1.Two or multiple resins were seamlessly integrated and joined through the molding process to improve the performance and properties of a final product . Examples such as flexible grip handles on power tools and kitchen utensils.
2.Overmolding Process Types,Two injection molding processes dominate the manufacture of overmolded products: insert molding and multi-shot(2K) injection molding.
2.1 Insert Molding
The most widely used process is insert molding, where a pre-molded insert is placed into a mold and the TPE(or other resin) is shot directly over it. the insert or substrate part was loaded into second shot by robot or by hand.
such molds were usually ran in a vertical injection machine with one upper mold half and 2 lower mold half to minimize the insert loading time.
2.2 Multiple Material Molding
Multiple material, also known as two-shot (or multi-shot,2k molding), molding requires a special injection molding machine that is equipped with two or more barrels, allowing two (or more) materials to be shot into the same mold during the same molding cycle.
there are 2 basic mold structures based on the overmolded part design:
A Rotating the B mold half: the part shape at the core side was exactly same in both substrate mold and overmold with a different shape at cavity side, the lower mold half(B half) was rotated by 180° to second shot after substrate molding.
Rotating the B side Core Block/Insert:part shape at core side was different in 1st and 2nd shot.
3. Steel used for mold:H13 or 420H.
4. If the inserts are molded and stored prior to the overmolding phase of the process, they should be well-protected from contamination sources like dirt, dust, etc. Skin oil can also affect the TPE adhesion to the insert, so hand protection should be worn when handling the inserts. Mold release should not be used in either the insert or the overmolding processes, as the lubricants can seriously degrade the TPEs bond to the insert.
5.The melt temperature of the TPE must be selected based on the substrate used and target bond level desired in the finished component.
6.Venting depth: 0.0005”.
7. The TPE thickness should be less than or equal to the thickness of the substrate to prevent warpage;especially for long, flat geometries.
8. EDM or machining mark on runner surface permitted to aid in release ;
9. the runner length <8” and full-round runners are best, with the modified trapezoid,the second;
10. the preferred gate:edge gate, hot sprue or Submarine Gate (thru pins not recommended);
11. the texture grade of TPE should be greater that of substrate when both have esthetics surfaces(because of TPE Shrinkage),for example:
substrate :MT11020, TPE:MT11030.
12. No polishing needed where the Overmolding will be processed.
13. The balanced block must be used in all the overmolding tool because of small shut off area/length compared to the overall mold size:
14. It’s desired that the wall thickness of second shot(soft material, such as TPE) should be between 1.3mm and 1.8mm(1.5mm best) to get a better plastics filling and prevent the short shot(<1.3mm) and sink mark(>1.8mm).
15. An balanced mold filling must be considered when deciding the gate size:
15.1 the gate design should be adjustable and can be resized easily after T0 mold trial;
15.2 Gate diameter should be less than Ø1.5mm for a bigger TPE area and Ø0.8mm for a small TPE area
15.3 The “O” size of sprue should be less than Ø2.5mm(the dia. of bigger end should be less than Ø6mm).
16. the balanced part ejection should always be considered to prevent the part damage and part deformation. An ABS or PMMA ejector head insert was preferred whenever possible.
17. the slide must be designed as it can be polished with Cavity block together, that is an additional bolt and screw holes was needed to assembly slide and cavity block together.
18 To get a good steel condition and prevent flash, the shut off length should be over 0.8mm for softer material and 1.00mm for a harder material.
19. the shot weight of 2nd is usually much smaller than the 1st shot, especially, the substrate part has a bigger size, so a compact mold layout/configuration of 2nd is very important considering machine cost, a small machine with enough shot size can decrease the part pcs cost significantly.
20.To prevent the deformation of substrate part and flash, an strong support under the overmolded area should be designed and should not be cleared. the preferred supporting size should be extended the overmolded area by 5mm-10mm.
21.Venting the air trap area, which can be determined by mold flow analysis.
22) Suggestions,such as adding the slot or dot pattern feature to substrate part, should be made to customer if an poor grip(cling) between overmolded resin and substrate part was expected
23) the following shut off style should be always employed if possible in 2nd shot:
24. the side wall of rib and boss feature of substrate part can be cleared by 0.1mm-0.15mm and the the bottom must be supported to stand the injection pressure.
25. the electrode for the shut off land of 2nd shot should be same as that of 1st shot(but in actual EDM machining, an stock of 0.02mm was often left for the tuning afterwards), special care should be given to the deformation of this electrode because of fragile/poor steel condition in any operation.
26. It’s desired to machine the electrode for rib/boss features and the major part appearance area in seperated one, not an oversized electrode encompassing all the feature together in case steel tuning was needed later.
27. the depth dim of key shut off area, boss or main parting line must be carefully checked and confirmed at the end of finish CNC & EDM machining. steel should not be removed in mold fitting stage before the relevant dim was measured and confirmed.
28. Mold fitting : the fitting of substrate should not be started before the fitting of main parting line and steel shut off was completed. A better fitting between a whole pcs of substrate part and steel of both cavity & core block should be got(the clearance between substrate part and steel should be less than 0.05mm, otherwise, the reworking of steel is needed before the fitting of substrate part), and then the substrate part can be cut into small pad with a size of about 25mm*25mm, this small pad will be used for substrate part fitting locally.
29. the securing & position of substrate part in the mold cavity should be carefully planned:
29.1 the substrate part can’t be moved in mold close stage;
29.2 the loading can be completed easily;
30. the overload between steel and substrate should be an appropriate one to prevent the damage of substrate part.
31. for the substrate part with GF additive may has a bigger part shape deformation, adding material to the shut off land of substrate part may help the fitting.
32. the ideal fitting between substrate part and steel should be :
32.1 no damage to substrate part, but a certain overload was applied to it;
32.2 no local part shape damage to the bottom of rib or boss feature;
32.3 a balanced imprint between substrate and steel
33. the fitting of slide should be started upon the completion of fitting between substrate and steel.
34. No polishing to the shut off land is needed(but removing the EDM texture mark may be needed, it must be done by hand using oil stone,no operation of grinder is allowed).