The gate type and location selection are also an important factor for proper mold design. Place gates at the heaviest cross section to allow for part packing and minimize voids & sink. Be sure that stress from the gate is in an area that will not affect part function or aesthetics. Gates vary in size and shape depending upon the type of plastic being molded and the size of the part. Large parts will require larger gates to provide a bigger flow of resin to shorten the mold time. Small gates have a better appearance but take longer time to mold or may need to have higher pressure to fill correctly.
6 - Production Considerations
Cycle Time: Small changes can make a big difference and cutting a few seconds from your cycle
Assembly: Design your part to minimize assembly.