It seems old-fashioned to load anything by hand these days, especially in a high-tech operation like injection molding, which has so many high-precision process controls. However, “hand loads”—mold inserts that are physically inserted into the mold and then removed when the part is finished—are essential for the production of certain parts.
Hand loads are used to create complex geometries like undercuts and threads that can’t be formed in the line of draw. Hand loads are typically preferred for two situations:
- A very low-volume application where it is not practical to spend extra money on automatic lifters, slides, or unscrewing mechanisms
When the part geometry is so complex it cannot be formed using automatic lifters, slides, or unscrewing mechanisms
Injection molding companies that refuse to consider hand loads in these situations will miss out on some simple solutions to complex design challenges—which are also opportunities to exceed client expectations for design and production.
For example, some molders may look at hand loads as an outdated way of doing things. As a result, they have stayed away from using hand loads and therefore have lost the expertise in using them. This is a big mistake, because hand loads have an important role in forming very complex geometries. Their use can also eliminate the need for secondary operations, thereby speeding up the production cycle, reducing production of scrap, decreasing the probability of rework, and overall saving money.
An example is the production of a latching body for an automotive client. The latching body is a very complex part that requires two angled air channels. The company initially expected to drill out the channels as a post-mold operation; instead hand-load inserts were used to create the channels. This eliminated the machining operation, which led to cost reduction and quality improvement.
The following points should be considered when using hand loads:
Always build multiple hand loads to allow for proper cooling between cycles while maintaining production cycle times (this way extras are also available if one gets damaged)
Utilize small rare-earth magnets to hold hand loads in place—this minimizes the chance of damage due to hand loads not being fully seated and accidentally shifting
Use soft steel or hard steel—soft steel (P-20) helps minimize damage if an insert is accidentally closed upon
Utilize a low-pressure mold close to minimize potential damage
For larger hand loads, design in gripping features so the hand load can be held more easily, which reduces dropping and related damage
Build an extraction fixture for hand loads having complex geometry
Hand loads do, however, require extra labor for installation and removal. This must be factored into the cost/benefit analysis, but may also be the only way to meet the customer’s needs.
Another concern is that too many hand-loaded inserts for a partcan result in morevariation in the molding process/cycle times. The molding process depends on consistency—if the assembly of the mold has too much time variation, it can affect part-to-part consistency and possibly result in bad product, high scrap rates, etc. If hand loads do result in delays between cycles, the probability for shorts of filling variations is increased. Every effort must be made with hand loads to keep the molding cycle as consistent as possible.
Remember that hand-loaded inserts are an extension of the mold and must maintain precise temperatures for molding and part consistency. Pre-heating or pre-cooling these hand-loaded inserts to a constant temperature that matches the mold temperature improves consistency of cycle times. It is also a good idea tomakeseveral extra sets so they can cool outside the mold while waiting their turn. Hand loads can be water-cooled or air-cooled. In some cases, heating the hand loads between cycles may be required, which can be done with a heating fixture.
Despite the extra care that is required with hand loads, they can be the best solution for producing complex parts and/or low-volume runs, at lower cost.